Geotechnical problems associated with the construction of Chater Station, Hong Kong.pdf

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  Geotechnical problems associated with the construction of Chater Station Hong Kong Richard Davies David Henkel Introduction The construction of Chater Station i nvolved a deep excavation in a congested urban area. This presented major problems in both engineering and planning as it was necessary to ensure minimal disturbance to the sur rounding area and to maintain traffic flow and services. Ground movements around the excavation were also of the utmost importance as the consequences of damage to adjacent building would be high and would incur substantial costs and delays. The problem of predicting ground movements was, however . complex as it involved the inter-relation of many fa ctors including ground conditions , methods of construction and groundwater control. Furthermore, the situation could be greatly influenced by factors which were difficult to assess, such as workmanship and geological and con structional details, and this placed severe limitations on the validity of predictions based on soil tests and theoretical analysis alone. Thus , during construction the performance of the excavation was under constant review and changes were made to the construc tion techniques as the actual behaviour of the excavation and surrounding ground became better understood. The contract Chater St ation formed part of Contract 106 of the H ong Kong Mass Transit Railway. The contract was awarded in early 1976 to the Metro Joint Venture, comprising Dragages DTP Sponsor) , Hochtief AG., Gammon HK) Ltd. and Senta b. In March 1976 Ove Arup and Partners were commissioned by the Joint Venture to carry out the design of the station and to provide geotechnical advice on matters arising from its construction. The Fig  1 Site location Fig 2 Plan of Chater Sta tion and adjacent buildings diaphragm wall described in this paper was constructed by the Bachy Soletanche Group. Main construction commenced in October 1976 and the station was handed over to the M ass Transit Railway Corporation in February 1980. Chater Station The station is situated in the heart of the commercial district of Hong Kong in an area of reclaimed land Fig. 1 ). Historically, this COURTS OF --- HO NG KONG CLUB CHATER ROAD H 7 ~ j 7 fa ;J:~A m ~O WATEf U ABLE ã2mP. D .; FILL MARINE DEPOSITS DE CO MPOSED T RACK JUSTI CE ,/ ~ r 1 - -_-1:::r-=- GRAN I TE 1 2m DIA PHRAGM - 21 .  m~   Sii D ijNiiGil ã t -- WALL ,' { #. t MANDARIN HOTEL l 2b S TORE Y l t 1 ~ :. ( ãIt IH ) CEST R. \I 1) 1, 11\ 1 I <t HUIS<, \\ ãISJ [] EJ r o RO A D area has always been the focus point of Hong Kong and is the site of important old colonial buildings as well as prestigious high rise blocks. At the site the water table is high and the ground conditions poor, with loose reclamation fill and marine deposits over a residual soil derived from the decomposition of granite. The latter material is basically a si lty sand and has been described in detail by Lumb 1. The station is shown in plan in Fig. 2 and in WALKWAY CHATER ROAD PRINCE ·s BUILDING I T STOREY l DR IV EN P ILES SI IN -21 .  m   . Y. IIIJ il JIIII DE COMPOSED GRANITE ~ GRANITE ROCK Fig 3 Section 1 1 10 20 -----d8 METRE S 4 '---------------   -   ~ '1 L=~5=~10~= ..... 2~=~3QMETRES Fig 4 Section 2-2 -   GRANITE ROCK  7 ~  I i :  1 ' ~  } .. b«nl oni tr slurr yf  ll t.d t ru c h o Co nstruct 12 m th ic k d aphr a jm fo t 1 trri ~~~  s~~ ~ ? ~ r ?m t ·e nch m am t ain e:d by exce ss h yd rostatic p res su re of b1m ton 1 e: slurry Fig 8 Construction sequence _j l, J b I ns ta ll weKs   co nstruct te:mpcrory d eck a nd ex co ¥ote f or roo f slob Cons truct roof slab section in Figs. 3 and 4 The station comprises three levels which form the concourse. track and sidings levels. Construction involved an excavation up to 27 m deep and approxi- mately 400 m long which took up almost the entire length and wi dth of Chater Road Fig . 5). At the eastern end, the excavation came within a few metres of the older buildin gs su ch as the Courts of Justi ce (Fig. 6) and even closer to the high rise blocks to the west (Fig. 7). c Ba cktllt abo ¥e roo f slab De wa te r a nd e x cn v at e t or co n ccur s e and tr ack s lab s  cas ti ng sl ab s as ex ccvct  On proc ee ds Excava te btlo w t  ac k slab St  ufl1 n9 w al l w here r equ ired Fig 5 General view of Chater Road during station construction Fig . 