Soil-cement column wall with wall-strut to minimize ground movement for a road tunnel construction in Bangkok subsoils

Transcription

1 Soil-cement column wall with wall-strut to minimize ground movement for a road tunnel construction in Bangkok subsoils Tanseng Pornpot 1 1 School of civil engineering, Suranaree University of Technology, Thailand ABSTRACT A two-lane access road tunnel in Sathorn district of Bangkok required a 6.m excavation in soft deposit. The tunnel wall is very close to the existing hotel which requires quiet environment during construction. The ground movements must also limit as the excavation is close to the existing hotel. This paper present design and construction of a soil-cement column (SCC) wall for using as retaining wall during excavation. The -struts are used instead of conventional bracing to minimize ground movement. The lateral ground movements behind are comparable to diaphragm wall with bracing. The performance of -strut used in this project is evaluated by comparing with a case history data. 1 INTRODUCTION This paper presents the design and construction of a two-lane private tunnel to link between an existing-underground access to an underground car park of a new luxurious condominium in Sathorn, Bangkok. Figure 1 shows cross section of the tunnel. The tunnel is 11m wide with 3.93m headroom clearance. Figure 2 shows the layout of the tunnel. One side of the tunnel is close to an existing four-story reinforced concrete hotel. The clear distance between the hotel and the tunnel structure is about meter. The opposite side of the hotel is a one-story reinforce concrete building which is 6 meter from the tunnel wall. The tunnel must be constructed while the hotel is still operated. The hotel requires that the construction method must create very low noise all the time. Therefore, the appropriate construction method must be chosen to meet the requirement. Another restriction is the ground movement. As the tunnel alignment is close to the existing hotel, the ground movement caused by tunnel construction may disturb the hotel structure. However, the foundation type and size of the existing hotel and building are not known. Therefore, the movement of the ground should be kept as small as possible. The tunnel structures lie in soft layer of Bangkok. The tunnel structure is supported with bored-pile foundation. To construct the tunnel, the open cut and cover method is selected. The earth retaining structure is required to resist earth pressure and to prevent ground movement which may cause damage to existing structures. The general require excavation depth is 6m but there is a sump pit which requires 7.6m deep excavation. Regarding construction matter, the noise during construction must be kept as low as possible. Therefore, the pile foundation was constructed with a drill and grout technique ( micro-pile ) which causes low noise and does not require any heavy equipment. However, for conventional retaining wall construction, i.e. steel sheet pile wall or diaphragm wall, requires heavy equipment and create loud noise during construction. Additionally, these walls are flexible which require some internal bracing to prevent flexure failure of the wall and to reduce ground movement. If the bracing is used, the lifting crane and installation also cause loud noise which are cannot be accepted. From the reason mentioned above, finally, soil-cement column (SCC) gravity wall constructed with low-pressure mechanically mix method had been chosen. The SCC has

2 been used for medium deep excavation in Bangkok subsoil since This method uses low speed mixing tool to mix soil with cement grout in-situ. The cement grout is prepared at batching plant outside the project then transport to the site by concrete truck. After the construction method was selected, the analysis and design for SCC gravity wall has to be done to ensure that the is stable during excavation and the excavation with this method does not create any excessive ground movement. The details will be discussed in the following sections. Four stories reinforce concrete building One story reinforced concrete building 13.3m Hotel side 2.m Tunnel roof Crust Column - Very soft Aesthetic Tunnel base slab Unknown pile type and size -1 Soft to medium stiff -strut Foundation pile - stiff Figure 1 Cross section of tunnel structure with soil protection system and existing structure. m Scale 1m Existing R.C building Existing one story R.C. building Existing R.C building I-1 -strut SCC buttress Connect to existing underground access Existing R.C building Hotel side Existing R.C retaining wall I-7 Existing four stories Hotel Connect to underground car park Figure 2 Layout of the tunnel and existing structures 2 SUBSOILS CONDITION At design stage, two boreholes are made to obtain soil profile and soil properties for analysis and design. Figure 3 shows subsoil condition with soil strength parameters. The soil profile consists of 2.m thick medium stiff crust overlying 12.m thick of soft. The stiff located at m from ground surface. The soil parameters are obtained from unconfined compression test on soft to medium stiff. For stiff, the soil parameters are correlated

