al Administration, Ca

anatysis is often considers ttlernsnt of road smban P

asis is therefore given to

SIS

erp van padopvullings word

ontledlng.

kument handel oor die roeline of eenvoudi rd. Vir meer ingewikkeMe gevalie m e t daar van spesialis geategniese ku

gter nie dat geotegnisse kund ord nie. Dit word benad Die gebruik van toep

beveel dat sekere for

nvenience the investigation a B

carried out in stages. These ra

sophisticated invest

the earlier stages in some detail

ith the more complex prob

not identified and not because th

ation and design

process described, although se

reassd geometric standards for modern roads havs led to much h

eased capital costs and utilisation have demande

these factors Rave led to the necess

votvgd in embankment most obvious and

atbn are necessarily c

B A S I C STEPS IN SYS'$EM P--

A . AIM OF INVE

M = METHODS OF INFORMATION

$ : EVALUATION C R l T E W l A

Q 2 DECISION

STAGES IN S Y S T E M p-.

S I T E l ! \ cVESTIGATIQfy --p----

L licl . d

I s ? S T A G E

~---.1.1

2 nd S T A G E

seen as a ration

nerally of increasi

er each of the sta

ective, of the invest

s construded ecommically.

reas of the country in

isture contents on slope stabiliB

Iso directly related to embankment heights. Hence, in

re probable than for low embankments. Cogni

e relative probabilities of distress. It is, never-tta

nce to the embankment height to the exten% that

mbartkments ar ere are for example, numerous embankments below 5m height which have

th excessive settlement a instability and, conversely,

ssibility of distress.

the process of evaluating the res

aluation is no more than straightf

because it is to

ms. In other cases, it may be

ses, the evaluation phase may be the

hasized that evalu

re,

separate formal pr

METHOD

ul lsy dlpt probably very thin

DECISION

DECISIO Signad A B Charles

nvestigatbn, ie pits, auger holes, reholes, etc. shouM th

reholes so that maimurn information is obta

embankments cross alluvial ds its there will probably be a

ation for the structur

e boreholes to provide informatbn for both the ernbankme

t the structure will have to be piled and information

he rock quality. These aspects are of little relevanc

ch information on the overtying soil is required. The type of the

then be defined by the requirements of the embankment,

to h piled, information on the soil cover is still required in order to choose the

sed embankments are fairly I e, information from conve

icient for the embankment de . If further boreholes or pro

sitbned with care. The topography and the arr photos, and even th

on as to where further investigation should be located. Onl

es at regular chainages and at standard offsets from the c

sitioning of boreholes grobabty reflects the most pessimist

o account to avoid excessive consewatism.

slope, for exampia, W

ith small crossfalls, ie

es of failure, are essentially similar to flat

ntinuities, such as fau&s or

to assess their

n and construction,

ossfallis, there are not likely to e large depths of

tx>t necessarily require such conditions.

use instability. Colluvium djacent to the main

of the investigation a appear to have a sat

e after construction, bue to a

arameters based on the initia

se sites are similar to those encountered at

ter course exists. It is likely that there

to the crossfali, and it is im

ent construction.

particularly aimed at ex

investigation. In partimlar, v

he same contrac? It is

ugh to the other; it coul

ught that further investigation ma

wide just sufficient information so fh

sy to achieve and all too often the invesli

01%; frequently, th re expensive, at? vious, mistake is

by excessive laboratory testing, the results from

, nor used in the design

o n the trial pits,

The method 01 descri

f that stratum then

t appropriate investig

example, a series

tes, a l thugh some borehol

rns are classified under stab~lrty and/or settlement. The relevant soil charaaeristics

th of the subsoil is required to estimate the stability of slopes. This can

of the material, or from tn-situ tests, such as the CPT, SPT and vane shear

evaluation of the tests and S equent analyses are

sign. if these indicate stability problem, re sophisticated test

ial testing. The shear strength of clays is

h, c, or the drained stre

ely the situations of a rapid applrcation of load,

licalion of bad, which

tests. The emphasis S

of the distribution

blern, then carefutly c

never be justified.

earth structures.

asic informatton can, hawev rtk

The analysis of th els by calculations of fact rs sf satev for stab~P~ty,

input parameters have been selected

ever, in defining accepta dsrs of safety an

erne level required tor the c

stabiffly factors of safet

ts, @g factor of safety not less than 1.5 or sett1

rticular circurnstanc

in the realms sf sp

f safety, it is likely to be the st

fety for road embankme

ntually back to the same

figure, is cakulated for

ki I V AvERrbGL PORE PRESSuWE R A T 1 6 C AWQWHLB S L I P S U R F A C E es ki >

T I M E

is unlikely that high pore

ankment. In some

intedace to allow rapid dissipation o

nt will be necessary.

