EnhanceI Ident of slope stabilityby vegetationA report commissioned by Comtec (UK)t by D.H. BARKER", CEng, BSc, MICE, MIHT

UNLIKE DEEPSEATED FAILURES, surfacefailures in the form of shallow rotational slipsor linear surface flows (see Fig. 1) may nothave the effect of preventing, to a greater orlesser extent, use of areas above and belowthe slope. However they can cause

and has been attributed to the followingseparate effects:1. Wind speed attenuation;2. Umbrella or shield effect;3. Surface flow retardation;4. Soil moisture content reduction by

(Linear

Fig. 1. Shallow failure modes

Circular

considerable inconvenience and expensewhen the necessary repairs are made. Deep-seated failure modes are now routinelyanalysed and designed against in the case ofnew earthworks and remedial measures, andare now relatively rare events. On the otherhand scant regard has been paid todesigning against shallow failure, withconsequences which are evident to users ofmotorways and trunk roads in the BritishIsles and overseas. Preliminary results of arecent survey by the UK Transport and RoadResearch Laboratory indicate that shallowslips have occurred on about 12% of thelength of motorway surveyed for cuttingsand embankments constructed in over-consolidated clays (Ref. 1).

The role of vegetation, i.e. grasses, shrubsand trees, in erosion control is well known

overtranspiration;5. Infiltration (it should be noted that this

effect can be either adverse orfavourable, depending on climatic andsoil conditions);

6. Root reinforcement.The net result of vegetation is to prevent

the degradation of a slope due to loss of sur-face particules leading to rill and gullyforma-tion (see Fig. 2).The entire slope, if untreated,eventually retreats and flattens as the spursbetween gullies collapse into them and a newcycle of rill and gully formation starts.

Apart from the superficial effects notedabove, the roots of trees, shrubs, grasses andlegume mechanically bind or reinforce thesoils they are growing in, often toconsiderable depths. Depending on soil,climatic conditions and to an extent on themaintenance regime, the roots of certaingrasses may commonly achieve depths of0.5 to 0.75m and often in excess of 1.5m.Thus a band of naturally reinforced soil

tComtec IUKI Ltd., Tunbndge Wells, Kent, TN3 9BT.'Geostructural Compos<tea Ltd.. Edenbndge, Kent,

trading as Geostructures Consulting.

a= 35'= 914mm

>s= 18.4kNlm*"o 138 kNim'=

41'5'74

194

can be established which has an apparentcohesion or an enhanced cohesion.

This increased shear resistance results in asignificant increase in the factor of safety forthe slope against shallow-seated failures.This effect has been established in

comprehensive field observations oftimbered slopes before and after clear felling(Ref. 2), laboratory strength measurementsof rooted soil (Refs. 3, 4 and 5) andengineering analyses (Ref. 6).

Even a small increase in cohesion or shearstrength will counter the loss of strength ofunsaturated soils caused by an increase in

pore-water pressure resulting from infiltra-tion due to prolonged rainfall or ground-water seepage. Figs. 3 and 4 illustrate theinfluence also of soil friction and "cohesion"of nominally cohesionless soils on the factorof safety of relatively shallow strata over-laying an inclined bedrock with a temporarygroundwater table due to infiltration.

It has been confirmed in field observationsof shallow slips of 0.5 to 2.0m depth in

London Clay embankments (Ref. 11) thatfailures were initiated by increases in porewater pressure. A follow-up investigationcarried out at City University, London,involved a series of special stress path testsand conventional CD triaxial compressiontests. The strength of soil was significantlyless at the small effective stressescorresponding to those in the slope at failurethan that inferred from the results ofconventional CD tests carried out at highereffective stresses assuming a linear failureenvelope (see Fig. 5).

The beneficial dual effect of vegetation in

Zto

2rn

I-rnZ«tts

I-LLI

rn

OrkO

n

Fig. 2. Progressive failure of slope due to erosion gullying

0 5 10 15COHESION test,

kN/m'ig.

