Challenges in Designing the Underpinning & Strengthening for Existing MRT Viaduct Structure to Facilitate Construction of DTL3 Expo Station

Transcription

1 Challenges in Designing the Underpinning & Strengthening for Existing MRT Viaduct Structure to Facilitate Construction of DTL3 Expo Station Abstract Neo Chee Wei Meinhardt Infrastructure Pte Ltd, Singapore The Downtown Line Stage 3 (DTL3) Expo Station is an interchange station located perpendicular to the existing East West Line (EWL) Expo Station. The proposed station is located within close proximity to several sensitive EWL MRT structures. Due to the site arrangement and the effect of excavation works for the proposed station, the existing MRT structures would require underpinning and strengthening so that the construction of the proposed station will not disrupted and compromised the functionality of existing MRT system. Existing MRT viaduct piers, P412E & P413W are to be underpinned and its foundation shall be transferred to the new proposed station structures with existing piles being removed and replaced with barrette piles. Pile strengthening is proposed for viaduct piers, P413E & P414W. The concept of the strengthening works is by introducing compensation piles with transfer beam similar to the underpinning scheme for P412E and P413W. Different options of underpinning system had been evaluated and the most feasible option was adopted for the design so to facilitate construction of the proposed station so that the functionality of existing MRT system is not disrupted and compromised. A more advance constitutive model namely, Hardening soil model was used to simulate the stress-strain behaviour. The basic feature of the hardening soil model is the stress dependency of soil stiffness which also contains both shear hardening and compression hardening. This paper describes in details the challenges in the design considerations for developing the strengthening and underpinning scheme for the project. This paper also describes a case history of similar underpinning works for construction of Circle Line Paya Lebar Interchange Station. 1. Introduction The proposed DT35 Expo Station is an interchange station allocated perpendicular to the existing Expo Station and it undercrossed the existing East West Line (EWL) at the junction of Changi South Ave 1 and Expo Drive. Figure 1 shows the overall view of the DTL3 alignment and the interface with the existing East West Line. Due to the close proximity to existing EWL structures and considering the effects of excavation works, two (2) numbers of viaduct piers (P413E & P414W) with foundation piles shorter than the proposed excavation required strengthening so that the viaduct and its existing piles could withstand the lateral movement induced during diaphragm wall installation and subsequent excavation of the station and two (2) numbers of the viaduct piers which are situated directly above the proposed station required underpinning. The location of these affected viaduct piers are shown in Figure 2. For the existing MRT station, although strengthening of piles is not required, however a 350mm micropile wall is been introduced in between the proposed station and existing station. The purpose of this micro-pile wall is to serve as a cut off wall in the event where the soil in diaphragm wall trench may be loosened during trenching, the cut off wall would serve as a physical barrier to protect the underground soils surrounding the existing EWL station piles such that they would not lost their geotechnical capacity. In this paper, different options of underpinning systems will be discussed and the most feasible option will be recommended.

2 Figure 1. Overall view of DTL3 Alignment and Interface with EWL Figure 2. Layout of Proposed Expo Station and Existing MRT Viaduct Structure The existing clear height between the viaduct and existing ground level is between 3.2m to 5.4m, construction activities would be hindered by this low headroom condition, as shown in Figure 3. Therefore in order to have sufficient space for construction activity, top soil will need to be trimmed by about 1m to 3m at different areas. Due to site constraints, the proposed horizontal alignment, as shown in Figure 2, is set very close to the existing station columns (STN-P). The distance measured from as-built record shows that the proposed station wall clashes with the existing escape staircase. The staircase shall be relocated temporary to facilitate construction of the proposed station. The foundation system for the two existing MRT viaduct columns (P412E & P413W) at the centre road median would be transferred and integrated with new station structure. The existing piles shall be cut away so that they would not obstruct passengers flow. From as-built drawings, the existing pile lengths for viaduct columns, P413E & P414W are short (approx. 10m to 12m). The piles of station columns STN-P are about the formation level of proposed new station. The Singapore Expo is about 30m away which is not likely be affected by the excavation and construction of the proposed new station.