3 Depth, m from standard penetration test. The stiffness parameters, i.e. Young s modulus, are obtained from empirical relationship between strength and stiffness. Crust, med. t = 18 kn/cu.m. s u = 2 kpa = 62 kpa su (t/sq.m) gamma (t/cu.m) SPT-N, blows/ft 2 4 Very soft t = 16 kn/cu.m. s u = 13 kpa = 32 kpa 1.3 Soft Soft t = 16 kn/cu.m. s u = 22 kpa = kpa t = 18 kn/cu.m. s u = 3 kpa = 87 kpa stiff t = 19 kn/cu.m. s u =98 kpa = 49 kpa BH-2 BH Figure 3 Subsoil profile of the project 3 SCC WALL ANALYSIS AND DESIGN Design of At the development design stage, the SCC gravity wall without reinforcement was selected as the site has sufficient space for thick gravity wall. The past experiences of using gravity in Bangkok subsoils shows that the without internal support can be used without any report of total collapse but the amount of ground movements are unpublished. The preliminary analysis with FEM shows that the lateral wall movement is large and cannot be accepted. Therefore, the temporary steel bracing was proposed to reduce lateral movement. However, the construction team does not want to use internal steel bracing as it creates noise during installation and need more time for installation and removal. The solution comes out with pre-installed below final excavation level instead of steel bracing. The using of pre-install concrete wall below the excavation depth was report by Wunsch et al. (2) and Ou et al. (26). In this project, the term -strut was used as this structure replaces conventional temporary steel bracing. The -struts are perpendicular to with 2.3m interval as shown in Figure 2. The wall-strut comprised of 7mm diameter SCC with 1mm overlapping. The wall-struts overlap with for effective load transfer. At the final design stage, the construction sequence was defined as shown in figure 4. The construction starts from the existing underground access side. The counter weight berm has to be left to increase stability and the base slab must be constructed portion by portion. This construction sequence can be modified if the observed performance of the wall shows that the is stable and lateral wall movement is acceptable. The criteria from FEM used for observational program is shown in Table 1.

4 Tunnel structure Final excavation with berm Bench Ramp Unexcavated Figure 4 Longitudinal section of proposed construction sequences Table 1 Criteria for observational program Level Criteria Trigger criteria Activity Safe N/A Reading < Safe curve Proceeded construction as planned Alarm 7% of FEM Safe curve reading < Alert curve Increase monitoring to every 2 days Alert 8% of FEM Alert curve reading < Action curve Monitor daily and prepare for contingency plan Action 9% of FEM Reading action curve Implement the prepared contingency plan and/or emergency plan. Follow up the event till risk is over Analysis by finite element method (FEM) For the design, finite element method is used to analyse the stress in soil and in. The FEM is used rather than conventional static analysis because movement of the ground is a major concern in this project and cannot be calculated with static analysis. The structure is a massive gravity wall which does not require reinforcement if the wall is thick enough. In this project, rows of SCC with tip in stiff are used at the hotel side. However, there are some manhole areas where rows of SCC cannot be installed; the SCC buttress wall is used to compensate the thickness of the wall. For the opposite side 6 rows of SCC are used. The wall is assume to be monolithic even it comprises of many single SCC with overlap. However, in the analysis, overlap is simulated with interface element which can model detach or slippage of the SCC at overlap zone. The shear strength of the overlap zone is assumed to be 8 per cent of SCC strength. Total stress analysis with total stress parameters is applied as the excavation duration is about 3 months. For low permeability soil, this short period still reasonable to consider as undrained condition. The parameters used in the analysis are shown in figure 3. The finite element mesh used for the analysis is shown in figure. Figure FEM mesh

5 Instrumentation and contingency plan During construction, the observational method was adopted. The horizontal movement criterion was used for assessing the safety level as shown in figure 6. The guideline contingency plan was setup and will be implemented if the horizontal movement read with inclinometer exceed action level. When the incident occurs, the soil must be backfilled to form counter weight berm in front of the and temporary steel bracing must also be installed. HOTEL SIDE Temporary bracing Sequence 1. Stop excavation immediately Backfill berm Backfill berm 2. Backfill the soil to -.m to create counter weight berm 3. Install temporary bracing at -4.m to stop further movements 4. Closely monitor until no significant increasing of ground movement observed * This contingency plan is a guide line only. The plan can be modify to suit construction activity. Figure 6 Contingency plan A monitoring program is setup for construction control. Figure 7 shows detail of instrumentation installation. Inclinometers tubes are installed in the ground at 1m away from to monitor outward movement during installation and inward movement monitor during excavation. Surface settlement arrays perpendicular to the are installed to monitor ground surface movement. There are also crack meters and settlement point installed on critical location on the building to monitor cracks and distortion of existing structure. The monitoring frequency is set to twice a week. The major criterion is lateral ground movement measured with inclinometers. Crust, med.. -2.m Inclinometer tube Inclinometer 1m Soil side Settlement point on 1m 1m 1m 1m Very soft to soft SCC WALL -.m Excavation side Excavation side SCC WALL Soil side Surface settlement array in ground stiff -18.m Depth of inclinometer tube Plan view of inclinometer position Figure 7 Inclinometer and settlement arrays Surface settlement array 4 PERFORMANCE OF THE SCC WALL AND SCC WALL-STRUT Before commencement of the excavation, the soil-cement was cored with triple core pack tube for strength test. The strengths of the cored SCC are all met the design undrained shear strength (3 kpa) as shown in figure 8. The maximum shear strength of SCC at age of 2 days with 2 kg of cement per 1 cubic meter of soil is about 1 kpa. It should be noted that the layer near stiff has low water content hence high strength. This cause difficulty

