~, I~WA S 8SA4SOME DESIGN AND CONSTRUCTION CONSIDERATIONS

by G. S. LITTLEJOHN, BSc, PhD, CEng, MICE, FGS; B. JACK, CEng, MIStructE; and Z. SLIWINSKI, CEng, MICE"

OF PARAMOUNT importance is theprovision of site investigation datawhich will facilitate anchor design andchoice of anchor construction tech-nique. The basic information required isillustrated in Table II.

Item

TABLE II

Data required

Borehole General soil profileGround water level

Undisturbed Shear strengthssample (fp, C„, c'nd fp')

DensityConsolidation andcompressibility indices

Disturbedsample

In situtest

Mechanical analysisChemical analysis

Standard penetrationor Dutch cone readingsVane test results

Construction Proximity of operationssuch as piling, blastingor freezing

sThe first part of this Paper, by B. J. Jack,dealt with wall design and appeared in theSeptember, 1971, issue of this journal, pp.14-17. The second, by Z. Sliwinski, coveredwall construction, in November, 1971, pp.18-21

In sands, the friction angle (fp) com-bined with the effective overburdenpressure enables the capacity of theanchor to be calculated since the re-sistance to pull-out of the anchor de-pends on the ground restraint whichcan be mobilised adjacent to the groutinjection zone.

Grading samples are invaluable sincethey enable the permeability and there-fore the groutability of the soil to beassessed, and in addition when thesamples are used in conjunction withstandard penetration tests to estimaterelative density, then tp values can be

ANCHORDESIGNby G. S. LITTLEJOHN

determined if these are not alreadyavailable.

Chemical analyses of the soil andgroundwater are important since sul-phate content and pH, for example, candictate the type of cement grout anddegree of corrosion protection.

Anchor locationSince the waling level and spacing of

the anchors is determined in the walldesign, the location of the top anchor isfixed and only the inclination and lengthof the anchor remain to be calculated.

Anchor inclination is kept small andideally should be less than 20'o thehorizontal. In many cases, however, thisis not possible due to the proximity ofadjacent foundations, and values of20'-40're common.

With regard to overall length, thefixed anchor must be embedded(a) deep enough to avoid the localised

passive failure of the soil associatedwith the failure condition for shal-low deadmen, and,

(b) far enough away from the wall toensure against a slip failure beneaththe toe of the wall and beyond thefixed anchor zone, at a lower factorof safety than the design specifica-tion allows.

A minimum depth of 5-6 m is nor-mally considered sufficient to guaranteea deep seated failure condition at pull-out, and, for initial guide purposes only,the "free" anchorage length may beestimated with the help of the construc-tion diagram shown in Fig. 9.

Overall stabilityAs a second step, the stability of the

whole system must be checked to as-certain whether the chosen anchorlengths are sufficient or not. Where thewaling loads on the wall have been de-signed in accordance with the principlesoutlined earlier, it is assumed that theanchor prestress introduced will pre-vent slip planes occurring between wall

and the fixed anchor zone. In otherwords, it is assumed that the prestress-ing of the anchors introduces a newstate of stress in the retained soil masswhere the normal stresses and conse-quently the shear strengths becomelarge enough to prevent the mobilisationof sliding surfaces ahead of the fixedanchor zone. The sliding surfaces whichare still possible will therefore pass be-yond the fixed anchor zone. As an addi-tional safety precaution the mid-pointsand not the ends of the fixed anchorsare usually arranged along the slidingsurfaces with the required safety factor,according to practice in continentalEurope. It is noteworthy, however, thatin the absence of detailed informationon fixed anchor/soil interaction, theauthors consider that the fixed anchorzone should be completely beyond theestimated slip plane.

The shape of the sliding surfacewhich will occur for systems with onlyone row of anchors is known throughthe work of Kranz (1953), Jelinek andOstermeyer (1966) and Ranke andOstermeyer (1968) and the procedurerecommended for consideration is amodified Kranz method suggested byLocher in 1969 (see Fig. 10).

