1.121
GEOLOGY AND GEOTECHNIQUE OF THE SCHELDT SURGE BARRIER, CHARACTERISTICS OF AN OVERCONSOLIDATED CLAY
J. SCHITTEKAT. 1'ractione 1,. Engineering. Dept of Boil mechanica. Bru8sel8 J.HENRIET, Geological Institute, State Univel'sity of Ghent and· N. VANDENBERGHE. Geological Dept of Belgium. 8r>U8sel.s. Belgiwn
ABSTRACT Preconsolidation pressure and past Durial depth of the Rupelian clay have been investi gated both by laboratory testing and in situ Flood protection of the upstream part of the testing, with seismic techoiques and the self Scheldt bastn, lncluding the city of Antwerp, boring pressuremeter. The obtained flgures requests the construction of the Scheldt are evaluated for their geological plausibi Surge Barrier. The geological sequence on the lity. dam site consists of three main units Qua ternary sandy deposlts, Late Tertlary glauco nitie sands and the Rupelian elay, known as INTRODUCTION the Boom elay. which ls a stiff. fis8ured Tertlary clay. Throughout time, the Scheldt basin has suffe Geotechnical propertles of these units have red from widespread, severe floods and the been invest!gated by extensive laboratory frequency of flooding has lncreased over the and field tests including seismie surveying, past cent.ury. cone penetration testing, self boring pressu remeter testing, MENARD pressuremeter testing Consequent.ly, t.he Depart.ment Waterways of t.he and sampling in borings. The main part of Minist.ry of Publie Works hss elaborat.ed t.he this testing programme has been confined to so-called Sigmaplan. This over-all plan aims the Rupelian clay. t.o protect the low-lying regions of the Scheldt. bastn against floods, generated by Some main aspects of the stratigraphie subdi rising North Sea waters, of ten due t.o the vision of the Rupelian elay on the dam site enhancement of spring tides by persistent are diseu8sed and related to observations on westerly or north-westerly winds. A key ele other sites. A few particular features of ment in this plan is the storm surge barrier this clay deserve some special attention with at Oosterweel, Antwerp (cfr. map, fig. 1). regard to eivil engineering works : Preliminary investigations of this project - large loaf-shaped earbonate concretions or have been eommitted to a joint venture, cal septaria, whieh have been investigated by led "STORMVLOEDKERING SCHELDE BEKKEN" or reflection seismies. boring and eone pene "SVKS" • tration testing, - elay diapirs, of which some geoteehnical At the design level of such a dam, which has properties have been investigated for the to account for severe dynamic loading from first time, storm waves, earthquakes and accidental - clay f1ssures, which deserve some attentioo shocks, major attention has to be paid to regarding their origin. extensive and deep foundations. The geological detail strueture aod geo For practical purposes, the Rupeliao clay has technical properties of the dam site have been subdivided ioto some main units, defined been investigated by extensive field tests : 00 base of lithological and geotechnieal pro s hlgh-resolut.ion seismic survey, 85 cone pe perties. netration tests, 3 self boring pres8uremeter A tentative correlation with some other sites test.s, 27 borings for the collection of is proposed. undlsturbed" samples for laboratory tests, MENARD pres8uremeter HEMIKSEM AARTSELAAR KONTICH I TERHAGEN I , NI1El 15TEKENE I :III r I, :I
"
DOEL
...,
·70 R ARRI R ...,