3 Figure 3. Site Photo of Existing EWL Expo Station and Viaduct Several technical papers on similar underpinning works has been reviewed and compared with the design carried out for the underpinning work of this project. Yeoh et al (2008) has presented in the paper that the underpinning and pile removal at operating SMRT Bishan Depot for the safe passage of a tunnel boring machine. The underpinning and pile removal was successful with minimum impact to the existing depot structure and operation. Ma et al (2008) presented in the paper that a shield machine has to cross under a bridge pile foundation at Shajinggang Bridge on Siping Road of Shanghai China. A new pile underpinning technology has been used in this project and it was shown to be successful in allowing the underpinning and pile removal works to be carried out without disturbing the heavy traffic flow of the major road. Chong et al (2006) has presented the underpinning works of the B2 link at existing Dhoby Ghaut Station (DBG) for the construction of Circle Line (CCL) interchange. The link structure at the existing DBG has to be temporarily supported by underpinning to enable construction of the new station box of CCL. This paper showed that hand-dug caissons under certain soil conditions can be employed in underpinning works even in tight spaces. Its robustness, large axial and bending capacity is suited for underpinning highly loaded columns and coupled with high stiffness. 2. Geotechnical Condition The soil investigation report indicated that the site consists of backfill material approximately at the top 3m and underlying by Old Alluvium (OA). The OA contains mainly sand and silt, SPT-N values ranging from 30 to 100 within excavation depth. Groundwater level is found very close to ground level. Figure 4 shows the soil profile for the site. Figure 4. Soil Profile at Proposed Expo Station

4 The interpretative subsurface profiles have four soil layers, namely Fill, Kallang Formation, Old Alluvium (OA) (N<50) and OA (N>50). The design parameters in Table 1 show the design parameters for the various soil types. The soil layer boundaries were interpolated to develop the subsurface profile. The subsurface profiles indicate that Fill and OA layers consistently exist but in varying thickness. Pockets of Kallang Formation are shown to be present in several locations. The engineering properties of OA material were comprehensively described by Wong et al (2001), Chiam et al (2003), Chu et al (2003) and Li et al (2001). Table 1. Design Parameters Material Unit Weight (kn/m 3 ) Strength Parameters Total Stress Effective Stress S u c' Undraine d Modulus E u (MN/m 2 ) Drained Modulus E (MN/m 2 ) Coefficient of Earth Pressure At-rest, K o Permeability (m/s) (kn/m 2 ) (kn/m 2 ) ( o ) Fill E z (20 S u 35) S u E U / F F2 19 M z (20 S u 50) 1.285z for 10 S u S u E U / Su E U / OA (E) (N<10) 20 5N E U / OA (D) (10 N<30) 20 5N N E U / OA (C) (30 N<50) 21 5N N E U / Old Alluvium OA (B) (50 N<100 ) OA (A) (N 100) 21 3N N+40 E U / E U /

5 3. Existing MRT Viaduct Structures In general the span length of the viaduct at this area is between 20m to 25m with circular pier and the viaduct beam is sited on bearings with ballast track. Details of the MRT structure are summarized in Table 2. Table 2. Summary of MRT Structure Details 4. Temporary Earth Retaining Structures for Expo Station In view of several sensitive MRT structures are within the proposed Expo station footprint, the earth retaining and stabilizing structures (ERSS) must have sufficient robustness so that the effects of construction on these sensitive structures would be reduced to its minimum. Top down construction sequences with diaphragm wall has been selected as the construction method within this area, as it would provide a more stable and robust system. Underpinning and pile strengthening would take priority in each phase of the construction sequences this include installing of diaphragm wall and casting of roof slab. The detail construction sequence is further discussed in the following section of the report with the recommended underpinning / strengthening system. 5. Permanent Underground Structures for Expo Station The proposed station is a Civil Defence (CD) station, the external hull of the station shall be specialized reinforced concrete (RC) structures designed and built to offer protection from direct and indirect weapons effects. The ERSS proposed is permanent diaphragm walls which will also be used as temporary earth retaining structure during excavation and construction of the station. The diaphragm wall will be combined with a layer of internal RC wall to meet the CD design requirements. Based on CD requirements, the design of external hull as primary protection zone shall be minimum 1.95m. Therefore, it is propose to adopt 1m diaphragm wall plus 1m internal RC wall to satisfy the