The earth pressure, E, on the verticalcut through the mid-point of the fixedanchor is calculated with a nominalfriction angle d„, and the resultant forceR„on the inclined plane of the slidingwedge must form the same angle gi„with the normal to the sliding plane. tp„has been correctly assumed if theweight G and the forces, E and Rn, arein equilibrium. If this is not the case,then tp„has to be altered and whenequilibrium is achieved the factor ofsafety is defined as

tan faF =tan tp„

where ff

is the actual angle of internal friction.This definition corresponds with the

concept of partial safety factors as pro-

Fig. 9. Location of fixed anchor,first estimate

12

Nrotaar//

/E,

/

A 'AiI

Rtrt„h

E,

Rn

F=-tan Btan 8„

G1

Fig. 10. Stability of a wall with one rowof anchors, modified Krantz'ethod

Spiral gt„ MMiMG,

i4 t~an

F. tan 8,'g. > >. Stability analysis —spiralshas aped sliding surfaces

posed by the late Prof. Brinch Hansen.lt is considered that the main attractionof this method is its simplicity, and al-though the forces acting are assumed tobe concurrent it is considered that the

/

value of F is a safe estimate since thestabilising passive resistance, availablefrom the embedded depth of soil withinthe excavation, is ignored in the calcu-lation.

For systems with several rows of an-chors, the shape of the sliding surface isnot known from experiments and thestability is evaluated using the circle orlogarithmic spiral method. A logarithmicspiral has the property that the radiusfrom the spiral centre to any point onthe curve forms a constant angle y withthe normal line to the curve. If a nomi-nal friction angle of the soil tp„ is em-ployed where

tan tptan tp n F

iiithen the line of action of the resultingforces on each part of the sliding sur-face will pass through the spiral centre.None of the forces along the sliding linewill therefore create a moment aroundthis point and they can therefore beneglected, when considering the equi-librium of moments around the point.

The safety factor F is correct whenthe moments of the remaining weightsand forces on the sliding body totalzero. Fig. 11 shows the principle, andagain by ignoring the passive resistance

. of the soil beneath the excavation when: the moments produced by G, and G„-'alance, a conservative value of F

equals tan y/tan 4„.In both the stability analyses de-

scribed, the basic assumption is made

that anchor prestress increases theshear strength of the sand, sufficientlyto displace the potential failure planebeyond the fixed anchor. Care shouldtherefore be taken not to apply thesemethods outside the range of cohesion-less soils. In stiff cohesive soils, forexample, it is clear that anchor prestresswill only increase the soil shear strengthgradually as consolidation occurs. Con-sequently, in this situation a conven-tional analysis of the overall stabilityshould be carried out, neglecting thepresence of the soil anchors, and thenthe fixed anchor must be located somedistance beyond the potential slip zoneto ensure that excessive pressures arenot transmitted across this zone, whichmight lead to premature failure.

Load carrying capacityIn fine to medium sized sand where

the permeability (K„,j ranges from10-'o

10-4 cm/sec, the fixed anchor formedconsists of a smooth grout cylindersince the sand does not allow permea-tion of the dilute cement grout (Fig.12).

For this type of anchor equation (3)is commonly used by specialist contrac-tors, to estimate the ultimate loadcarrying capacity.

T,=L.n'tantt .....................(3)where L is the fixed anchor length in ft(m), n' 4-5 tons/ft (13-16.5 t/m),and tp is the angle of internal friction. Inequation (3), n'utomatically takes ac-count of the depth of overburden abovethe fixed anchor, h = 20-30ft (6.1-9.2 m), fixed anchor diameter, D7-8 in (180-200 mm) and the range offixed anchor lengths, L = 3-20ft (0.9-3.7m) over which the rule has beentested.

In general, however, practising engi-neers require an empirical rule whichrelates anchor pull-out capacity withminor dimensions and soil parameters.Equation (4) for vertical anchors is re-commended for consideration.Tr .——

L '/rAy(h+ —wDLtany + Byh —(D' d')

2 4(side resistance) + (end resistance)"""""""" (4)

where A = ratio of the contact pres-sure at the fixed anchor/soilinterface to the effectivepressure of the overburden

B = bearing capacity factor7 = unit weight of soil over-

burden (submerged unitweight beneath the watertable)

h = depth of overburden to topof fixed anchor

L = length of fixed anchorD = effective diameter of fixed

anchord = effective diameter of grout

shaft or columnA normally lies within the range 1-2

but the actual value depends to a greatextent on the anchor installation proce-dure, i.e. drilling method and grout in-jection pressure. The bearing capacityfactor B depends on the angle of shear-ing resistance of the soil and the ratioh/D. It is noteworthy that in compactfine to medium sand (4 = 35') values

Fig. 12. Section of fixed anchor formedin fine-medium sized sand

of 1.4 and 31 for A and B respectivelyhave been measured where rotary per-cussive drilling techniques have beenemployed with grout injection pressuresof 50 Ib/in'350 kN/m') to give a ratioL/D = 27. Where anchors are formed infine cohesionless soil using cementgrout, safe working loads are usuallylimited to 40 tons.