ANTWERP ." _ L-.. JAN DE > .l>l NI ElJ O_4(l(XlIo, Fig. 1 General north-south geological section Bnd locallsatlon map. tests. 51nce most of these tests have been The Rupelian strata have 8 gentIe dip of performed in the Boom elay. their interpre about 40/00 towards north-east. tation might contrlbute to a better knowledge Although the elay thickens to the north of thls formation. (e.g. 76 Ut at Doel, 127 Ut at Meer), it is probable that it has been partly eroded in the vbole area south of the West Netherlands GEOLOGY Basin and the Netherlands Central Craben (KEIZER' LETSCH 1963). The subsol! of the dam site 18 built up of a In former studies, VANDENBERGHE (1978) has series of Rupelian clays, about 70 1D thick, correlated the clay sequences eropping out in covered by Neogene ssoda, 3 to 7 IR thick the Waasland, Boom and southern Kempen and under the river chanDel end about 8 m thick establlshed 8 stratlgraphic subdivision of under the hanks (f1g. 2). 'nlese Bands are the lower part of the Boom clay (lower 55 m). known as -Kattendijk- Bands (DE MEUTER & LAGA The denomination of the lower septeria levels 1976), belonging to the Lover Pliocene. (levels "1" tot "8") and the subdivision into These sediments are covered by Quaternary "grey c1ey" and "bleek clay" (fig. 1) are formations with a thickness varying between 1 borrowed from this work. and 6 m in the river chenne1 eod ranging up to 20 man the banks (fig. 2). SPECIAL STRUCTURAL FEATURES The Rupelian elay, or Boom elay is a marine deposit of Hiddle Oligocene age (35 m.y.). The total thiekness of the originel elay for Some particular structural features of the mation could have been weIl above 100 m as elay eould have an important influenee on the suggested by the 150 to 200 m thiek Boom elay foundation design. in the South Netherlands • just north of the Belgian boundary (PANNEKOEK 1954). '" SmalI sized fissures oeeur in the heavy Later erosion has removed part of the Boom clay, generally displaying pol1shed surfaces clay to Ieave typieal thiekness of 69 to 72 m end strietions or slickensides. In the Boom in Oosterweel. Furthermore. there is eviden area, such smooth end grooved shear surfaces ce that in Neogene times, it was covered by have also been observed in septaria . This thieker deposits than those left today. Con might suggest that the fiasures originated at sequently, the Boom clay may be considered as a compaction stage closely approaching the an overconsolidated clay. lower plasticity condaion, the septaria however still being in a deformable state. 1.123 o .\0 o • o o ~ 0 ~ • . . ATERNARY ~ SANOS ::::::::::::::: S KATTENDIJK SAND5 'K' ------BI(Q==-=--'K'-- BK' -30 ------'" ------BK' --- o boung ----- BK' cont pe:nelfallon test o Z5 50 'i'S lOOm C plessuremetel test 'ig. 2 Geologlcal cross seetion over Scheldt river at the dam site end geomechanical subdlvision. * Septaria (fig. 3) are loaf shaped càrbonate A frequency analysis of septaria lndications concretions. showing internally nearly ver over more than 40 cone penetratlon tests and ticalopen cracks. closing towards the rlm 20 borings is presented in table 1. end sometlmes water bearing. They occur in distlnct horizons, characterized by a relati On flg. 4, the bulk of septaria ind1cations vely higher carbonate content. These septa has been grouped lnto depth beIts. each of ria levels farm stratigraphical key horlzons whlch representlng a major eoneentration of which can be used for correlatlon purposes observations. The depth range (height) of (VANDENBERGHE 1978). Individual concretions the beIts could elther be due to a seatter 1n may have a diameter up to 1 mand a thickness depth observatlons of a slngular level or to ranging between 10 and 30 cm. a laek of resolutlon of proxlmate levels, the latter hypothesis belng eorroborated by same further evidenee. Septaria level 8 for instanee turned out to be composed of two distlnet levels. The lower one ls sideritie and may be eorrelated wlth septaria level 8 of the Boom elay stratigraphy by VANDENBERGHE (1978). Septaria beIts 7 and 6 eorrespond to levels 7 and 6, recognized by the same author . ._...... "." Fig. 4 Septaria belts Fig- 3 Septaria fragment, he1ght 20 cm. * Diapir struetures in the top zone of the On the dam site, septaria have been sampled Boom elay have already been reported by seve in many borings and probably account for most ral authors. LAGA (1966) deseribed 8 diapir peak values in the cone