Curren: dirac:ice inc esic nineear:a re:aininc s:rue:uresby C. J. F. P. JONES", BSc, MSc, Pho, CEng, MICE

IntroductionTHE BUILDING RESEARCH ESTABLISH-MENT, together with the Department ofTransport recently commissioned a reviewof the foundation and sub-structure meth-ods used in the design. of small andmedium span bridges. The study, whichwas based upon a series of exhaustiveinterviews with practising engineers, givesa comprehensive view of the design of

sAssistsnt Director Structural Engineering, Direct-orate of Planning, Engineering and Transportation,West Yorkshire Metropolitan County Council,

This article is based on a talk given by theauthor earlier to an informal meeting of the BritishGeotechnical Society at the Institution of CivilEngineers

conventional earth retaining structures(BRE, 1977).

In an opening commentary it is arguedthat our retaining wall design is basedupon experience of structures going back150 years. Indeed, most modern earth re-taining structures are based upon deriva-tions of the classical theories of Coulomb(1773) and Rankine (1857) and althoughfield evidence shows that the assumptionsmade with these theories are demonstra-tively incorrect, there are few retainingwal'I failures associated with modern con-struction.

At the same time, application of ourcurrent design rules to those walls which

are 100/150 years old suggest that theyshould not stand up and, indeed, there isgrowing evidence that many are reachingthe end of their life. Further inspectionshows that the Victorians hed very dif-ferent ideas and used different techniqueswhen bui'Iding walls and it is perhaps rele-vant to question whether we are basingour current practice on 150 years worthof experience or whether modern methodsare such as to make any comparison withthe past impossible.

Retairing wall typesMany of the types of retaining walls

constructed today are il'Iustrated in Fig. 1.Mass, reinforced concrete cantilever, sheetpiled, counterfort and anchor walls arecommon as are reinforced earth wallsabroad, but reticulated and Tsagarelistructures are somewhat rare.

The I'ndustrial Revolution produced an

Mass Cantilever Cantilever

Counterfort Anchor Fig. 2. Form of Victorian masonry wa//s

,:Mt

M, y

I

Reinforced earth Tsagareli

eticulated

Fig. 1. Types of modern retaining wal/s

40 Ground Engineering

Fig. 3. Typical example of wall reachingthe limiting point of stability

unprecedented construction boom and wehave inherited a great number of retainingwalls of various forms. Many are asso-ciated with the railways, docks and her-bours, and canals; arguably the greatestconcentration of retaining walls can befound in the woollen district of Yorkshirewhere the nature of the dales is such thata majority of structures have to be builton side iong ground. Stone is plentifuland the dry stone retaining wall is themost common feature of the landscape;when used as earth retaining structuresthey frequently take the form illustrated

j in Fig. 2.Reference to Fig. 2 poses a prob'lem as

the form of many of the Victorian wallsdoes rrot appear to fall within the typesof wall shown in Fig. 1, although it shouldif we base part of our current practice onthis form of structure. Logically it shouldresem'ble a mass wall, but even a cursoryinspection indicates that the constructionis too narrow.

The 'behaviour of many of our Victorianstone walls is interesting in that we areseeing an increasing number of wali fail-ures. These failures are due in part to thedeterioration of the stone forming thestructure and also to a tendency for thewalls to change shape; ultimately thisleads to an unstable condition, Fig. 3. Thefailure of a stone wall of the latter sortis unpredictable and is often preceeded bya long period in which the wall retains

a bowed shape, Fig. 4. The reasons whythe wall becomes mis-shapen are complex;the vibration from modern vehicular trafficis certainly a contributory factor in someinstances, but many failures occur in areasremoved from vehicular traffic. When fail-ure does occur it is normal practice toreplace the wall using the same stone,but using a construction similar to thatshown 'in Fig. 5. Fig. 5, clearly, is a masswall.