Factors of safetyHaving established the ultimate load

holding capacity of the anchor usingeither equation (3) or (4), it is neces-sary to apply a factor of safety toguarantee the performance of the indi-vidual anchor. In multi-anchor systemswhere progressive failure must be pre-vented, the minimum factor of safety(S,) normally employed is 1.6. Sincethe local soil properties are not normallyknown with the degree of accuracy as-sociated with the steel components ofthe cable and top anchorage, a value of2 is common for fixed anchor design incohesionless soil both for temporaryand permanent works.

In order to cHeck and possibly opti-mise the fixed anchor design at the be-ginning of the contract a minimum ofthree test anchors pulled to failure isrecommended where the fixed anchorlength is varied and the cable is de-signed in each case to ensure that fail-ure occurs at the fixed anchor/soilinterface.

With regard to overall stability, afactor of safety F = 1.5 is customary,but as in all designs the choice is basedon how accurately the relevant charac-teristics are known, whether the systemis temporary or permanent and the con-sequences if failure occurs.

Ground Engineering 13

ANCHOR CONSTRUCTION by G. S. LITTLEJOHN

The method which is employed foranchorages in sand entails a number ofworking operations, as follows:(i) A casing, 2-6 in (50-150 mm) nomi-

nal diameter, is driven through the wall

and the retained soil mass to the desireddepth, using rotary, rotary-percussiveor vibro-driving techniques.

Anchor hole formation is aided byvarious flushing techniques. In sandsand gravels, for example, water flushingwidens and cleans the hole and ensuresa better bond at the grout-soil interface.(ii) The cable, which consists of high

tensile strands, wires or bar is homed,the length of cable above the fixed an-

chor being decoupled from the ground

by some form of sheathing.(iii) Grout, consisting of neat cementand water, is injected into the holeunder pressure as the casing is with-

drawn over the fixed anchor length. Thehole is then topped up with grout andallowed to set following complete with-drawal of the casing. Grout W/c ratiosranging from 0.4 to 0.65 are recom-mended and the injection pressures mayvary from 30-1 000 kN/m'.

Care should be taken not to exceedthe theoretical overburden pressure,since this could cause fissuring in theground and possibly lead to groundheave at the surface as well as possibledamage to existing anchors. During thegrouting stage, therefore, a careful noteof injection pressure is required toge-ther with grout consumption.

Where the ground is variable, highalumina cement is often employed sinceit enables the anchor to be tensionedwithin 24 hours. Consequently, if theground conditions have deterioratedlocally without being observed, the ten-sioning stage will indicate a redi)cedcapacity, and remedial measures can betaken immediately.(iv) Within six hours of grouting, the

+IVII'"fl

~~ ix

'~'" '4ii'NiiiW'jl

Fig. 14. Temporary corrosion protectionof top anchorage using Stripceal

co

30-

ao+6 20-

10

grout column filling the hole is flushedback using air and water, to within, say,5ft (1.5m) of the top of the fixedanchor.(v) When the fixed anchor grout hashardened (minimum crushing strengthof 28 N/mms is normally specified), thecable is post-tensioned to the desiredload.

Thus the anchorage is based on groutinjection and consists basically of acable which is bonded into a groutedzone of alluvium (the fixed anchorage).The rest of the cable is encased in aprotective sheath to prevent the cablefrom coming into contact with the sur-rounding ground and also to provide asafeguard against corrosion.

Post tensioningThe post-tensioning operation pre-

tests the anchor, thus ensuring itssafety. To establish a measured factorof safety against withdrawal of theanchor it is necessary to apply a tem-porary test loading on site. However,the allowable test load (T,) is limited

by the elastic limit of the steel cable,and the maximum recommended testload is equal to 80% of the breakingload (T„). Thus, for a cable workingat 62.5% T, the maximum measuredsafety factor which can be pro-vided is S,„= T,/T,v = 1.28, whereT„, is the working load.