resistance graphs. structure observed during exc8vations on the measured in the eley. right bank of the Scheldt 1n Antwerp. TABLE 1 1.124 SEPTARIA FREQUENCY Septaria belt Freguency C.P.T. Frequency borings Total freguency (%) 12 8/41 12/24 31 11 13/41 5/24 28 10 5/40 4/24 14 9 9/30 5/19 19 8 8/27 4/15 29 7 11/27 1/15 29 6 11/26 4/15 37 Subbottom profiling by WARTEL (1980) confir This hypothesis might be corroborated by the med the occurence of diapirs under the observation af analogous diapirs of Ieper or Scheldt. Seismie reflection profiling with a London clay, piercing into Quaternary sands high resolution boomer souree showed how such in the Thames valley or in paleovalleys in diapir structures fade out with depth, unde the North Sea (HENRIET e.a· 1982). formed reflections generally being faund Diapirism might be geomechanically interpre between 25 and 35 below chart datum (HENRIET. ted in terms of the TRESCA yield criterion (~ SCHITTEKAT and HELDENS 1983). A typical • 0 and Cu), the horizontal stress lnherited diapir structure is shown on fig. 5. This from past burial conditions largely exceeding picture has been recorded upstream Antwerp the vertical stress after erosion and hence during a reconnaissance seismie reflection inducing vertical clay flow. survey on behalf of the Rijksinstituut voor Grondmechanica and the Bijzondere Studie As shown by BEDBERG (1974), over-pressure in dienst Pre-Metro (1982). clay might also be enhanced by methane gas generation, the development of a gas phase in As an additional observation on this picture the pore fluid impeding to same extent fluid it may he observed how septaria horizons expulsion and nonnal consolidation. In the elearly stand out as alignments of diffrac ne1ghbourhood of the dam site, same seismic tion hyperbola, each eoncretion aeting as a sections displaying noise bursts flanked by singular point reflector. uplifted or down bending reflection termina tions could suggest the presence of deforma Curiously enough, comparable updoming elay tions of diapiric nature. The axis of one of struetures have never been observed in aoy of these structures has been tested by cone pe the numerous clay pits in the Waas land, Boom netration. but the derived geomechanical pro and Southern Kempen regions . It is hence perties turned out to be not significantly thought that diapirism is related to the different from neighbouring undisturbed Scheldt river incision and the associated regions • clay ~elaxation phenomenon. GEOMECHANICAL SUBDIVISION NKD ;~;;;;;~;~~fii;;~~~~S= SCHELDTBOTTOM -10- - SEPT ARIA From a geomechanical point of view, the Boom HORIZONS clay can be subdivided into th. follow1ng units (fig. 2). _"SB"SEPTARIA * A weathered top layer, with a thickness va - LEVEL rying between a few decimeters end 4 m. In this top layer, the clay is more light colou red and it is characterized by same degree of degradation in geomechanical properties . It is called BKO and is found between 18 and 22 m below chart datum, which is the zero level -30- of NKD ("Nieuw Krijgsdepot"). * A complex unit is situated between -22 m sc and -35 m. It is called BKl and consists of a banded sequence of mainly silty and some "DIAPIR clayey horizons. This complex unit is cha racterized by geomechanical properties which are definitely more favourable than those of Fig. 5 Diapir structure under the Scheldt the underlying unit. river. upstream Antwerp. 