UNDERREAMED.GROUND.ANCHORSAncrage d.ans le sol avec élargissement

by

Richard H. BASSETTUniversity of London King's College - Civil engineering Strand-London WC 2R 2LS - Great Britain

SOMMAIRE

Les ancrages dans le sol acquièrent une impor­tance grandissante pour les appuis de murs desoutènement, aussi bien en terrains cohérents qu'enterrains non-cohérents. Pour les sols cohérents, lerendement de la portance d'un ancrage augmenteavec la construction d'élargissements sur le fût.Les formules empiriques qui permettent de calculercette charge utile proviennent de la théorie despieux. L'étude examine, au moyen d'expériencesen laboratoire, les caractéristiques suivantes desancrages à élargissements de diamètre: influencedes distances entre les' élargissements,· influencedu nombre de ceux-ci et relation entre rigidité dusol et rigidité' du câble de l'ancrage. L'observationdes mécanismes de rupture montre que les élé­ments individuels de la formule empirique sontjustifiés, ce qui permet de suggérer des valeurspour les dimensionner. .

SUMMARY

Ground anchors are becoming increasingly impor­tant for supporting retaining walls in both cohesiveand non-cohesive soils. In cohesive soils theefficiency of an anchor's carrying capacity isimproved by forming underreams on the shaft.Empirical formulae for calculating their load capacityhave been developed from piling philosophy. Thepaper examines by controlled laboratory experi­ments the following features of underreamedanchors: The influence of the spacing of under­reams, the influence of the number of underreamsand' the relationship between the soil sti·ffness andthe anchor tendon stiffness. The observed faituremechanisms show that the format of the empiricalformula is justified and values for design constantsare suggested.

INTRODUCTION

The rapid increase in the economic importance ofholding back permanent retaining wall structures by theuse of anchor ties was indicated by Ostermayer (1974).The exponential increase suggested has continuedunabated and in the author's personal experienceappears to be coupled with a continuous demand forincreased load capacity. Practical anchoring techniquesfall into two distinct catagories,

a) Pressure' grouted or regrouted shafts, relying on theadhesion bond between the face of a grout. mortarbody and the soil (most commonly used in cohesion­less soils and rock), or

b) Open drilled shafts in which the anchorage zoneis deliberately expanded in diameter to form a seriesof bells or underreams (most commonly used in

self supporting soils from fine cohesive clays tomoderately hard rocks).

Although a considerable body of 'literature has nowbeen collected, Littlejohn (1975), these predominantlyconcern either practical examples, wall-anchor inter­action studies or fundamental laboratory tests of anchorplate systems in dry sand. Little fundamental study,with the exception of Ostermayer (1974), appears to'be available to justify the empirical design formula(Littlejohn 1970a 1970b) recominended for the realconstruction methods.

The author has been working on both groups. Butthis paper will present only' data obtained from acontrolled laboratpry examination of the behaviour ofmulti-underreamed anchors in. a very low permeability,saturated, uniform, cohesive material.

THE UNDERREAMED ANCHOR PRINCIPLE

Straight shaft, pressure grouted anchors have beenused in cohesive materials but boring the holeinvariably results in the remoulding of a thin layer ofsoil adjacent to the final anchor member. The consi­derable, consequent reduction in strength can rarely becompensated for even by pressure grouting. Under­reams cut out iuto undisturbed soil at intervals along

11

the shaft induce a cylindrical shear failure althoughintact· soil (fig. 1). Various patented cutting tech­niques are employed in the U.1(. the blade cutterof Universal Anchorage Limited and the, expandingbrush of the Cementation Company are typîcalinstances.

Dr. Littlejohn (1970a 1970b) presented an empiricalformula (1) for estimating the ultimate capacity of thistype of anchor based quite logically on piling philosophy

rcW ult == Ne . (D2 - d}) 4 .Cu + rc D . lu . Cu +

ts.rcds.ls.cu (1)end bearing + capacity of underream length + capacityof shaft length.