Every anchor should be tested to 80%T„and representative anchors (1 in 10,say) should be constructed with extracable where T„= 50% T„ to give ameasured S = 1.6.

In fine to medium sized sand, fixedanchor displacement during initial ten-sioning is fairly common and this shouldnot be associated with failure, sincesome relative displacement at thegrout/sand interface may be necessaryto mobilise the load (Fig. 13). In thesecircumstances the load-carrying capa-city of the anchor is established from asecond tensioning cycle. The fixed an-

chor movement should then be simplydue to small elastic deformations, pro-vided that the working load is notgreater than 80% of the maximum testload.

It should be emphasised that thesetest loads are only applied for shortperiods, but experience has shown thatthe decrease in anchor carrying capa-city under long term loading is relativelymodest for most cohesionless soils, i.e.loss of prestress due to fixed anchordisplacement is not greater than 5%.

Corrosion protectionWhere ground conditions are not hos-

tile and the working life is less than twoyears, a greased tape decoupling sheathover the elastic length is normally speci-fied with the normal grouting procedurewhich gives a cement grout cover overthe fixed anchor. On completion of thestressing stage, the top or movableanchorage and the protruding cable maybe painted with a removable plasticcoating such as "stripceal" (Fig. 14).

For permanent anchors, individualstrands making up the cable can begreased and sheathed with extrudedpolypropylene under factory controlledconditions. The fixed anchor length ofthe cable is then stripped and de-greased before casting into a corrugatedplastic tube using a high strength syn-thetic resin.

This fully protected restressablecable is homed and grouted in the nor-mal way and, after stressing, the topanchorage can be enclosed by a steelor rigid plastic cover filled with greaseor bitumen.

Full scale sustained load tests havebeen carried out on resin bonded strandanchors of this type (Fig. 15) and theresults obtained from a test anchor pre-stressed to 220 ton (70% U.T.S.) areillustrated in Fig. 16. Creep of the lowerend of the fixed anchor amounted to3.5/1000 in (0.09 mm), which occurredwithin 25 days, whilst the movement at

0 1 I I i I I 1 I I

0 1 2 3 4 5 6 7 6 9 10

Vertical displacement of movable anchorage, in

Fig. 13. DESIGN DETAILS

Depth of anchorage = 30 ft. Length of fixed anchor = 12 ft. Length of column = 0.Dia. of casing = 4 in. Cable (No. of $ in dia. strands) = 12. Average injection pressure = 75 fb/inr

Water/cement ratio of grout = 0.64. Quantity of cement = 4.5 cwt.Ground conditions: Compact fine/med. sand Iffy = 35'). Water table is 6 ft 6 in below surface.

I y

i

i

]

I II

d1

t

i

the jack amounted to 35/1000 in (0.9mm) after 30 days. This latter creeprepresents a load of prestress in thelaboratory system of less than 5% due

to cable relaxation and partial debond-

ing in the fixed anchors. In practice,where the "free" or elastic length is

commonly 30 ft (9.1 m), the loss of pre-

stress would be 1%.

Lateral movements and settlementsWith regard to permissible lateral

movement of the wall and settlementsof tile retained soil mass, it is consid-

ered that the engineer has to answer

two questions. In the first place he has

to evaluate the allowable differentialsettlements the existing structure adja-

cent to the excavation can withstandwithout experiencing damage, and in

the second place to predict the differ-

ential settlement which can be expectedat a given site.

Concerning the first problem, field

observations have clearly shown thatdistortional settlement which will causedamage to a building cannot be pre-dicted from a theoretical computation.This is due to the fact that the actualbehaviour of a building is greatly influ-

enced by a number of factors which arenot considered in a theoretical calcula-tion, such as interaction betweenstructural and secondary elements, thetime factor and the redistribution of theloads.

The allowable settlements have there-fore to be decided on the basis ofprevious experience derived from ob-servations on similar types of structure.In this connection, a considerable studyof limiting settlements and differentialsettlements has been made by Skemp-ton and MacDonald (1956). This papershows that, in practice, the allowablesettlement is governed more by theavoidance of cracking in the panels andfinishes than by the overstressing of thestanchions and beams. Consequently,the allowable distortion will in almostevery case be governed by a considera-tion of factors which are related to thepractical use of the building.

