l .aPers In situ and stiffness seven of this data is compared with that obtained from routine ground stress at investigation and laboratory testing techniques and also that overconsolidated clay and weak rock obtained from recent research. sites — Part 1 Ground conditions by AS O'rien'J Forbes-King2 PA Gildeaa and P Sharpa General — For reasons of length this paper is to be published in This paper concentrates on data obtained from three sites two in three parts. Part 1 describes the sites which were London Clay and one in Weald Clay (Figures 1 to 3). Additional data investigated and data on in situ stress is also presented for other heavily over-consolidated clays (Glacial Till, Oxford, Gault and Kimmeridge Clays) and a weak rock measurements is presented. Data on undrained (Keuper Marl). stiffness is presented in Part 2. Part 3 presents data A summary of the index parameters for the seven sites which have on drained stiffness, conclusions and references for been investigated is given on Table l. the entire paper are also given. Weathering and recent geological and 'man Introduction made'tress history Many engineering designs are influenced by the degree of An understanding of the degree of weathering and its variation with foundation movement which is expected to occur during and after depth is an essential element in the reliable interpretation of in situ construction. Overconservative assessment of these movements stress and stiffness at overconsolidated clay and weak rock sites. will lead to unnecessarily expensive structural design. For projects Appreciation of the recent geological and 'man made'tress history in urban areas the accurate assessment of ground movements is is also important. For the three sites under detailed review, the often of fundamental importance, and their prediction depends on weathering and stress history is quite varied. The weathering the appropriate selection of in situ stress and stiflness parameters. schemes which were utilised at these sites are based on work by However, experienced engineers realise that routine ground Chandler (1969)and Chandler and Apted (1988). investigation techniques do not generally provide reliable data on At the London Clay site 1 (LCS1) the weathering profile is not ground stiffness nor in situ stress. well developed, with only a thin layer of weathered London Clay During the early 1980s new sampling, laboratory and in situ overlying unweathered material (Figure 1).Boyn Hill Terrace testing methods became commercially available. In the past five Gravel and made ground overlies the London Clay. At certain years the authors have gained considerable experience with these locations within LCS1 cuttings and tunnels are present. new techniques, in a wide range of ground conditions. This paper At London Clay site 2 (LCS2) weathering is more pronounced reviews the assessment and interpretation of data for the selection than that at LCS1, with a 7.5m thick layer of partially weathered, of appropriate ground stiffness and in situ stress parameters. Some grade 2, London Clay (Figure 2). The grade 2 London Clay contains frequent lenses of silt and sand. The London Clay at LCS2 'Mott MacDonald Ltd, Formerly WS Atkins Consultants Ltd is overlain by the younger Flood Plain Terrace Gravel, and has 2 WS Atkins Consultants Ltd, Geotechnical Division therefore undergone a greater number of cycles of erosion and Table 1:Summary ofindex parameters for all sites. SITE ClAT/ROCK TYPE DEPTH NEL(NI QMlMI SsmLING TECWIQK PlASTICITT INDEX IMHNINED SMEAR STRENGTII GEOLOGICAL PERI(XI LEVEL (X) IN TRIAXIAL C(SPRESSI(NI (m) (kPA) SITE 1 LONDON CLAY (Grade I) 10.0 to 30.0 Push-in, thin vail 30 to 50 110 to 195 TERTIART (Eocene Epoch> SITE 2 LONDON CLAY (Grade 2) Push-in, thin vail 30 to 45 60 to 100 6.5 to 25.0 TERTIARY (Eocene (Grade 1) Push-in, thin vail Approx.45 100 to 150 Epoch) SITE 3 VEALD CLAT (Grade 3) 1.5 to 4.0 Push-in, thin vali 25 to 40 55 to 70 (Grade 2) 4.0 to 15.0 Push-in, thin vali 20 to 35 130 to 240 LOVER CRETACE(XIS (Grade I) 15.0 to 30.0 Rotary cored 20 to 25 270 to 400 SITE 4 GLACIAL TILL 1.0 to 2.0 Hand cut block 15 to 20 90 to 120 QUATERNARY SITE 5 GLACIAL TILL 1.5 to 21.5 Rotary cored 15 to 25 65 to 380 QUATERNARY OXFORD CLAY 21.5 to 44.0 Rotary cored 25 to 35 200 to 250 JURASSIC (Callorian Age) SITE 6 GAULT CLAY 3.5 to 6.0 Rotary cored 40 to 5S 70 to 90 LONER CRETACEOUS (Albion Age) KIHHERIDGE CLAY 19.5 to 45.0 Rotary cored 30 to 45 85 to 530 UPPER JURASSIC (Kimseridgian Age) SITE 7 KEUPER HARL (Grade 8) 1S.O to 25.0 Rotary cored 10 to 15 750 