3. Influence ofcohesion on the stability ofa sandy residual soil resting on an inclinedbedrock contact. H„= 0.5H. (From Gray &Leiser, 1982, Ref. 8J

April 1986 11

ix.

Zotry

in 2Z«c

IIJic«fin4.OIcO 1cy«tIc

8=36'l

~ 914mm

ysv 18.4kWm'*

13.8kWm'o*

o

compensating for this reduction in soilstrength at shallow depth by providing areinforcing matrix of roots, increasing soilsuction i.e. reducing pore water pressuresand the weight of unstable soil mass byevapo-transpiration, speaks for itself. Carefulselection of suitable deep-rooted grasseswhich will thrive in the particular soil type,climatic conditions and maintenance regimeexisting at a given slope or set of slopes alonga road, say, will provide a sound basis forenhancing the stability of a slope. In somecases, e.g. for short-term protection over awinter period, cereal crops having well-developed root systems, such as rye, havepotential in this role.

However grasses are generally moreeffective since once they are established theyprovide a dense complete cover. Grasses arealso a very successful family of plants.Covering 250/o of the world's land surface,they have adapted to virtually all types ofsoils and climates. Specially bred varietieswith deep-root systems and short swards arenow beginning to be produced in commercialquantities, thus realising the potential ofgrasses to reinforce slopes and at the sametime minimising maintenance requirementsand end-of-dry season fire risk.

I

0 5 10 15COHESION (Cs),

kWm'ig.

4. Influence ofcohesion on the stability ofa sandy residual soil resting on an inclinedbedrock contact. H„= H. (From Gray &Leiser,1982, Ref. 8j

Fig. 5. MohrsCirclescorresponding topeak failurestates ibr the stresspath tests (AfterAtkinson & Farrar,Ref. 11j

40

—300oy

~~ 20ity

'E 10z

20 40 60NORMAL STRESS (0')kPa

80 100

influences incorporated was the purelymechanical effect of roots in soil.

Lopez-Tello was able to show in a study ofa 10m high 1 in 1 cut slope in clay that on thebasis of an average root density of5 OOOKg/Ha (or 1%of volume of soil) a 33%increase in the factor of safety resulted fromvegetation cover.

As a matter of interest, other elementsincorporated into this complex model were:(i) Slope hydrology;(ii) Soil fissure development;(iii) Solar radiation.(iv) Climatological variations;(v) Surface runoff.

In order to develop the concept of rootreinforcement in more detail, simple modelshave been proposed to predict the influenceof unidirectional reinforcement of soils byboth fibres/roots and strips. To date thesemodels have only been tested under a limitedrange of conditions. Even so they are usefuland, over the next few years, as more work isdone in design of ealthworks on shallow-seated or surface stability of slopes, asopposed to only deep-seated circular orwedge failure considerations, feed-backfrom the field and laboratory tests of rootedand denuded slopes should allow increasedconfidence in their use over an extendedrange of applications.

Mechanics of rootreinforcement

The analogy of soil reinforcement naturallywith roots of vegetation to the 'modern'on-cept of reinforced soil, i.e. soil containinglayers of synthetic reinforcing inclusion, isobvious. Though many theories can be foundin the literature as to the mechanism of soilreinforcement, one generally accepted is theequivalent confining stress increase due tostrain restraint in the direction of reinforce-ment. This in turn mobilises additional shear

Determination of the reinforcedeffect and enhanced factor ofsafety of a vegetated slope

Some interesting research has beencarried out by Kassif and Kopelovitz (Ref. 3)and Waldron (Refs. 4 and 5) in order todetermine the efficacy of root reinforcementin binding soil and increasing its shearstrength.

Kassif and Kopelovitz measured thereinforcement provided by plastic-fibre incompacted samples of both clayey sandsand loess in direct shear tests. It was shownthat the shear strength increase, LIIS„, varieddirectly with the root area ratio (A„/A). Formost soil-fibre systems with A„/A less than1 to 2% increases of 250%were measured inshear strength of fibre reinforced soil overthe values for the same soil without fibres.