Gloser 'inspection of dry stone wallsin Yorksh'ire reveals a constructional tech-nique which had few variations. Thestones used for the face were of medium/large size, carefully graded and placed byhand to fit; behind these, smaller flatstones were used, laid in an horizontalplane grading back from the face, Fig. 6.The structures are porous and essentiallyflexible and must have been built in layersusing relatively unskilled labour and noconstruction plant; in particular no moderncompaction equipment, Fig 7a. The near-

est approach to this form of constructionwould be a partly reinforced earth struc-ture as shown in Fig. 7b in which thesmall stones are replaced by layers of fab-ric reinforcement. As with any reinforcedearth structure the structure in Fig. 7awould have to be constructed in layersand because of its narrowness it wouldnot be possible to use eny large compac-tion equipment. Recent research at Cam-bridge suggests that the stability of sucha structure can be guaranteed, providedthe thickness to height ratio (I: h) isgreater than 3:10 (Smith, 1977).

Field measurements of retainingwalls

Reverting to modem structures and mod-ern methods of design it is worth con-sidering how actual performance com-pares with the design assumptions. In anattempt to answer part of this questionWest Yorkshire Metropolitan CountyCouncil/Department of Transport have

i.,t, I

„,@y)N!tI

Bursting ~ Potential failure plane

Fig. 4. Mode of failure of Victorian mas-onry wal/s

Fig. 6. Masonry wall after failure showing typical constructional details

Victonan wall Partial reinforced earth

i

MortaredMasonry ~

Concrete

Large stones Small stones ih.

Fabric reinforcement

Drain ~'ftOin

Constructed in layers.No compaction plant.

(a)

Porous.Constructed in layers.No construction plant.

(bl

Porous.

Fig. 5. Example of modern interpretationof Victorian wall

Fig. 7. Method of construction of (aj Victorian masonry wa/I and (b) its modernequivalent

September, 1979 41

recorded the actual pressures generatedon a series of motorway structures onM1, M62 and M18. Results of these testshave been reported elsewhere (Sims etal, 1970; Jones 8r Sims, 1974, 1975; andJones, 1973).

Some of the results from the workundertaken on the M1 motorway nearRotherham (see Fig. 8) are shown in Fig.9. Fig. 9 shows the vertical and lateralpressures acting on a 40ft high retainingwall at three levels at a point where themotorway is retained close to the top ofthe wall. Reference to the results shows thatrecorded pressures are markedly differ-ent to the earth pressures assumed duringthe design of the structure. The designwas based upon a normal equivalent fluidpressure assumption derived from theCoulomb/Rankine theories. The main con-clusion from these tests is that theory andpractice do not coincide.

Fig. 8. Instrumented retaining wa/I on M1 motorway

'Level 1

o----o Vertical pressurer—x Lateral pressure

30-

c= 20 .ClI

o-10-

CO

Lu

01968 69 70 71

.0,0,0o'

Theoretical vertical pressure (t = 90lb/ft')

Theoretical lateral pressure (K, = 0.22)

72 73 74 75 76 1977

"Level 20—--o Vertical pressure

s—"Lateral pressure

30cC

= 204)

10

EO

UJ

,a/

Theoretical vertical pressure (t = 90lb/ft')

Theoretical lateral pressure (K, = 0.22)

01968 69 70 71 72 73 74 75 76 1977

Comparison between assumed andactual behaviour

The retaining wall used in the M1motorway tests was designed as a con-ventional cantilever with an assumed tri-angular lateral pressure distribution basedupon the development of an active earthpressure condition. To achieve this situa-tion the wall has to be able to move,either bodily on the foundation or thewall stem must flex; for the latter situa-tion it is necessary for the wall to bepropped during the back-filling operation.This is seldom, if ever, done on modernconstruction sites, Fig. 10a.

The actual construction method is shownin Fig. 10b. After the wall has been con-structed the fill is placed in layers withthe aid of compaction plant. As filling pro-ceeds the wall deflects progressively for-ward and the degree of deflection is afunction of the compactive effort, thebackfill materiel properties and the stiff-ness of the wall. The actual lateral pres-sure acting against the wall is a residuallateral pressure, Fig. 11a.

