INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Panel discussion: The prediction and the control of displacements around deep excavations in completely decomposed granite Débat de spécialistes: Prévision et contrôle des déformations autour d’une fouille profonde dans le granite complètement décomposé

A. W. Malone - Geotechnical Engineering Office, , People's Republic of China C. W. W. Ng - Hong Kong University of Science and Technology, People's Republic of China J.W. Pappin - Ove Arup and Partners, Hong Kong, People's Republic o f China

S y n o p s is : This contribution covers two subjects: the construction methods employed in Hong Kong to keep displacements around deep excavations to a minimum and the prediction o f displacements using small strain models. O f importance for the prediction o f displacements, it is shown that completely decomposed granite o f the Pluton exhibits small strain stiffness behaviour typical o f sedimentary soils. In the case history presented, finite element computations with a small strain stiffness model gave good predictions for lateral wall displacement but the vertical settlement predictions were less good, possibly due to the effect o f remnant structure in the completely decomposed granite not allowed for in the model.

M in imizin g displacements profile overlain by thin layers of loose sandy fill and marine deposits. SPT N-values increase from about 10 at 5m depth to about Experience in Hong Kong shows that excavations deep below the 100 at 30m depth. The groundwater table is about 1.5m beneath the watertable in completely decomposed granite (i.e. material that ‘can ground surface. Unlike many sites in urban Hong Kong, this is a be broken down between the fingers to its constituent grains and ‘virgin’ site, as it is believed that the ground beneath the site had not slakes in water’) can cause large settlements o f the adjacent ground. experienced any substantial stress changes resulting from At sites sensitive to displacements, top-down excav ation within a construction dewatering activities since it was reclaimed in the concrete diaphragm wall box is now the favoured construction 1920s (Lui & Yau, 1995). Adjacent to the site there are sensitive method for excavation depths greater than around 12m. But structures, damage to which had to be avoided, and therefore installation o f the diaphragm wall needs to be carried out with care. precautions were taken to limit displacements during basement In the early days o f diaphragm walling in completely decomposed construction. To achieve an effective groundwater cut-off, a deep granite in Hong Kong, in the late 1970s and early 1980s, large diaphragm wall was installed, to ‘engineering rockhead, Grade 3 movements were commonly recorded during diaphragm w all rock’ at 47-62m below ground level. Grade 3 rock is defined as construction (Figure 1). Practitioners in Hong Kong have since ‘material that cannot usually be broken by hand, is stained learnt that, to limit displacements, it is necessary to use short trench throughout and gives a dull ring when struck by ham mer’. Curtain panels, take care in excavating and to maintain the slurry pressure grouting was carried to a depth o f some 10m beneath the wall. A well above the ground water pressure, i.e. rather more than that waterstop was installed between trench panels to 30m depth and a needed for theoretical static stability. By this means, installation milled hydrofraise formed on panel edges beneath this depth. Care settlements can be limited to say less than about 10mm. But it was taken in excavation o f the panels, which were 4m in length, and remains a challenge to keep displacements due to excavation and this clearly paid o ff as relatively small settlements o f the adjacent dewatering within acceptable limits. A case history will help to illustrate the measures to be taken to minimize displacements. The 26m deep excavation illustrated in Figure 2 was carried out by top-down construction within a concrete diaphragm w all box 107m by 67m in plan (Lui & Yau, 1995). Ground conditions at Dragon Centre, located in an area o f deep weathering in the district o f northern Kowloon, comprise a deeply weathered granite

Distance from Excavation Max. Trench Depth 0.5 1.0 1.5 2.0 2.5 ■

-cST'

o Hong Kong t Bell Common 0.1 Excavations ■ South Cove ± London NPY ♦ Studenterlunden

Settlem ent Max. Trench Depth Figure 1. Summary of measured settlements caused by the installation o f concrete diaphragm w alls, after Clough & O’Rourke (1990).

