SMART-Earth Retaining Systems

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International Conference and Exhibition on Trenchless Technology and Tunneling,7th – 9th March 2006, Hotel Sheraton Subang, Subang Jaya, Malaysia.

EARTH RETAINING SYSTEMS FOR THE SHAFT

EXCAVATION OF SMART TUNNEL

Siow Meng, TAN; Chee Siong, LIM and Toong Woh, CHANG.

SSP Geotechnics Sdn Bhd

([email protected])

ABSTRACT

This paper presents the temporary earth retaining systems used for the lateral support

of deep shaft excavations for the SMART tunnel. The tunnel has a total of six shafts,one each at both ends of tunnel for TBM retrieval, and two shafts, namely North andSouth Junction Boxes, at the interface of cut and cover road tunnel merging into themain TBM tunnel, one 150m long shaft for TBM launching and one ventilation shaft.The deepest excavation was about 27m. This paper presents four shafts whereexcavation have reached the final level. The two shafts at both ends of the tunnel willnot be covered as they are currently being constructed.

The entire length of the SMART tunnel is in Kuala Lumpur Limestone formation which isfamous for its high variation in limestone level and karstic features. In view of this, theretaining systems proposed should be highly flexible, robust and safe to cater for largevariation in limestone level that could only be fully revealed during construction. Thiswould minimise idling time and design amendment.

The two main types of retaining wall adopted were cantilever RC wall and bored pilewall. The former was for location where rock level was not more than 6m deep. Elsewhere, bored pile wall, either contiguous or secant, tied back by ground anchorages wasadopted.

Probing to determine the rock depth variation along the wall lines was performed using asteel beam before construction. This was useful to forewarn the need of designadjustment. Although certain variation in rock depth was anticipated, surprises were stillencountered as the rock depth was revealed during construction.

This paper presents the design concept of the retaining systems, the problemsencountered during the implementation of the retaining systems for the four shafts, theexperience learnt and the performance of the retaining wall.



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INTRODUCTION

The proposed Stormwater Management AndRoad Transport (SMART) tunnel project aimsto address the flooding problem as well as

traffic congestion problem at Kuala Lumpur. Aspecial report of the project was covered byT&TI (2004). The project consists of a 10kmlong, 11.8m internal diameter tunnel formed bytwo tunnel boring machines spanning acrossthe eastern side of Kuala Lumpur in north-south direction. The main tunnel alignment issuperimposed on the Kuala Lumpur streetmap as shown in Figure 1.

The excavation of four shafts out of a total of

six have been completed. They are NJB andSJB (North and South Junction Boxes), NVSand SVS (North and South Ventilation Shafts).The depth of excavation was about 27m.

SITE TOPOGRAPHY AND SUBSOIL CONDITIONS

As shown in Figure 2, the alignment of theSMART tunnel is in Kuala Lumpur limestone

formation which is well known for its highlyerratic karstic features. Detailed descriptionon geology of the site, features of thelimestone and its engineering properties arepresented in another paper by the first authorfor the same conference.

DESIGN CONCEPT

In the selection and design of the retainingsystems, it was aimed to be highly flexible,safety and robust to cater for the largevariation in limestone depth which would onlybe fully revealed during construction. It washoped that idling time and design adjustmentwould be minimum.

During the design development stage, few options, namely diaphragm wall, secantbored pile wall, contiguous bored pile wall, sheet pile wall, cantilever RC wall and opencut were studied. Although it is the most economical temporary work solution, open cut

Figure 1 SMART Tunnel Alignment and Site LocationPlan

NJB

NVS

SVS

SJB

NJB

NVS

SVS

SJB

QUARTZITE &PHYLITE

GRANITE

LIMESTONE

SMART

Figure 2 Geological map (GSM, 1995)



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slope was ruled out due to the uneven rock profile making it impractical in constructionplanning. Further, there was space constraint. Diaphragm wall was not adopted as it isnot cost effective and difficult to key into rock unless special method like toe pin is used.For the case where the excavation is deeper than the rock profile, the inability of the wallto key into rock will pose instability when the toe exposed. Furthermore, there is the risk

of soil loss at the interface of the rock and overburden soil since diaphragm wall hasdifficulty to penetrate into rock to form a good cut-off.