Cracking of the panels of framebuildings of the traditional type, or ofthe walls in load-bearing wall buildings,is likely to occur if the angular distor-tion exceeds 1/300. Structural damageto the stanchions and beams is likely tooccur if the angular distortion exceeds1/'150.

From an examination of a number ofsettlement records, it is found that thegreatest differential settlements in abuilding, associated with a maximuman9ular distortion of 1/300 is typicallyabout 1$ in (44mm) and 1$ in (32mm)f«a foundation on clay or sand fillingrespectively.

Dther workers, including Po'. Toker, Thomas, Meyerhof and Bjehave also studied this tyPe of P«and their recommended angulartions at structural crack~~9 " "reviewed by Bjerrum in Fjgtable. 8/L = 1/500 is recommende

safe limit for buildings where cra kmgasais not permissible.

lt should be emphasised that in prac-tice the use of the review of obse v

Fig. 15. Diagram of the gun barreland ancillary equipment

1. Jack dial gauge2. Anchor movement

dial gauges3. Gun barrel4. VSL jack

5. Loading plate6. Comb casting7. Pressure gauge8. Hand pump9. Jack control valve

Corrugatedtube Resin bonded

Grout strands

Fig 16 Time deflectioncurves for anchors andjacking dial gauges

kki waaoa k-sac+

Scale for anchorend curve,x 0.0001 m

MO Scale for jackingend curve,x 0.0001 in

for lackmg end

for peripheral strandanchored end

for central strandanchored end

0 10 00 00

Time, days

tions presented requires detailed con-sideration of a number of relevantfactors. Bjerrum (1963) considers thatone of the most important is probablythe time factor. The slower the settle-ments occur, the larger the distortionalsettlements a building is able to with-stand without experiencing damage.This, together with the fact that settle-ments are more evenly distributed in

clays compared with sands, explainsthe different settlement criteria forbuildings on the two materials.

However, it should be pointed outthat structural settlements which mayoccur adjacent to an excavation as aresult of wall displacements are extrato those already existing, and totalsettlements must be obtained by addi-tion. The practical assessment of limit-ing values of settlement due to walldisplacement should therefore take intoaccount the settlements existing beforeoperations commenced. These may al-

ready be approaching limiting values.In order to assess what further differen-tial settlements can be tolerated by anadjacent structure, where no settlementhistory is available, a survey of thefoundation or first storey levels mayprovide some useful data on existingangular distortions.

Finally, it is necessary to take into

Angular distortion dlL

1 1 1

100 000 0001 1 1

400 500 0001 1 1 1

100 MO 900 1000I

Limit where disiculties withmachinery sensitive to settlementsare to be feared

Limit of danger for frames with diagonnls

consideration that cracks inevitably ap-pear in all buildings due to reasons otherthan differential settlements.

The second problem which the engi-neer faces concerns the estimate of thedifferential settlement which he can ex-pect at a given site.

Assuming good workmanship and aproperly designed and executed wallconstruction and excavation procedure,the settlement depends primarily on thesoil properties and dimensions of theexcavation.

Peck (1970) has assessed the avail-able data on excavations using standardsoldier piles or sheet piles supportedwith internal bracing or prestressed an-

~ Sate limit tor buildmgs where cracking is not permissible

Limit where first cracking in panel walls is to be expected

Ir- Limitd where dihiculties with over liead cranes are to be expected

Limit where tilting of high, rigid buildings might become visible

r.considerable cracking m panel walls and brick walls

Safe limitfor flewble brick walls, hIL<g

~-Limit where structural damage of general buildings is to be feared

carnage

criteria

Fig. 17

Ground Eilgineering 15

chors, and states that if the sand isabove the water table or if the groundwater has been lowerecf and broughtunder complete control, adjacent settle-ment of dense sand appears generallyto be inconsequential. Settlements as-sociated with loose sands and gravelsmay be of the order of 0.5% of theexcavated depth, H, at the edge of theexcavation, diminishing to negligiblevalues within a distance to 2 to 3 Hfrom the excavation. Whilst this orderof settlement, 5, is probably acceptablefor excavations down to 25 ft (7.6 m),i.e. 3 = 1$ in (38 mm) and

1 15/L ——

400 600, especially where the ad-

jacent buildings are separated from theexcavation by a street, larger move-ments can give rise to excessivedamage to street services or buildings.