2325 ground were recorded during diaphragm wall installation (<10mm), decay curves and other material properties were determined from compared with those shown in Figure 1. Cambridge self-boring pressuremeter tests carried out in completely On completion o f the diaphragm wall, a multiple well pumping test decomposed granite at a site in Kowloon Bay, some 5km to the east was carried out to test the effectiveness o f the groundwater cut-off. o f Dragon Centre, and triaxial testing (p'-constant with local strain A pumping test is now standard practice in Hong Kong for measurement) o f soil from the same site (Ng et al, 1998). The excavations deeper than about 8-10m at sensitive sites. During the completely decomposed granite at Kow loon Bay is a similar soil to pumping test the groundwater within the diaphragm w all box was that at Dragon Centre, the sites being within the same granite body drawn down to 24-30m below ground level. The associated (a very uniform equigranular medium-grained biotite monzogranite reduction in head in standpipes installed in the ad jacent ground o f the Kowloon Pluton) and the soil showing similar fines content outside the box averaged 0.5m, indicating that a good seal had been (15-30%). The material exhibits pronounced strain-dependent achieved over most o f the site. A s a contingency measure, recharge stiffness (Figure 3). The S-shaped curve used in the model is the wells had been installed to maintain water levels, but these were not mean o f the measured data, with Gmax/ p' set at 1500. used during the pumping test; some were operated during The measured and computed wall displacements due to the excavation. During the works, measurements were taken o f surface pumping test are illustrated on Figure 4. Inclinometers 13,14 and settlements and lateral wall displacements (Lui & Yau, 1995). 16 are at the mid-points o f three sides o f the diagrapham wall box. Large wall displacements occurred during the pumping test (Figure Along with the BRICK predictions, an elastic Mohr Coulomb model 4) and these might have been less had the top floor slab been cast prediction is also illustrated. For the elastic Mohr Coulomb before carrying out the pumping test. This procedure is now being analysis, Young's Modulus is assumed equal to lxN (SPT) value in introduced for sensitive sites. MPa, following the normal assumption in Hong Kong. It is seen that the BRICK prediction is good but the elastic M ohr Coulomb model over-predicts. Wall displacements at the final excavation PREDICTION OF DISPLACEMENTS to B5 level are shown in Figure 5. The BRICK prediction is again good, in terms o f magnitude and curvature, but the elastic Mohr Now let us compare the observed displacements with those Coulomb model is less good. Figure 6 shows the associated computed by small strain modelling. The sequence o f operations incremental settlements due to excavation to B5 level on survey modelled in the finite element analysis comprised the pumping test, followed by staged excavation to B5 level. The modelling used the SA FE finite element program with Simpson’s ‘bricks on strings’ soil Lateral Deflection (mm) model (Simpson, 1992). This model has the ability to capture the 100 50 0 -50 effect o f changes in stress path and strain dependent soil stiffness, 10 14 16 13 from a given modulus decay curve (S-shaped curve). The modulus 0

-10

-20

\VW Reduced '30 ''\\\\ Level (mPD) -40

-50 ------Me asured (3 sections) -60 ------BR JC K ...... Me hr Coulomb -70 Figure 4. Lateral deflection o f the diaphragm wall in the multiple well pumping test.

Shear Strain (%)

Figure 3a. Self boring pressuremeter, Kowloon Bay. Lateral Deflection (mm) 100 50 0 -50 10 14 16 13 0 / -10 A' . y

-20 ('^ L B 5 CT \ Reduced '30 Level (mPD) -40 % ' Y\ -50 ------Me asured Y\\ (3 >ections) \ -60 ------BR JCK \ ------Me hr Coulorab Shear Strain (%) -70 Figure 3b. Drained constant p’ triaxial tests with local strain Figure 5. Lateral deflection of the diaphragm wall during measurements, Kowloon Bay (Ng et al, 1998). excavation to B5 level.