The author, Bassett (1970) in his discussion to thispaper and in Bassett (1971) had derived a similarformula (2) based on the full scale testing of sorne500 production clay anchors but the author included areduction coefficient tu to be applied to the capacity ofthe underream length and questioned Littlejohn's Nevalue (see Littlejohn's reply (1970) c).

Il"Vu1t == Ne . (02~ d}) 4 .Cu + tu· II . D .lu· Cu +

ts . II . ds . ls . Cu (2)

Vnder the author's direction Potts (1973) set out toinvestigate both the fundamental components of thisempirical equation and possible alternative analyticalapproaches. His work on the following is presentedhere:-(i) The examination of the behaviour of a single under­

ream in order to measure the Terzaghi bearingcapacity factor Ne and to assess the influence ofthe cavity which forms below an underream onrapid loading.

1) ADHESION FAILURE UNSUITABLEON SHAFT STRATA

SUITABLEii) END BEARING FAILURE BEARING

IN CLAY STRATA

(iJOCOHESIVE FAiLURETHROUGH CLAY

SHAFT DIAMETER

d.

Fig. 1. - Diagram of an underream anchor.

(ii) The examination of the mechanism of failure tojustify the format of t~e empirical equation (2).

(iii) The examination of the influence of varying thespacing of underream bells.

(iv) The effect on the load capacity of increasing thenumber of underreams and to relate this effect tothe ratio between the stiffness of the anchor tendonsystem and the equivalent response stiffness of thesoil deformation mechanism.

EQUIPMENT AND TEST PROCEDURE

AlI the tests described wer~ performed on a saturat­ed, remoulded, reconsolidated, London clay with thefollowing specification;

Water content 43 0/0LL . 86 °10PL 23 °10PI 63 °10Liquidity Index 0.35V ndrained triaxial shearstrength 40 KN/m2 •

This is in good agreement with Skempton and Nor­they (1953). The data indicated an undrained E(at 1/2% strain) == 15 X 105 N/m2•

The anchor was formed of 30 mm diameter, stiff,aluminium discs sorne of which incorporated a straingauged load Jransducer. These discs represented theunderream bells and were connected by various lengthsof rod, hollow hypodermic tube, or stiff springsto represent the tendon system. The load carryingzone formed by the discs was deeply embedded (depth/diameter > 15) in the clay contained in a 380 mmdia. X 900 mm high cylindrical container (fig. 2).The lateral pressure in the soil was initialIy fixed andmonitored by instrumented bolts.

AlI tests were performed at a constant pull out rate of1.7 X 10-2 mm/s. For saturated, remoulded Londonclay this is effectively rapid and undrained.

Fig. 2. - Photograph of the test equipment.

12

THE SINGLE UNDERREAM

(ii)

Two initial tests were performed on single discs thefirst with a hollow tube tendon venting the area belowthe disc to atmosphere, the second with a solid stem.The results are shown in figure 3 as curve A and curveB respectively.

The vented disc, A, reaches its initial failure in endbearing at approximately 1 mm displacement. Curve Bshows a similar basic format the pressure differencebetween Curve A and B representing the suction onthe underside of the disc is shown in figure 4.

where Ps is the pressure required to expand the cavityC

and - is the ratio of the cavity volume to the volumea .of the plastic zone.

• C ( E )l • e . a = "(1 - v) 2 Cu .

for v == 0.5 (undrained), Cu 40 kN/m2,

E == 15 X· 105 N/m2

Ps == 188 kN/m2

Cur~ a #6fut"O/ suclionbe/ow undvf'&Jm.

I~O

2.

100

/_----4__" _

Fig. 3. - Load-displacement for a singleunderream.

(}5 /'0

DI.sp/acemet1f in ",,,,.

2.0 3'0.

Fig. 4. - Suction-displacement belowa single underream.

2

---"~.

1 .~.