to 850 UPPER 'IRIASSIC (Grade C/0) 20.0 to 30.0 Rotary cored 5000 to 8000 26 GROUND ENGINEERING AUGUST 1992 TYPICAL LOG Moisture Content (S(r) Bulk Density (Mg/ms) Undrained Shear Strength (kPa) Q 2Q 4Q 60 SQ IQQ 1-841881 88 I ssxax os Q 100 200 300 400 Made Ground Boyn Hill TERRACE GRAVEL Weathered London CLAY Profile from l standard penetration tests (assuming Cu = 4 4 x 'N' X X X IO X X 12- X E Unweathered London CLAY Stiff becommg very stiff with depth, dark 14- grey-brown fissured silty CLAY with partings sl ot sand and fine silt with occasional pyrites nodules. c 16 a 18 a 01 m 20 22 X X 24- X 0 26 X 28 Key 0 Oi 0 Unconsolidated undrained 30 triaxial tests 32 London CLAY Bosement Beds 34 PL WC LL Note London Cloy is underdroinsd, U = 30kPo ot its surface, P5kPo at 6 Sm, 130 kPa at 14 Sm and 80kPa at 20m Figure 1:Summary ofground conditions —London Clay site 1. TYPICAL LOG Moisture Content (%) Bulk Density (Mg/ms) Undrained Shear Strength (kPa) 0 20 40 60 80 100 Ice 188188Iessazas 11 100 200 300 Sand and gravel with bnck trapments MADE GROUND m Firm to stilt slightly gravelly CLAY with roots ( BRICKEARTH X Medium dense brown very sandy fine to coarse X GRAVEL. FLOOD PLAIN GRAVEL L Firm to stiff brown veined orange brown intact to extremely closely fissured,shghtly sandy very sihy CLAY, with pockets of sand ond gravel. Weathered LONDON CLAY (Grade 3) Profile from standard penetration tests ( assummg Cu = 4.4*'N' Stiff dork grey brown extremely closely fissured sliphtly sandy Mlty CLAY with occasional pyrites X nodules and freauent lenses ot fina and medium sand IO (upto 10mmthick) Portmlly weathered LONDON CLAY xx (Grade 2) X X 5 12 X very stiff dork prey fissured sxty cLAY with sl occasional gravel seed pyrites nodules and sand x x 14 sized shea frapments. Clay sometimes thinnly bedded or thickly laminated Occasional Cloystone nodules vs olid bands. c 16 a X ip X 18 X a si 20 ilia cz 22 Urweathered LONDON CLAY (Grade I ) X X 24 26 Key 28 0 Unconsolidated undrained triaxiol tests. 30 (strain rate, t = I'yv /day) 32 34 0 tfoie Groundwater able ot 2mbslow pround level. PL WC LL Pore water pressure vance hydrostatically with depth Figure 2: Summary ofground condi ti ons —London Clay site 2. 27 GROUND ENGINEERING AUGUST . 1992 TYPICAL LDG Moisture Content l %%uo) Bulk Density (Mg/m ) Undrained Shear Strength (kPa) 0 20 40 60 60 IOO I 94 1 99 1 92 1 9920 9 04 0 100 200 300 400 500 Firm ta stiff hght brown mottled orange brown X slightly silty CLAY WEALD CLAY~(Grade 3b) 0 XX Stiff light and green brown extremely closely o 0 0 grey X fissured silty CLAY with mad>tone fragments. X 0 0 ': WEALD CLAY, (Grade 3a ) XX shtf ta very stitf brown grey becammg very mottled, 0 X closely fissured silty CLAY, (Grade 2b) X 0: X Occasional bands of madstone X X X X Very stiff, becoming hard, dark greV,>hahtlf X laminated fissured very wlty CLAY, with bands of X cemented sandy clay and shattered shell fragments 0 X Occasional mudstaae bands, becoming more X IO— 0 X Profile from freaaent with depth. 0 X standard penetration 0 1 1 tests — XX X WEALD CLAY, (Grade 2a) 0 Cu=4.51(N 12 (assuming ) E 0 X 0 X 14- Very weak, becoming weak, green grey Indurated 0 X iv and fnable fissured MUDSTONE. 0 X 0 X xct 16 0 WEALD CLAY, (Grade I) ~ 16 L 0 0 X m 0 20 X Key m 22 O Unconsolidated undrained trioxial tests. = 24 (strain rate,E I %%uo /day) 0 Unconsolidated undrained 26 triaxial tests, = (strain rote,t 0 5'yv to 2'/o/min ) 26- 30— 32- 34 Note Groundwater table at 2.2m below ground level. PL LL Pore water pressure vance hydrostatically wbt depth deposition compared with that at LCS1. A study of this site's depth (Figure 3). The weathering scheme used for this site (Sharp geological history revealed that a periglacial river system ran close 1988) is summarised on Table 2. At the WCS superficial deposits to the site. Man made features are absent. are absent. Hence the clay's most recent stress path has been one The Weald Clay (WCS) outcrops at the ground surface and varies of unloading, rather than the reloading which has occurred at both from a firm completely weathered clay to a weak mudstone at London Clay sites. Man made features are also absent at the WCS. ABOVE: Figure WEATHERING GRADE 3:Summary Weathered state Completely Highly Weathered,2b Partially Virtually Unweathered,16 ofground and subgrade weathered,3b weathered,3a weathered,2a unweathered,1b conditions, Orange/brown Orange Grey/brown Grey/green or Dark grey or Weald VISUAL Colour mottled /brown/grey or grey/blue grey/blue dark grey/green As for 1b mottled Clay site.