The 'lateral pressure (P) at a depth (Z)produced by a loose soil placed behind anunyielding structure can be given by theexpression

K, = (2)

P = (K,yZ)

where K„= coefficient of earth pressureat rest related to Poisson's ratio by theexpression

vel 3

0----0 Vertical pressure

x i Lateral pressure

The application of a uniform verticalpressure Pr over the entire backfill sur-face increases the lateral pressure tobecome

30-P = K, (yZ + Pv) (3)

8Theoretical vertical pressure (y = 90lb/ft')—20

8

o.10.

0V

~COO.O.o--~

~~~Theoretical lateral pressure (K, = 0.22)

O

Fig. 9. Results of earth pressure measurements on M1 motorway

42 Ground Engineering

01968 69 70 71 72 73 74 75 76 1977

If the structure yields then the elasticdeformation of the soil mass will reducethe lateral pressure. The limit of the re-duction in P will be the active state whenthe sail is at the point of failure, and theamount of yield required to induce theactive state is dependent upon the degreeof compaction within the soil mass. It isarguable th'at the yield at the bottom ofa cantilever wail is sufficient to generatethis situation, Figs. 11b and c.

Instead of a uniform pressure over anentire backfill, modern compaction meth-

ods apply a high pressure over a limitedarea. Thus the lateral pressure immediatelybeneath the compacting device may beequal to P in eqn. 3; elsewhere the pressurewill be less. As the backfill is compactedin layers the average pressure will besome fraction of that suggested by eqn. 3and will approach the lateral pressure re-maining in the soil after compaction iscomplete. This suggests that there will

be a relatively uniform lateral pressuredistribution up the back of the structure,and this has been observed in practice(Sims at al).

If the backfill soil- is assumed to bemade up of individual incompressible par-ticles, compaction takes place by amovement of the particles relative to oneanother. Defining the direction of thismovement as s with respect to thed'irection of P», the vertical pressure, andif the angle of friction between the partic-les is ss the coefficient of earth pressureat rest K, is

(a)Assumed

(b)Actual

Tempro

uivalentid pressurekN/

Tension observedon front faceduring filling

Compaction plantP/ÃX/XX/EA

Fill in layers

I

Designed as a fixed cantileverX

BM = ffWx/2 dx0

Deflects progressively forward during compaction(As the wall deflects movement at the bottom of the wallstem is sufficient to produce active conditions).

Fig. 10. Assumed (a) and actual (b) construction methods for a cantilever retaining wall

P tan (s-ss)Ktt

P, tan s(4) 1.0

If the vertical compacting pressure isreduced to P„'after the passage of thecompaction plant), then the soil tends torecover its original volume reversing thefrictional force on the plane of movement.The ratio of the horizontal to the verticalpressure becomes

itial

l—After period

of time

0.5 .

Ko

K,

P'an(s+ 4)K„' —=

P»'an a0

0 .001 .002 .003 .004 .005

P 's the residual lateral pressure and K,'s

the coefficient of the residual lateralpressure against an unyielding structure.

The residual lateral pressure may beinfluenced by friction developed betweenthe soil and the structure. During com-paction the soil moves downwards againstthe structure developing frict)on. Whenthe compacting pressure is releasedthe upward movement is restricted byfriction so full expansion cannot takeplace. This may cause the lateral pressureadjacent to the structure to be differentfrom that in the surrounding fill material.The resultant pressure of the fill will stillbe downwards, causing compression onthe backface of the structure. This verti-cal earth pressure component is ignoredin normal design although it can be suffi-cient to cause tension forces to developon the front face of reinforced concretewalls, Fig. 10b, at a point just below thelevel of the compacting plant (Jones,1973).

The main conclusion derived from theM1, M18 and M62 field tests was thatthe method and type of construction usedtoday is the dominant factor in the genera-tion of the lateral earth pressures whichthe wall must withstand. This lateral pres-sure is a composite of the )ateral residualcompaction pressure, the active and theat rest pressures, Fig. 11d. It is interestingto note t'hat similar pressure diagramshave been obtained in centrifuge experi-ments on reinforced earth, F:g. 11e(Bolton, 1977).