6 Depth, m for thorough mixing, therefore, SCC strength at this level is lower than the upper part which has higher water content and softer. 3.7 Crust, med. A B C D E F su (t/sq.m) Very soft 1 Soft stiff Design value Untreated soil 2 kg/m^3 - lab 2 kg/m^3 - lab 3 kg/m^3- lab 2 kg/m^3 - coring in field 2 days Figure 8 Strength of mixed soil-cement The excavation near inclinometer I-1 is used for a full scale test section to observe the retaining system performance. The observed performance can ensure the construction team and consultant in the method. The construction sequence may be adjusted according to the observed performance. The performance of the compared with FEM calculations are shown in figure 9. It can be seen that the maximum horizontal wall movement at final excavation is 2mm and does not increase with time. The observed movement is only onethird of the maximum values predicted with FEM. There is no attempt to modify the analysis to change the control criteria. However, the construction sequence is change to full depth excavation without the counter weight berm instead. This revise construction sequence reduces construction time and difficulty in construction sequence significantly. The maximum lateral movement observed with inclinometer near the four-stories hotel is less than 17mm this movement less than the opposite side due to shallower excavation depth. Additionally, after the tunnel structure is constructed the summary of the monitoring points on the building show zero settlement and distortion and cracks width monitored with cracks meter are also zero. To evaluate the performance of the, the reference proposed by Tanseng (1997) is used. The relationship between normalized maximum wall movement and factor of safety against base heave is used to compare performance of retaining wall system. The references are taken from steel sheet pile wall and diaphragm wall with temporary bracing. The results from monitoring data are plotted as shown in figure 1

7 Hmax/H (%) Horizotal movment (mm) Horizotal movment (mm) Elev. ~ -1.m (sump) Maximum Action Alert Alarm Trip 37 Trip 39 Trip 38 Trip 4 Maximum Action Alert Alarm I-7 (2/9/1) I-7 (6/9/1) I-7 (9/9/1) - -2 Criteria for inclinometer opposite to hotel side Criteria for inclinometer hotel side -2 Figure 9 Horizontal ground movements compared with criteria of the analysis form FEM H H max Sheet pile with bracing (Tanseng, 1997) Diaphragm wall with bracing (Tanseng, 1997) SCC without bracing - this study B D H Mana and Clough (1981) D t su Firm soil 1.7su F.S. H su t D Factor of safety against basal heave Figure 1 Normalized lateral movement versus factor of safety against base heave (Sheet pile and diaphragm wall data from Tanseng (1997) Figure 1 shows that the H / H max value of this project ranges from.3 to.6 per cent. These values are comparable to the excavation with braced diaphragm wall. It should be noted that the factor of safety was calculated with the mechanism propose by Terzaghi, 1943 to give the consistent with the data from sheet pile and diaphragm wall. However, the factor of safety calculated with FEM is 1.38 which is higher than the one calculated with Terzaghi s method as the failure mechanism is difference from Terzaghi s mechanism as shown in figure 11. To evaluate the performance of the wall-strut, the case of excavation in Bangkok subsoils using without wall-strut is selected for comparison as shown in figure 12 b. The maximum excavation is 6.4m and four rows of are use with tip of in very stiff. The lateral ground movement at the bottom of excavation in case A and case B are 12mm and 26 mm, respectively. The movement in case A is in bulging shape mode; however, the cantilever mode of ground movement is observed in case B. Moreover, the ground movement increase with time and it stop when the base slab is constructed. This comparison can confirm the effectiveness of the -strut.

8 Figure 11 Failure plane after full failure mechanism occur Crust, med. Horizonatal movement (mm) Elev. -1.m Silty (fill) Horizonatal movement (mm) m Very soft Soft stiff Case A Actual FEM Trip 33 Trip 34 Trip 3 Trip 36 Trip 37 FEM Inclinometer tube SCC wall -8.6m strut Soft to medium Soft to medium Figure 12 Horizontal movement of retained soil; case A: with wall-strut; case B: without wall strut Very stiff Feb 11 (I-27) 3 Mar 11 (I-27) 1 Mar 11 (I-27) 17 Mar 11 (I-27) 28 Mar 11 FEM Case B Actual FEM Inclinometer tube -6.4m -1.m CONCLUSION The without reinforcement can be used as retaining wall without any bracing successfully. Cost and time of construction are reduced. No damage to existing structure is reported during excavation. The amount of lateral movement observed with instruments is relatively low compared to the conventional sheet pile and diaphragm wall. -strut can reduce ground movement when compare with without -strut 6 REFERENCES K. Karlsrud, Ø. Engelstad, R. Wunsch, and D. Svärd (2), Diaphragm walls with crosswalls used to prevent bottom heave in soft for lot 2 of Lilla Bommen tunnel in Gothenburg; Geotechnical Aspects of Underground Construction in Soft Ground Proc. th Int. Symp.TC28. Amsterdam, the Netherlands Tanseng P.(1997). Instrumented deep excavation in Bangkok subsoils, Master thesis, Asian institute of technology, Thailand. C. Y. Ou, Y. L. Lin, and P. G. Hsieh (26), Case record of an excavation with cross walls and buttress walls, Journal of GeoEngineering, Vol. 1. No. 2, pp