Document number SANS 1936-2 - Geotechnical Division 96-10 _DSS_ 1936-2.pdf · DRAFT SOUTH AFRICAN STANDARD (DSS): PUBLIC ENQUIRY STAGE Document number SANS 1936-2 Reference 7135/1936-2/SP

DRAFT SOUTH AFRICAN STANDARD (DSS):

PUBLIC ENQUIRY STAGE

Document number SANS 1936-2

Reference 7135/1936-2/SP

Date of circulation 2011-12-20

Closing date 2012-02-21

Number and title: SANS 1936-2: DEVELOPMENT OF DOLOMITE LAND — PART 2: GEOTECHNICAL INVESTIGATIONS AND DETERMINATIONS Remarks:

PLEASE NOTE:

• The technical committee, SABS SC 59P: Construction standards – Geotechnical standards responsible for the preparation of this standard has reached consensus that the attached document should become a South African standard. It is now made available by way of public enquiry to all interested and affected parties for public comment, and to the technical committee members for record purposes. Any comments should be sent by the indicated closing date, either by mail, or by fax, or by e-mail to

SABS Standards Division Attention: Compliance and Development department Private Bag X191 Pretoria 0001 Fax No.: (012) 344-1568 (for attention: dsscomments) E-mail: [email protected]

Any comment on the draft must contain in its heading the number of the clause/subclause to which it refers. A comment shall be well motivated and, where applicable, contain the proposed amended text.

• The public enquiry stage will be repeated if the technical committee agrees to significant technical

changes to the document as a result of public comment. Less urgent technical comments will be considered at the time of the next amendment.

THIS DOCUMENT IS A DRAFT CIRCULATED FOR PUBLIC COMMENT. IT MAY NOT BE REFERRED TO AS A

SOUTH AFRICAN STANDARD UNTIL PUBLISHED AS SUCH.

IN ADDITION TO THEIR EVALUATION AS BEING ACCEPTABLE FOR INDUSTRIAL, TECHNOLOGICAL,

COMMERCIAL AND USER PURPOSES, DRAFT SOUTH AFRICAN STANDARDS MAY ON OCCASION HAVE TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL TO BECOME STANDARDS TO WHICH REFERENCE MAY BE MADE IN LAW.

AZ96.10 2008/08/08 sabs pta

ISBN 978-0-626- SANS 1936-2:2012 Edition 1

SOUTH AFRICAN NATIONAL STANDARD

Development of dolomite land

Part 2: Geotechnical investigations and determinations

Published by SABS Standards Division 1 Dr Lategan Road Groenkloof �� Private Bag X191 Pretoria 0001 Tel: +27 12 428 7911 Fax: +27 12 344 1568 www.sabs.co.za SABS

SANS 1936-2:2012 Edition 1

Table of changes

Change No. Date Scope

Acknowledgement The SABS Standards Division wishes to acknowledge the work of the National Department of Public Works, the South African Civil Engineering, Engineering Geological and Geotechnical Engineering Fraternity, and the National Dolomite Risk Management Working Committee established on instruction of the Cabinet Committee on Governance and Administration in developing this document. Foreword This South African standard was approved by National Committee SABS SC 59P, Construction standards – Geotechnical standards, in accordance with procedures of the SABS Standards Division, in compliance with annex 3 of the WTO/TBT agreement. This document was published in xxxx 2012. Reference is made in 3.10 to the "relevant national legislation". In South Africa this means the Engineering Profession Act, 2000 (Act No. 46 of 2000) or the Natural Scientific Professions Act, 2003 (Act No. 27 of 2003). Reference is made in B.4.1.6 and C.3.4 to the "relevant national department". In South Africa this means the Department of Water Affairs. SANS 1936 consists of the following parts, under the general title Development of dolomite land: Part 1: General principles and requirements. Part 2: Geotechnical investigations and determinations. Part 3: Design and construction of buildings, structures and infrastructure. Part 4: Risk management. Annexes A, B and C form an integral part of this document.


1

Introduction The development of dolomite land continues to present a challenge in South Africa. While opportunities exist in the development of such land, the adverse effects relating to the formation of sinkholes and subsidences, whether naturally or as a result of the development, cannot be ignored. In the absence of risk mitigation measures, sinkhole formation can result in loss of life. In addition, sinkholes and subsidences can cause severe damage to buildings and infrastructure and affect their serviceability. Avoiding the hazard associated with dolomite land by prohibiting development of any kind on such land is not practical as between four and five million South Africans currently reside or work on such land. Twenty-five per cent of Gauteng, the commercial, mining and manufacturing centre of South Africa, is located on dolomite land. At the other end of the spectrum, undue acceptance of risk is not an option given the potential severity of the consequences and the Government’s obligations in terms of the Bill of Rights. Systematic risk mitigation measures are therefore required. South African research shows that 96 % of sinkholes and subsidences that have occurred to date were man-induced, generated by ingress of water from leaking water-bearing infrastructure, poor stormwater management, etc. or due to artificial lowering of the groundwater level. Consequently, intervention through an integrated, comprehensive and pro-active dolomite risk management strategy has the potential to reduce the incidences of ground instability events (sinkhole and subsidence formation) by reducing the likelihood of water gaining entry into the subsurface profile, or controlling de-watering/recharging of the dolomite aquifer. The objective of SANS 1936 is to set requirements for the development of dolomite land in order to ensure that people live and work in an environment that is seen by society to be acceptably safe, where loss of assets is within tolerable limits, and where cost-effective and sustainable land usage is achieved.


2

Contents

Page Acknowledgement Foreword Introduction.................................................................................................................................. 1 1 Scope ..................................................................................................................................... 3 2 Normative references ............................................................................................................. 3 3 Definitions .............................................................................................................................. 3 4 Requirements ......................................................................................................................... 6 4.1 General ........................................................................................................................ 6 4.2 Feasibility-level dolomite stability investigations .......................................................... 7 4.3 Design-level investigations .......................................................................................... 11 4.4 Investigations during installation of services................................................................. 12 4.5 Inherent hazard classification of dolomite land............................................................. 12 Annex A (normative) Additional requirements for specific types of development ...................... 15 Annex B (normative) Methodology for the determination of the inherent hazard by the method of scenario supposition ............................................................... 20 Annex C (normative) Compilation of a geotechnical model ....................................................... 28 Bibliography .............................................................................................................................. 33


3

Development of dolomite land Part 2: Geotechnical investigations and determinations 1 Scope This part of SANS 1936 establishes requirements for a) feasibility-level geotechnical investigations to determine the general suitability of dolomite land

for development; b) design-level investigations, including footprint investigations, where necessary; c) the determination of the inherent hazard class of dolomite land, to be used in conjunction with

other parts of SANS 1936 to ascertain the dolomite area designation and requirements for the development of such land; and

d) inspection and verification during the implementation phase of a project on dolomite land. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. Information on currently valid national and international standards can be obtained from the SABS Standards Division. SANS 633, Soil profiling and rotary percussion borehole logging on dolomite land in Southern Africa for engineering purposes. SANS 1936-1:2012, Development of dolomite land – Part 1: General principles and requirements. SANS 1936-3, Development of dolomite land – Part 3: Design and construction of buildings, structures and infrastructure. SANS 1936-4, Development of dolomite land – Part 4: Risk management. 3 Definitions For the purposes of this document, the following definitions apply. 3.1 competent geophysicist geophysicist who is qualified, by virtue of his experience, training and contextual knowledge of dolomite environments in South Africa, to plan, conduct and interpret geophysical surveys on dolomite land