6 requirement. Whereas for the entrances, it would be appropriate to proposed 0.8m diaphragm wall with 0.5m of internal wall. The brief summary of the permanent structural element is summarized in Table 3. Table 3. Summary of details for permanent underground structure for Expo Station 6. Underpinning Options for Piers P412E and P413W As discussed in the previous section, due to the undercrossing of the proposed station over existing Viaduct pier P412E and P413W, the existing pile foundation of the Viaduct has to be abandoned and the viaduct load has to be transferred to the proposed station box. In the subsequent sections, the following options will be discussed and the most appropriate system will be recommended with full design: Option 1: Nine (9) piles/columns with 3m thick transfer slab without Jacking system, six (6) temporary piles will be cut away after the final stage of load transfer is achieved. Load transfer (from 9 piles to 3 piles) will be simulated in analytical model. Option 2: Option 3: Three (3) piles/columns with 3m thick transfer slab with Jacking system Barrette piles/columns with 2.5m thick transfer slab with Jacking system 7. Assumptions used in the Options Study The analysis approach used in predicting of settlement for the piers is base on a wish-in place 3-D FEM analysis. For underpinning system with load transfer (Option 1), analysis was carried out considering different stage of construction and estimating the settlement stage by stage. In a single stage load transfer system (Option 2 and 3), single stage analysis was carried out to estimate the settlement. The following loadings have been considered: Dead Load: Viaduct load: This include the Viaduct beam, cross head, pier and pile cap. The dimensions of the structural elements are obtained from the as-built drawings. Station: All external diaphragm walls are 1m thick with 1m/0.5m skin wall, 2m thick base slab, 0.7m thick concourse slab and 1.6m subway slab. As the thickness of the roof slab (transfer slab) varies for different option, hence it will be mentioned separately in each option. Superimposed Dead load: Viaduct: Ballast load, assume 24kN/m 2 ; Station: 100mm finishes and 1kN/m2 for services (including ceiling) Live Load: Train Load, Traffic Load above station Station: 5kN/m 2 for subway level, 15kN/m 2 for concourse level and 25kN/m 2 for base slab For calculation of pile settlement and pile spring, the pile / barrette design is based on the following assumption of shaft friction and end bearing: i) Shaft Friction: f s = 2.0 N, fs is limiting to 200kN/m 2 ; ii) End Bearing: f b = 60 N, f b is limiting to 6000kN/m 2, where N = SPT-N Value

7 The above assumption is subject to pile load test. The pile spring is derived based on the estimated load divided by the estimated settlement. The pile head settlement is based on the recommendation method by M. J. Tomlinson. The same method was also used to determine the spring constant for barrette piles and diaphragm walls. Formula for pile settlement: Long term material properties were used in analysis. The concrete grade proposed for the underpinning: Barrette piles: Grade 40 concrete with a thickness of 1000mm; Bored piles: Grade 50 concrete with pile size of 2m diameter. From BS5400, the long term material properties such as the Young s Modulus are taken as 75% of the short term value, i.e 25.5 kn/mm Option 1 9 Piles/Columns with 3m thick Transfer Slab without Jacking In this option, the proposed bored piles are set at reasonable distance from the viaduct (see Figures 5a and 5b) so that the underpinning works would cause only minimum disturbance to the existing foundation system. Furthermore to prevent soil collapse within the borehole as the surrounding soils are under high stress, temporary casing will be pushed into the bored hole segment by segment until a depth that it would not influence the existing pile. The proposed 9 numbers of bored piles have been designed to secure the pair of viaduct piers and also the piles would be design to allow for additional capacity that is required to resist additional stress caused by lateral soil movement during excavation. The 9 numbers of piles will remained until the entire station box section is completed. 6 bored piles will be cut and left only 3 piles in the middle row to act as permanent columns to support the new station as well as the existing viaduct structures. Figure 5a. Plan for Option 1