6 The Courts of Justice Fig 7 The Mandarin Hot el and Princes Building Photos: Richard Davies) f ~  : _ ~;:';' : @ l ;,.~ d Const ruc t sidings s lli b Switch off wells The sequence of construcuon is shown in Fig. 8 A diaphragm wall was constructed around the perimeter of the station before the main excava tion commenced, the station roof and floors being cast as excavation proceeded. The diaphragm wall formed both temporary support to the excavation and the permanent wall for the station. During con- struction the ex cavation was kept dry by pumping from a series of deep wells installed along the centre line of the excavation. Preliminary assessment of construction method In view of the proximity of adjacent buildings the need to control both ground movements and the stability of the excavation was of major importance, although quantifying the problem was d ifficu lt. Prior to construction of the Hong Kong Mass Transit System very l itt le was known abo ut actual movements around deep excavations in these geological conditio ns and virtually no data existed on the behaviour of decomposed granite under conditions of stress relief. The constructio n method was, however, one of the most positive and stiffest methods available and had been successfully used for the con- struction of deep basements and subway systems in Europe and the United States where control of groun d movements was important . Even so, movements of the ground surrounding the excava tio n were to be expected as a result of the diaphragm wall construction, dew atering and excavation. a) D i phr gm w ll construction Construction of the diaphragm wall involved excavating and concreting a series of panels between 2.7 and 6 1 m long up to 37 m deep adjacent to existing building foundations. Experience elsewhere suggested that the main problem would be ensuring stability of the trench and, provided this was satisfied, settlements would be small. During excavation, the trench is supported by bentonite slurry which normally has a specific gravity only slightly greater than water. For stability of the trench in a soil such as a silty sand where negative pore pressures dissipate during the period of excavation, the earth pressure and horizontal pressure due to surcharge have to be supported by the effective slurry pressure (i.e. the difference between the slurry pressure inside the trench and the external static water pressure). Thus, in situations such as Chater Road where the water table is high, the elevation of the slurry head above the water table is a critical factor and stability is highly sensitive to small changes in slurry head. The problem is illustrated in Fig. 9 In the example, the earth pressure and horizontal surcharge pressure have been calculated using arching theory. In plan, it is assumed that the soil forms a se mi-circular arch around the trench and. with depth . the horizontal pre ss ure is reduced by vertic al arching in a similar manner to silo theory (Fig. 10) . A theoretical s tudy was carried out to examine the problem in detail for each of the building s. For the majority of the high ri se buildings it was possible to show that, whilst 5  the surcharge pressure could not be supported by the effective slurry pressure, the structure and foundation of the buildings could redistribute the load. For the older buildings such as the Courts of Justice and the Hong Kong Club the problem was less severe since the surcharge loading was considerably less. Thus, in the majority of cases, provided the bentonite level was kept high and the panel length relatively short , construction using conventional methods was possible. This was not the case, however, for con struction of the wall adjacent to Swire House at the western end of Chater Road. This building is 22 storeys high and is founded on small individual pile caps beneath each column with piles driven just to the top of the decomposed granite. The superstructure is also relatively flexible and it was clear that either the building had to be underpinned to rock or major changes