Little information is available on therelationship between lateral wall dis-placements and adjacent settlements,although at Sheffield University experi-mental work by Matallana on model re-taining walls supported by prestressedanchors in dry sand has indicated aratio ofsettlement of sand at edge of wall (5jlateral displacement at top of wall (Ejequal to $.

In similarly ideal conditions, Bassett3

has stated that the ratio —approxi-

mates to 1/5 (range = 1/3 —1/10).These ratios at least give some guid-ance in the absence of field data and the

5low ratio of —is probably explained

by a combination of bulking and arch-ing, especially in compact ground. Inloose cohesionless soil where largerwall displacements may be expected, itis considered that the settlement profilemay approximately reflect the wall dis-placement profile, i e. 2 = 3. It isanticipated that this more conservativerelationship will be employed in themajority of cases to predict angulardistortions in the retained soil.

In a properly designed tie-back sys-tem, reduction in settlement can beachieved most effectively by reducingthe lateral movements of the walls intothe excavation.

With regard to the influence of an-chor prestress on lateral displacement,a tie-back system for bracing thesoldier-pile walls of an excavation 37ft(11.3m) deep in dense sands overlainby a layer of loose sand is shown inFig. 18a (Rizzo et al, 1968). The tie-backs consisted of driven H-piles pre-stressed to approximately 50% of theload calculated for the condition ofactive earth pressure. The walesthrough which the tie-backs transferredtheir forces to the soldier piles werelocated at the third points of the heightof the wall, and the soldier piles and theupper set of tie-backs were installed atthe bottom of an initial excavation10ft (3 m) deep.

The lateral displacement of the wallat the end of the excavation period isshown in Fig. 18b. Subsequent move-

Lateralmovement, in

012 3—0

Loose sandsome silt

—10

D

Dean

0 10 20i, I I

Feet

(a)Fig. 18

Soldier piles

(b)

ments were negligible. The soldier pilesmoved into the excavation by amountsdecreasing with depth from a little over2 in (50mm) at the ground surface toabout zero at the bottom of the cut, thehorizontal displacement of the top an-chor being about 1 in (25 mm).

At another section in the same cutexcavated to 21 ft (6.4m), a similarbracing geometry was used, but theanchors were prestressed to 110% ofthe load calculated on the basis of earth

Rue RoyaleLevel 58.00 A t

i

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Level 36.00

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49.50 I

76.66

t '+e 'I

lv!

8 1Level 30.0

I1

I

1

I

I

I

I

I

I I

I I

I48.07

35.00

30.00

40.00'6.00

18.80

Fig. 19. Left: Plan of diaphragm wallin the centre of Brussels

Fig. 20. Above: Section A-B of thewallin Fig. 19

Fig. 21. Below: Lateral movementsof the wall

After two stages ofexcavation

After finalexcavation

Original level58.30

0.97 1.00 t.fig mm

4.26 4.26 4.26 4.26

No.15 No.16 No.17 No.18

Observationline 47.70

Anchorline

Observationline 40.90

~Anchorline

36.00

9x~~~

1.00 1.20mm

1.05 1.081.03 I 1.001

0.90 1.15mm

r I K~~~ w"cc cr 'cc'co, c N~ cR w"cr "cd

No.15 No.16 No.17 No.18

pressure at rest. With this prestress, thehorizontal displacements of the top andbottom anchors were only about 0.2 inand 0.1 in (5mm and 25mm) respec-tively.

These results were achieved withaverage workmanship, but it should benoted that inferior workmanship caneasily lead to larger settlements thanthose inevitably associated with a giventype of construction and a given soil.

As already indicated, the principaladvantages of diaphragm walls con-structed under bentonite slurry are theability of the slurry to reduce loss ofground during wall construction com-bined with the elimination of vibrationand disturbance associated with piledriving.

In addition, the walls are semi-rigid,and although they do not possess suffi-cient rigidity to prevent yielding, theamount of movement, according toPeck, is less than that experienced withsteel or timber supports.

In view of these observations andcomments it is considered that diaph-ragm walls supported by prestressedanchors provide an attractive solutionwhere later wall movements and adja-cent ground settlements must be keptto a minimum.