Sheet pile wall has been successfully used in deep excavation in Kuala LumpurLimestone according to Chua et al. (1992). The sheet pile wall was driven to stop inoverburden soil. The toe of the wall was stabilized by toe pin formed of H-piles installedin bored holes. This method should be economical provided the sheet pile and the H-piles can be retrieved after the completion ofthe work. However, to enhance stability,bored hole for H-pile should be grouted.Furthermore, when the overburden soilthickness is deep, say more than 20m, largesheet pile sections that are not commonshould be used. It will be less cost effective.

Bored pile wall, either contiguous or secant, iseffective to deal with erratic rock profile.Bored piles are able to key in sufficient depthinto rock. Although secant pile wall offersbetter watertightness to prevent ground loss,it was not attractive in the beginning due tovery few contractors are available in Malaysia.

Reinforced concrete (RC) wall is the mostcost effective when rock level is not more than6m deep and there is space to form thetemporary cut for wall construction. Fordeeper rock level, contiguous bored pile(CBP) wall, tied back by ground anchorageswas adopted. There were different designs for the RC and CBP walls for various rockdepth, so is the ground anchor tie back. The standard details of the RC wall and CBPare shown in Figure 3 and 4.

Figure 3 Typical RC wall details

Figure 4 Typical CBP wall and gap sealing details



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Single size 800mm diameter bored piles at 850mm spacing were adopted. Ascontiguous bored piles are not overlapped, there are gaps between bored piles. Thegaps are prone to water seeping and losing of soil material. Cement bentonite piles intwo arrangements (Type A and Type B, Figure 4) were proposed to seal the gap.Cement bentonite piles shall be installed prior to the installation of bored piles so that

better watertightness can be achieved.

The design ground water table immediately behind wall was assumed would drop to 1/3of the overburden soil thickness or excavation height measured from ground level,whichever is lesser, during each stage excavation. This is due to the fact that it isdifficult to prevent water seeping via anchor holes as well as from the well jointed rockcut below the wall. The potential problem for dewatering is forming of sinkholes andground subsidence.

DETAILED DESCRIPTION OF THE SHAFTS

This section describes the implementation of the retaining system for the shafts, theexperience learnt, and the performance of the retaining system.

NORTH VENTILATION SHAFT (NVS)

NVS is strategically located at the mid way ofthe SMART tunnel. NVS consists of two

shafts, South TBM Drive shaft and North Driveshaft. It will serve as a temporary launchingshaft for the north and south TBM drive and aventilation shaft for permanent usage. Theshaft is the pioneer work for the temporaryretaining system of the SMART project. Thelayout of the temporary retaining wall for theshaft is shown in Figure 5. Figure 6 showsthe subsoil profile from the boreholes wherelocations are indicated in Figure 5. From theborehole, the anticipated rock profile is noterratic and the thickness of the overburden soilranges from 1.5m to 7.6m. The overburdensoil mainly consists of sandy silty soil withsome clay. As such, the CBP wall wasdesigned based on the overburden thicknessof maximum 15m.

To establish the wall type to be used, probingusing 10m long universal steel beam wascarried out along the wall. Deep depression of rock profile was detected at the North-

East corner of the North Drive Shaft, and South-East corner and West side of the South

RC Wall

CBP Wall

Tunnel

Original CBP wall

Relocated CBP wall

Deep Depression Zone

Figure 5 Layout plan of NVS

Deep Depression Zone



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Drive Shaft. It was decided that CBP wall to be used for the shafts and RC wall to beused for the cut and cover stretch between the shafts.