4

3.2 competent person geo-professional who is qualified by virtue of his experience, qualifications, training and in-depth contextual knowledge of development on dolomite land to a) plan and conduct geotechnical site investigations for the development of dolomite land, evaluate factual data, develop a geotechnical model, derive interpretative data and formulate an opinion relating to the outcomes of such investigations; b) develop and inspect for compliance the necessary precautionary measures required on dolomite land to enable safe developments to take place; c) develop dolomite risk management strategies; or d) investigate the cause of an event and participate in the development of the remedial measures required 3.3 dolomite area designation classification of dolomite areas in terms of the extent of mitigation required to achieve and maintain a tolerable hazard 3.4 dolomite land land underlain by dolomite or limestone residuum or bedrock (or both), within the Malmani Subgroup and Campbell Rand Subgroup, typically at depths of no more than a) 60 m in areas where no de-watering has taken place and the local authority has jurisdiction, is monitoring and has control over the groundwater levels in the areas under consideration; or b) 100 m in areas where de-watering has taken place or where the local authority has no jurisdiction or control over groundwater levels NOTE For more information on dolomite land in South Africa, see annex B of SANS 1936-1:2011. 3.5 dolomite residuum weathering product derived from the weathering and decomposition of dolomite 3.6 dolomite risk management strategy process of using scientific, planning, engineering and social processes, procedures and measures to manage an environmental hazard, and encompasses policies and procedures set in place to reduce the likelihood of events (sinkholes and subsidences) occurring on dolomite land 3.7 dwelling house single dwelling unit and any garage and other domestic outbuildings thereto, situated on its own property 3.8 dwelling unit unit containing one or more habitable rooms and provided with sanitary and cooking facilities


5

3.9 event occurrence of a sinkhole or subsidence 3.10 geo-professional practitioner of geotechnical engineering or engineering geology who is registered in any category of registration provided for in the relevant national legislation (see foreword) 3.11 hazard source of potential harm NOTE A hazard can be a risk source, i.e. an element which alone or in combination has the intrinsic potential to give rise to risk 3.12 hazard rating number of events that can potentially occur per hectare over a 20-year period due to development NOTE A tolerable hazard rating is one that complies with the requirements for a tolerable hazard (see 3.24). 3.13 infrastructure roads, railway lines, runways, liquid-retaining structures, stormwater systems, power lines, pipelines and associated structures, including water, sewer, fuel and gas lines, reservoirs, public swimming pools, attenuation and retention ponds for stormwater management, pump stations, dams, reservoirs and artificial lakes 3.14 inherent hazard potential for an event (sinkhole or subsidence) to develop in a particular ground profile on dolomite land 3.15 inherent hazard class IHC classification system whereby a site is characterized in terms of eight standard inherent hazard classes, denoting the likelihood of an event (sinkhole or subsidence) occurring, as well as its predicted size (diameter) 3.16 original groundwater level OWL mean groundwater level in the dolomite aquifer, expressed as a depth below natural ground level or elevation above sea level (or both), about which the seasonal natural fluctuation occurs 3.17 parcel of land tract of land, comprising one or more farm portions or properties registered in a deeds registry, and identified for the purpose of development 3.18 rational assessment assessment by a competent person of the adequacy of the performance of a solution in relation to requirements including as necessary, a process of reasoning, calculation and consideration of


6

accepted analytical principles, based on a combination of deductions from available information, research and data, appropriate testing and service experience NOTE In the context of this part of SANS 1936, rational assessment is a method permitted for assessment of the parameters that determine the inherent hazard class of dolomite land 3.19 return period recurrence interval estimate of the average interval of time between events of a certain size 3.20 risk effect of uncertainty on objectives NOTE Risk is often expressed in terms of a combination of the consequences of an event and the associated likelihood of occurrence. 3.21 scenario supposition context or deductive process in which a general set of factors can be used as a checklist, culminating in the stability characterization of a site or structure footprint 3.22 sinkhole feature that occurs suddenly and manifests itself as a hole in the ground 3.23 subsidence shallow, enclosed depression NOTE Most South African literature previously used the term “doline” when referring to subsidence as defined above. The use of the term “subsidence” is in line with international literature and practice. 3.24 tolerable hazard where the number of events experienced is less than 0,1 events per hectare per 20 years (preferably tending to 0 per hectare), i.e. a return period of an event occurring on 1 ha of more than 200 years NOTE Mitigating measures might need to be implemented in order to achieve a tolerable hazard. 4 Requirements 4.1 General 4.1.1 Geotechnical site investigations on dolomite land shall comprise investigations, as relevant, providing the information required for a) the identification and quantification of any hazard that might impact upon the development of the land; b) the determination of the inherent hazard class(es) of the site; c) the determination of appropriate dolomite area designation(s);


7

d) the selection and design of foundations, structures and infrastructure appropriate for the inherent hazard class and dolomite area designation;

e) the determination of precautionary measures; and f) the determination of the risk management required to achieve and sustain a tolerable hazard rating. 4.1.2 Geotechnical site investigations shall be undertaken under the direction of one or more competent persons. Such persons shall be directly involved in the planning of the investigation, the gathering and interpretation of factual data, the determination the inherent hazard class, the drafting of reports, inspections during implementation and any interaction with the statutory councils and authorities that may be required. 4.1.3 The competent person(s) shall formulate and document all determinations and do so in a manner that a peer review, if conducted on the same data, will arrive at substantially similar determinations. 4.1.4 An application for land use rights, made to any relevant authority, shall include a sufficient level of information to provide confidence in the presented determination(s). 4.1.5 Certain types of development have investigation requirements additional to those in 4.2 and 4.3. These developments include a) township developments; b) residential stands proposed for infill development (subdivision or second dwelling rights); c) residential stands proposed for rezoning and multiple dwelling rights; d) major roads, railway lines and runways; e) bulk pipelines; f) pump stations and water care works; and g) attenuation dams, reservoirs and public swimming pools. Additional requirements for the above types of development shall be in accordance with annex A. 4.2 Feasibility-level dolomite stability investigations 4.2.1 General 4.2.1.1 A feasibility-level investigation shall be carried out by a competent person for all developments on dolomite land to form the basis of development planning and broadly delineate the site into inherent hazard zones which are considered suitable or unsuitable for various types of development. 4.2.1.2 A feasibility-level investigation shall include sourcing, and consideration of available information. 4.2.1.3 Depending on the conditions encountered and the nature of the development, the feasibility investigation might provide sufficient information for development to proceed.


8

4.2.1.4 During this phase of the investigation, the competent person shall a) assess the basic geology and geomorphological features of the site, as well as any sinkholes and subsidences within and in close proximity to the site; b) conduct a geotechnical site investigation, typically comprising, as a minimum, a gravity survey in accordance with the requirements of 4.2.2, and borehole drilling in accordance with the requirements of 4.2.3; c) compile a geotechnical model for the site, which should include, as a minimum, an assessment of the bedrock morphology, the subsurface profile from ground surface to dolomite bedrock, and the geohydrological conditions; d) determine the inherent hazard class of the site or portions thereof, the dolomite area designation of the site as defined in table 1 of SANS 1936-1:2011, and the permissible land usage as defined in table 2 of SANS 1936-1:2011; and e) prepare a geotechnical report in accordance with the requirements of 4.2.6. 4.2.2 Geophysical survey 4.2.2.1 The geophysical survey should include a gravity survey undertaken by a competent geophysicist. Field observations may be made by a suitably qualified and experienced geotechnician under the direction of the competent geophysicist. 4.2.2.2 In executing a gravity survey, a) A gravity map shall be produced and used to determine borehole positions. A residual gravity map shall be produced after completion of the borehole drilling. On large projects, a preliminary residual gravity map may be produced after the initial phase of drilling and then refined after all drilling is completed. The final residual gravity map shall be incorporated into a report to be prepared by the competent geophysicist. b) The accuracy of reduced observations on a relative basis shall be at least 0,01 mGal or better. c) Contour intervals on gravity maps shall not exceed 0,1 mGal. 4.2.2.3 Depending on the geology, other non-destructive geophysical methods (electromagnetic, resistivity, seismic surveys, etc.) or remote-sensing techniques may be used to establish, for example, the contact between dolomite and other material such as intrusives or Karoo mudrock. 4.2.3 Borehole drilling 4.2.3.1 Boreholes shall be drilled on the site, typically using rotary percussion drilling methods, at a frequency not less than that given in table 1, unless the geological and development setting necessitates a higher frequency. 4.2.3.2 The position of the boreholes shall be chosen by taking into account the geophysical, geological, geohydrological and geomorphological data. 4.2.3.3 Boreholes shall be drilled at least 6 m into dolomite bedrock, except as indicated in 4.2.3.4. 4.2.3.4 In instances where very deep dolomite bedrock is expected (deeper than 60 m), a selection of boreholes shall be drilled into dolomite bedrock on gravity high anomalies to determine the shallowest bedrock depth. In instances where the shallowest depth is 60 m or more on gravity high anomalies, boreholes drilled on gravity low anomalies need not be drilled into dolomite bedrock. In de-watered areas, a representative selection of boreholes shall be drilled to 100 m or into dolomite bedrock to obtain a perspective of subsurface conditions below the OWL.