1.125 * This underlying unit, situated between -35m * Between -80 m aod -90 m, the Boom elay pro and -50 m, is called BK2 and is more clayey gressively grades int0 the underlying sands than BKl. through 8 transitional sequenee of silty to clayey fine sands. * Between -50 mand -sa m, one distinguishes a banded sequence of silty and clayey hori It ie remarkable that the Boom eiay sequence zons, called BK3. This unit has not been investigated in a boring performed on behalf thoroughly investigated on the dam site, but of the Studiecentrum voor Kernergie in Mol its properties are weU known from outerops (DETHY e.a. 1983) displays a groas zonation or suberops in the vieinity of Boom. It of some ma!n physlcal properties (natural includes the "grey elay" and the lower part gamma, reslstlvlty, unit welght of dry 8011, of the "blaek elay", described by grain s1ze fraction smaller than 2 um) whieh VANDENBERGHE (fig. 1). might fit the geomechanical subdivision pro posed at Oosterweel, 8S 8uggested by fig. 6. GAMMA Lex> RES1SllVI1Y UNll WEIGHT (J' DRY SOIL GRAIN SIZE FRACllON < 2 fm 50 lOOetlUl'ltlooS 0 10 20 Om f~·I. DEP1H H 16 " 11 k NArt3 2tI XI '0 50 60 170 180 180 200 BK' 210 _._.--_._. -_.------. ------.-.------_._-- 220 230 2L BK' lSO 260 BKL Fig. 6 Possible correlation between the geomechanical subdivision at Oosterweel and the zonation of same main properties of the Boom elay in the S. C. K. boring. Mol. ties of the different geomeehanical units are 1.126 Most units iqentified at Oosterweel have been subject of geotechnical investigations on va summarized in table 2. The variation with ·rious sites, shown on fig. 1. depth of the grain size fraetions smaller than 2,....um aod 20~. the plastic limit, the Unit BK3 has been investigated near Aartse liquid limit and the plasticity index is laar and Reet. shown on fig. 7. Unit BK2 has been intensively investigated U.U. triaxia1 tests tor some major eivil engineering works in the neighbourhood of Antwerp (E3 tunnel, Jan de Unconsolidated undrained (U.U.) trlaxia1 Voslei, Edegem) (DE BEER 1967). It has also tests have been performed on a large number been described in the clay pits of Kruibeke of undisturbed elay samples, most of which ("black clay"). On the borehole logs in Mol. having a diameter of 10 cm and a height of BK2 is characterlzed by a higher content of 20 cm. The undrained shear strength (cu) particles smaller than 2.)1m and a somewhat values, derived from these experiments. are lower unit welght of dry soi1, confirming its plotted versus depth (z) on fig. 8. more c1ayey eharacter (e.g. compared with Samples having undergone brittle rupture are BK3) , but it is curiously enough al80 eha indicated by a special symbol. racterized by a somewhat 10wer natural radio The resulting set of Cu values displays a activity. large amount of scatter. A l1near regression on all data yie1ds the Unit BKi has a thickness of about 20 m in f01lowing re1ationship Mol. At Oosterweel. erosion has stripped 80me of the top meters. leaving a thickness Cu • 175 + 0.8 % (kPa) ( 1) of about 17 m (including BRO). In Doel, somewhat further north of Oosterweel (fig.I), i ts thickness amounts al80 to 20 m. Only a C.U. triaxial tests few meters of the base of BKl are found on the site of the E3 tunnel, forming the very Consolidated undralned (C.U.) triaxia1 tests top of the Boom clay on that spot. hsve been performed on undisturbed samples w!th a diameter of 3.8 cm and a height of 10 cm. Resulting effective shear parameter LABORATORY TESTING values are presented in table 3. Fig. 9 shows the p, q-diagrams for the BKl and BK2 units, il1ustrating again the important Varlous laboratory tests have been performed amount of scatter on the data. by the Rijksinstituut voor Grondmechanica, the Laboratoire du Génie eivil de Louvain la FIELD TESTING Neuve aod the Laboratoire des Matériaux de Construction de l'Université de Liège. Static cone penetration tests (C.P.T.) Identification tests With exception of a few mechanical statie pe Current identificatioq testing has been netration tests (type M4) performed on the performed on more than hundred disturbed and left bank, all statie cone penetration tests undlsturbed samples. have been performed with an electrical cone. Resulting mean values of the physical proper- Only the latter measurements will deserve further attention. --_.