- -\- - - - - :..::::.~.~-:-- - - - ~.:..:--• .::--=-' - - - - - - - - - - - - ~

./.,.-- , ...-.....-V

Suc1ion & /owuncklYmm I~? oJ:notpheres

A suction. pressure of one atmosphere is developedafter only 0.15 mm displacement and reaches a peakin excess of this at 0.25 mm there after progressivelydecreasing. This is presumably due to evapouration atthe free clay surface and a reduction in suction aswater vapour fills the cavity. 1t is significant thata vacuum cannot be maintained even at this high rateof strain.

Pùtts (1973) adopted the expanding plastic cavityapproach to provide a theoretical peak value for theultimate load capacity of this single underream. Headopted from the work of Bishop, Hill and Mott (1945)the equation:

Ps = +Cu ( 1- 3 loge +) (i)

13

and over the area of the underream disc for a halfcavity is equivalent to a force of: - 137 N.

This value is drawn on figure 3 and is the theoreticalestimate for the maximum value of curve A. The.agreement is seen to be reasonably good. Swain (1976)has developed a similar theoretical argument but useda more extended approach to give the complete load­displacement relationship up to the initial ,failure.

The Conventional Terzaghi bearing capacity and thepiling approach to this problem suggests a value ofNe == 9. (This value of Ne is used by Littlejohn 1970a1970b). The experimental value for Curve A isNe == 5.4 and for the total load including the suctioncomponent in Curve B is Ne == 8.6..

VERTIFICATION Of THE MECHANISM POSTULATE IN THE EMPIRICAL FORMULAEAND THE INFLUENCE OF UNDERREAM SPACING:

As all the subsequent test series described in thispaper allowed no venting below the bottom underreamthe higher value of Ne == 8.4 was a~opted to includethe suction component.

To check the failure mechanism an underreamspacing of i.5 D, consistent with practice, was adopted.This model was tested using a split sample, the claysurface being marked with a fine horizontal grid.During the test this grid was examined and typicalphotographs are shown in figure 5.

The ruptures seemed ta justify the empirical approach,assuming two major components (1) end bearing .on the.top underream and (11) a waisted plug like componentfor the m'ain anchorage length. The pictures did howe­ver show a distinct indication of an end bearing passivecone above aIl the underreams before the plug failurewas fully forlned. This observation suggested that atransfer from the plug failure mechanism to a num.berof separate end bearing failures \vould almost cer­tainly occur when the load capacity between twounderreams reached the value of an end bearing failure,perhaps even earlier.

i.e. if the distance between two underreams L is thoughtof as a dünentionless ratio X diameter of underream(D)

. ~ L t1 . e . L == (1)\ X D

then for equal capacity 0lf a plug failure and an endbearing .failure

tu . Cu . TI . D. (~) . . D == Ne . Cu . (D2 - ds2). TI4ent

(L \ Ne ( D2 - d/ )

or D)crit = 4fu

• D2, L

as ds is small cOlnpared to D then D .cnt

Section 5 of this paper will indicate the determi­nation of tu == 0.63 and for this value and Ne == 8.5

Fig. 5a.- End bearing cones.

e

7

6

5

z

14

Fig. 5 b. - Plug failure.

the critical spacing ratio. is equal to 3.4. Using the

fully vented cavity value ( DL) . reduces to 2.1.ent

A series of unvented, rapid loading tests were carriedout using 3 underream discs at spacings of 1.5 D:2 D: 2.25 D: 3 D and 4 D. The results are shown infigure 6.

ToT;J1... Lo~D

1: /OlN.

/

~D 3D 4~

Spac/n'j 0+ Undvru;mJ

Fig. 6. - Load capacity-underream spacing.

Fig. 7. - Load-displacementcharacteristics for 3 under­reams spaced at 3D.