9

4.2.3.5 The drilling method should be adapted to provide optimum sample return, typically from every metre drilled. Casing might be required to minimize contamination of the sample. In some cases reverse circulation, Odex or Symmetrix type drilling might be required. 4.2.3.6 The use of data loggers to record drilling parameters is encouraged, and downhole borehole cameras may be considered should conditions dictate. 4.2.3.7 The borehole shall be drilled and a driller’s log prepared in accordance with SANS 633. Logging shall be carried out in accordance with SANS 633 by a competent person. 4.2.3.8 All boreholes shall be suitably backfilled. NOTE 1 Backfilling of the borehole should prevent the hole acting as a preferential flow path through which water might enter the profile either from the surface or from perched groundwater. NOTE 2 In all but D1 dolomite designation areas, the backfilling should incorporate a plug of low permeability material (e.g. a slurry formed by mixing cement and bentonite with the drilling chips) near the ground surface and at the depth of any low permeability layer on which perched groundwater might form.

Table 1 — Minimum frequency of boreholes in dolomite areas for a feasibility-level investigation

1 2

Study area

ha Minimum number of holes per hectare

< 1,0 3

> 1,0 but < 2,5 2

> 2,5 but < 10,0 1

> 10,0 but < 50,0 0,5

> 50,0 but < 100,0 0,3

> 100,0 but < 200,0 0,2

> 200,0 0,15

NOTE The frequencies recommended above are a realistic indication of the minimum number of holes required. The competent person should be able to demonstrate compelling reasons for adopting a frequency that is lower than that stipulated.

4.2.4 Near-surface investigations Near-surface investigations shall be undertaken where deemed appropriate by the competent person, particularly where there are indications of shallow dolomite or where information on the properties of the shallow overburden is required, either for the design of foundations or for the purposes of assessing the inherent hazard class. NOTE Near-surface investigation includes the excavation of test pits or exploratory trenches. 4.2.5 Gathering of geohydrological data 4.2.5.1 Geohydrological data should be obtained from a relevant authority or reputable sources in the industry. Wherever practically feasible, the following data should be obtained and recorded: a) the presence of groundwater compartments in the region and the name of the compartments; b) the National Quaternary catchment number in which the site is located;


10

c) original groundwater level (OWL) in metres above mean sea level; d) coordinates, collar elevation, groundwater level, pumping equipment installed in the borehole and water use for all existing boreholes on or in close proximity to the property being investigated; e) the presence of any large scale groundwater abstraction or de-watering activities in the vicinity of the property being investigated, e.g. centre pivot irrigation, large scale groundwater abstraction for mining, municipal, domestic or irrigation use, etc. as these could impact on or influence groundwater levels; f) any known planned future water abstraction activities in the area; g) the presence of any regular groundwater-related monitoring activities (e.g. water levels, pumping records) and the custodian of such data; h) location and yield of springs on or in close proximity to the property being investigated and the likely reason for their existence; and i) location of pans, state of pans, variation in water level and the likely reason for the existence of the pans. 4.2.5.2 Where groundwater is encountered in a new borehole, the following shall be recorded: a) borehole coordinates stating how these were determined (survey, handheld GPS, interpolated from maps or plans, etc.); b) borehole collar elevation (metres above mean sea level) and a note on how the elevation was determined, e.g. survey, handheld GPS reading or interpolated from contour plan; c) depth drilled (metres below collar elevation); d) depths at which groundwater strikes were encountered and an estimate of yields at each water strike e.g. weak, medium or strong; NOTE On completion of drilling, a total blow yield may be estimated and recorded. e) static groundwater level measured at least 24 h after completion of drilling as a depth below collar; and f) date of each observation. NOTE If the borehole is found to be dry, it should be reported as “borehole dry”. If the borehole has (partially) collapsed during this period, this should also be recorded, including to what depth. 4.2.6 Report 4.2.6.1 The investigator shall document and report all findings and determinations in a written report that a) provides a description of the site and defines its extent and boundaries; b) describes the investigation carried out and presents the results thereof in detail; c) establishes the geotechnical model for the site;


11

d) establishes the nature, fluctuations and compartmentalization of groundwater, and original groundwater levels from geohydrological data; e) assesses the effect of changes to the groundwater level; f) determines the inherent hazard class of the site or of the various portions thereof; g) determines the dolomite area designation and appropriateness of proposed land usage in accordance with the requirements of SANS 1936-1; h) provides the information needed for identification of the precautionary measures required in accordance with SANS 1936-3, and the risk management strategies required in accordance with SANS 1936-4; and i) identifies any outstanding information to be determined or confirmatory investigation to be undertaken during the design-level investigation, particularly in respect of verifying or refining the inherent hazard class of the site or parts thereof. 4.2.6.2 Drawings shall be to a suitable scale, legible and easily reviewed. All drawings shall be fully annotated and show a coordinate grid. The coordinate system used (e.g. Cape, WGS84, local, etc.) shall be stated in the drawing. NOTE The preferred coordinate system for new projects is the WGS84 coordinate system. 4.2.6.3 Borehole coordinates and collar elevations shall be shown on the borehole logs and shall be listed in the drawings. 4.3 Design-level investigations 4.3.1 General Design-level investigations, including footprint investigations, shall be carried out a) where required in terms of table 2 of SANS 1936-1:2011; b) where additional investigation is required for particular types of development (see 4.1.5), if not included in the feasibility-level investigation; c) where the feasibility-level investigation identifies the need for additional information or confirmation of the findings of that investigation, particularly with regard to the inherent hazard class of the site or parts thereof; d) where further geotechnical information is required for the design of foundations, structures or infrastructure; or e) where deemed necessary by the competent person. 4.3.2 Requirements 4.3.2.1 Design-level investigations required for particular types of development (see 4.1.5) shall comply with the requirements of annex A. 4.3.2.2 The requirements for and methods used during all other design-level investigations shall be determined by the competent person to ensure that the investigation fulfils its intended purpose.