Published data on movements asso-ciated with anchored diaphragm wallsin sand are limited, but two interesting

I

1

I'

'- r'.'a

16

+0.00

SurchargeA=1 mm

~—5.00

i'etained soil.'

g—10.00

ir —30'=

0= 2t/m>

Flg. 22. Above: Diaphragm wa/Iarrangemant for the Emobuilding, Madrid

Fig. 23. Right: Section through one ofthe instrumented panels on theGuildhall scheme

case histories have recently been de-scribed by Vander Linden and Maestreat the Speciality Session No. 14 of theSeventh International Conference onSoil Mechanics and Foundation Engi-neering in Mexico.

Vander Linden illustrates the excava-tion for the new headquarters of theSociete Generale de Belgique in thecentre of Brussels (Fig. 19), where an800 mm thick diaphragm wall has beenconstructed through compact fine sand(Brussels stage) overlying fairly com-pact clayey sands (Ypres stage), which

::in turn is underlaid by low permeabilityz clay. Tall buildings (foundation loads =

12-15 t/mz) surrounded the area, butthanks to the careful stressing of theanchors Vander Linden states thatmeasured wall displacements were lessthan 1 or 2mm.

At the section CD, the wall was heldback by two rows of anchors (see Fig.20), each anchor being prestressed togive a horizontal working load of 36tonnes. Lateral movements of the wallat the two excavation stages are shownin Fig. 21.

Maestre describes an interesting inci-dent which occurred during the excava-tion for the new Emo building in Madrid(see Fig. 22) . Here a 750 mm thickdiaphragm wall, constructed to retain15m of compact Miocene clayey sand.was overloaded by a 5 m surcharge ofsand. The maximum displacement re-corded at the top of the wall was only7mm and on removal of the surcharge

(n "the wall recovered 5mm, thus illustrat-ing the good holding power of theanchored wall structure.

Vertical and horizontal wall move-, ments have also been recorded recently'at the Guildhall Precincts RedeveloP-

ment site in London (see James andPh'i, F.

hi)lips). At the wall section shown inig 23, two rows of prestressed anchors

. ', supported the 500 mm thick wall, thePPer and lower anchors carrying nomi-'nal working loads of 5Q and 20 tonnes

respectively.Th rveys were carried ofirst

andxcavation. and

hi d hen the excavation4 3 nd 6.7 m respec, u h ut the wo«ve"ica„t of th wall were negligi

ments

4. Morgenstern, N. R.. and Amir-Tahmasseb,I,, 1965, The stability of slurry trenches incohesionless soils, Geotechnique XI/, No.4,387-395.

5. Nash, J. K. T. L., and Jones, G. K., 1963,Ths support of trenches using fluid mud,Proc. Symp. Grouts and Drilling Muds inEngineering Practice, pp. 177-180. London,Buttsrworth.

6. Schnsebeli, G., 1964, "La stabilitfi destranchses profondes forges en presence debouc", Houille blance, 19:7:815-820.

7. Veder, C., 1963, Excavation of trenches inthe presence of bentonite suspensions forthe construction of impermeable snd loadbearing diaphragms., Proc. Symp. Groutsand Drilling Muds in Engineering Practice,p. 181. London, Butterworth.

8. Wielicka, H., and Malasiewicz, A., 1967,Wplyw zjawisk fizyko chemicznych naWlasnosci zawievin ilowych stosowanychdo glevienia waskprzestrzennyeh wykopowArchiwum Hydrotschniki 1967.

9. Elson, W. K., 1968i Experimental investiga-tion of the stability of slurry trenches",Georechnique XVIII No. 1 pp. 37-49.

10. Florsntin, J., 1969, Les Parois Moulees DansLe Sol, Proceedings of the Seventh Inter-national Conference on Soil Mechanics andFoundation Engineering, Speciality Session14, Mexico, 1969, Vol. 3, pp. 507-12.

11. Courteille, G., 1969, Actron stabilisatricedes suspensions thixotropique sur le paroisde fouilles, Communication de 14me Ses-sion Speciale, Mexico, 1969.

12. Road Research Laboratory, 1950, Design ofconcrete mixes, Road Note No. 4. HMSO,London.

13. Kranz, E., 1953, Ueber die Verankerung vonSpundwanden. W. Ernst & Sohn, Berlin.14. Jelinsk, R., and Ostermeyer, H., 1966,Verankerung von Baugrubenumschliessun-gen, Vortrage der Baugrundtagung, 1966, inMunchen, Deutsche Gesellsc)taft fiir Erd,und Grundbau e.V.Essen.