2326 Distance from Excavation (m) This hypothesis needs to be tested at other sites and by numerical experiments using a modulus decay curve representative o f soil with structure intact at small strain and suffering progressive destructuring at larger strain, in a continuum model capable of handling progressive destructuring. These observations, soil tests and computations have yet to be carried out.

C onclusions

Regarding control of displacement (conclusions from general practice): (i) ‘top-down’ construction within a diaphragm wall box is favoured for very deep excavations at sensitive sites, Figure 6. Settlements during excavation to B5 level. (ii) precautions need to be taken during diaphragm wall installation, and (iii) a sophisticated ground water cut-off system is required. Distance from Excavation (m)

0 20 40 60 80 100 120 Regarding prediction o f displacement (conclusions from this case history): 14 (i) completely decomposed granite of the Kowloon Pluton shows small strain stiffness behaviour typical o f sedimentary 10 y soils, / /- / (ii) the finite element predictions with small strain stiffness were / / good for lateral wall displacement, ------Measured (iii) but less good for vertical settlement, possibly due to the (3 sections) / effect o f remnant structure not allowed for in the model, and ------BRICK (iv) the elastic Mohr Coulomb displacement predictions were not ...... Mohr Coulomb as good as the small strain predictions.

Settlement (mm) Figure 7. Settlements during the multiple well pumping test. A cknowledgements

The assistance o f James Lui, Paul Yau and Jessie Kw ong in the lines normal to the wall at the positions o f inclinometers 13,14 and preparation o f this contribution is gratefully acknowledged. 16. The measured settlement trough is narrower and closer to the w all than might have been expected for sedimentary sands but the maximum settlement is within the range o f expected values, being R eferences about 0.15% o f excavation depth and about 60% o f maximum wall movement. Neither model predicts the settlement very well. Figure Clough, G.W. and O’Rourke, T.D. (1990). Construction induced 7 shows settlements during the pumping test. Measured settlements movements o f in situ walls. Proceedings o f ASCE Conference are surprisingly low, given a wall deflection o f 50mm or more. Both on Design and Performance o f Earth Retaining Structures. predictions are poor. Cornell University, Geotechnical Special Publication 25, 439- The poor BRICK settlement prediction requires some explanation. 470. Why should the BRICK model over-predict settlements but give Lui, J.Y.H. & Yau, P.K.F. (1995) The performance of the deep good predictions for wall movement for both the pumping test and basement for Dragon Centre. In Proceedings o fthe 1994 Annual the excavation to B5 level? The Ko value at this site is expected to Seminar of the Geotechnical Division of the Hong Kong be roughly 0.3 to 0.4, hence one contributory facto r may be stress- Institution of Engineers, Instrumentation in Geotechnical induced stiffness anisotropy. However, further computation Engineering, Hong Kong Institution o f Engineers, 183-201. suggests that this factor is unable to fully explain the measurements. Ng, C.W.W., Sun, Y.F. and K.M. Lee (1998). Laborato ry Therefore the hypothesis is put forward that the stiff behaviour o f measurements o f small strain stiffness o f granitic saprolites. the ground in the active zone during the well pumping test is due to Geotechnical Engineering, Southeast Asian Geotechnical remnant structure in the completely decomposed granite. That is, Society (in press). structure present in the soil in the ground but not allowed for in the Simpson, B. (1992). The Thirty-second Rankine Lecture : Retaining model. Completely decomposed granite has a remarkably fragile structures : displacement and design. Geotechnique, 42, No. 4, structure. This makes it difficult to take undisturbed samples and to 541-576. carry out insitu and laboratory soil testing without damaging the soil structure. It is hypothesised that soil structure in the main body o f ground in the active zone survived wall installation (<10mm settlement) and the pumping test (up to 5-15mm extra settlement) but, adjacent to the excavation, was substantially broken down by staged excavation to B5 level (40mm extra settlement). The structure o f the soil within the box is unlikely to have remained intact during piling and dewatering during the pumping tests (estimated settlement roughly 75mm). The resulting low stiffness is represented in the model, which therefore gives a good prediction o f wall displacement.

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