Lecture jijij

8/10/2019 Lecture jijij

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CIVL6077 Ground Investigation and Soil Testing

Lecture 3

Dr. Fiona KwokRoom 521, Haking Wong Building

The University of Hong Kong

Tel: (852) 2859 2655

Email: [email protected]


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CIVL6077 Ground Investigation and Soil Testing

Sampling, Boring and Drilling


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3.1 Introduction


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Soil sampling and classification on MARS!

(Grotzinger & Vasavada, 2012)


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Ground Investigation (2)Aims of ground investigation:

Determine the sequence, thickness and lateral extent of soil strata and the level

of bedrock.

Obtain representative samples of soils and rocks for identification and

classification.

If necessary, carry put in-situ tests on site to assess soil characteristics.

If necessary, perform laboratory tests on high quality undisturbed sample to

determine relevant soil parameters.

Identify the groundwater conditions and any contamination locations

(Knappett and Craig, 2012; Lehane, 2010)

Stage 1: Research + Walkover Desk study report

Stage 2: Site investigation In-situ/ Lab testing Interpretation Report

Stage 3: Construction & Post-construction monitoring

Stages of si te investigation:


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Site Investigation

(AGS, 2006)

Field characterisation methods:

1. Drilling and sampling

Trial pits

Soil borings

Drive/ rotary samplers

2. In-situ tests

Standard Penetration Test (SPT)

Cone Penetration Test (CPT)

Dilatometer Test (DMT) Pressuremeter

Vane Shear Test (VST)

3. Geophysical methods (coveredlast lecture)

Mechanical waves (S-wave, P-wave)

Electromagnetic (radar,resistivity)


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3.2 Sampling


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What quality of samples are required? 1) Undisturbed 2) Disturbed

High quality undisturbed samples are important for determination of shear

strength and consolidation properties, as engineering properties of soil are

connected tightly with its structure and fabric. Disturbed samples are used

principally for classification and description only.

Undisturbed samples are generally taken by cutting blocks of soil or rock, or

by pushing or driving tubes into the ground.

Disturbed samples are taken from cuttings produced by the drilling process.

Disturbed samples will have the same particle size distribution as the in-situsoils, but the soil structure will have been significantly damaged. Also, the water

content may be different from that of the in-situ soil.

It is impossible to obtain 100% undisturbed samples, as any sampling

techniques (no matter how precise it is) will induce a certain degree of

disturbance.

It is generally believed that undisturbed sampling is not possible in granular

soils. In addition, soft clays are extremely sensitive to sampling disturbance, the

effects being more pronounced in low plasticity clays.

Disturbance of samples causes underestimation of strength, overestimation ofsettlements and makes a mockery of laboratory permeability testing.

Sampling (1)


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(Clayton et al, 1995)

Driven samplers: pushed into the soil without rotation; displaces soil as they

penetrates

Rotary samplers: rotated and pushed downwards, while cutting and grindingsoils beneath it.

Sampling (2)

Sample quality classif ication (BS5930:1999):

Undisturbed

Disturbed

No geometric distortion

Geometric distortion

Density altered

Density & water content altered

Particle size distribution altered


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(Geoguide 2; BS 5930)

Sample Quality Class


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Disturbed Sampling Disturbed samples can be taken by boring tools or excavation by hand or

equipment.

Disturbed samples can be obtained to great depth but their quality are usually

Class 3 or poorer.

Drive Sampling Drive samplers are samplers which are either pushed or driven into the soil

without rotation. The volume of soil corresponding to the thickness of thesampler wall is displaced into the surrounding soil, which is either compacted or

compressed.

Drive samplers can be divided into 2 broad groups: open-drive samplers and

piston drive samplers.

Open-drive samplers consist of a tube which is open at its lower end, whilepiston drive samplers have a movable piston located within the sampler tube.

Piston samplers can be pushed through a soft soil to the desired sampling level,

but open-drive samplers will admit soil as soon as they are brought into contact

with, for example, the bottom of a borehole.


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Normally 50 mm, 76 mm and 100 mm in diameter and 450 mm in length.

It is a robust sampler but the driving action (either by dynamic or static) meansintroduces some disturbance.