CondusionsA list of actions required to ensure the

most economic construction of an earthretaining structure is given in the BRE

44 Ground Engineering

Observed lateralpressure distribution

(a)

h/iH~

(b)

(c) (d)

Reinforced earth centrifugeexperiments (Bolton)

(e)

Fig. 11.Theoretical lateral earth pressure acting on a retaining wall

'dualalsure

K,

K,) K,

draft commentary on modern practice in

the design of bridge foundations and sub-structures:

(i) Use simple, cheap and readily ava'il-able materials.

(ii) Construct as much as possible withplant at ground levels.

(iii) Design the structure to be stableat all stages of construction with-out propping.

(iv) Excavate with foundation horizontal.

(v) Use foundation of simple shape.(vi) Construct in one plane at a time.

(vii) Form all the surfaces horizontal andvertical and without skew.

(viii) Make walls wide enough for a manto get inside between reinforcementand capable of being constructedin single pour.

(ix) Detail reinforcement so that it canbe fixed 'in one plane at 8 time,avoiding the use of very small orvery large reinforcement bars.

Unfortunately the above list does notconsider the action of the compactionplant, although this single item may domost to determine the stresses locatedin any particular earth retaining structure.Reference to Fig. 7 shows that the Vic-torians followed these nine points wherethey applied. The modern design assump-tions and techniques do not compare fav-ourably, the exception being the modernform of reinforced earth for which theBRE nine~oint pl'an reads like 8 specifi-cation.

In conclusion it would appear that Vic-torian engineers constructed their retain-ing walls in a manner which reflected theassumptions made in the design. Withthe construction of modern earth retainingstructures we appear to ignore the con-struction method although 'it is intrinsicin the performance of any earth retainingstructure, and base our design on modelor pilot tests and assumptions which 'have

little relevance to present day construc-tional techniques.

The BRE review suggests that majorsavings can be made in retaining well de-sign provided there is a change in attitudein the engineering world and providedthat consideration during the design anddetailing of any structure is given to theglobal conditions. These global conditionsmust include the consideration of our con-structional tec'hniques.

From the preliminary vibration investi-gation results given in Fig. 2, it can beseen that this acceleration will arise outof blasting at a scaled distance, S, of

55m/Vs'kgFrom eqn. 2, S =D/J'W, the value of

safe proximity, D, may be calculated forany charge weight per delay, W. For con-venience this relationship can be showngraphically by plotting D against W for arange of practicable delayed chargeweights 'as shown on the design chartFig. 4. From this chart it can readily beseen that the closest safe proximity forblasting adjacent to the existing slopeis 55m, for a charge weight per delay of1kg. Now this charge weight is unlikelyto be efficient for quarrying. To be able toachieve an efficient blasting intensity andyet still extract limestone up to 55m fromthe lagoon dam, the embankment wouldhave to 'be strengthened.

The predicted effect of various remed-ial measures are also shown on the designchart (Fig. 4), for rapid assessment pur-poses. These relationships are computedin the same manner as outlined for theoriginal slope.

For quarrying adjacent to this spoil heap,a commercial appraisal is necessary. Thisexercise 'is best carried out by the quarryowner who Iis in the best position tochoose the optimum solution. He canmaximise quarry proceeds yet maintain thesafety of adjacent slopes and structures.

ConclusionThe effect of ground vibration on slope

stability is detrimental. The author out-lines'a practical method to produce de-sign charts for quarry blasting so that theexplosion-induced vibration may be pre-dicted and the effect on soil slopes as-sessed.