12

4.3.2.3 Where required, footprint investigations shall be carried out to confirm, refine or amend inherent hazard class below the footprint of the structure and the associated dolomite area designations. 4.3.2.4 During the design-level investigation, the competent person shall compare the geotechnical conditions encountered with those expected on the basis of the feasibility-level investigation. Where discrepancies or variations exist, sufficient additional work shall be undertaken to determine the extent and effect of such discrepancies and the inherent hazard class of the site or part thereof shall be amended accordingly. 4.3.3 Report Reporting shall be generally in accordance with 4.2.6, as relevant. 4.4 Investigations during installation of services 4.4.1 The developer shall appoint a competent person(s) to assist with the implementation of the project. Such person should preferably be one (or more) of the competent persons responsible for the preceding investigations of the site. 4.4.2 The competent person so appointed shall a) interact with suitably qualified professionals and the person responsible for the development of the parcel of land or the relevant portions thereof, concerning planning, design of infrastructure, buildings, structures, and residential units; b) certify that the layout of the development conforms to the dolomite stability zoning of the site and confirm such certification by signing the layout drawings for the development; c) establish formal inspection procedures with the person responsible for the installation of the services so that the available trenching and works are optimally utilized within the construction framework and programme for inspection purposes; d) coordinate activities associated with the inspection procedures; e) inspect the service trenches and open works to verify the stability zoning (inherent hazard classes and dolomite area designation) established in the preceding investigations and to check for palaeo-karst structures; f) investigate potential palaeo-structures wherever they are exposed; g) undertake, where justified, additional geotechnical site investigations; and h) confirm or refine the inherent hazard class of the site. 4.4.3 The competent person shall document the implementation phase requirements by preparing a report or drawings (or both) that record such findings and recommendations. 4.5 Inherent hazard classification of dolomite land 4.5.1 Classification method 4.5.1.1 All dolomite land on which development is to take place shall be classified in terms of a) its susceptibility to an event; and b) the anticipated size and type of event.


13

4.5.1.2 The inherent hazard shall be assessed by means of a) a rational assessment in accordance with 4.5.1.3, or b) the method of scenario supposition, as set out in annex B. 4.5.1.3 Rational assessment shall be based on a geotechnical model of the site prepared in accordance with annex C and on a reasoned assessment of all factual, statistical and other data on the site and its surroundings. The reasoning processes, calculations, analytical principles, deductions, research, test results and experience employed in the assessment shall be fully documented in a manner that can be used to defend the outcome of the assessment. 4.5.1.4 The size of the sinkhole shall be determined in broad categories using the descriptors given in table 2.

Table 2 — Terminology used to describe size of sinkholes

1 2

Maximum diameter of surface manifestation,

m

Descriptor

< 2 Small sinkhole

2 to 5 Medium-size sinkhole

5 to 15 Large sinkhole

> 15 Very large sinkhole

4.5.1.5 The inherent hazard shall be determined in accordance with the categories given in table 3.

Table 3 — Inherent hazard categories

1 2

Inherent hazard Anticipated events per hectare per 20 years a

Low Typically less than 0,1 events, but occurrence of events cannot be excluded. Return period of an event occurring in an area of 1 ha is greater than 200 years.

Medium Typically between 0,1 and 1,0 events. Return period of an event occurring in an area of 1 ha is between 200 years and 20 years.

High Typically greater than 1,0 events. Return period of an event occurring in an area of 1 ha is less than 20 years.

a Based on events that have occurred per hectare in a 20-year period in the "type" area with no special precautions taken in respect of the design and maintenance of water-borne services

4.5.1.6 The susceptibility of, size and type of event should, as a minimum, be assessed with respect to ingress water and artificial groundwater drawdown. NOTE 1 In a non-de-watered situation, water ingress is likely to be the primary trigger mechanism for sinkhole or subsidence formation. NOTE 2 In a de-watered situation, the effect of groundwater level drawdown and the effect of exposure of the profile thereby increasing the depth over which subterranean erosion can take place (i.e. lowering the base level of erosion) should both be considered.


14

4.5.2 Inherent hazard classes 4.5.2.1 The inherent hazard class on dolomite land shall be determined in accordance with table 4.

Table 4 — Statistical occurrences of inherent hazard classes of subsidences and specified-size sinkholes

1 2 3 4 5 6

Statistical occurrences of sinkholes and subsidences

Small sinkhole Medium sinkhole Large sinkhole Very large sinkhole

Inherent hazard class

< 2 m 2 m to 5 m 5 m to 15 m > 15 m Subsidence

1 Low Low Low Low Low

2 Medium Low Low Low Medium

3 Medium Medium Low Low Medium

4 Medium Medium Medium Low Medium

5 High Low Low Low High

6 High High Low Low High

7 High High High Low High

8 High High High High High

NOTE The statistical occurrence of the event/hectare over a 20-year period is in the following ranges: – low: 0 < 0,1 (return period is greater than 200 years) – medium: > 0,1 < 1,0 (return period is between 200 and 20 years) – high: > 1,0 (return period is less than 20 years)

4.5.2.2 The inherent hazard class may apply to the entire site or, more typically, will vary across the site requiring zoning of the site into different inherent hazard classes. 4.5.2.3 The inherent hazard class and the boundaries of the zone to which this applies shall be shown in the drawings.


15

Annex A (normative)

Additional requirements for specific types of development

A.1 General A.1.1 The requirements of this annex apply in addition to the requirements in 4.1 to 4.5 (inclusive) in respect of the type of developments listed in 4.1.5. A.1.2 The additional requirements of this annex may be complied with as part of the feasibility-level investigation or the design-level investigation, as deemed appropriate by the competent person, except where the context indicates otherwise. A.2 Additional requirements for township development A.2.1 Feasibility-level investigations The feasibility-level investigation of areas demarcated for township development shall, as a minimum, include a) a feasibility-level dolomite stability investigation in accordance with the requirements of 4.1 and 4.2; b) a supplementary investigation, which might be required to reduce the uncertainty and increase the confidence in the dolomite stability assessment, particularly on sites with potentially highly variable conditions; c) the typical frequency of appropriate data points for such a supplementary investigation, as given in table A.1; NOTE Appropriate data points include, for example, boreholes, trenches, test pits and auger holes. d) interaction with suitably qualified professionals and the person responsible for the development of the parcel of land or the relevant portions thereof, concerning planning, design of infrastructure, buildings, structures, and residential units; and e) certification that the layout of the development conforms to the dolomite stability zoning of the site.

Table A.1 — Typical frequency of appropriate data points in dolomite areas for supplementary investigations of township development

1 2

Inherent hazard class Typical number of data points per area

1 1 per 7 000 m2

2 1 per 5 000 m2

3 1 per 3 000 m2

4 1 per 3 000 m2

5 1 per 1 000 m2

6

7

8

Required numbers of data points to be agreed upon by geo-professional and all peer reviewers


16

A.2.2 Design-level investigations The design-level investigation of areas demarcated for township development shall, as a minimum, include a) a design-level dolomite stability investigation in accordance with the requirements of 4.1 and 4.3, as applicable; b) appropriate data points at the minimum frequency as given in table A.2; NOTE Appropriate data points include, for example, boreholes, trenches, test pits and auger holes. c) a final dolomite area designation, which might differ from that indicated by the feasibility-level investigation.

Table A.2 — Minimum frequency of data points for a design-level investigation

Category Type of development Inherent

hazard class Minimum number of appropriate

data points

Building (brick or concrete) 3 per structure

Factory (steel frame) 4 per ha or 4 per structure

Quarry or borrow area 2 per ha

Tower or mast 1 per 25 m2

Reservoir 1 per 100 m2

Bridge 2 per abutment and 1 per pier

Substation 4 per ha

Water care works and pump stations 4 per ha

Compact

Public swimming pools 1 per 50 m2

Pipeline 4 per km

Road/rail/conveyor 5 per km

Canal 4 per km

Power transmission 4 per km

Tunnels 5 per km

Power plant 10 per ha or 5 per structure

Linear

Dam

1 to 8

1 per 10 ha within the basin and 10 per km of wall

1 and 2 1 per 1 000 m2

3 and 4 1 per 500 m2

5 1 per 300 m2

Housing complex (RL and RH in accordance with SANS 1936-1)

6, 7 and 8 1 per 150 m2

Airport 1 per ha or 5 per structure

Industrial complex 10 per ha or 5 per structure

Power plant 10 per ha or 5 per structure

Large

Dam

1 to 8

1 per 10 ha within the basin and 10 per km of wall

NOTE Under special circumstances, the competent person may elect a lower frequency, provided adequate motivation for such a decision can be put forward.