Waldron investigated the effects of plantroots on the shear strength of soil. He tested25cm diameter root permeated 60cm longcolumns on a special direct shear rig. Planttypes were alfalfa, barley and yellow pinetested at various depths and shear displace-ments in both a homogeneous silty clay loamand stratified soil profiles (see Fig. 6).

Alfalfa roots were generally the mosteffective reinforcement for all types of soil.The relative strength increase for alfalfa was290%—very much greater than other plants,which did not include grasses. No details ofroot concentration or root area ratio weregiven. However data on shear resistanceincrease against root area for barley rootedsoils was provided: a linear relationship wasobserved as shown in Fig. 7. It can also beseen that a relatively small root cross-sectionarea (A„/A = 0.1%) is required to mobiliseincreased shear resistance (see also Fig. 6).

Theoretical models ofstabilisation by use ofvegetation

In research by Lopez-Tello (Ref. 9) intomodelling and quantifying the stabilisingeffects of plants on slopes one of the six

0—

c 30-

SOiL PROFILES

300Profile(-

30cm depth

IVE

ioo Ifo

Profile ((-30cm depth

aO

eicxc0vt

aicc

F0SI

Iry

60

Symbol Porticle s ze,percent luSDA)

gravel sand silt cloy

Bulk Ders tygxcms

Texture

12016 52 32

16 52 32

86 9 561 31 5 3

silty clay loam

silty clay loam,compacted I 50

I 66loamy sand

graVel, Sandaea I I 90

Fig. 6. Experimental soil profiles and results of direct shearsoils. (After Waldron, 1977, Ref. 4j

Prof(let(-30cm depth

Profile 57-45cm depth

<AlfalfaPine-

«it 6 itI' vtO 30 50 10 30 50

Shear Displocement, mm

tests on root-permeated and fallow

cb

EE

lnIVI

N0

p c

S 0«8

Icc

8in

Io—

1, Silty Cloy Loam

m, Silty Clay Loam

ftt, Stiiy Cloy Loom-pmvel interface

ta, Gravel mix

ttt, Silty Clay Loam

ttt, Silty Clay Loam-prolal interface

~ 3r5 ~ IO o,-3 9i -092

02 04 06 0.8 I 0

o„Root Cross Section per Shear Cross Section,e

~mI 0«

cme

Fig. 7. Shear strength increase vs. root arearatio at 25mm shear displacement for barley-root-permeated soils. (From Waldron, 1977/

12 Ground Engineering

20—

z 15

V)CI

K 10Z

4jKI-ro

5clLLIZV)

.IO .I5ROOT AREA RATIO, ~A - %

DOUG.

I Soilom. roots

.20

75E

I- 50

tn 25

0 I I

0 1 2 3 4 5 6 7

SHEAR STRAIN (%)Fig. 8 (Left). Examples ofshear strengthincreases resulting from root orfibre reinforcement of various soils. (From Gray & Lei ser, 1982, Ref. 8)Fig. 9 (Above). Stress-strain response under shear of unreinforced soilvs. soil-fibre composite dune sand reinforced with 1.75mm diameterreed fibres. (After Gray & Leiser 1982, Ref. 8)

resistance beyond that which would begenerated by the externally applied confiningstrength alone.

Another marked effect is to make soilsmore resilient or tougher —i.e. able to resistcontinued deformation without loss ofresidual strength (see Fig. 9) —the idealperformance sought from reinforced soil.Beyond a certain limit, the equivalent confin-ing stress mobilised by the roots levels off ata value called the critical confining stress.The shear resistance provided by thereinforcement is then a constant and is afunction of following properties of the soil-root (or fibre) system:

Tensile strengthModulusLength/diameterFrictional

characteristics

of the roots or fibresof the roots or fibres

ratio of the roots or fibresof the roots or fibres

and soil

Shown in Fig. 10 is a perpendicular rootreinforcement model consisting of a flexibileelastic root aligned perpendicularly to theshear surface at the onset of shear. This is anidealisation of the case in which vertical roots(taproots and sinker roots) extend across apotential sliding surface within a slope. In

this model, the force developed in the root asthe soil is sheared is resolved with atangential component resisting shear and anormal component increasing the confinedstress and hence frictional resistance alongthe shear plane. It is assumed that the roots/fibres fall in tension rather than by loss ofanchorage or bond. The soil friction angle (Ip)is assumed to be unaffected by thereinforcement. This mobilisation of thetensile resistance in the roots can berepresented as an increase ttSR in the shearstrength of the soil given by;

This equation states that the maximumpossible root contribution to soil strengthcan be estimated by measuring the tensilestrength of the roots TR and the fraction ofpotential sliding plane occupied by rootsAR/A. AR can be determined by counting thenumber of roots in different size classes (n) ina given soil cross-sectional area (A), and thensumming the product of the root numbers ineach size class times their mean crosssectional area (aJ for that size class,

i.e. tR ——TRE A-. (9)

Since for natural root systems, the roottensile strength tends to vary with the size ordiameter of the root, the last equation can bere-written:

tR=E ATn;a; -. (4)

The only unknown in the equations is theangle of shear rotation or distortion (9).Thisvaries with the thickness of shear zone IZ)and the amount of shear displacement XResults of tests carried out by Waldron (Ref.4) confirm |) varied between 45'-50 . Fieldobservations by Wu (Ref. 6) of failures in rootpermeated masses on slopes, indicated that() varied at most between 45 and 70'. Thevalue of the bracketed term in equation 1

varies between 1 to 1.3for 25'<Ip <40'and40 < |)<70.

Hence, adopting a median value of 1.15asthe most likely value of the bracketed term,equation 1 may be re-written:

i.e.

hSR = 1.15tR ". (5aj

bSR = 1.15TH(AR/Aj ...(5bj

LISR = 1,15E 'A' ...(Scji.e.LISR = tx(cos () tan qt + sin 8) ...(1)

As stated above, more refined rootreinforcement models have been developedto mirror inclined roots etc. The simplevertical model should provide sufficientlyaccurate estimates for the rootreinforcement of a soil for most situations.

The above root reinforcement modelassumes that the failure mode is a tensilefailure of the root fibres i.e. their tensilestrength is fully mobilised. For thisassumption to be valid pullout or bond failuremode must be prevented. To meet thiscondition the roots must have a combinationof sufficient length beyond the failure zoneand sufficient roughness so that the bondbetween the root and soil exceeds the tensilestrength of the root.

The minimum length, L min in mm, of rootsof uniform thickness, dR (mm), required toprevent pullout or bond failure is given by thefollowing equation:

) TRdRLmm

where TR = tensile strength of root, kN/m'maximum bond stress orpullout resistance betweenroot and soil,

kN/m'tability

analysisIn order to illustrate the analytical process

involved in quantifying the effect of plantroots in stabilising a slope, we shall considerthe case shown in Fig. 11 where a shallowsoil layer supports vegetation on an infiniteslope. The potential failure plane is assumedto be just above the interface of the soil andthe bedrock.

Under steady state conditions of seepagethe factor of safety F1 is given by:

Ss+ bSR -. (5)T

where itSR ——

8

shear strength increase fromroot or fibre reinforcement, kN

angle of shear rotationangle of internal frictionaverage tensile strength of rootor fibre per unit area of soil, kN.

intactroot

1tII

deformed root

~

T

Average tensile strength of roots or fibresper unit area of soil tR is given by:

tR——TR (AR/Aj —.(2)

where TR = average tensile stength of rootor fibre, kN

and AR/A = root area ratio of fraction of soilcross-sectional area occupiedby roots.