Figure 7 shows the as-built lengthof the bored piles. The rock is

erratic which was not detectedfrom the boreholes. The thickestoverburden is about 34m, locatedat the west side of the South DriveShaft. The rock level is about 20mat the North-East corner of theNorth Drive Shaft and South-Eastcorner of South Drive Shaft.

A deep depression zone at the South Drive Shaft exists based on the as-built length ofbored piles. This zone will intersect with the south wall of the shaft and long pile length

is anticipated at this wall. To avoid encroachment of the pile toe into the tunnel crown,the wall has to be relocated to location shown in Figure 5. This decision was made afterfew original bored piles at the wall have been installed.

The bored piles have to be re-designed due to the erratic rock profile. The design lengthof the piles was extended to cater for the overburden soil thickness of 34m for anexcavation depth of 27m. Due to the constraint on pile size, two layers of bored pileswere adopted at the deepest rock depression, at the West side of South Drive Shaft.The wall was tied back by nine layers of ground anchors.

The CBP wall has to be tied back by anchors in the standard design. However, at theNorth-East of North Drive shaft, a second deep depression zone was detected whichhas unfavorable rock dipping down behind the wall. Long anchor length is anticipated.

As such, internal corner strutting using universal steel beams was designed for thiscorner. The wall was braced by 5 layers of strut (Figure 8).

Figure 6 Subsoil profile from borehole logs

South Drive, Tunnel Crown

South Drive,West Wall

North Drive Shaft

Figure 7 As-built retaining wall for NVS

South Drive Shaft



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During the excavation, ground water seeped through the contiguous bored piles and thelimestone. Water flow through the soil/rock interface and concentrated at the limestonedepressions. The amount of water from one discharge point, which is at the North-Eastcorner of North Drive Shaft, was estimated about 200 litre per minute (see Figure 9).With this amount of water flow, concern on the ground subsidence arised. Obviously,

the gap sealing using cement-bentonite pile was not effective to reduce the water flowthrough the gap between bored piles. After some trial and error, fast set chemicalgrouting was successfully used to reduce the volume of the seeping water from thegaps.

Some water springs were found seeping fromthe joints of limestone and inter-connectedsolution channels. To mitigate ground waterloss, rock fissure grouting and cavity groutingwere performed along the retaining wall facingJalan Cheras road. Holes were drilled atcertain spacing. When cavities wereencountered, cement mortar or concrete willbe pumped down to fill up the cavity. Thedrilling continued until reaching required depthwhich is 3m to 5m below the final excavationlevel. During drilling, if the loss of water fromthe limestone was significant, cement groutwas carried out by descending stages.Packers are used to seal the hole at eachgrouting stage so that pressure can be built

up to enable grout to permeate into the rockfissures or joints. The hole was then re-drilledand the grouting process was repeated untilthe required depth was reached. The spacingof the grout hole was adjusted based on theintake of the grout and where the rock isfracture. Figure 8 Corner strut at North-East Corner of

North Drive Shaft

Figure 9 Spring of seeping water from CBPwall

Figure 10 Exposed fracture rock boulder

Tunnel Crown





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were used to seal the gaps betweencontiguous bored piles (see Figure 12).

With the above precautious measures, theimplementation of the retaining wall system

was a lot more satisfactory than the NVS.Some pile toes were hanging above therock mass as pile termination was misledby the material encountered at the toe.Such pile toes were strengthened byreinforced concrete mass (see Figures 13& 14).

SOUTH VENTILATION SHAFT (SVS)

SVS is the smallest shaft amongst the four.It is 28m x 42m in rectangular shape (see

Figure 15). Even though the shaft is small, a16m deep depression was found at thesouth-west side of the shaft. Other than that,the rock level is shallower than 4m deep.The contractor proposed a ‘modified’ secantpile wall for the depression zone. The pileswere 900mm diameter and spaced at850mm. The piles were overlapped by50mm so that there is no gap in between thepiles. This eliminates the needs of using jetgrouting columns to seal the gap.