17

A.2.3 Inspections during implementation Inspections during implementation on areas demarcated for township development shall, as a minimum, a) include the requirements of 4.4; and b) certify that the inherent hazard class and the dolomite area designation are correct for each stand on which a dwelling house is to be erected, within the limits of the investigation.

A.3 Additional requirements for infill development on residential stands In addition to the applicable requirements of 4.1 to 4.5 (inclusive), the competent person shall, in the investigation of infill development on residential stands, including subdivision or second dwelling rights within proclaimed townships in dolomite areas, a) source available information on the historical performance with respect to the dolomite stability of the township as a whole and the surrounding stands in particular; b) certify that the proposed subdivision/second dwelling application complies with SANS 1936-1; c) determine the appropriate dolomite area designation and the potential nominal loss of support

due to sinkhole or subsidence formation below a foundation; d) inspect the open works to verify the stability zone; and e) determine the monitoring area designations in accordance with SANS 1936-4. A.4 Additional requirements for rezoning and multiple dwelling rights on residential stands In addition to the applicable requirements of 4.1 to 4.5 (inclusive), the competent person shall, in the investigation of residential stands proposed for rezoning and multiple dwelling rights within proclaimed townships in dolomite areas, a) source available information on the historical performance with respect to the dolomite stability of the township as a whole and the surrounding stands in particular; b) undertake trench and surface mapping in dolomite outcrop areas; c) certify that the proposed rezoning, with its associated dwelling density, complies with

SANS 1936-1; d) determine the appropriate dolomite area designation and the potential nominal loss of support

due to sinkhole or subsidence formation below a foundation; e) inspect the open works to verify the inherent hazard classification of the site; and f) determine the monitoring area designations in accordance with SANS 1936-4. A.5 Additional requirements for major roads, railway lines and runways The investigation of major roads, railway lines and runways in dolomite areas shall, as a minimum, a) comply with the requirements of 4.1, 4.2 and 4.3, as applicable;


18

b) investigate and prepare a dolomite stability report covering the width of the development and a corridor on both sides of the development equal in width to the average overburden thickness, to the extent permitted by surrounding development; c) comply with the minimum frequency of data points given in table A.2; d) provide for interaction with relevant suitably qualified professionals concerning planning, water

precautionary measures, and the design of culverts, bridges and overpasses; e) comply with the requirements for inspection during implementation given in 4.4; and f) develop a dolomite risk management strategy specific to the infrastructure in accordance with SANS 1936-4. A.6 Additional requirements for bulk pipelines The investigation of bulk pipelines in dolomite areas shall, as a minimum, a) comply with the requirements of 4.1, 4.2 and 4.3, as applicable. b) investigate and prepare a dolomite stability report for the pipeline and a corridor on both sides of the pipeline equal in width to the average overburden thickness, to the extent permitted by surrounding development; c) comply with the minimum frequency of data points given in table A.2; d) provide for interaction with suitably qualified professionals concerning planning, water precautionary measures and the design of the pipeline; e) establish formal inspection procedures with the person responsible for the installation of the pipeline so that the available trenching and works are optimally utilized within the construction programme for inspection purposes; f) comply with the requirements for inspection during implementation given in 4.4; and g) develop a dolomite risk management strategy specific to the pipeline in accordance with SANS 1936-4.

A.7 Additional requirements for pump stations and water care works The investigation of pump stations and water care works in dolomite areas shall, as a minimum, a) comply with the requirements of 4.1, 4.2 and 4.3, as applicable; b) comply with the minimum frequency of data points given in table A.2; c) include interaction with the relevant suitably qualified professionals concerning planning, water precautionary measures and design of structures; d) comply with the requirements for inspection during implementation given in 4.4; and e) develop a dolomite risk management strategy specific to the development in accordance with SANS 1936-4


19

A.8 Additional requirements for attenuation dams, retention dams, reservoirs and public swimming pools The investigation of dams, reservoirs and public swimming pools in dolomite areas shall, as a minimum, a) comply with the requirements of 4.1, 4.2 and 4.3, as applicable; b) comply with the minimum frequency of data points given in table A.2; c) comply with the requirements for inspection during implementation given in 4.4; and d) develop a dolomite risk management strategy specific to the development in accordance with SANS 1936-4. NOTE The proposed site for dams and reservoirs should be at least 100 m from the nearest housing or commercial development unless stringent precautions are taken and the permission of the local authority has been obtained.


20

Annex B (normative)

Methodology for the determination of the inherent hazard

by the method of scenario supposition B.1 General The inherent hazard associated with dolomite land can be evaluated using a simple generalized set of factors (circumstances, facts or influences contributing to a result) based on the risk management principles contained in SANS 1936-4, and the idealized three-dimensional model (see B.2), by applying the method of scenario supposition as outlined in table B.1 and described in B.3 and B.4. The method of scenario supposition defines the context or deductive process in which a general set of factors can be used as a checklist, culminating in the stability characterization of a site or structure footprint. The method provides a reference framework for the evaluation of ground-surface stability on dolomite land. It requires hypothesising the impact of man's future activities on the likelihood of sinkhole and subsidence formation in the context of a de-watering or non-de-watering scenario. In this method, the hazard associated with dolomite land is evaluated using a simple generalized set of factors, which include receptacle development, nature of the blanketing layer, mobilizing agencies, mobilization potential, and potential development space. These factors are determined by accumulated knowledge or site-specific testing. The method requires consideration of both the principle mobilization agencies, namely ingress water, and groundwater level drawdown.

Table B.1 — An overview of the activities associated with the method of scenario supposition

1 2

Activity Description

1 Perform a field reconnaissance and desk study of site.

2 Undertake preliminary zoning, utilizing tools such as air photograph interpretation and geophysics.

3 Characterize preliminary zoning, based on an examination of boreholes.

Review individual borehole profiles and other geological ,geohydrological and geotechnicala data within the context of the selected scenarios, including evaluation factors such as:

Sinkhole formation Subsidence formation

4

• Mobilization agency/agents • Receptacle development • Potential development space (related to potential

sinkhole size) • Nature of blanketing layer(s) • Mobilization potential of blanketing layer(s)

• Mobilizing agency • Nature of blanketing layer(s) • Mobilization potential • Lateral extent • Dolomite bedrock topography

5 Collate individual borehole characterizations and amend preliminary zoning, taking historical information into account. Determine the lateral extent of the conditions providing the hazard characterization and develop composite inherent hazard characterization using the subsurface conditions represented by the various boreholes in conjunction with geophysical data, karst and bedrock morphology, and geohydrology.

6 Finalize dolomite stability zoning in terms of the susceptibility of a particular event (sinkhole or subsidence) of a certain size.

a Field and laboratory tests may be undertaken to obtain a general perspective of the geotechnical characteristics of the predominant horizons constituting the blanketing layer.

Complicated vertical and lateral variations typical of dolomite profiles will often necessitate broad characterization of the geotechnical properties of the blanketing layer to evaluate the potential in-


21

service behaviour of the profile. Where conditions are more uniform, laboratory or field testing (or both) may prove useful in predicting potential behaviour. B.2 Idealized three-dimensional model for evaluating inherent hazard B.2.1 General Postulated mechanisms of sinkhole and subsidence formation involve different processes, geological settings, and triggering agents. They may, however, be simplified and generalized for the purpose of evaluating the inherent hazard in terms of an idealized three-dimensional model as shown in figure B.1.