A realistic example is given whichdemonstrates a determination of the safeproximity for quarry blasting adjacent toa colliery lagoon embankment. The choiceof a suitable factor of safety is the keyto safe proximity determination. The meth-od outlawed. is suitable for all soil slopes

and would be of use for spoil heaps andlagoon dame, opencast overburden dumpsand natural slopes.

The costs of carrying out these neces-sary investigations and analyses can us-ually be met by increasing the saleablereserves of the quarry which would other-wise be sterilised by the use ~of conser-vative approximations, or, at worst, byavoiding compensation payments result-ing from a slope fai1ure.

AcknowledgementsThe author is thankful to Mr. G. Hayes,

the Area Director of the South Yorksh'ireArea NCB, for permission to submit thisPaper; and to Mr. J. Evison, Mr. A. Baconand Dr. N. Goulty for their assistancethroughout the study. Reference to Derby-shire in this Paper implies the county andnot the NCB Area.

The views and opinions expressed in

this Paper are those of the author andnot necessarily those of the National CoalBoard.

This Paper is one of those presentedat the 1978 Cooling Prize Competition andthe author gratefully acknowledges theencouragement given by the British Geo-technical Society.

ReferencesDuvell, W. I. & Devine W, (1966): "Effect ofcharge weight on vibration levels from quarryblasting". US Bureau of Mines Reprint of Inves-t I g S'I I0h S .Duvall, W. l. & Fogelson, D. E. (1962): "Reviewof criteria for estimating damage to residencesfrom blasting vibrations' US Bureau of Mines,Report Rl 5968.Ganguli, D. K., Misre, G. B. & Sengupts, S.1977): "Ground vibrations from open-pit blasts".ournal of Mines, Metals and Fuels, August.

Lengefors, U. & Kihlstrom, B: Rock Blasting.Wiley.National Coal Board (1970): Spoil Heaps andLagoons (Technical Handbook) NCB.Plleider E. P. (1968): Surface Mining. AmericanInstitute of Mining, Metallurgical and PetroleumEngineers Inc.Serms, S. K. (1973): "Stability analysis of em-bankments and slopes". Geotechnique 13, No. 3,423-433,Sarme, S. K. (1975): "Seismic stability of earthdame and embankments". Geotechnique 15, No.4, 743-761.Stagy, K, G. & Zienkiewcz, O. C. (1968): RockMechanics in Engineering Practice. WIley.Thoenen, J. B. & Windes, S. L. (1942): "Seismiceffect of quarry blasting". US Bureau of Mines,Bulletin 442.

254 3 2

I

A determination of the safe proximity for quarryblasting adjacent to slopes (continued from page 32)

ReferencesBuilding Research Establishment (1977): "Draftcommentary on current practice in design of bridgefoundations and substructures". Department ofthe Environment, Msy. This document precededthe full report "Bridge Foundations and Sub-structures" which hss recently been publishedand is available from HMSO, PO Box 269, LondonSE1.

Coulomb (1773): "Essai sur une application desregles de msximis et minimis a quelques prob-lems des statique re.'atifs a L'Architecture". Math-ematic Memoirs presented to the AcademicRoyale des Sciences. I.

Jones, C. J. F, P. (1973): "Earth retaining struc-tures", PhD Thesis, University of Leeds.

Jones, C. J. F, P., & aims, F. A. (1974): "Com-parison between theoretical snd measured earthpressures acting on s large motorway retainingwall". Jour. Inst. High. Eng., Dec.

Jones, C. J. F. P., 8 Sims, F. A. (1975): "Earthpressures against the Abutments and Wingwsllsof Standard Motorway Bridges". Geotechnique Vol.1S No. 4

20

eeuL

15+

UlID

ts 10O

00 20 40 60 80 100 120

1. The existing slope

2. Remedial works onthe existing slopeto draw down thewater table by Sm

3. Regraded slope to1 in 3.5

4. Regraded slope to1 in 4

I I

140 160 180

Rankins, W. J. H. (1857): "Theory on the stabil-ity of loose earth based on the Ellipse ofstresses". Phil. Trna. Royal Soc. 147.

Sims, F. A., Forrester, G. R. & Jones, C. J. F. P.1970): "Lateral pressures on Retaining Walls",ourn. Inst. High, Eng., June.

Distance in metres between the slope and the quarry blasting

Fig. 4. Design chart example: to determine the charge weight per delay for quarryblasting adjacent to a lagoon embankment in Derbyshire, at various distances, whilstmaintaining a factor of safety of 1.5

September, 1979 45