8 Figure 5b. Section A-A of Option 1 The disadvantage of the option is no preloading are proposed for the new piles in the design as it deems to be unnecessary, any increase in settlement during construction could be rectified by adjusting the ballast. Cutting of redundant piles (stage construction) may cause sudden increase in deflection (although the overall settlement will still be within tolerance and it could be controlled by additional propping system). This has raised Client s concerns during concept design stage. Base on the preliminary study the induced settlement at the pile cap top is about 12.47mm (without preloading), which included stage construction. 9. Option 2 3 Piles/Columns with 3m thick Transfer Slab with Jacking Considering the shortcoming of Option 1, in this option modification was carried out to option 1 by considering jacking system and eliminating the necessity of stage construction (cutting of redundant piles). In order to make this possible the transfer slab is lowered down to the soffit of the existing pile cap so that side jacking could be considered. Also, instead of having 9 numbers of piles, 3 numbers of bigger piles are introduced (see Figure 6a and 6b). In order to satisfy the deflection requirement the transfer slab thickness has been increased to 3m thick. Although this option has overcome the shortcoming of Option 1, however during the construction of the adjacent roof slab, having only one row of piles at the middle would not provide equivalent stiffness compared to Option 1. Deep excavation is also required during the casting of the transfer slab and this would cause disturbance and weaken the existing piles before the viaduct has been underpinned, addition disturbance and construction difficulty due to the used of large diameter bored pile could also be an issue. Based on the preliminary study the induced settlement at the pile cap top is about 13.32mm.

9 Figure 6a. Plan for Option 2 Figure 6b. Section A-A of Option Option 3 Barrette Piles/Columns with 2.5m thick Transfer Slab with Jacking It is the preference of construction team to replace bored piles with Barrette piles as to minimize the type of construction on site. The Barrette piles are set out at a similar distance to the pile option (see Figures 7a and 7b), so that the disturbance during installation of Barrette piles would be kept to an acceptable level, in addition the setting out has also taken into account of the available space for M&E rooms. Base on the study a 2.5m thick transfer slab would satisfy the settlement requirement.

10 Figure 7a. Plan for Option 3 Figure 7b. Section A-A for Option 3 Although the disturbance to existing piles during the installation of Barrette pile would be higher than Option 1, by providing sufficient clear distance (minimum 3.5m) between the existing piles would reduced such effect to its minimum. Furthermore with a reduction in slab thickness compared to option 2, the disturbance to the existing piles could also be reduced. Jacking could be introduced as a contingency measure. Base on the study the induced settlement at the pile cap top is about 10.9mm vertically and 14mm horizontally. From the above study, Option 3 is the most preferred option by client.