-TAllLE 2 IDENTIFICATION TESTS BKO BKl BK2 BK3 % particles 20 I""' % - BO BO BO % particles 2,....., % - 50 57 54 Natural water content W % - 26,93 29,40 29,50 Liquid l1mi t Wl % - 66 73 60 Plasticity index - 40 44 35 Unit weight of dry soil ~ kN/m 3 - 15,31 14,6 14,74 Unit weight of so11 r kN/m 3 19,31 19,42 19,17 19,31 Unit weight of solid partieles r. kN/m 3 - 26,54 26,52 26,52 1,127 0,,10 '0,,,,,30 '0 SO 60 70, '/, 0,,,,10 '0 30 '0, SO,,60 0 10 30 ./, 10 NKO '0 0, '0 30 '0 SO 60 70 80 90 'I. 0 10 '0 30 '0 SO 60 70 80 90 '/, ·10 , ,,,,,, , , ~. \ ·IS c ;. ", ·'0 BKO -2S BKl ·30 , \ -lS , f \ 'S \ ,-' < , <:...... I "0 , \ · ,,1 , I I , / ' ,1 BK' ( , <, \ I: , , , i , : .4S \1 , , , , , -SO PLASTIC GRAIN SIZE GRAIN SIZE PlA5TIC1TY LlCU1D LIMIT FRACTIQN FRACTION INDEX LIMIT Wp <20~m <'I'm lp Wl Fig. 7 Profiles of scme main sediment properties at Oosterweel. MEAN ENVELOPE 0 lOC Mean and minimum values of the cone resistan .'., • .. ce qc and mean values of the local skin =• • . BK1 -30 20 -::;1> .. '. "- friction is are summarized in table 4. An : . example of a cone penetration graph is pre '. · • sented on fig. 10. •• .'.. .' ·• It should he reaarked tbat ,on the base of • Boae foraer lnvestlgatlons, it was generally . adm1tted that cone resist.nce values should .' •• Bteed!ly increase with depth ln the Boom • elay. At Ooaterweel, none of aore thsn 50 electrlcal aod mechanical eone penetration tests displays an lnerease of cone Testetanee • ·. with depth, at least abave the level of -50 m BK3 _60 50 (i.e. within BKI aod BK2). Below -50 m. CODe ~ Cu.175.(lJlZ resistance do steadily increase with depth. a trend which is continued down to the bottolll ., of the fonution. It should he noted that a •· similar absence of increase of cone res1s- tance vith depth in the considered clay units had already been reported on some other sites tOOt such as at Kallo end Jan de Voslei (fig. 11) , MENARD pressuremeter tests BK' II to,l"... wun 10'51" lMfCMtnCl"ons • t",tllc fO,h,lfC Borings for the MENARD pressuremeter tests Fig. 8 have been performed by the direct flush method. a bentonite slurry being used as Undrained sheaT strength versus depth drilling mud. The pressuremeter tests have U.U. triax1al tests. been carried out with a MENARD type B equip ment. in accordance wlth standard procedures. 1.128 • •. · q • "•• · •. .. • MPa •• • .. • 02 • ...... • • • d' • ." . • •• BK' • .. • . • • • • • 0,1 p. '--- 0,1L- 02"-- 0,3"-- 0.'~ US_'__ 0.6_"_ ..i_0.7 __MPa p Fig. 9 p.q,- diagra~s for C.U. triaxial tests. 61.r ~3.r = principal stresses at rupture. TULE 3 EFFECTlVE SHEAR PARAMETERS Number of samples c'(kN/m2 ) 0' c'min (kN/m2 ) 0' min. BKO - - - - BKI 327 22 25,04° 13 17, 00° BK2 162 40 17 ,05 0 21 12,92° BK3 IB 4B 20,11 0 43 15,32° KALLO JAN DE VOSLEI 1.129 , L B 12 16 MPa MP, " .. :l COOlE 'lESISUHCE SKIN .IIIClI0~ ." , , , NKO .. • ~...... 0. , " :~ " ·20 . ~ :\: ·5 -25 " -;1--- ·10 :>- i " .l<) · , -15 I T ;i ~ " · f- - · ., -20 < -i~-- ~ ----- ~ ·La . & ... ." r . ~ --1,-- • " , -50 '" " ------t~· · ." -~ '1 ·55 L ·60 Fig _ 10 Typical statie cone penet!ation test ,OS at Oosterweel. ." ----TABLE 4 Fig. 11 Statie cone penetration tests at Kallo and Jan de Voslei, Antwerp. C.P.T RESULTS r I 1I'"c,min (MP.) (MP.) (MP.) BKO 3,1 3,7 0,20 with I--- -- qc the mean value of the cone BKl 3,2 4,2 0,22 resistance within the considered layer BK2 2,6 3,6 0,16 BK3 2,8 4.6 3,88 5,45 0.20 the meao value of the local skin friction withio the considered layer 1.130 It turned out that the results could he NKD -20 influenced by the drilling procedure. Dril ling in short stages (4 to 6 m) and execution of pressuremeter tests immediately af ter MEAN LIMIT PRESSURE l MENARD drilling yielded higher pressure modulus -25 values than when drilling was performed in IMlT PRESSURE (Sap) long runs or when testing gat delayed. Lea ving 8 borehole open for about 12 hours BKI NVELOPE ! MENARD 1 yielded a limit pressure of 1.05 MPa, while the mean value of 8 measurements carried out shortly .after drilling