15. Ranks, A., and Ostermeyer, H., 1968, Con-tribution to the investigation of stability ofmulti-tied walls, Bautechnik, 10, 341-50.

16. Sksmpton, A. W., and MacDonald, D. H.,1956, Allowable settlements of buildings,Proc. Inst. Civ. Eng., 5, 727-68.

17. Polshin, D. E., and Toker, R. A., 1957,Maximum allowable non-uniform settlementof structures, Proc. 4th Int. Conf. SoilMech., 1, 402—5.

18. Thomas, F. G., 1953, The strenttth of brick-work, The Structural Engineer, 31, 35-46.

19. Meyerhof, G..G., 1953, Some recent founda-tion research and its application to design,The Structural Engineer, 31, 151-67.

20. Bjerrum. L., 1963, Discussion, Proc. 5th Int.Conf. Soil Mech., 11, 135-7.

21. Peck, R. B., 1969, Deep excavations andtunnelling in soft ground, Proc. 7th Int.Conf. Soil Mech., State of the Art, 225-290.

22. Matsllanai G. A., 1969, An experimentalinvestigation of anchored model retainingwalls", MEng thesis, University of Sheffield.

23. Bassett, R. H., 1971, Private communicationreferring to PhD thesis by I. A. A. Smithentitled "Lateral pressures on rigid wallsconstrained to fail under rotating activeconditions" (to be published).

24. Rizzo, P. C., et ai, Prestressed tie-backwalls for two deep excavations in Buffalo,New York, Paper presented before ASCE,Pittsburg, Sept. 30, 1969.

25. Vender Linden, J., 1969, Controls desmouvements horizontaux d'une paroimoullss dans le sol avec ancrages prfi-contraints, Speciality Session No. 14,103-4, Proc. 7th Int. Conf. Soil Mech.

26. Maestre, M., 1969, Incident following over-loading of an anchored diaphragm wall,Speciality Session No. 14, 43-4, proc. 7thInt. Conf. Soil Mech.

27. James, E. L., and Phillips, S. H. E., 1971,Movement of a tied diaphragm retainingwall during excavation, Ground Engineer-ing, Vol. 4, No. 4, pp. 14-16.

I

0.'0

, 0'.

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the apparent overall effect of anchoringwas to pull the wall into the retainedsoil mass, i.e. away from the excavation.Lateral movements at the top of thewall during the second and third sur-veys were 4mm and 7'm respec-tively. Since no heave was apparent inthe retained soil, the inward movementmay have represented compaction ofground loosened by the slurry trenchingprocess —or possibly compression ofthe filter cake.

Although the case histories describedlack quantitative data on soil properties,and wall and anchor designs, it is con-sidered that the examples quoted illus-trate that wall movements and thereforesettlements can be kept within tolerablelimits.

As Peck has suggested, the absenceof settlement records at least suggeststhe absence of serious settlements.

REFERENCES

1. Rows, P. W., 1956, Sheet pile walls atfailure, Proc. I.C.E., Part 1, Vol. 5, p. 276.

2. Rows, P. W., and Briggs, A., 1961, Mea-surements on model strutted sheet pileexcavations, 5th Int. Conf. on Soil Mach.and Found. Eng., Vol. II, pp. 473-78.

3. Golder, H. Q., Gould, J. P., Lambe, T. W.,Tschebatarioff, G. P., and Wilson, S. D.,1970, Predicted performance of braced ex-cavation, Proc. Amer. Soc. of Civ. Engrs.(S M. & F E. Div ), Vol. 96, No. SM3(May).

SITE INVESTIGATION IN THE WASH

A SCHEME FOR fresh water storage inthe Wash is being examined by Binnieand Partners, for the Water ResourcesBoard. Soil Mechanics, Ltd., have car-ried out a preliminary site investigationof an area off the southern shore of theWash between the mouths of the RiverNene and Great Ouse for the formationof embankments for reservoirs and rivertraining. The investigation, carried outin tidal conditions up to 2 miles(3.2 km) from shore, included boringsand Dutch soundings in estuarine siltand sand and underlying glacial de-posits to depths of 30 m.

i ii )i)II~: .,(4 lp sr

Ground Engineering 17