Fig. 10. Model offlexible, elastic rootextending verticallyacross a horizontalshear zone. (FromGray & Leiser,1982, Ref. 8)

,I I

IIIi

shear

son ~

April 1966 13

Typlcal spreadsystems of

. and depth

savannahsPecies ofvarious

s of

environment9 wn in the s

.e.Plants rp

. Prairie i

are shown' arne sollln ig 12

onC)i BianShallow seated sip

occurrencesope failures arere common

Ning slopes. Theiry embankment and

tp highwaprevention is of co

stabilis'

engineers L'koncern

resetion of ex'.. i ewise

xistin a„e

effes and s illrvoir slope

9 nd new earth f.

cts of overto P inP ways against the

's of current'g nd flood dInterest

isc arges

e work of severaresearchers

ral field and I

and coshpws that the d

boratory

o centrationof'th, stren th

Iy to the stability can contribute

to shallow''y of slopes with r

e

ef trans)at jpha I fresPect

e icial action ariailures, This

mechanical reies primarily from

e increased spil s~

'I by roots

P t transpirat'on resulting f

Thion.

rom

e developmentinteraction

of models pf

box t, together with ao soil-root

ox tests of root dassociated sh

ide the basis fopain soil samplese and I

ear-

abilising effect pquantification pf th

'ncreased use fgetation. Thro

with t.9 certain grasse

" rieties

Inpotential toestabl

s and legumeish dee

onditions, itwillboils and clim t;

harness oepossibleforen

'ic

onomical and effec'nalytical basis

enhancing theive natural me

to shalloestabllltyof slo es

means of

ow translatlo I fP withrespect

ailures

1 1. For es orces on a soil mass aboabout to slide (Aft he s ier Bachehe & /IrfacAskill

here Ss

he s i, 1984, Ref. 10j

w is sheann rr e area Lx

g resistance ofx 1 (unit width)

increased

y„ is the uniti weight of wa

resistance de due to

, are as shown in F

root T bl

is the o

a e I (Ref. 10effect of vegetation, in this cactor of safet f

in stead y p ge.Th

Th

i on a modifi d f

e downslo e o

Eq io 9

es o g ssT

e substitution

ypical values fos for followin ro systems are:

ensile stren g

S H b2.7m

mia.-99.7N/mm'uglas

Fir

Concentrations ofTypicalBmm dia.—

rees. dominantn value 70--113 roots/ 'fH nceA/A ratio of 0.14 to 0.

Megahan et )n ain Douglasg s Fir 0.05 to

Q1 t QB/c —Waldron (Se Re ef. 4)

7 = Ws.sing

where Ws is unit w'

is the bulk unit w 'tweight of th

is the slope angle

The total shshear strengthd b

f h''df f Ch t th.

Reference. Parsons, A.W. Et Per,

stability problems In ageing hP S

a works, pp.63-68. I

2 G DH (1978inforcing soils and

Pm. S .S'il ReinforcInniques in Engr. Practice, p .25

S = (Ss+ hSR) + a'.tR 0 .ta nip

where S is thes 'hear stren throot-free soil,is the e

lp'stheeffet'ffective norm al stress,

gective an lee soil.

Incorporating this ef b

1(c' tts)L + Wss . cos P . tan

W . I P

where Ws' oyant w ''sthe buoyant w 'yantweight of the

c's thee effective coh

hs s ren thoot-f e so'I,

is the shear stren theng increment

Y, = 18.0 kN/ms

y, = 20.0 kN/my„= 9.8 kN/m

I) = 35', +h,=09m

tt S(kN/m')

o sistent w th/m Wuu et al. (1979)

= 1.22 m

P, = Weight of trees 'o trees per unit sloppe area

assumed

h,(m)

h2

(m)pi

(kN/m')FStw

(kN/m')

Case 1

uncovered slope 0.4(assumed)

0.5(assumed)

0.0(no plant on sl

0.0s ope)

0.0

6.0 3.8 0.1(Wu et al. 1979)

Case 2plant growth 1.930.7

(assumed) (assumed)B ey expressing the forweight d t kiking the width of th

ollowing e ue soil bl

equations areCase 3dense growth 0.9

(maximum)0.0 6.0 3.8 0.1 2.18

3 306

FFECT OF VEGTABLE I: EEPAGE CALCULATIOIONS ARE BASED 0

, 1981)AS MODIFIED0

Fixed d

ION OF TWITH Fl

Y

ata

REES FOR GRA SSES. (AFTER

c'= 5.3 kN /m'