Figure B.1 — Schematic three-dimensional model showing various components of dolomite land

B.2.2 Blanketing layer Dolomite overburden comprises all the materials that occur between the ground surface and the dolomite bedrock surface. It typically includes transported soils, residual dolomite soils (wad and chert rubble), fresh and weathered intrusive sills, layers of Karoo sedimentary rocks and Quaternary deposits. The blanketing layer is the component of the dolomite overburden that overlies the potential receptacles. Two blanketing layers are shown in figure B.1, one of which comprises the full thickness of dolomite overburden (see MPS 1 in figure B.1), while the other is relatively thin and overlies interconnected openings within the overburden (see MPS 2 in figure B.1). B.2.3 Receptacles Receptacles can occur either as small disseminated and interconnected openings in the overburden (especially where chert rubble is present), or as substantial openings (cavities) in the bedrock. Both types of openings can receive mobilized (eroded or slumped) materials from overlying horizons.


22

B.2.4 Mobilization and mobilizing agencies In the dolomite context, mobilization is the movement of the blanketing layer by subsurface erosion, slumping, or ravelling by an external agency. Mobilizing agencies include water ingress, ground vibrations, water level drawdown, or any activity or process that can induce mobilization of the material within the blanketing layer under the force of gravity. In a non-de-watering scenario, the static groundwater level is not an agent but a positive, mitigating factor as it is the base level for subterranean erosion. B.2.5 Maximum potential development space Maximum potential development space (MPS) is a simplified estimation of the maximum size sinkhole that can be expected to develop in a particular profile, provided that the available space is fully exploited by a mobilizing agency (see figure B.1). The available space depends on the depth below ground surface to the throat of a receptacle or disseminated receptacle, and the angle of draw in the various blanketing materials. MPS 1 in figure B.1 shows a profile with a deep groundwater level situated within the bedrock. The blanketing layer, and hence the potential development space, is fully exposed to the potential activities of extraneous mobilizing agents. Figure B.1 also shows a significant layer of intrusive material with a low mobilization potential, i.e. low inherent hazard. This horizon acts as either an aquitard or an aquiclude that prevents mobilization and movement of materials into the receptacle. The material within the development space is thus protected from the mobilization agency. MPS 2 in figure B.1 reveals the presence of potential disseminated receptacles above the intrusive horizon displaying the low mobilization potential, i.e. low inherent hazard. A smaller potential development space is thus available for exploitation by a mobilizing agency. B.3 Determination of the hazard B.3.1 Sinkhole size B.3.1.1 The maximum size of a sinkhole is assessed by estimating the potential development space within the blanketing layer based on a) the estimated depth below the ground surface to the potential throat of either the receptacle or

disseminated receptacles (i.e. the thickness of the blanketing layer); b) the size of the throat or potential throat which might relate to the width of a solution opening

(fissure); and c) the estimated angle of draw in the various horizons of the blanketing layer. NOTE The angle of draw is the angle of a metastable slope to that which a particular mobilizing agency will produce in that material. The material within the cone defined by the angle of draw can potentially be mobilized by being moved or drawn into the conduit at the base of the cone. Typical angles of draw can range from 45° for silty clay (wad) to 90° for shale, mudrock or c hert. The specific values for a given profile are dependent on local conditions, observation of actual sinkhole sidewalls in the immediate area, if available, and, more importantly, geotechnical information gathered during field investigations. In certain instances, practical constraints and depth might preclude testing. B.3.1.2 The potential development space should be assessed by considering the presence of receptacles or disseminated receptacles. Any suppositions made should be elaborated on in the reporting stage of an investigation giving reasons based on experience and data. Conservative assumptions should be reported. For example, it might be assumed that receptacles are present within the bedrock, even if not encountered during drilling.


23

B.3.1.3 The depth below ground surface of the potential receptacles should be determined. The presence of disseminated receptacles occurring above the dolomite bedrock should also be considered. It might be necessary to assess or accept the worst possible situation. B.3.1.4 The thickness of the various horizons that constitute the blanketing layer should be considered. The depth to the potential receptacle should be obtained from borehole information, while the radius of the potential development space on the surface should be obtained from a simplified diagrammatic approximation. The angle of draw of the various materials and the depth of the receptacle may be used to project and estimate the radius. B.3.1.5 The size of a sinkhole should be recorded in terms of the broad categories of potential development space using the descriptors contained in table 2. NOTE The size of the receptacles will determine to what extent the potential development space can be utilized. Thus for each receptacle there is a potential development space that can be fully realized or exploited, thereby creating the maximum size sinkhole, provided that a) the receptacle is large enough to accommodate all the mobilized material from the potential development space, b) all the materials within the blanketing layer can be mobilized, and c) a sustained mobilizing agency is present. As there is no efficient technique available at present to ascertain the volume of receptacles, it is generally assumed that receptacles of adequate volume are present. B.3.2 Subsidence size The hazard relating to subsidence formation should be expressed as a) subsidence and large sinkhole; or b) subsidence and medium-size sinkhole, etc. NOTE Subsidence size is generally predictable if formed by the process of premature termination of sinkhole formation. Where a subsidence is formed by consolidation of overburden material due to de-watering or saturation of near-surface materials, the size is difficult to determine. However, the use of gravity and borehole data might give an indication of the potential subsidence scale. Typical sizes range from several metres to kilometres in width and length. Consequently, for subsidences, the intention is mainly to determine the likelihood of their formation. B.4 Determination of the inherent hazard B.4.1 Sinkholes B.4.1.1 The inherent susceptibility of sinkhole formation is a reflection of the geotechnical characteristics of the materials in the blanketing layer and depends mainly on the susceptibility (mobilizing potential) of materials to exploitation and mobilization under the influence of a mobilizing agency. For the selection of this hypothetical mobilizing agency, the assumption should be made that the site will be "abused" through inappropriate land use, poor management of stormwater and water-bearing infrastructure during future development, and the worst-case scenario (with respect to the mobilizing agents): groundwater level drawdown and water ingress. B.4.1.2 The susceptibility of the blanketing layer to mobilization and the formation of a sinkhole should be expressed using the descriptors given in table B.2 in terms of inherent hazard, which is classified as a) low,


24

b) medium, or c) high. NOTE The inherent hazard of a site is related to its inherent geological setting. Through the practice of dolomite risk management, however, the development placed on the site is managed in order to present a tolerable hazard rating.

Table B.2 — Descriptors for assessing the inherent susceptibility of the blanketing layer to mobilization

1 2

Inherent susceptibility Descriptors

Low

The profile displays no voids. No air loss or sample loss is recorded during drilling operations. Either a very shallow groundwater level or a substantial horizon of materials with a low potential susceptibility to mobilization might be present within the blanketing layer (e.g. continuous intrusive features or shale material). Depth to potential receptacle is typically great and the nature of the blanketing layer is not conducive to mobilization.

Medium

This type of profile is characterized by the absence of a substantial protective horizon and has a blanketing layer of materials potentially susceptible to mobilization by extraneous mobilization agents. The groundwater level might be below the blanketing layer. Certain geological settings of shallow, continuous outcropping and sub-outcropping dolomite bedrock, with limited weathering and fissure development, might be characterized by favourable inherent hazard conditions.

High

The blanketing layer reflects a great susceptibility to mobilization. A void might be present and is interpreted to be very likely, within the potential development space, indicating that the process of sinkhole formation might have already started. Boreholes might register large cavities, sample loss, air loss, etc. Convincing evidence exists of cavernous subsurface conditions, which might act as receptacles. The groundwater level might be below or within the blanketing layer. In a de-watering scenario, the lowering of a shallow groundwater level might not only generate sinkholes or subsidences due to dissipation of pore pressure and increase in effective stress in the then exposed soils, but also exposes a deeper profile to the action of ingress water.