Suitable to sample fine and coarse-grained soil but unsuitable to sample very

coarse soil. Can be used to considerable depth, the highest quality samples obtained is Class

2.

Advantages:

Cheap

Simple to operate

Disadvantages:

Poor cleaning of the borehole before sampling, or collapse of sides of the boreholeafter cleaning may mean that much of the recovered soil is not only highlydisturbed, but also non-representative.

The use of a large area ratio can induce soil displaced by the sampler drive andcausing large-scale remoulding of the sample.

Open-drive Samplers



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SPT Liner Sampler

This is the sample collected while performing the Standard Penetration Test

(SPT).

It takes 35 mm samplers to recover small samples, thereby avoiding to use the

larger diameter open tube sampler.

The first 150 mm drive is not counted. The SPT N-value is the blow counts for

subsequent 300 mm penetration.

The test is terminated if N > 100.

It recovers Class 3 or Class 4 samples (disturbed).



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Driven Tube Sampler (U100) (1)

Waxed and sealed afterremoval from ground

100 mm diameter tube (metal/ plastic)

(Binns, 2007)


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Driven Tube Sampler (U100) (2)

Heavy shear distortion for laminated, soft clays

(Craig, 2004; Binns, 2007)

How disturbed is undisturbed?

Reasonably good for firm and stiff clays

How is disturbance measured?

The area ratio of the sampler

determines the volume of soil

displaced by the sampler as a

proportion of the sample volume.

The lower the area ratio, the lower the

degree of sample disturbance

Area ratio =

(Good samples: < 10%)

U100 characteristics

Easy to use, cheap, simple to

operate

Ideal for sites with limited access

Damages material soil

structure disturbed/ destroyed

Sample class 2 or 3 can be

achieved


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(GEO Guide 2)

OpenDriveSampler


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Piston-drive Samplers

It is a thin wall sampler where a piston inside a barrel remains stationary when the

barrel is driven down by static thrust.

The sample size are 76 mm, 100 mm and up to 250 mm.

Suitable to sample very soft to firm soils to considerable depth.

The highest quality samples obtained is Class 1.

Pistons have been included in sampler designs in order:

a) to prevent soil entering the sampler tube before the sampling position is reached.

b) to reduce losses of samples, by providing an efficient airtight seal to the top of the

soil in the tube during withdrawal. Any tendency of the sample to slide out of the

tube is counteracted by pressure decrease above the sample.

c) to reduce the entry of excess soil into the tube during the early stages of sampling,

as a result of using a relatively high area ratio, and to prevent too little soil entering

the sampler at the end of the drive, as a result of the build-up of internal friction

d) to increase the acceptable length to diameter ratio. Adhesion between the tube

and the soil entering it will tend to reduce recovery once large length/diameter

ratios are reached, but the movement of the top of the sample away from the

underside of the piston will form a vacuum which will tend to increase the recovery.



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Piston Drive Sampler (1)

(http://www.esnnw.com/discrete.html)

How does it work?

(A) The piston sampler enters the borehole with a sealed drive point locked in place. This

prevents the soil from entering the tube while it is driven to the top of the desired

depth.(B) The pin is released from the surface with an extension rod, releasing the drive point.

(C) The sampling tube is pushed ahead into the soil keeping the piston fixed in ground

and then collecting the sample in the tube.

(D) The core sample is vacuum sealed and recovered using the pressure developed

during the sample advancement.


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Piston Drive Sampler (2)

(Binns, 2007)

Piston sampler characteristics

U100 sample in soft laminated clay

Piston sample of soft laminated clay from the same location

Useful for collecting undisturbed samples from soft to stiff clay and sand

Piston sampling prevents the entry of debris before sampling, they reduce the entry

of excess soil during sampling and they largely eliminate sample losses

Have low area ratios

Can produce Class 1 samples

Piston sampling can guarantee

Class 1 undisturbed samples in

suitable soils


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(GEO Guide 2)Thin-walled Stationary Piston Drive Sampler


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The aim is to sample a cylindrical column of soil into a core barrel by removing

the surrounding soil by the rotating action of the drill bit.

Drilling fluid is pumped down to the drill rod to lubricate, cool down the drill bit

and flushes the drill debris up the borehole.

The drill hole is normally protected from collapse by casing in sampling soil.

The core barrel can be single-tube, double-tube or triple tube.