11 11. Pile Strengthening System for Piers P413E and P414W In view of the close proximity of the excavation works to Viaduct pier P413E / P414W and also the recommendation of the Building Damaged and Protective Work Report, pile strengthening works are considered. The concept of the strengthening works is by introducing compensation piles with transfer beam similar to the underpinning scheme for P412E and P413W. The arrangement of new pile is also similar such that they form a structural frame that is rigid and can resist lateral and vertical forces induced by excavation. The additional piles are to be in place before the installation of diaphragm wall and excavation for station works at the Viaduct pier area. Transfer beam shall be cast to combine all the new and old piles together. Since the new piles will be installed to a much deeper depth so that any subsequent excavation will not affect the bearing capacity of the new foundation system. The following options have also been looked into in the design of the pile strengthen system: - Option 1: Bored Pile with transfer beam - Option 2: Barrette piles with transfer beam (similar location to piles option) - Option 3: Barrette piles with transfer beam spanning transversely 12. Study of Different Pile Strengthening Options for viaduct piers P413E and P414W All three options in general are of the same pile strengthening concept, the difference between these options are referred to the supporting system or arrangement. Option 1: By using bored piles of 1400mm diameter as a supporting system (see Figure 8) would provide lesser disturbances to the existing piles, however the piles need to be set at a distance from the viaduct as it required higher headroom machine near to the viaducts. The client also preferred to adopt the same system (diaphragm wall) to save time and cost. Figure 8. Option 1 Option 2: Having Barrette pile of 2800mm by 1000mm thick as a supporting option (see Figure 9) is also a viable option and low headroom machine could be utilised. Furthermore, the Barrette piles could be integrated as the station earth retaining system.

12 Figure 9. Option 2 Option 3: Similar to Option 2, however in this option the transfer beam is span in the transfer direction (see Figure 10). Although this would provide a shorter load transfer path, however the risk of having the existing piles being disturbed would be comparatively higher than Option 1 and 2. Also due to the existing piles, placing of reinforcement would be an issue. Figure 10. Option 3 From the above comparison, Option 1 is most preferred from design point of view but from construction point of view, Option 2 is preferred. As such, Option 2 was selected. 13. Settlement Analysis Considering Station Excavation In view of the excavation works to be carried out for the proposed station, each stage of excavation involved would have induced a certain amount of movement to the adjacent piers. In order to capture this movement due to excavation, stage analysis using PLAXIS has been carried out. In the following sections, various analyses will carried out to simulate the behavior of the viaduct movement due to staged excavation, both with and without recommended underpinning/strengthening.

13 14. Analysis Approach While using PLAXIS 2D for the analyses to predict the viaduct movement due to staged excavation and construction, both drained and un-drained analysis have been carried out. Loading considered was 20kPa surcharge load for construction machineries and viaduct load. Water pressure on the retaining walls was calculated by carrying out steady-state seepage analysis with ground water table at ground level by assuming no drawdown. Because the use of elastic-perfectly plastic Mohr-Coulomb (MC) constitutive model leads excessive heave, a more appropriate constitutive model namely, Hardening Soil (HS) model was used to simulate the stress-strain behavior. The basic feature of the HS model is the stress dependency of soil stiffness which also contains both shear hardening and compression hardening. Figure 11 shows the PLAXIS model used to study the behavior of the piers due to the excavation for the proposed Expo Station. 15. Analysis without Consideration of Underpinning and Strengthening In order to determine the necessity of pile strengthening, a stage analysis was carried out to investigate the behavior of pier P413E / P414W during the proposed excavation phases. The result of the analysis for viaduct piere P413E / P414W movement is shown in Figures 12a-c. From the analysis without pile strengthening minor movement of about 3mm was observed at excavation to subway level stage. However as the excavation proceed to S5 strut level, substantial movement was observed. The estimate maximum movement obtains form the analysis is 40mm vertically and 3mm horizontally. Figure 11. PLAXIS model used to study the behavior of the piers due to excavation 16. Analysis with Consideration of Underpinning and Strengthening With similar sequence of construction as above section, the recommended underpinning system was introduced into the analysis. The movement of viaduct pier P413E / P414W is shown in Figure 12a-c. The viaduct pier experience similar trend of movement in the initial stage of excavation, however after the transfer beam is been cast the movement was restrain form moving further, with the final estimated settlement of 10mm.