B.4.1.3 The potential mobilizing agents should be considered. In the evaluation and reporting procedure, the particular external mobilizing agents or combination of agents to which the blanketing layer might be subjected in future should be clearly indicated. Substantiation should be provided for assumptions made in the deductive process. B.4.1.4 The mobilizing potential of the blanketing layer should be considered. Boreholes drilled during a site investigation should be individually evaluated and their profiles should be characterized by being abstractly subjected to the activity of an assumed mobilizing agency within the context of the selected scenario. This borehole information should also be considered in conjunction with geophysical information. The gravity method should be used as a tool in delineating potentially problematic areas with respect to ground settlement. If, for example, it is assumed that the profile will be subjected to a mobilizing agency in the form of ingress of water, then the potential susceptibility to erosion of the materials within the blanketing layer should be assessed. The susceptibility to consolidation and subsurface erosion, including piping erosion, should be carefully argued, considering aspects such as grading, density and permeability.


25

B.4.1.5 If de-watering of the local dolomite aquifer is possible during the lifetime of the development, then the de-watering scenario should be considered. In a non-de-watering situation, where subsurface erosion is caused by ingress of water, the premature termination of the process might result in subsidence formation rather than sinkhole formation. B.4.1.6 The stability assessment of a site should take into consideration the impact of groundwater level drawdown regardless of the likelihood of the drawdown. A perspective should be obtained of the potential impact of groundwater level drawdown on stability, even within those groundwater compartments not subject to de-watering at the time of investigation. NOTE 1 The potential impact of future de-watering in such instances will assist in compiling an appropriate dolomite risk management strategy, involving monitoring of groundwater levels to preclude such an event occurring. NOTE 2 Areas of shallow groundwater level (less than 30 m below ground surface) are more negatively impacted on by groundwater level drawdown than areas where the groundwater level is deep (greater than 30 m). The positive contribution towards dolomite stability of shallow groundwater levels should only be taken into account where a) the compartmentalization of dolomite groundwater and aerial extent of the compartments have been

determined; b) all groundwater users are identified and the maximum quantities abstracted are known; c) there is evidence to show licensed pumping does not exceed normal natural replenishment; d) there are data showing the groundwater level behaviour patterns during periods of drought and recovery; e) the groundwater level is not located within the dolomite residuum; and f) the local authority or relevant national department (see foreword) has regularly monitored the groundwater

level in the past and continues to do so. B.4.1.7 The characterizations of the individual boreholes within a potential zone should be collectively considered. If several boreholes confirm a particular inherent hazard characterization, that zone should be defined accordingly. The subsurface conditions represented by the various boreholes should be used in conjunction with geophysical data, anticipated karst and dolomite bedrock morphology, and geohydrology to develop a holistic perspective of a sub-area and a composite inherent hazard characterization of the zone. If there are marked deviations, the zoning should be modified by the creation of separate zones, always erring in favour of a conservative (worst-case) assessment. In some instances, it might be necessary to combine different hazard classes. B.4.2 Subsidences B.4.2.1 The susceptibility of the soil material to mobilization, i.e. consolidation settlement under the influence of the mobilizing agency (groundwater level drawdown or surface water ingress), should be characterized using the indicators given in table B.3. B.4.2.2 Isopach or contour maps and cross sections should be considered as useful tools in the evaluation of larger sites (greater than 100 ha). These maps may include a) dolomite bedrock morphology (i.e. elevation of bedrock head) with the OWL (elevation); b) groundwater depths, in metres; c) thicknesses of aquitards, such as Karoo horizons;


26

d) thicknesses of problematic materials, such as dolomite residuum (wad); e) depth to the top of problematic horizons; and f) thickness of overburden below the OWL. Such maps should be contoured based on the gravity contour map, taking the gravity trends into account. Contour maps and cross sections should take into account the presence of groundwater barriers (groundwater compartment barriers).

Table B.3 — Indicators for assessing the inherent susceptibility of the blanketing layer to mobilization

1 2

Inherent susceptibility Indicators

Low

In this type of profile, the groundwater level can be a) above the bedrock and at a shallow depth, thereby reducing the likelihood of ingress of water

eroding the blanket layer; b) in the dolomite bedrock, negating the effect of groundwater level drawdown; or c) in soil material with geotechnical characteristics reflecting a low susceptibility to consolidation

settlement, i.e. material with a high density, low void ratio and a low compression index (e.g. Karoo mudrock) or stiff or dense blanketing material.

Medium

This type of profile is characterized by the absence of a substantial protective horizon and has a blanketing layer of materials potentially susceptible to mobilization by ingress of water. The groundwater level is within the dolomite bedrock or at a depth within the blanketing layer. Voids and disseminated voids might be present above the bedrock, indicating the susceptibility to subsidence formation. Areas of continuous, sub-outcropping or outcropping dolomite bedrock characterized by limited leaching or fissure development (or both).

High

Thin blanketing layer covering intensely leached and jointed or fractured outcropping and sub-outcropping dolomite bedrock

Or

The blanketing layer reflects a high susceptibility to mobilization. The groundwater level is above the dolomite bedrock in soil material with a low dry density, high void ratio, and a high compression index. Any lowering of the groundwater level will change the state of these materials thereby inducing consolidation. Residual dolomite soils, namely wad and ferroan soils, have a high potential for dramatic ground settlement.

Or

The blanketing layer is heterogeneous, not only vertically, but also from one borehole to the next, and is of a moderate to high permeability. Variable, often rapid, drilling penetration times through the overburden are encountered with substantial sample and air loss. Cavernous conditions, cavities, thick deposits of Quaternary material, especially wind-blown sands (suggesting the presence of palaeo-sinkhole conditions), steep gravity gradients, troughs within gravity plateaus, groundwater level within the dolomite residuum, and a lack of control of groundwater level drawdown are typically encountered.

B.4.2.3 Cross sections should be based on the following considerations: a) gravity anomalies (cross sections traversing gravity high and gravity low anomalies);


27

b) orientation of macro-structures and possible displacements (often discernible on the gravity contour map); c) presence of groundwater levels and potential barriers; and d) ground surface topography versus dolomite bedrock topography. B.4.3 Characterization of dolomite land in terms of inherent hazard B.4.3.1 Dolomite land shall be characterized in accordance with 4.5, using a) the sinkhole size determined in accordance with B.4.1; b) the inherent hazard for sinkhole formation established in accordance with B.4.1; c) the inherent hazard of subsidence formation established in accordance with B.4.2. B.4.3.2 Inherent hazard is defined in terms of ingress water and groundwater level drawdown. B.4.3.3 Potential mobilizing agencies (water ingress, ground vibrations, water level drawdown or any activity or process that can induce mobilization of the material within the blanketing layer under the force of gravity) are described in B.2.4. For simplicity, ground vibrations as a mobilizing agency need only be described where applicable. The impact of the mobilizing agencies of ingress water and water level drawdown should be considered in both a de-watering and a non-de-watering scenario. However in a non-de-watering scenario, a shallow static groundwater level is not an agent but potentially a positive, mitigating factor. Consequently, the hazard assessment as set out in the report should address the following: a) Hazard classification with ingress water as a primary triggering mechanism. This classification is based on an OWL that has not fluctuated beyond seasonal limits.

b) Hazard classification with respect to ingress water in the event that the groundwater level/base level of erosion is lowered. This classification indicates the anticipated susceptibility of the profile to the potential action of ingress water in the event that the groundwater level should be lowered at some future date.

c) Hazard classification with groundwater level drawdown as a primary triggering mechanism. This classification is based on an OWL that has fluctuated or is fluctuating beyond seasonal limits. EXAMPLE The classifications above may be reflected as follows: a[b]//c, for example 3[6]//7, where:

a is the inherent hazard class where water ingress is considered as the primary mobilizing agent as described in B.4.3.3(a);

b is the inherent hazard class with respect to water ingress in the event of groundwater lowering as

described in B.4.3.3(b); and c is the inherent hazard class where groundwater drawdown is considered as the primary mobilizing

agent as described in B.4.3.3(c). B.4.3.4 The inherent hazard class and the boundaries of the zone to which it refers should be shown in the drawings.