CBP Piles

900mm diameter jet grouting piles

Figure 13 Exposed pile toe Figure 14 Concrete mass to cover pile toe

Figure 12 Typical gap sealing details with jet grouting piles

Main Tunnel

Bored PileWall

RC Wall

SVS

Figure 15 Site planof SVS



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SOUTH JUNCTION BOX (SJB)

Similar to NJB, SJB is a merging shaftat the south at the existing KualaLumpur – Seremban Highway. The

layout plan of the shaft is shown inFigure 16. Modified secant pile wallwas used for the three sides of theshaft whereas RC wall was adopted foranother side. At the West side of theshaft, bored pile wall cut through theembankment slope of an existing roadbridge. One stretch of the bored pilewall was running parallel to the wingwall of the bridge at a distance ofabout 8m. As such, ground anchors

can not be used to tie the wall as theywill hit the wing wall.

The use of corner strutting wasassessed. However, it was rejecteddue to the unfavorable geometry of theshaft at the corner. Soil nails were finallyadopted. They were arranged in fivelayers. The nails were 6m long and150mm diameter. The estimated

maximum soil retaining height H was 9m.The ratio of nail length to the H is 0.67.Figure 17 shows the cross section of thesoil nailed bored pile wall.

Commercial finite element software,namely PLAXIS, was used to analyzethe behavior of the wall. Plane straineffective stress analysis with Mohr-Coulomb soil model was adopted. Thebored pile wall was modeled based on

Mindlin’s beam theory. From theanalysis, the estimated working load ofthe nails was 45kN.

Three pull out tests were performed on a preliminary soil nail and two working nails.The preliminary nail was tested to a maximum load of 135kN which is three times thenail working load. The maximum displacement recorded was 4.4mm at the peak testload. Two other tests were performed on the working nails to the maximum test load of2 times the nail working load. Tested nails were acceptable. Figure 18 shows thecondition of the soil nailed wall during excavation.

RC WallExistingRoadBridge

Wing Wall

Bored Pile Walltied back by soil

nails

TBM Tunnel

To South In ress / E ress

Figure 16 Site plan of SJB

Figure 17 Typical cross section of soil nailed bored pile wall



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CONCLUSIONS

The temporary retaining system for the shaft excavation was presented. From theexperience learnt from the implementation and design of the retaining wall system, thefollowings are recommended:

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Limestone rock profile is difficult to predict from limited numbers of boreholes.Probing should be carried out along the wall to predetermine the rock level. Even inthe smallest shaft of 28m x 42m, a deep depression of 16m was encountered whilethe general rock profile was less than 4m.

• Contiguous bored pile wall and modified secant pile wall can be successful retainingwall for deep excavations in limestone. The adoption of suitable gap sealing methodis important for contiguous bored pile wall. Jet grouting columns are recommended.Bored piles can be overlapped instead of gap sealing with jet grouting.

• The bracing system for the retaining system should be reviewed after the installation

of bored piles. This is important to achieve economical design and to suit the localsite constraint. The retaining wall was braced by ground anchors in standard.Corner strutting and soil nails were used to retain the wall at localized area.

• Deep depression zones in limestone are weak points which are prone to seepage ofground water. Rock fissure grouting and cavity grouting for the jointed rock andcavities should be implemented prior to a deep excavation to prevent excessive lossof groundwater.

Figure 18 Soil nailed bored pile wall



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REFERENCES

Chua T. S., Liang K. M., V. Ganeshan and Chong, T. S. (1992), “A Deep Excavation Work in

Kuala Lumpur Limestone”, 2nd

International Conference on Deep Foundation Practice

incorporating PILETALK, Singapore, November, pp. 63 - 72.

T&T International, “Smart Solution to Kuala Lumpur’s Flooding”, May 2004, pg 16 – pg 19.

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SMART-Earth Retaining Systems