14 Figure 12a. Summary of Resultant Displacement for Viaduct Pier P413E / P414W Figure 12b. Summary of Horizontal Displacement for Viaduct Pier P413E / P414W

15 Figure 12c. Summary of Vertical Displacement for Viaduct Pier P413E / P414W 17. Construction Sequence at Existing Viaduct and Station at Pier Locations In general the sequence of construction at existing viaduct and station pier location are governed by pile underpinning / strengthening works and road diversion. By taking into consideration of the two governing criteria and limited working space the following proposed construction sequence is been established. Stage 1- Barrette and Micro-pile Installation 1. Divert traffic 2. Cut down the ground level to Install the barrette piles (For P412E/P413W and P413E/P414W) 4. Install micro pile wall The following activity could be carried out simultaneously with step 1 to 3 5. Construct temporary escape staircase for existing EXPO station 6. Construct proposed transmission tower 7. Demolish existing transmission tower after completion of proposed transmission tower 8. Demolish existing escape staircase after completion of proposed temporary escape staircase Stage 2- Underpinning works for P412E/P413W 1. Excavate to formation level of transfer beam/slab at Cast transfer beam or slab. 3. Backfill slope area to Stage 3 - Underpinning works for P413E/P414W 1. Excavate to formation level of transfer beam at Cast transfer beam 3. Backfill to Stage 4 & 5- Casting of roof slab at GL A1-E/ Install Diaphragm wall and king post at GL A1-E/ Cast Capping beam for micro pile wall with diaphragm wall 3. Excavate to S1-1 Strut level 4. Install and preload S1-1 Strut 5. Excavate to roof slab formation level at Cast roof slab and remove strut S Install decking and divert road

16 Stage 6, 7 & 8 - Casting of roof slab at GL 6/ D-E 1. Install diaphragm wall and king post at GL 6/ D-E 2. Excavate to S1-1 Strut level 3. Install and pre-load S1-1 Strut 4. Excavate to roof slab formation level at Cast roof slab and remove strut S Install decking and divert traffic Once the roof slab has been cast, top-down construction can proceed by excavation and casting the structural slabs, level by level until the base slab is completed (with relevant strut). After completion of all the structural slabs the casting of columns and walls will follow till the basic station box is completed. 18. Pre-loading and Debonding of Existing Pile In-order to ensure proper load transfer is achieved between existing viaduct foundation and transfer roof slab, pre-loading through jacking would be necessity procedures and jacks could also reduce the magnitude of settlement under unforeseen circumstances. Different jacking locations were studied and the major concern was that the jacking system should not increase the excavation depth of the transfer beam, as the existing piles of the viaduct was relatively short and by exposing too much of the pile would reduced the skin friction of the existing piles. With the above consideration side jacking was introduced as shown in Figures 13a-c. Figure 13a. Layout Plan of Hydraulic Jack Location Figure 13b. Section A-A Figure 13c. Section B-B

17 The proposed pre-loading procedure mention below is a general requirement form the design view point a detail method statement will still be required form the contractor base on the availability of jacking product and equipment. However the following would form the basis of the method statement; 1. After casting of transfer slab with existing piles de-bond (see next section), readings of all instrumentation on viaduct and viaduct pier must be recorded before the commencement of next stage of construction activities. 2. Install side jacking frame, all connections are to be drilled-in and to ensure full bonding strength is achieved between bolts and existing concrete. 3. Install 100T (jacking load) hydraulic jacking system, the system must be able to carry out synchronize jacking and also have sufficient flexibility to apply different jack force on each jack (to rectify minor pier tilting if necessary). 4. Proceed with pre-loading in stages after transfer slab has achieved cube strength of 25N/mm2. Each stage of jacking forces should not be more than 10% of the specified pre-loading forces of 30T per jack. Each increment of load to be maintained for 60 minutes. 5. Within each stage of jacking, monitoring of settlement / movement on the viaduct pier and stress within the barrette piles shall be recorded. 6. Once pre-loading forces are achieved, pressure grouting shall be done between Viaduct footing and transfer slab. 7. Proceed with the construction of the station and the frequency of monitoring shall be as per specified in instrumentation drawing. 8. If further jacking is required to adjust settlement or tilting of the viaduct, which breach the allowable limit given in Code of Practice for Railway Protection (CPRP), the maximum amount of jacking forces could be applied is an additional 35T per jack. 9. After station basic structural (slab and structural wall) is completed, pressure grouting shall be done between Viaduct footing and transfer slab. Due to the necessity of jacking between existing viaduct pile-cap and transfer roof slab, debonding of existing piles within the transfer slab area shall be provided. The de-bonding detail is shown in Figures 14a and b of this report. Additional reinforcement shall be provided to prevent debris from falling under CD design requirements. It is necessary to maintain that the pier is able to transfer horizontal train load to the roof slab when debonding of existing piles is taken place for pre-loading. Diagonal steel members connecting to transfer slab to act as a transfer mechanism will be provided in this stage, as shown in Figure 13. Figure 14a. Typical Debonding Detail for Existing Viaduct Pier - 1