ER BACHE Et

Ws'= . h—Yi . i + (Y2 Yw)h2

Ws=y .hi'+Y2'h2L = LAa/cpSAa p(LAa ——1 aS u 't

Ini ength)

where y, and arey, are the unit wei habove and bt ble respe

t'low thc ively

14 Grround Engineering

Case 4cut-overimmediate

Case 5aftercut-over

Case 6long aftercut-over

0.9

0.4(case 1)

0.0

0.0

0.5(case 1)

0.9(maximum)

6.0(fresh roots)

3.0(50%)

0.0 0.0 0.0

0.0 0.0(plants removed)

0.0 0.0 1.75

1.13

3. Kassif, G. & Kopelovitz, A. (1968); "Strengthproperties of soil-root systems". TechnionInstitute of Technology, Haifa

4. Waldron, LJ. (1977):"The shearresistanceofroot-permeated homogeneous and stratifiedsoil". Soil Sci. Soc. Am. J. 41, pp.843-849

5. Waldran, L.J. & Dakessian, S. (1982): "Theeffect of grass, legume and tree roots on soilshearing resistance". Soil Sci. Soc. Am. j. 46,pp.894-897

6. Wu, TH., McKinnel, WP. & Swarston, D.i(L(1979);"Strength of tree roots and landslideson Prince-of-Wales Island, Alaska". CanadianGeotechnical Journal 16, pp.19-33

7. Brand, E.W. (1981):"Some thoughts on rain-induced slope failures". X Int. Conf. S.M. &F.E., pp.373-376, Stockholm

8. Gray, D.H. & Leiser, A. T. (1982):-Biotechnicalslope protection and erosion control". VanNostrand Reinhold, New York

9. Lopez-Tello, L.F. (1977): "Plantaciones entaludes efecto estabilizador (Stabilisingeffects of plants on slopes)". Rev. ObrasPublicas, N3148, pp.659-668, Madrid

10. Bache, D.H. & MacAskill, I. A. (1984):'Vegetation in civil and landscapeengineering". Grenada, London

1 1. Atkinson, J.H. & Farrar, D.M. (1 985): "Stresspath tests to measure soil strength parametersfor shallow landslips" XI Int. Conf. S.M. & F.E.,San Francisco

Bibliography1. Gray, D.H., Leiser, A.T. & White, CA. (1980):

"Combined vegetative-structural slopestabilisation". Civil Engineering-ASCE, Jan.,pp.82-85

2. "The influence of vegetation on clays".Symposium in Print, Geotechnique, June1983 and ICE 1984

3. Kobashi, S. (1 971):"Erosion &surface stratumfailure of steep slopes & their preventionmethods". Proc. Fourth Asian Region. Conf.Soil Mech. & Found. Eng., pp.433-436,Bangkok

ft

3-

2I

6-

yiii((

b

(

(

(Y I,

/

«tm''.',I'k"

4. Van Zyl D.JA. & Jaaback, G. (1975): "Anappraisal of two qualitative field trialscomparing methods of erosion control onearth slopes". Proc. Sixth Region Conf. forAfrica Soil Mech. & Found. Eng., pp.271-279,Durban

5. Blong, R.J. & Humphreys, G.S. (1 982):"Erosion of road batters in Papua NewGuinea". The Institution of Engineers,Engineering Transactions, pp.62-68, Australia

6. Lee, K.L.,Adams, R.D. & Vagneron,J J. (1973):"Reinforced earth retaining walls". Soil Mech& Found. Div. J. ASCE, Oct.

7. Prellwitz, R.W., Howard, TR. & Wilson, WD.(1983): "Landslide analysis concepts for

management of forest lands on residual andcolluvial soils". Transp. Res. Rec., n919,pp.27-36, Washington DC