Rotary corebarrels designed to sample the harder materials encountered during

site investigations can be classified into 3 broad groups:

a) Refracted corebarrel: Corebarrels with retracted inner barrels, such as theconventional double-tube swivel type corebarrel.

b) Protruding corebarrel: Corebarrels where the inner barrel protrudes ahead of

the outer barrel, in an attempt to protect the ground being sampled from the

deleterious effects of flush fluid, such as the triple-tube Denison corebarrel.

c) Retractor corebarrel: Corebarrels where the inner barrel is spring mounted, so

that it protrudes in relatively soft ground, but retracts when harder layers are

encountered, such as the triple-tube Mazier corebarrel.

Rotary Sampling



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Most rugged, least expensive

Consists of head section, recovery tube, reamer shell and

cutting bit

They are seldomly used as the corebarrel rotates directly

against the core

The core recovery is usually unsatisfactory.

It is used in hand portable rotary core drills for coring

through concrete and masonry walls horizontally to enable

wall thickness and backfill materials to be determined(disturbed samples).

Single-tube Barrel


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Double-tube barrel is the basic standard.

Outer barrel rotates with the cutting bits. Inner barrel is either fixed or swivel

type (with bearings) that retains the core sample.

Double-tube barrel does not protect the core from drilling fluid. Core is extracted from core barrel by hydraulic action.

It is normally used for good quality moderately weathered to fresh rock.

Unsuitable for soil sampling except in coring large boulders.

Double-tube Barrel


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Non-retractable triple-tube barrel incorporates a third tube to protect the core

even further from drilling action/ fluid damage. It can have either a split tube,

which is removed, or a plastic tube to provide longer term protection. A less

effective (cheaper) alternative is to incorporate a nylon liner in a double tube. It

collects up to 100 mm samples. High quality Class 1 samples can be obtained from colluvium using large

diameter triple-tube corebarrels in conjunction with air foam as flushing medium.

Retractable (Mazier) triple-tube barrel is a variation where the inner tube isattached to a retractor and can extend beyond the cutting edge. This gives

complete protection to the core in softer rock whilst in harder rock where this is

not necessary, it retracts to become a standard triple tube. This is used in

alternating soft/hard rock, typical of a weathered profile. It collects up to 74 mm

samples. Samples of Class 1-2 can also be obtained using the Mazier sampler in

conjunction with air foam or water as flushing medium.

Unsuitable to sample loose sand and soft clay.

Triple-tube Barrel


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Sampling tools for different types of soil and

rock


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Undisturbed Sampling

A high quality undisturbed (Class 1) sampling can:

a) preserve soil texture - identification & classification

b) preserve soil structure - relict joints, kaolin veins etc.

c) preserve soil engineering properties - shear strength, stiffness, density,

compressibility, permeability, etc.


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Block Sampling

(Lehane, 2011)

Normally obtained from excavations or exposures or test pits

Firm through stiff to very stiff cohesive materials can be sampled

300 mm cube sample recovered

Block sample characteristics

Large volume of material can be obtained

Class 1 sample can be retrieved, even in soft clays and weathered rock

Multiple test types can be performed

Very expensive and timely to perform block sampling

Skilled workers required

Hand-held sampling toolsBlock sampling in excavations Retrieved cube samples wrapped to

avoid loss of water content


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3.3 In-situ Subsurface Exploration


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Subsurface Exploration: In-situ methods (1)


Geophysical (indirect) methods may be useful for investigating the lateral

variability of the ground, but their results can only be used in qualitative rather

than quantitative terms (although they are by far the best method for probing

ground stiffness).

In most practice, parameters for engineering design are derived from in-situ(direct) tests carried out in boreholes or from self-penetrating probe. Most

profiling is done on the basis of soil and rock descriptions, carried out either on

samples obtained from boreholes, or on the faces of trial pits or shafts. And the

majority of classification and index testing is carried out on samples taken from

boreholes and trial pits. In-situ methods provide the opportunity to obtain samples for visual description,

index testing, parameter determination and installation of instrumentation.

There are, in general, 5 categories of in-situ sub-surface exploration methods:

1. Boring

2. Drilling

3. Coring

4. Probing

5. Trial Pits and Trenches


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Subsurface Exploration: In-situ methods (2)


1. Boring is carried out in the relatively soft and uncemented ground (engineering

soil) which is normally found close to ground surface.