28

Annex C (normative)

Compilation of a geotechnical model

C.1 General The inherent hazard associated with dolomite land may be evaluated using the scenario supposition method (see annex B) or by rational assessment. Irrespective of the method used, the geotechnical model prevailing across the site should be determined by means of a site investigation so that the dolomite area designation can be ascertained. The geotechnical model forms the basis for the eventual land use, engineering design and precautionary measures to be adopted on a specific site (see figure C.1). C.2 Site investigation A primary aim of any site investigation on dolomite land should be to collect data for the compilation of geological, overburden and geohydrological models. Geophysical surveys, remote-sensing techniques, field information (which might comprise geological mapping, a literature search of previous subsidence, site observations, etc.) may be used to determine the location of data points (boreholes, test pits, etc.) to characterize the site in terms of inherent hazard. After the assessment of all appropriate data, the data should be combined to refine the inherent hazard class zones and zone boundaries. C.3 Geotechnical model C.3.1 General The geotechnical model forms the basis for the determination of land-use type, foundation and infrastructure designs, and the implementation of appropriate precautionary measures. The geotechnical model is made up of three components: a) the geological model; b) the overburden model; and c) the geohydrological model. Compilation of the geotechnical model is based on sound scientific principles and inductive reasoning, and includes processes such as precedent, empiricism, and experience. C.3.2 Geological model The geological model is a three-dimensional presentation of the distribution of the various geological materials occurring on and in close proximity to the site or area to be developed.


29

Figure C.1 — Geotechnical model and inherent hazard class determination

Geology Overburden Geohydrology

Geotechnical model

(see annex C)

Hazard determination

(see tables 2 and 3)

Inherent hazard class

(see table 4)

Dolomite area designation and appropriate development

(see SANS 1936-1)

Risk

Implementation

Foundation design measures

Water precautionary

measures (see SANS 1936-3)

Risk management plan

(see SANS 1936-4)

Monitoring (see SANS 1936-4)


30

The geological model should include at least the following: a) geological setting, dynamics and site geology; b) geomorphology; c) bedrock lithology and karst features; d) presence and distribution of intrusions; e) presence and distribution of historic karst features (sinkholes, subsidences, swallow holes, etc.); f) presence and extent of voids in overburden and bedrock; g) overburden thickness and composition; h) weathering and weathering products; and i) drainage. The data obtained from available information and site-specific investigations are combined to produce a 3-D model of the distribution of geological materials. This model is typically presented in the form of a geological map of the site and idealized geological sections. C.3.3 Overburden model C.3.3.1 The overburden model depicts the vertical distribution of materials overlying the bedrock and is closely linked to the geological model. The properties of the overburden or groundwater (or both) are of significant importance in determining the inherent hazard pertaining to a site. These material properties will influence the percolation of groundwater through the soil profile, the erodability, the angle of repose, and eventual maximum potential size of the surface karst features. One of the more important factors in the assessment of the susceptibility of the subsurface dolomite profile to sinkhole or subsidence formation is the performance of ‘type’ profiles under the influence of various mobilizing agencies. Consequently, the experience of the practitioner is critical to the correct analysis of the anticipated performance of materials constituting the blanketing layer (portion of the overburden overlying potential receptacles). C.3.3.2 The following properties of the overburden are important in assessing the inherent hazard of the profile: a) index properties (grading and Atterberg limits); b) shear strength (particularly cohesion); c) consolidation characteristics; d) permeability; e) erodability; f) dispersiveness; g) mineralogical composition; and h) microstructure.


31

C.3.3.3 In instances where the geological setting is such that the inherent hazard class selection is marginal, laboratory or field testing might may prove useful. For example, where alluvium constitutes a large proportion of the overburden, testing is required to verify whether the geotechnical conditions are inherent hazard class 1 or inherent hazard class 4. The need for field and laboratory tests to determine these properties is left to the discretion of the competent person executing the investigation. C.3.3.4 The execution of field testing and the selection of samples for laboratory testing is often complicated by the significant horizontal and vertical variation in the nature, composition and geotechnical characteristics of soil and rock materials constituting the reworked and in-situ overburden. For example, over very short distances (metres) vertically and laterally in a profile, dolomite residuum typically can vary in colour, moisture content, composition (manganiferous to ferruginous in content), density, grading, soil fabric (in non-reworked soils this varies according to the nature of the original dolomite parent rock, typically laminated or massive), etc. Testing can further be complicated by the presence of chert gravels and slabs in the residual dolomite horizons. The destructive nature of percussion drilling precludes tests on samples from percussion drilled holes and compromises down-the-hole testing due to alteration of the borehole face by disturbance, shearing, smearing, fracturing, etc. The excavation of trenches, often in different directions across the site, can assist in the assessment of the distribution, properties and variability of the upper overburden zone and with the collection of disturbed or undisturbed samples for laboratory testing. C.3.3.5 Useful field tests include a) seepage/permeability tests conducted at various depths, and b) plate load tests in trial holes. C.3.3.6 Useful laboratory tests include a) foundation indicators, b) permeability testing, c) oedometer tests, d) shear box tests, e) triaxial tests, and f) X-ray diffraction. C.3.3.7 Testing of the profile for foundation design purposes should be in accordance with the Site investigation code of practice (see bibliography). C.3.4 Geohydrological model The geohydrological model is a three-dimensional presentation of the distribution and geohydrological properties of the various material types in the overburden occurring on and in close proximity to the site or area to be developed. The critical parameters forming part of this model are a) depth to groundwater;


32

b) position of perched and regional groundwater with respect to bedrock; c) variation of groundwater level over time, taking the likelihood of future de-watering into account; and d) groundwater abstraction and recharge conditions. Geohydrological data can be obtained from the relevant national department (see foreword). Published maps on the location of various groundwater compartments, and the past and present water extraction with historic groundwater movements are available. All the requirements in 4.2.5 shall be complied with, as applicable.


33

Bibliography Standards ISO 31000, Risk management – Principles and guidelines. Other publications Buttrick, D.B., Kleywegt, R.J., Trollip, N., Van Schalkwyk, A., and Watermeyer, R.B. Proposed method for dolomite land hazard and risk assessment in South Africa. Journal of the South African Institution of Civil Engineering. No. 43. 2001. Buttrick, D., Trollip, N., Watermeyer, R., Pieterse, N., Gerber A. ‘A performance based approach to dolomite risk management’ in Environmental Earth Sciences, Springer-Verlag Volume 64, Issue 4, Page 1127-1138. 2011. Site investigation code of practice 1st Edition. South African Institution of Civil Engineering – Geotechnical Division. January 2010. Wagener, F. von M. Dolomites: problem soils in South Africa – State of the Art. Vol 27 Issue 7, Civil Engineer in South Africa. 1985.

© SABS

Documents

Document number SANS 1936-2 - Geotechnical Division 96-10 _DSS_ 1936-2.pdf · DRAFT SOUTH AFRICAN STANDARD (DSS): PUBLIC ENQUIRY STAGE Document number SANS 1936-2 Reference 7135/1936-2/SP