in the same depth interval amounted to 1.86 MPa. The results of the MENARD pres8uremeter tests are summarlzed in Tahle 5. Fig. 12 shows the variation with depth of the limit pressure (mean value and envelope of data) ; self bo ring pressuremeter data, discussed in the next paragraph, have been added for compa BK2 rison purposes. -45 Self boring pressuremeter tests (S.B.P.) -50 Lateral in situ stress and in situ shear modulus being of obvious relevance for the design of friction piles and tor predictive tinite element model1ng, 1t was decided to -55 BK3 carry out sel! boring pressuremeter tests on the left bank of the Scheldt, down to a depth of 80 m (NKD-72). The contractor was Fig. 12 MENARD pressuremeter tests. PRESSUREMETER IN SITU TECHNIQUES (P.M.LT. , Comparison with eelf boring Cambrldge, U.K.). A staodard undrained self pressuremeter data (SBP). boring pressuremeter test directly yields three soil parameters (WINDLE & WROTH 1977) : the in situ total lateral stress, the undrained shear strength and the shear modulus. ----TABLE 5 PRESSUREMETER RESULTS I I I I PI -tr (MPB) pi (MPB) ËM,min(MPa) EM(MPB) I- BKO - 1,97 BKl 1,54 1,97 20 60 BK2 1,46 1,82 17 60 BK3 - 1,83 with PI -Ci'- the mean limit pressure value leas one standard deviation PI ~ the mean value of the limit pressure irM,min - the mean pressuremeter modulus, when clay is disturbed (drilling with long runs) EM - the mean pressuremeter modulus without clay disturbance. Ambient pore water pressure has not been mea the expression 1.131 sured. Under the assumption that the pore ~ water pressure distribution is hydrostatic, e u 97 + 3,32 z kPa (4) the effective lateral stress a-h may be de rived too. The values of the shear modulus G do not show any significant variation with depth (fig. CLARKE (1981) and WROTH (1982) have shown 13) the mean value amounts to 37 MPa. that optimal positioning of the self boring pressuremeter cutter might yield a minimum Limit pressure data may be inferred from the disturbance of the surrounding medium. HANDY stress displacement diagrams of the self bo e.a. (1982) also found a good agreement ring pressuremeter tests. Limit pressure va between the determination of in situ horizon lues defined as the total pressure eorrespon tal stress with the S.H.P. aod with the lowa ding with an axial displaeement of 10 % are stepped blades • The profile of the lateral shown on fig. 11. It appears from this plot stress determined through S.H.P. testing that these S.B.P. limit pressures are confi should thus, with a fair amount of confi ned between the mean value and the upper en dence, be considered as representative for velope boundary of the 1imit pressures, de the variation of the in situ horizontal termined with the MENARD technique. stress with depth. Fig. 13 presents the pro file of the effective lateral stress data A synopsis of profiles of some major geo versus depth. technical properties on the dam site (cone resistance, pressuremeter modulus and effec SElF BCJlING PRESSUREMETEll - lEFl BANK tive shear parameters) is schematically pre ~" ~ 600 700 llOO 9OOkF'll 100 200 m 'OOkPc 20000 sented on fig. 14. N_~ OEP.T~ ~~g OEPT~ -20 z • '" '" ~"f"""'" PAST BURIAL DEPTH .," .. .. .," * Continuous , single channe1 reflection pro '" '" filing has been carried out 1n PVC-lined bo reholes. Tube wave reflection patterns thus '" generated have been interpreted in terms of EFFECliVE LATERAl STRESS' ~.~ lJNOIlillNED SHEAR SHEA!l MODUlUS G shear wave velocities (HENRIET, SCHITTEKAT & STRENGT~ C~ HELDENS 1983). Shear waves in marine sediments are koown to Fig. 13. Self boring pressuremeter data displayastrong veloeity gradient in the first tens or hundred meters depth. Following an empirica1 relation between shear wave ve 10city snd depth in marine silts and clays A linesr regression yields the relation (HAMILTDN 1976), the most representative va ship : lue of shear wave velocity (355 mis st the average level of -28 m) suggests a maximum ~'h - 344 + 5,64 z KP. (2) burial depth of about 83 m in clayey or silty sediments· For comparison purposes , the profile of ef fective verticsl stress versus depth has been * Over a hundred oedometer meter tests have added to this graph been carried out on undisturbed clay samples and have been processed through a graphica1 ~v .. 