2. Drilling has traditionally been used in the more competent and cemented,

deeper deposits (engineering rock). It is now also widely used to obtain high-

quality samples of heavily overconsolidated clays, for specialist laboratorytesting. Both of the above methods can produce holes to great depths, which

can be used for in situ tests as well as for sampling, and can allow the

installation of instrumentation (for example, to measure groundwater pressures).

3. Coring is used to extract continuous intact cores of hard rock and building

materials (e.g. concrete) for further testing or quality check. It could not provideinformation about lateral variability.

4. Probing is increasingly being used as a cheap alternative to boring and drilling.

It is used as a qualitative guide to the variation of ground conditions, and is

particularly valuable for profiling. The techniques used are often fast, but they

cannot be used to obtain samples or to install instruments.

5. Examination in situ, by trial pits and shafts, provides by far the best method of

recording both the vertical and lateral ground conditions. Borehole methods

generally only take restricted samples, perhaps at every metre or so of depth,

for engineering description.


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Soil Boring and Drilling

Excavation using

hand and shovel

Power-assistedexcavation drill


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3.4 Boring


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1. Boring


The principal methods used for advancing boreholes to obtain samples or detailsof soil strata include:

a) Light percussion boring

b) Power augering

c) Hand-operated augering

d) Washboring

1a) Light percussion boring

Tripod rig towed by standard four-wheel drive vehicle.

The rope from the winch drum passes over the pulley at the top of the rig frame

and is used to raise and lower a series of weighted tools on to the soil being

drilled.

For clays, progress is made by dropping a steel tube known as a claycutter intothe soil.

For granular materials, such as sands or gravels, a shell is used. The shell is

bored with water surging to loosen the soil while it is being advanced.

This method is capable of reaching over 60 m in stiff clays.


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Light Percussion Boring (1)


For soil exploration.

Limited use in rock.

Cheaper than rotary

drilling.

Severe restrictions in

Hong Kong due to

common occurrence of

boulders and

corestones which wouldneed to be chiselled.


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Light Percussion Boring (2)


Light percussion boring tools


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Power Augering (1)


1b) Power augering

Bucket augers consist of an open-topped cylinder which has a base plate with

one or two slots reinforced with cutting teeth, which break up the soil and allow

it to enter the bucket as it is rotated.

The top of the bucket is connected to a rod (called the Kelly rod) whichtransmits the torque and downward pressure from the rig at ground level to the

base of the hole.

Bucket augers are used for subsurface exploration in the USA, but are rarely

used in Hong Kong because of relatively hard rock in the city.


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Power Augering (2)


Flight augers may be classified as short-flight augers or continuous- orconveyor-flight augers.

Short-flight augers consist of only a few turns of flight above cutting teeth or a

hardened steel edge.

The principal limitation of short-flight augering is that the hole depth is restrictedto the length of Kelly rod which the rig can handle (3-6m).


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Power Augering (3)


The problems of deep drilling with shortaugers are largely overcome by the use of

continuous or conveyor augers.

Continuous augers can be classed as: (i)

solid stem continuous-flight augers or (ii)hollow stem continuous-flight augers.

Solid-stem continuous-flight augers allow

much deeper holes to be drilled with fewer

problems.

It presents a serious problem in siteinvestigation because soil moving up from the

base of the hole is free to mix with the soil at

higher levels on the edge of the borehole.

Mobile small-diameter solid

stem continuous-flight auger

(CFA) rig


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Power Augering (4)


Hollow-stem augers consist of an outer spiral continuous flight with a separateinner rod which blocks off the base of the hole when the auger is being

advanced.

When samples are required, the inner rods and plug are removed and samples

can be taken from the material below the base of the auger.


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Power Augering (5)


Advantages of Hollow-stem CFA:

Ideal method of producing site investigation holes, because it is often fast and

reliable.

No drilling fluid is used.

Soil sampling and testing through the auger

Natural gamma logging of formation through augers

Limited, controlled hole caving

Fast and efficient drilling and sampling

Disadvantages of Hollow-stem CFA:

The small samples obtained are not suitable for the determination of undrained

shear strength and consolidation properties for fissured clays or soils with fabric.