53

. The theoretical increase in ultimate load up to

~ = 3.4 is also show. The predicted form is closely

adhered to but deviation to an end bearing failure isearlier than expected i.e. at approx. 2.9 D. This wasin fact expected as the full Ne value of 8.5 should not

.apply to the 2nd and 3rd underreams in end bearingbecause their capacities will obviously be reduced bythe suction zone forming below the disc above. At

the experimental ( ~). . of 2.9 this suggests that. ifcnt

Ne == 8.5 applies to the top underream plate a value ofonly 6.6 is possible on the two lower plates. Separatemeasurements of the loads on each underream at aspacing of 3 D confirm this figure 7.

240 N are carried on the top plate and a total of 380on the lower two. It should be noted from this graphthat the peak capacity of a L == 3 X D set up wasnot achieved until a displacement of 4 mm hadoccured - whereas study of aIl Potts (1973) datashoyved that a 1.5 D spacing achieved peak afterapproximately a 2 Inm displacement.

The maxiInun1 effective spacing appeared to be 2.9 Dbut in view of the earlier comments on loss of capacity,Ne reducing to 5.4, with time as seepage relieves thevacuum and in order to limit displacements under load,a practical spacing of 2 - 2.25 D should be adopted.This conclusion is in very close agreement with Mohan,et al (1969) on Inulti-underream piles.

THE MAGNITUDE OF THE FRICTION tu & INFLUENCE Of TENDON 5TIFFNE55

In view of the data in section 4 the dual mechanism. L

was certainly mobilised at D ratios of less than 2.• 1

A further serIes of tests were therefore performed atL '

the D ratio of 1.5 with 2, 4, 6, 8 and 10 underream

plates connected with(i) 'solid rods, and (ii) stiff coil springs.

The peak results of the solid rod series are shown infigure 8. The dotted line represents the load capacitygiven by Littlejohhn's equation (1), full Cu mobilisedon the whole underream plug i.e. tu in equation (2) == 1.

The experimental data indicates a reduction to an tuof between 0.63 and 0.79.

This result appears to justify the authors originalpostulation in (1970) but it does not identify the cause.The waisted shape, i.e. the reduced D, of the failureplugs seen in figure 5b and the influence of the under­ream tips seem possible causes but detailed .work bySwain (1976) on the strain patterns round underreamssuggests that the rapid migration of .excess porepressure above the underreams to the pltig shearingzone causes noticeable softening, even at rapid loadingrates.

Figure 8 does however indicate a progressive andvirtually linear rise in ultimate capacity with each

8

,4

/'

/

2

Numbcr 0+ UnJer~

l5po~ "5D)

Fig. 8. - Load capacity. Number of underreams.

15

IJi.splDcernent:

~ undUrYlOrns

Load

l---

.s , 7 • 9Humber of Uncl~rNl.Dms.

Fig. 9. - Load capacity at various displacements.

2.

4

1

12.

,

40.JJJ.Jp/acl!menf ln mm.

JO

(*) The stiffness ratio of K soil : Ktendon for the rods was1: 20 where K soil is the mean load-displacement slopefor the soil at 1/2 mm displacement and Ktendon == theelastic load-displacement value.

_0 .-

additional underream. If howerver additional datashowing .the load capacity at various displacements isadded to figure 9; then at an overall safety factor of2 it is apparent that very little is gained by the use ofmore than 6 underreams.

.1t was suspected that the significance of displacementwould be further accentuated in very stiff soils. In theabsence of equipment capable of testing marI or shalethe soil properties were kept consistent and this featurewas examineci-.·· by reducing the stiffness of the inter..connecting tendon member.

The test series described above was repeated,-

~ = 1.5, with stiff coil springs in place of the solid

rods (*).

Fig. Il. - Idealised load-displacement for 2, 3 and 4 un­derream anchors.

10

---.--. Ff'!j Sf1mr oIJ I{n ..----- --"

.;+' R'iJ Spaclflg. __.~ Sr'Ï>q ]J~forrrx/ÎàtJ/L ..~ J

1 ----1 --

O,

z

7

4

5

lOta1 Lood 011

111111

1

/TI End ~(UJrjn9 Load: on Top Undvrmm

(_._._-.L.__ _ 1··

'~2 .5'hta~ lLJad 01) "\ • -. -r _. -.n"'r1k ~~I7?_"_"-:,:+_I'-'-''- __

"" l ,"'> ,/..: ,<,'''; 1 1

"".. 1";' / ~ P/ug Sf1to,. LoaCl onV ~_,-.,,,, . :Botfom.l1ndvrr.am~- 1

Fig. 12. - Experimental load-dis­placement curves for anchors con­nected by spring units.