18 Figure 14b. Typical Debonding Detail for Existing Viaduct Pier Predicted Pier Movement for Underpinned Viaduct Pier In order to study the movement of the Viaduct pier under both construction stage and permanent stage, both Stage Analysis and Wish-in-place Analysis were combined and the movement extracted from the stage analysis is compared with load case 4 of the Wish-in-place analysis and add with load case 5. The resultant movement = 17.74mm while the horizontal movement = 14.0mm and vertical movement = 10.9mm. The predicted movements of the viaduct piers are within the allowable limit as specified in the Code of Practice for Railway Protection (CPRP). 20. Case Study of Underpinning of Viaduct Piers for Construction of Circle Line Paya Lebar Interchange Station A case study on a the construction of the underground Circle Line (CCL) Paya Lebar Interchange Station which requires two existing viaduct piers of an operating Mass Rapid Transit (MRT) line to be underpinned was carried out to compare with the proposed underpinning design. This case history was reported in papers by Lim et al (2007) and Cai et al (2003). The paper by Lim et al (2007) describes the design of the underpinning works, its construction and the monitoring of the nearby existing MRT viaduct structure including the measures that were implemented to minimize movements. The loads from each viaduct pier were transferred from their existing piled foundation to a new support system using hydraulic jacks and a transfer beam supported at both ends on barrette piles.

19 Figure 15. Photograph showing the location of viaduct piers to be underpinned during construction of Circle Line The two existing EWL MRT piers, as shown in Figure 15, were designed to be underpinned by transfer beams spanning between barrette piles. The loads from the existing MRT piers were to be transferred to the new transfer beams using hydraulic jacks. Figure 16 shows the cross section of the underpinning area. The 8m (L) x 1m (W) barrette piles provide support for the 18m (L) x 8m (W) x 3.1m (D) transfer beams. To allow adequate working space for jack set-up, the transfer beams were located 1m below the soffit of the pile caps. Figure 16. Cross section of the Underpinning Works for Viaduct Piers

20 The sequence adopted for underpinning construction, as shown in Figure 17, is as follow: a) Installation of barrette piles to about 50m be-low ground level b) Installation of lean concrete walls to about 15m below ground level c) Strutted excavation in 4 layers to 10m below ground level d) Construction of transfer beams across barrette piles e) De-bonding of existing piles f) Construction of corbels at four corners of each pile cap g) Excavation to 2m below transfer beams to allow deflection by self-weight h) Installation of jacks and jacking i) Cutting of 8 nos of 1m diameter bored piles at each pier j) Continue strutted excavation to formation level (about 5m) k) Cast station base slab and build up structure in stages up to roof level l) Encasement of pile cap and jacks Figure 17a. Sensitivity study for optimum jack load Deflection of existing pier Figure 17b. Sensitivity study for optimum jack load Deflection of new transfer beam In order to derive the optimum percentage of jack pre-loading, a sensitivity study was carried out using different percentages of pier dead load. The predicted vertical movements of the pier and new transfer beam are shown in Figure 17a and 17b respectively. From the sensitivity analysis, a jack pre-loading of 50% of pier dead load is optimal as it limits the vertical movement of piers to be less than 0.5mm. Pre-loading of jacks to 50% of the existing pier dead load was then adopted. Figure 18. Instrumentation plan and monitoring results