There are considerable dangers of disturbing soil ahead of the auger. Heavydownward thrust may cause the auger to be forced into the soil, displacing

material ahead of it instead of boring through it.


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Hand-operated Augering (1)


1c) Hand augering Hand augers provides a light, portable method of sampling soft to stiff soils near the

ground surface.

The method is cheap because of the simplicity of the equipment.

The most commonly used auger for site investigation is the Iwan auger. This is

normally used at diameters of between 100 and 200 mm. Small helical augers arequite effective in stiff clays, but become difficult to use once the water table isreached.

Barrel augers are now rarely seen, but were formerly used with the light percussionrig when progress through clays was made using a shell. They allowed the base of

the borehole to be very effectively cleaned before sampling took place. In stiff or very stiff clays, hand-auger progress will be very slow, and the depth of

boring may have to be limited to about 5 m. When such clays contain gravel, cobblesor boulders it will not normally be possible to advance the hole at all. In uncementedsands or gravels, it will not be possible to advance the hole below the water table,since casing cannot be used and the hole will collapse either on top of the auger

(which makes it difficult to recover the auger from the hole) or when the auger is beingremoved.

Only samples of very limited size can be obtained from the hole. In addition, it will notbe possible to carry out standard penetration tests without a frame to lift the triphammer and weight, so that no idea of the relative density of granular deposits can be

obtained.


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Hand-operated Augering (2)


Not popular in Hong Kong because of boulders and corestones


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Washboring (1)


1d) Washboring

Washboring is a relatively old method of boring small-diameter exploratory holes

in fine-grained cohesive and non-cohesive soils.

A very light tripod is erected, and a sheave is hung from it. In its simplest form

there are no motorized winches and the drilling water is pumped either by hand,or by a small petrol-driven water pump.

Hollow drilling rods are connected to the pump via a flexible hose, and the drilling

crew lift the string of rods by hand, or using a rotating steel drum.

Progress is made by jetting water out of a bit at the base of the rods. These are

continuously turned using a tiller, whilst being surged up and down by the drilling

crew.

Cuttings of soil are carried up the hole by the drilling water (the flush) and

emerge from a casing T-piece, being deposited in a sump.

The method should not be used above ground-water level when undisturbedsamples are desired of the soil above this level, since the water will enter the soil

below the bottom of the hole and change its water content.


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Washboring (2)



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Washboring (3)


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3.5 Drilling

2 D illi


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2. Drilling

a) Rotary Drilling

Drilling has traditionally been used in the more competent and cemented, deeper

deposits (engineering rock).

It is now also widely used to obtain high-quality samples of heavily

overconsolidated clays, for specialist laboratory testing. Both of the above methods can produce holes to great depths, which can be

used for in-situ tests as well as for sampling, and can allow the installation of

instrumentation (for example, to measure groundwater pressures).

The drill bit or casing shoe is rotated on the bottom of the borehole.

It is the most common method of subsurface exploration method in Hong Kong.

The drilling fluid, which is pumped down to the bit through hollow drill rods,

lubricates the bit and lushes the debris up the borehole.

Types:

Open hole (or full hole) drilling (no need of a sample)

Core drilling (need of a sample)

R t D illi (1)


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Rotary Drilling (1)


Rotary drilling requires a combination of a number of elements:

a drilling machine or rotary rig, at the ground surface, which delivers torque and

thrust;

a flush pump, which pumps flush fluid down the hole, in order to cool the

mechanical parts and lift the cuttings of rock to the ground surface as drillingproceeds;

a string of hollow drill rods, which transmit the torque and thrust from the rig, and

the flush fluid from the flush pump to the bottom of the hole; and

a drilling tool (e.g. a corebarrel), which grinds away the rock, and in addition may

be designed to take a sample.

Rotary drilling uses a rotary action combined with downward force to grind away the

material in which a hole is being made.

Rotary methods may be applied to soil or rock, but are generally easier to use in

strong intact rock than in the weak weathered rocks and soils.


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(GEO Guide 2)


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Open hole Drilling


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Open-hole Drilling


The formation of a hole in the subsoil without taking intact samples is known asopen-holing.