Fig. 10. - Idealised load-displace­ment for a 3 underream anchor.

Loo.d.

16

An idealised summation of the behaviour of a 3underream anchor with weak spring connection isshown in figure 10. (Potts 1973).

The total load curve shows three distinct changesin slope as each successive underream fails. In figureIl the concept is extended to 2, 3 and 4 underreamgroups.

The corresponding experimental data is given infigure 12. The correct trend is observed, little ofsignificance resulting from the use of more than two

underreams. The displacements in this series are someten times larger than those previously measured. Thisis due to the unrealistically soft spring tendon members,the stiffness ratio is Ksoil : Ktendon == 7 : 1 (*).

ln real practice the stiffness ratio of say LondonClay to cable anchor tendon is thought to be between1 : 5 and 1: 3 and of Keuper MarI to cable anchortendon nearer 2 : 1 or 1 : 1. So the indications from thesoft spring tests may in fact ·be more representativethan those of the solid bar series.

CONCLUSIONS

The tests have indicated the validity of the compo-

site empirical equation (2) provided that the .~spacing of the underream bells is maintained definitelybélow 2.9 and if displacements are a limiting factor it

is recommended that a value of ~ of approximately

2 should be adopted. 1ts use also implies that thestiffness ratio between the soil and the anchor tendonallows the \vhole system of underreams to be deve­loping the failure mechanism when the initial end

bearing failure condition is reached by the topunderream

The end bearing factor Ne appears to be approxi­mately 8.5 for rapid short duration loads but it isrecommended that a value reduced .to 6 should beadopted for permanently loaded situations.

The research described above continues to be deve­loped to broaden its scope. Aiso several real problelnsstill remain unanswered. A particularly important oneis the effects of cyclic loading and very long termcreep «settlement» on the carrying capacity and·deformations and th~se are now being investigated.

REFERENCES

BASSETT (R.H.). - Contribution to the Proc. of theConf. on «Ground Engineering», p. 89-94.· ICELondon (1970). .

BASSETT (R.H.). ---:- Contribution to Session IV ofthe 5th European Conf. on Soil Mech & Found.Eng., Vol.· 2, p. 330-334, Madrid (1971).

BISHOP (R.F.), HILL (R.) & MOTT (N.F.). - «TheTheory of Indentation and Hardness Tests», Proc.Phys. Soc., Vol. 57, Part 3 (1945).

LITTLEJOHN (G.S.). - (1970) a) «Soil Anchors»,Proc. of the Conf. on «Ground Engineering», ICELondon. (1970) b) Contribution to the Consult­ing Engineers. May 1970. (1970) c) Author's.reply to the Contributions to the Conf. on «GroundEngin~ering», ICE London.

17

LITTLEJOHN (G.S.) (1975). - «List of Ground An­chorage References» supplied privately to theauthor.

MOHAN (D.), MURTHY (V.N.S.) & JAIN (G.S.). ­«Design & Construction of Multi-Underream Piles»,Proc. 7th, Int. Conf. on Soil Mech. & Pound. Eng.,Mexico (1969).

OSTERMAYER (H.). - «Construction, Carrying Beha­viour and Creep Characteristics of Ground An­chors». Proc. of Conf. on Diaphragm Wall &Anchorages. LCE, London (1974).

POTTS (D.M.). - «The Performance of Multi­Underreamed Ground Anchor in Clay Soils»,Part III Research Project Dept. of Civil Eng.,King's College, London (1973) .

SWAIN (A.). - «Model Ground Anchors in Clay»,Ph.D. Thesis .. Cambridge (1976).