21 The instrumentation plan and summary of monitoring results for the underpinning process is shown in Figure 18. The same monitoring plan used during jacking was implemented to measure settlements and deflections during pile cutting. At the end of jacking, no measurable movement was observed on the piers. At the end of pile cutting, 1.4mm settlement of the pier was recorded. 21. Conclusions and Recommendations Based on the literature review and detail evaluation of various option for this underpinning works, we have concluded that the option with barrette piles and transfer slab/beam to be the most effective and practical solution. The predicted vertical and horizontal movement of the existing viaduct is 14mm and 10.9mm respectively, which is within the allowable limit as specified in the CPRP. This demonstrates that the proposed underpinning and strengthening works for the existing viaducts is possible to limit the disruption to the operation of the existing MRT system. This is of ultimate importance as the MRT system cannot afford to be interrupted during operational hours. At the time of preparing this paper, the underpinning works has not been implemented yet and therefore no monitoring results can be presented for discussion and to justify the design approach. However, the author has confidence that the actual behavior of the underpinning works will be close to the design assumption after a detailed case study of similar underpinning works at Paya Lebar station presented by Lim et al (2007) which shows that the magnitude of movement induced by the underpinning works are within the acceptable limit. Nevertheless, the author recognizes the importance of monitoring of the existing viaduct structure and hence, a comprehensive monitoring is recommended to monitor the performance of the structure during the underpinning process. The monitoring results can be used for verification of the design assumption and when necessary, modification to the design and construction can be made based on observational method and back analysis. During implementation stage, proper construction planning, close monitoring of works and anticipation of problems is crucial in minimizing risks to the underpinning works. When the underpinning is completed successfully, it will be a precious experience to be referred for future projects.

22 References: [1] Code of Practice for Railway Protection, [2] J. Chu, P.P. Goh, S.C. Pek and I.H, Wong Engineering Properties of the Old Alluvium Soil. Proceedings Underground Singapore [3] K.M. Yeoh, Steve Liew & K.N. Lok, Underpinning and Pile Extraction of an Operational Depot. International Conference on Deep Excavations (ICDE 2008). [4] K.S. Wong, W.Li, J.N. Shirlaw, J.C.W. Ong, D.Wen and J.C.W. Hsu Old Alluvium: Engineering Properties and Braced Excavation Performance. Proceedings Underground Singapore [5] Min, Cai & Boey S Design Consideration for Paya Lebar Interchange Station, Circle Line. RTS Conference, Singapore. [6] Public Works Department, Singapore Geology of the Republic of Singapore. [7] Q.W. Xu, X.F. Ma & Z.Z. Ma, Application of Pile Underpinning Technology on Shield Machine Crossing Through Pile Foundation of Road Bridge. [8] Ronnie Chong, Oskar Sigl & J.A.D.M. Jayasinghe, Design and Construction of the Underpinning of the B2 Link at Dhoby Ghaut Station on Contract 825. International Conference on Deep Excavation, Singapore [9] S.L. Chiam, K.S. Wong, T.S. Tan, Q. Ni, K.S. Khoo and J.Chu. 2003, The Old Alluvium. Proceedings Underground Singapore [10] T.F. Lim, B.S. Tan, W.A. Chang, K. Kusano and R. Chakravarthy Underpinning Two Viaduct Piers of an Operating Mass Rapit Transit Line for The Construction of Singapore s Circle Line Paya Lebar Interchange Station. Proceedings World Transit Conference [11] W.W. Li and K.S. Wong Geotechnical Properties of the Old Alluvium in Singapore. Journal of The Institution of Engineers, Singapore Vol 41, No