Such methods are usually used to drill through soft deposits, which have been

previously sampled by light percussion or auger rigs.

The drill bit cuts all the material upon contact and within the diameter of theborehole.

Sampling during open-holing is usually limited to collecting the material abraded

away at the bottom of the borehole, termed cuttings, as it emerges mixed with

flush fluid at the top of the hole.

Drill debris brought to the surface in the flushing medium can only provide anindication of the soil/rock material and ground conditions being encountered.

It is useful for rapid advancement of a borehole required for field testing or

instrument installation.

Drill bits for open-holing

Core Drilling


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Core Drilling


The most common use of rotary coring in ground investigations is to obtain intactsamples of the rock being drilled, at the same time as advancing the borehole.

To do this a corebarrel, fitted with a corebit at its lower end, is rotated and grinds

away an annulus of rock.

The stick of rock, the core, in the centre of the annulus passes up into thecorebarrel, and is subsequently removed from the borehole when the corebarrel

is full.

The length of core drilled before it becomes necessary to remove and empty the

corebarrel is termed a run.

The basic components required for test or core borings include:

a) Drilling machine

b) Casing

c) Drill rods

d) Barrels

e) Drilling bits

Drilling Machine


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Drilling Machine Drilling machines consist of a power source, a mast for lifting apparatus, and a

pump for circulating water or mud (or a compressor for air drilling) to lower,

rotate, and raise the drilling tools to advance the hole and obtain samples.

Test borings for obtaining representative or undisturbed samples under all

conditions are normally made by rotary drills, and under certain conditions, with

the tripod, block, and tackle.


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Drilling in urban areas in Hong Kong

Rotary drilling rigs are available in a

wide range of weights and powerratings in Hong Kong.

Normally skid mounted

Working platform for drilling on slope

(GEO Guide 2)

Casing


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Casing Casing is used to retain the hole in the normal drilling operation, with tripods or

rotary machines, at the beginning of the hole and under Hole Stabilization.

Drilling cost is related directly to casing and hole size.

Drill Rods


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Drill Rods Drill rods connect the drilling machine to the drill bits or sampler during the normal

test or core boring operation with rotary machines.

Selection is a function of anticipated boring depth, sampler types, and rock-core

diameter, and must be related to machine capacity.

The more common diameters are as follows: A rod is normally used in wash boring or shallow-

depth rotary drilling to take SPT samples.

B rod is often used for shallow rotary core drilling,

especially with light drilling machines.

N rod is the normal rod size for use with large

machines for all sampling and coring operations. It is

especially necessary for deep core drilling (above 20

m).

H rod is used in deep core borings in fractured rocksince it is heavier and stiffer than N rod and will

permit better core recoveries.

Barrels


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Barrels

The corebarrel is the normal equipment for recovering samples of rock in siteinvestigation.

In its simplest form, the corebarrel consists of a single tube with an abrasive

lower edge which is loaded and rotated while a flush fluid is passed around the

bit under pressure.

First, the core inside the barrel is subjected to rotational forces due to the friction

of the inside of the barrel against the outside of the core, because the core (being

attached to the parent material) does not rotate.

Secondly, the flush fluid passes over the surface of the core continuously while it

is inside the barrel during drilling. When the flush fluid passes continuously over the core inside the barrel, erosion

will occur. To counteract these two effects the double-tube, swivel type corebarrel

is now commonly used as standard.

Triple-tube barrels are identical to double-tube barrels except that a tight-fitting

liner tube is used inside the inner barrel.


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Triple-tube Corebarrel (1)


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(GEO Guide 2)

Triple tube Corebarrel (1)

Mazier-type triple-tube corebarrel

Triple-tube Corebarrel


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(GEO Guide 2)

Triple tube Corebarrel

Non-retractable triple-tube corebarrel

Wireline Drilling


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g

Wireline drilling is a technique commonly used nowadays,principally because it reduces the trip time (i.e. the timenecessary to extract the corebarrel from the bottom of the hole,empty the core and replace the barrel).

This technique has become well established on high qualityground investigations, and has proved particularly effective inthe coring of relatively difficult deposits, such asoverconsolidated clays, chalks, and interlayered sands, gravels,limestones and clays.

Wireline drilling does not use any outer casing, but instead usesan outer barrel which extends at full diameter to ground level.