39,6 + 9,5 z kPa (3) approach (CASAGRANDE 1936) in order to deter mine the preconsolidation pressure. This profile has been constructed sssuming 3 a submerged unit weight jf 9,3 kN/m for the Various processes, aod not the least the sam Boom clay, of 9,5 kN/m for the overlying pling proeess itself and subsequent manipula ssturated sands and a unit weight of 16,0 3 tions, might have caused some 10ss of precon kN/m for the dry sand cover. solidation but in no case any gain. Hence, those samples displaying the highest By further processing of the self boring measured preconsolidation might be considered pressureroeter data) the undrained shear as to represent the closest available appro strength C of the clay has been determined ximation of the real preconsolidation. Pro in accordance with the method GIBSON aod cessing of the data shown on fig. 15 yields a ANDERS ON (1961). The resulting profile is past burial depth of at least 88 m for 1 %of shown on fig. 13 and might be approximated by the samples and of at least 69 m for 5 % of 1.132 EFFECTlVE "0 SHEA~ PARAM!:TERS NKO 0 5 12 ------10 Z< 11 2.31 331 Fig. 14 - 10 , -80 '" Synopsis of geomechanical properties on the dam site. the samples. _,,:200;::.~4:,:0~0~600~i800~Jl';'OOO:.:.....:1.;.200:.:..._'~40,,:0_k_r'+fn' NKO °fl .... * The latera! stress in the Boom clay has been determined by the self boring pressure -20 ...... ;. meter tes~s (equation 2). !bis latersl .. stress crb eannot he larger than the upper -. limit stress. whieh presumably should he the ...~-""'" passive earth pressure a-~: o • -30 o '"" '0 " , ..• 0 ""h "'''"p - Kp' O"v+(Kp-1)c' cotg 0' (5) 8 ç" wlth Kp ~ the coefficient of passive earth ...... 10. ol • preS8ure •• Z -LO - tg (45" + 0'/2) and ~~ defined by equation (3). W.-88m Considering that for BKl : -50 . . c' • 22 kN/m2 _60 and thus Kp • 2,46 equat10n (5) might be wr1tten as cr~ ~ -70 0"; - 166,3 + 23,4 z (kPa) (6) Under the banks of the rlver, the depth of the top of the clay amounts to about 28 m, which value, 1ntroduced 1n equat10ns (6) and -80 (2), respect1vely yields NK~I:1~:-' v_:_~_~_~_IT_Y_V_~_~_~~_:_T_Y__~_~DU_p_oL_'U_S '_M_P_O_'__--:=,.-<~:::c_ ::::::::::::::= -10 ZK 600 1100 ------1700 -20 BKO mo '50 340- 0.480 '80 BKI lEf - -JO 1t.40 '70 355 _ J250 Q.L68 7J5 ------BK2 1660 J ·LO 1560 to 1720 -50 BKJ -60 FIG.16 SYNOPSIS OF SEISMIC WAVE VELOCITIES AND DYNAMIC ELASTIC CONSTANTS ON THE DAM SITE. c.ity. YOUNG' s modulus gat determined trom the shear modulus and POISSON's ratio. A synopsis of seismie wave veloelties aod dy namic elastic constants is presented on fig: 16. P-waves veloelties of 600 to 1100 mIst measu red below the water tabie, are deflnltely sboormal for saturated condltlons (the P-wave veloclty in water amounts to 1480-1500 mts). 'l'hey have to be related to the presenee of sediment gas (methane). As a matter of fact, two gas eruptloDS have been observed durlog penetratlon testlng 00 the left bank, between 10 and 18 m depth. In th"e Boom clay ltself , a P wave veloclty low of appreximately 1440 mis should also be traced te the presence of a minor gas concen tration, probably trapped in the silty hori zons of BKl and related to the presenee of organie matter ln the elay. ACKNOWLEDGMENTS The authors are grateful to the Director General, Antwerpse Zeediensten (Ministry of Public Works, Belglum) and the joint venture S.V.K.S. for permission to publ1sh this paper. The authors are indebted to Mr. Maertens (Ministry of Publlc Works, Belgium) for valuable diseusslons. 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