8. Holy, M. (1980); "Erosion & environment".Pergamon Press, Oxford

9. "Construction on Slopes Manual" (1984 Ed.),Geotechnical Control Office, Hong Kong

10. Schiechtl, H. (1980):"Bioengineering for landreclamation and conservation". University ofAlberta Press, Edmonton, Canada

1 1. Hewlett, H. W M., Boorman, L.A., Bramley, M.E.& Whitehead, E. (1985): "Reinforcement ofsteep grassed waterways". Technical Note120, CIRIA, London

7-Fig. 12.Differencesin spread and depth ofroot systems ofvarious species ofprairie plants grownin the same soil environment. (Fmm Weaver 19191. Reproduced from Soil Water PlantRelationships by Kramer. Used with the permission ofMcGrawHillBook Company. (From Gray& Leiser 1982, Ref. 8)

&Trade Literature

Piling Engineering, by W.G.K. Fleming,A.J. Weltman, M.F. Randolph Et W.K. Elson.Published by Surrey University Press, amember of the Blackie Group, WesterCleddens Road, Bishopbriggs, Glasgow G642NZ. 1 57mm x 240mm; 392pp.; illus.; priceE35.

Liked the book. Pity about the jacket. Whyillustrate such a good title with a picture ofhow not to engineer a pile? All the authorshave been prolific, so inevitably much of thetext and illustration can and have been seenin diverse previous publications, except formarine and offshore piling which is ignoredthroughout the book —except in thereferences. The short chapter on siteinvestigation for piling is very welcome,drawing attention to the particular needs forsuccessful piling. All the more strange thatthe importance of groundwater informationis accorded only half a dozen generalisedlines.

The long chapter on basic piling methodscontains too much of too general a nature,

with too little insight fora book of this quality.The chapter on problems in pile constructionalso repeats a lot of previously publishedmaterial and photographs, and even heredisposes of the effect of water in boreholes intwo short paragraphs. In a comprehensivechapter on the design of piles, it is good tosee design methods brought up to date withvery well-detailed references, and taking intoaccount modern developments in theory andin pile installation processes.

Another good chapter follows on retainingwalls, unusual but very necessary in a bookon piling, this being an all too common areaof failure.

The short chapter on integrity testing alsobrings these improved methods right up todate, together with a very clear andcomprehensive chapter on load testing ofpiles. The TNO dynamic test creeps into thelatter chapter as well as the former, probablybecause it is an integrity or a load testaccording to how its results are interpreted.The integrity tests are clearly explained andthe authors have not shirked mentioning thedrawbacks associated with each test, withparticular emphasis on the importance anddifficulties of interpretation.

Choice of piling method warrants a bookon its own and so in the space available thisfinal chapter can only generalise, but doesinclude some modern views. A peculiaromission here is any reference to groundimprovement processes, which now beingwidely available cannot be isolated from adiscussion on foundation choice and

economy of method.The diagrams and photographs are not

given the space afforded in some less priceybooks. If graphs are to be useful they shouldbe large enough and graduated for values tobe read off the curves reasonably accurately,or at least to clarify at a suitable scale thepoints they intend to illustrate.

The book is generally soundly referenced,but for a book which is making a seriousattempt to bring design and theory up to datewith current practices and knowledge, someof the references are incorrect or outdated, orso remote, or ordinary, as to be of little weightor relevance. I am also suspicious of the valuefor money of a book where an author putsmany more references under his own namethan any of the giants who have dominatedfoundation mechanics over the last 40 years(score: Randolph 19, Bishop 3).

Nevertheless, the book is a worthwhilecollection of many aspects of piling, andbrings together several levels of knowledgeof considerable assistance in understandingthis most complex of design andconstruction problems. Anyone studying thebook would be a better piling engineer at theend of it. R.J.C.

Application of Centrifuge Modellingto Geotechnical Design, edited by W.H.Craig. Published by A.A. Balkema, PO Box1675, 3000 BR Rotterdam, TheNetherlands. 180mm x 253mm; 501pp;illus.; price (sterling) f31.

April 1986 15