The inner barrel is lowered through the full length of the outer

barrel, on a wire line. When it reaches the bottom of the hole it latches inside the

outer barrel, in the correct vertical position. The outer barrel isthen turned by the rig, as flush is pumped down it. The latchingmechanism holds the inner barrel down, but does not fix it sothat it must rotate with the outer barrel.

When the outer and inner barrels have been drilled for thelength of the run, the wire line is winched upwards, and thelatching mechanism automatically disengages the inner barrelfrom the outer.

The inner barrel and core are hoisted to ground surface, wherethe core is extracted and a new length of outer barrel is addedto the string.



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Drilling Bits


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Drilling bits are used to cut soil or rock. Chopping bits are used for wash borings.

Drag bits (fishtail or bladed bits) are used for rotary soil boring. They are provided

with passages or jets through which is pumped the drilling fluid that serves to

clean the cutting blades. The jets must be designed to prevent the fluid stream

from directly impinging on the hole walls and creating cavities, or from directingthe stream straight downward and disturbing the soil at sampling depth.

Rock bits (tricone, roller bit, or Hughes bit) are used for rock drilling.

Core bits (tungsten carbide teeth or diamonds) are used for rock coring while

advancing the hole.

The variables in a corebit design are:

face contour;

cutting material; diamond types, grades and sizes;

mounting matrix;

waterway size, shape and position; and

kerf width.

Geometric relation of casing to drilling bits


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(GEO Guide 2)

Flush Fluids


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Flush fluid is passed around the bit while drilling proceeds. The purpose of the fluid is:

a) to remove the cuttings from the borehole

b) to cool the drilling bit, and drill rods;

c) to reduce mechanical and fluid friction; andd) to help to retain an open hole wherever possible, without the use of casing.

A large number of different types of flush fluid are in use:

a) water-based (e.g. water, bentonite/water (drilling mud))b) oil-based

c) air (or mist)

d) stable foam.

Water is commonly used as a flushing medium in rotary drilling but may have adeleterious effect on both the stability of the surrounding ground and on the

samples obtained. Its use must be carefully considered.

Replacement of water by air foam may be required but more expensive.



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Drilling Mud as Flushing Agent


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Drilling mud = a thin mixture of water and bentoniteAdvantages:

It is more viscous and can lift cuttings adequately at a lower velocity.

It will cake the edges of the borehole, and the outside of the core, and will largely

eliminate the seepage of water out of the borehole, thus reducing problems of

loss of return.

Smaller volumes of flush fluid will be required and the fluid may be recirculated

via a settling tank (where the cuttings are allowed to drop out of suspension).

The cake formed on the outside of the borehole has the effect of considerably

improving the stability of the borehole, provided the flush fluid head is maintainedhigher than that of the groundwater.

Disadvantages:

Drilling mud are difficult to dispose of, at the end of drilling a borehole. The mud

cannot simply be tipped on the site, and it cannot be discharged into nearbysewers.

Bentonite mud must be properly mixed, using appropriate equipment, in order to

ensure that it is of the correct consistency and does not contain unmixed dry

bentonite lumps, capable of clogging flush ports in the corebarrel.


Portable Drilling Equipment


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These drilling equipment are very portable. They can be mobilized/set up within1-2 days.

Light-weight, can be mobilized quickly without the need for heavy scaffolding

Lower in cost, gets more holes within the same budget and quicker than

conventional drilling methods

Capable of drilling down to 15 m for in soil and rock with the installation of casing

Creates minimum impact to the environment

(GEO Guide 2)


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(Hunt, 2007)


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3.6 Coring

3. Coring


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(GEO, 2010)

Rotary core drills are useful for coring through concrete or masonry retainingwalls, as well as some naturally occurring substance (e.g. rock).

It is mounted horizontally to core for determining the wall thickness and backfill

materials.

In the coring process, the sample is pushed more or less intact into the tube.

Removed from the tube in the laboratory, it is inspected and analyzed by different

techniques and equipment depending on the type of data desired.


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3.7 Probing

4. Probing Probing is increasingly being used as a cheap alternative to boring and drilling


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Probing is increasingly being used as a cheap alternative to boring and drilling.

It is used as a qualitative guide to the variation of ground conditions, and is

particularly valuable for profiling.