ts of construction muM be ntrolled il necessary, the am

struction situatio

en be not less th

the check analyse

snt variable, ie it re -water level, it is

of safety. Such embankment

S. The acceptable factors

,O for the initial asse

re conditions. From

actor of safety shouM

Is to express these variable the form of simple critsri

il is necessary to use ju

ankment, in terms of th amount and time. Thes

roximate limits, from the initial or sscond-stage assessmen

, embankment settlement predictions mu

tcs five years. Ali such estimates

s are acceptable; in so

, so that time is

rary placing of a higher fill than

nt in the time available than

surcharge is primarily dependent on th

eneral situatbn only and each c

uires fairly sophisticated investigation a

urn surcharge height will be in

worth the additional pr

torn the stntdurs

ankments over soft

e slopes and gulleys. Such em

y firm, the large pressures caus

e the cross-fall, and thus heigh

cceptable settlement shuM be

from the preliminary a

riod, it is necessary to procee

ns of the paramete

r that the Izictclr of safety is extremely dependerlt an %he

cohesion T h e former, however, has a fairly smalf ran

The ar~alysis 1s Wherefore most sensitive to variations In the pore pressure ratm

to assess for ail but the most s~rnple slo stablllty analyses. This

of reailistcally nmdelling the geology and rhe

PhaP can occur qtdrte f ~ e uently m scsuttlern

the usual sensiPivrPy analyses, 1s "rat of large c

n-sr'tu subsorls These may be stiff at the time af investig

eaker under an embankment in these cases

lgEhs of t h e wefled sods. {and &his 1s d[f%tcuM becabrse ass

difticultf or to analyre ?he em ankmenl an the basi of %he effective stress

B are already familiar with the a plication of prcsba

nl constn~ctron, so the advantages of these rnetho

l LIT

,

ents settle, but very fs

ility of many embankments.

stages, of increasing levels of so

+case of most road embankments

of the fill can be

f appropriate soil

35

le of the Pibt a

th of the day layer is assumed to = 1, then the factor of s

sensitivity analysis can very e

oukl then de~rease to 2,10.

to check the influenc

t fits of using berms.

rri

I a factor of safety les S

spts.

solidated clays, c' is t ' varies within a r

endent variable (See sect

factor of safety cha

asis of the constmd

of construction cannot be

e uses the same

st be emphasized that this is a very crude $9

ssessrnent by an experienced person.

&hough this test was originally devised for th

een cone pressures, Q, and

se can be expressed by the folio

= Cone Factor and varies from 10 to 30

y =3 density of the subsoil

D = depth of subsoil

Far recent alluvial y be taken as 14 fort

is usually small co

rehinary analyses, t

arc!, in that shear

fry of the vane, ie.

hence 0f the shear stren

tisns of the DCP test and the asse

mare sophisticate

menil assessments,

ssessment of the future

M

n of embankment settle

s order of maagnitu sefltlement, viz B0 m

words, in the majorsy of cases

tests.

The methods of estimating settlements are subsections in the order in which

firs! action rs ta define the

late the requtred reit

nt s n the rnagr~~tuds of the

the other considerations

ramme wi%! asst~lt in the pavemen! layers $:eing built a year or

B1 there are no stwc h $h@ particular embarakment,

ss is to consider t

er of magnitude of t

le standard materi

Rcaw@ver, there is ask settlement equation may be use

E

where 6 -- settlement

o = increase in stress due to embankment

H = thickness of compressible stratum

E = modulus = I f q ,

= coefficient of cornpressibility

e , tf the serbsaii is deseri ed as medium dense sand and is I Q m thick undertyi

out 20 MPa, U is about 200 kPa since the embankment dens

20 k ~ / d , therefore:

then be necessaq to valuate the consequences of this estimated se

ankment, and it can be constmcte

fy satisfactory, teadin

x cutvert passed throu

te of the coefficient of

er)/ f~ssured, with frequent

10 rn2iyeai in this case the time

be smfficsent, say 2 years

pie 1, therefore is 0.5 years, and only 50 rnrn seillemenl will occur aller this perbd

further invsstigation wiiB !hen be re

onstralg initial estimates based on n more than t@!Ea56e

al descriptions sl th

I f , as a result of the init1 1 assessment of settlemen

reqwred, Bhrs wsll generally take 181

and hence seR-tlements e

( a ) SPT The tables m t

T values given a

t a refers ia the mean increase rn stress ue to the embankment !@ad at the centre

l-

erlying clomprcssible strata

ate thicknesses of corn

the thickness 01 subsoil is lar

ation:

se in veflical eflective stress at centre of stratum

ressure at centre af stratum

ives continuous readings of cone pressure, it is comrraon to su

gnitude of cone pressures, as well

nr is then carried out in a tabular form and rna

rnative method is the use of a set?lernent chart (Figure 1 1 )

uisman-de Beer equation given a

th sunicient accuracy f

e average cone press

f these cases m r e W histicated in-situ t , such as the piezo

ring piessuremeter (S

er cases, undisfu&e

ocedrnent to describe either this testin

recl in the geotechnical literature on the subject.

here more sophisticated testi methods are required, it

h suggested in this document, it is likely that such expertise wouM have

e of the investigation, since the planning is itself a fu

ation is organized, it should never result in the g

le logs and consolidation test resuhs, and asked to

. Institute for Mini

lies such as OM infilled chan

ne limitation of the seismic techn us is that the relatkrely small energy in

dudes reliable investigation at depths in excess of

extent, be overcome y using more sophisticated equipment or by using

recommended that the work shou only be carried out by personnel with

training.

sistivity methods may also be used. These re ecialist input, sinc

e field operatbn techniques appear to be si

very useful for detecti anomalies such as intrus

of marked changes in materials and for detecting water.

(W in many cases sa

rcient information for the first-stags inve

on of the subsoil in a relatively ondistu

in art excavation dee

is used primarily on satur

re many variations in the rnetho

e more usual methods.

chniques can be divided into so

number of different mac

necessary to obtain a

De from SPTs or CPTs. This is unusu

ion of the embankmen

ignificant pmblem i

as by fieM tests, such

that the more clayey samples are

the csnsistency

criptions are inter

nse