would have to be made to the construction method. To gain better understanding of the behaviour of the ground adjacent to a diaphragm wall excavation, an instrumented test panel excavation was carried out. The details of the test and results are described by Stroud and Sweeny 2. The test generally confirmed that , except for Swire House, stability of the trench could be maintained, provided careful control was exercised during construction. (b) ewatering Settlement due to dewatering occurs as a result of an increase in effective stress in the ground. To quantify the problem it is necessary to know the drawdown / depth profile outside the excavation and the compressibility of the soil. Although reasonable estimates of the com pressibility of the soil can be made from in situ penetration tests, laboratory tests, and experience with foundation settlements, estimating the drawdown/depth profile is rather more difficult. Much depends on the local permeability profile and geological variations and, where rock is high , the detailed connection between the base of the diaphragm wall and rock. The latter con sideration was of particular concern at Chater Road since the rock level varied substantially over very short distances and it was doubtful if a satisfactory cut-off could be achieved. To attempt to quantify drawdown , a series of parametric studies were carried out using a finite element seepage programme assuming various conditions of permeability profile and cut-off. Some data was available from borehole tests on relative permeability but the test results were difficult to rely on with confidence. Some of the results of these studies are shown in Fig. 12. The study clearly indicated that wide variations in drawdown could occur and if unfavourable situations arose, dewatering settlements, particularly of the older buildings, could be excessive. Estimating the possible dewatering settle ment of the piled buildings presented a special problem. Settlement of the ground relative to the pile causes negative skin friction which can increase the load on the pile. The latter factor is important for end bearing piles when the penetration into the bearing strata is small and, in these circum stances, it is possible for piled buildings to settle almost as much as the ground surface. Large settlements due to dewatering where pile penetration into the bearing strata was small, have for example, been observed by Lumb 3 in the Mongkok area of Hong Kong. To illustrate the problem for the buildings at the west end of Chater Road, the preliminary estimates of settlement due to dewatering are shown in Fig. 11. (c) Excavation Movements of the ground around the station 6 were anticipated as a result of vertical and Ho ri zont al elft ct1v t pr u.su re ( kN / m2 ) Horizontal l?ff tct iv e prl:Ssurt ( kN/m2) stur ry filled trench _ {SG ·I OS ) E15 .,... - / c. 0 20 JO surcharge 50kN / m2 leng th of trench 6,\m \ \ ---- effe ctiv e. slur ry \ pressu r2 \ t he oretical \ to tal hom :on tal \ pr essure to be suppor te d \ by effective sl urr y pr l S sure \ \ () Oh' I kN/ m I slurry f1 h ad tr e nch ISG· I.OSJ P 2P 3P ,p I(} / t ~r el cal total t] he r 1z on tal pressur e to be &Jppori td ~ure ; e~ :~v s . sure / i VliJ~~: slurry 25 - 3 r I 35 0 IQ ,b jJ ii) Oh ( kN/m 1J { a .) EX CE SS SLUR RY HEAD 2 · L5m I b E XC E SS SLURRY HEAD Sm Fig. 9 Stability considerations for diaphragm wall excavation Fig. 10 Model of soil arching for preliminary assessment of earth pressures around slurry trench Fig.11 Preliminary estimate of settlement of piled building due to dewatering Fig.12 Some results of parametric study of dewatering problem 80 -   ,<> a, q estimated q_ :S po ss ible range of ~0 setf1ement q & 8 g IB drawdown in decompo sed grani te ( m) distanct from oxcavation (m) 4P .¥J 1fl 19 q -Omf'O Fill ( k ã 1(T'm/stc.) diaphragm · wall Compltltly dtcompostd granih ( k ã 1Cf m/stc Modtratoly dtcomposid gr nite r k·'IO'm sec) tq.Jipoltnt ial lin s distanct from avation (m) P 2? 