The techniques used are often fast, and are generally cheaper than boring and

drilling, but they cannot be used to obtain samples or to install instruments.

The objective is to provide a profile of penetration resistance with depth, in order

to give an assessment of the variability of a site.

It produces simple results, in terms of blows per unit depth of penetration, which

are generally plotted as blowcount/depth graphs.



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Dynamic Probing4b) Dynamic Probing


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4b) Dynamic Probing

Dynamic probing involves driving a solid cone into

the ground, using repeated blows of a hammer with

a fixed mass falling through a fixed distance.

Typically the rate of driving is between 15 and 30

blows per minute.

As the cone is driven into the ground the number of

blows required to drive it each increment (typically

100 mm) is recorded.

The blow count is plotted against depth to provide acontinuous profile of penetration resistance with

depth.

Any build-up of friction between the rods and the

surrounding soil will clearly influence the measured

penetration resistance, and so several tests makeprovision for this to be reduced, either by pouring

mud or water down the outside of the rods, or by

pumping mud down the rods to a flush port just

above the tip, or by the use of casing.



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3.8 Examination In-situ

5. Examination In-situ/ Trial Pit This is the most direct method of examining the soil condition by excavating to


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g y g

the required point of interest for direct inspection and sampling.

5a) Trial Pit

Trial pits provide the best method of obtaining very detailed information on

strength, stratification, pre-existing shear surfaces, and discontinuities in soil.

Very high quality block samples can be taken only from trial pits (and therefore

expensive) .

Trial pits may be excavated by either hand digging or machine excavation.

In general, machine excavation is used for shallow pits, whereas handexcavation is used for deep pits which must be supported.

In the limited space of a trial pit, which is often 1.5m x 3m in plan area at ground

level, it is usually impossible to place supports as machine excavation proceeds.

Shallow trial pits provide a cheap method of examining near-surface deposits insitu, but the cost increases dramatically with depth, because of the need to

support.


Trial Pit It is noted that every year many people are killed during the collapse of


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unsupported trenches (high risk to workers!) so do not enter trenches or pits

more than 1.2m deep without either supporting the sides or battering back the

sides.

Trial-pitting permits the soil to be examined in-situ and allows undisturbed block

sample to be obtained. The pit should be supported to prevent soil collapse and water should never be

allowed to get into the pit.

The pit should be properly backfilled in compacted layers.



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(GEO Guide 2)

Examp

leofTrialPitLog


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Slope Surface Stripping5b) Slope surface stripping


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Many slopes are protected with a thin layer of chunam (a lean soil-cement mix) or

shotcrete.

Stripping of this surface protection is required for examination/logging of soil

underneath.

Light scaffolding and working platform are necessary to enable stripping and

logging to be carried out.(GEO Guide 2)


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Slope Surface Stripping in Hong Kong General Specification of Civil Engineering Works (CEDD) set out the guidelines

f l f t i i S Cl 7 34


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for slope surface stripping. See Clause 7.34.

The stripping should be at least 500 mm wide extending to the maximum height

of the slope, and located to pass through areas of different surface features such

as boulders protruding from the face, or where seepage can be seen. The underlying Common Ground shall be excavated to a minimum depth of 100

mm and up to 300 mm as instructed by the Engineer.

All excavation shall be terminated if boulders, rock or hard strata are

encountered.

(GS, 2007)


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Large bored shafts (2)


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(GEO Guide 2)

Examp

leofLogSheetfor110mdee

ppitinUK

Borehole Cameras5d) Borehole Cameras

TV d b h l b l d i id l ti l


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TV and borehole cameras can be placed inside a relatively

small hole (75-150mm) and can, therefore, be used with

conventional drilling methods to examine deep features.

TV cameras are usually used to examine the sides of a

borehole for jointing and other features, and to investigatethe extent of old mine workings.

Simple borehole cameras can be constructed by placing

fairly conventional photographic equipment within a drill

barrel, but this type of equipment is only of use in

examining subsurface cavities.

Optical methods of examining the sides of boreholes or

drillholes can only be relied on under the most favourable

conditions, when the hole is dry.

These devices cannot give results in the muddy conditionswhich normally exist in water-filled holes.



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(SAM Manual, 2011)

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