1ll Fill I k - xr mlsoc. l -30 'diQpr,agm - OmP.0 wall Marino ( k ã l()~ ·  m_l_s_oc_ l .J'.l --1() oquipot<nt ial lints IMPERMEABLE - --:I  horizontal stress relief due to excavation. The amount and mode of movement depends to a large degree on the deflection and stiffness of the wall and support system (Fig. 13 ). A review of available data, concerning the movements around similar well-supported excavations elsewhere in the world where stiff diaphragm walls have been used, suggested that lateral movements and settle ments of the ground were likely to be of similar magnitudes with maximum move ments of about 0.15 to 0.2% of the depth of excavation. This represented movements of around 40 to 50 mm. Also, movements could be expected to occur at least to a distance away from the excavation equal to its depth. Movement of ground _Movement of ground / ___ ___ __ / ____ Del lection of 'Mill -Deflection of wall Fig. 13 Examples of ground movements around an excavation -~, ~ ~~  _:_--:;:__ I STAGE 1 f 1 1 i n I I oncrettd pontls r c E ~4 j I , OOURTS OF JUST I CE I ' I POINT F POINT E PO NT lo I ~1 c ' .__ . ... - . -\ COURTS OF J USTI CE a) Settlement of Courts of Ju stice \ PRINCE '5 BUILDING 27 STORIES ON PLEO RAFT distance fro] diaphra  ~ wall ~ ]0 o 1 2 6 ti :f : : : : b) Settlement of Prince's Building 6Q I_ I l 11 12 n 14 15 1s 11 ·1a 19 20 21 22 23 24 · 25 Fig. 14 week nos ( 1977 ) Development of settlement due to diaphragm wall construction A theoretical study of the problem was also carried out by modelling the stiffness of the wall and support system with assumed earth pressure coefficients and soil stiffness. This gave similar results for the magnitude of deflection of the wall although l ittle was known about the actual in situ properties of the soil. From the preliminary assessment of the stability of the excavation and possible ground movements it was clear that many problems had to be resolved, some of which could only be tackled during construction after initial measurements of the actual behaviour of the ground became available. Also , the con  struction period was extremely short and there was little time to carry out extensive field and laboratory tests to supplement the tender information. A programme of instrumentation was therefore devised to monitor settlements of buildings , horizontal ground movements and changes in pore pressure and, if necessary, measures could be taken to overcome the problems during construction. Fig.15 Settlement of buildings due to diaphragm wall co nstru ction Monitoring Throughout the period of construction, the settlement of all buildings was measured at least once a week and, during critical periods, up to twice a day. Pi ezometers were installed at various depths adjacent to all the buildings. In clinometers to measure horizontal move ments were also installed adjacent to the Courts of Justice at the eastern end of the station and adjacent to Swire Hou se to the west. Construction of the diaphragm wall Construction of the diaphragm wall com  menced in October 1976 and by March 1977 approximately half of the northern wall had been completed . This was followed by construction of the wall adjacent to Princess Building and the Courts of Justice on the southern side of the station . During construction of the diaphragm wall relatively large settlements of the buildings occurred , with maximum settlements of 38 mm at the Hong Kong Club , 78 mm at the Courts of Justice and 21 mm at Princes Building. The magnitude of settlement was considerably larger than anticipated and, as far as the authors are aware, very much greater than has been reported elsewhere in the world as a result of diaphragm wall construction. The development of settlement was also unusual. Most of the settlement did not occur during excavation of a single panel as might be expected if problems of instability were arising but , as shown in Fig. 14, occurred during the construction of a series of adjacent panels. By the time the wall was completed settlements were observed up to 50 m away from the wall (Fig. 15). It was also observed that construction of the north wall had resulted in a rise in the water table which would have reduced the effective slurry pressure supporting the subsequent south wall excavations. The magnitude of the observed settlements showed a marked similarity to the local horizontal movements in the dec omposed granite measured in the test panel excavation and was dependent on 7
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