Effects of tunnel construction on nearby pile foundations 2

CHAPTER INTRODUCTION 1.1 Background Pile foundation subjected to passive loading from soil movement has been an important issue in recent years especially with concern towards its performance. Such soil movement can be caused by the construction of embankment (Stewart, 1992; Ellis & Springman, 2001), deep excavation (Poulos & Chen, 1997; Goh et al., 2003), tunnelling (Loganathan, 1999; Jacobsz et al., 2002) or slope movement (Fukuoka, 1977; Chen & Martin, 2002). In this thesis, the effects of tunnel construction on nearby pile foundation are investigated. The increased demand for underground systems in urban areas particularly for mass transportation can be seen in big cities like Singapore, Hong Kong and London. This has led to many tunnels being constructed in proximity to structures. Extensive research has been carried out in the United Kingdom particularly on the Jubilee Line Extension (Burland et al., 2002) on the effects of tunnelling on nearby structures. However, most of the structures in the studies are supported on shallow foundations and very little work has been carried out on structures supported on pile foundations. This is due to the fact that most structures were built long before the tunnels are planned. As a result, instrumentation inside existing pile foundation is difficult to install for further investigation. 1.1.1 Tunnel construction near pile foundation Among all the passive loading caused by soil movement, the one caused by tunnelling is perhaps by far the most complicated. This is due to the complex tunnelling processes particularly for shield tunnelling, which comprises shield machine advancement, application of face pressure, tail void grouting and lining installation. These caused significant disturbance to the surrounding soil with shearing (during shield advancement), loading (application of face pressure and grout pressure) and unloading (soil stress relief) mechanism. Since the structures usually exist in an urban environment long before a tunnel is planned, engineers only have the choice of aligning tunnel position relative to the nearby pile foundation. Figure 1.1 shows a schematic illustration of two typical situations being encountered in practice: (a) tunnelling under pile foundation and (b) tunnelling adjacent to pile foundation. During tunnelling, stress relief will occur in the surrounding soil. When the tunnel is constructed under a pile, it is likely that the pile base resistance will be first reduced and in turn leads to pile settlement. To maintain equilibrium of load, the base resistance is transferred to the pile shaft. If the tunnel stress relief is great enough to fully mobilise the pile shaft resistance, a larger pile settlement would be anticipated. However, pile lateral response is unlikely to be of significance in this situation. In the second situation where the tunnel is constructed adjacent to a pile foundation, a different mechanism is observed. The stress relief due to tunnelling would cause soil settlement above tunnel. This in turn causes negative skin friction (NSF) to act along the pile shaft above the tunnel level. In order for the force to be in equilibrium, the pile shaft below tunnel level (which is not subjected to settlement) would support the dragload from NSF above the tunnel level. Only when the positive shaft resistance and pile base are fully mobilised, settlement would become a problem. This depends on the availability of pile length extension below tunnel level. It is also to be checked that the dragload would not cause the pile to overstress. In addition, the lateral pile response can be significant since horizontal soil movement is largest near tunnel. Besides the two possible situations, other possibilities of tunnel alignments near pile foundation were identified from reported case histories and can be grouped into four different categories as shown in Figure 1.2. The categories are based on the relative position of tunnel to pile foundation. The pile responses due to tunnelling in each category requires further investigation owing to the varying observations by previous researchers as presented in Chapter 2. The ignorance of not taking into account the additional loading caused by soil movement as mentioned above could lead to excessive pile settlement or pile structural capacity being exceeded. Under-designed pile foundations will be reflected on the superstructure such as cracks on beam, column or wall and ultimately collapse if the damage is large. Aside from potential damage, poor understanding of the mechanism will also lead to an expensive protective or mitigation work. Therefore, further studies would be required to develop a better understanding of the problem and contribute to an economic design. 1.1.2 Current design and construction approach The current codes of practice not provide guidance and basis for the design of piles subjected to soil movement caused by tunnelling. The design requirements are usually stipulated by the local authorities. To date, there are a few design approaches available to analyse the problem. Figure 1.3 summarises the approaches obtained from literature review. In the direct method, pile responses are computed from some analytical and numerical analyses. These include finite element analysis, boundary element analysis and soil-spring analysis. Besides, design charts are also available. In the indirect method, pile responses are not computed. For example, the ‘risk of damage to building’ assessment method categorises the overall building damage, whereas methods used by Jacobsz et al. (2005) assumed the pile to response similarly to greenfield soil movement. Furthermore, Nakajima et al. (1992) and Inose et al. (1992) check the factor of safety of pile bearing capacity by hand calculation. More details of the methods mentioned are described in Chapter 2. Despite the design and assessment carried out, some local authorities also impose restrictions on the tunnelling activities near critical structures. In Singapore, the Land Transport Authority (LTA) imposes a criterion for the design of deep foundation to allow for an additional movement of 15mm in the short term and 25mm in the long term to account for future development (LTA, 2002). In addition, LTA defines an area of first reserve which is typically 6m away from the tunnel extrados. Within this area, the activities should be dealt with care. In Japan, the provision of tunnelling work adjacent to foundation can be classified into three zones (Figure 1.4) as according to Fujita (1989). If the tunnel lies within Zone 1, there is no work restriction. In Zone 2, the tunnelling work shall proceed with prudently selected methods and techniques. In Zone 3, auxiliary measures and prediction should be carried out. Similar classification was also adopted by Moroto et al. (1995). 1.2 Objectives of the study With the current understanding of the effects of tunnelling near pile foundation based on the limited available case histories, laboratory tests and numerical studies, the design of tunnel near pile foundation is yet to be understood. Despite the increasing demand for underground systems, the constraint in the congested area inevitably creates pressure for tunnelling engineers to make sure that the construction of tunnel does not cause detrimental effects to adjacent pile foundations. The thought and potentially wide scope have naturally stimulated the eagerness to undertake the current study. The aim of the thesis is to develop a better understanding of the effects of tunnel construction on nearby piles and hence contribute to the design and analysis of such piles. This aim is achieved through the study using various methods as outlined below. As mentioned above, placing instrumentation inside existing pile foundation is not feasible, which restricts further understanding of pile responses caused by tunnelling. Therefore, it is one of the objectives of this research to carry out field study with in-pile instrumentation. A Mass Rapid Transit (MRT) line constructed in Singapore, North-East Line (NEL) C704 was identified for this study. In the NEL C704, twelve working piles were instrumented with extensive strain gauges which allow the bending moment and axial force developed during tunnelling to be monitored. The case study will add to the existing knowledge of field monitoring of pile responses during actual tunnelling. The case study was only limited to specific range of tunnel-pile configurations, tunnelling process and soil type. Further understanding of the problem outside the range is therefore unknown. To overcome the limitation, three-dimensional (3-D) finite element (FE) studies were performed. The FE model allows varying parameters and configurations to be studied. In order to a general study, a reliable finite element model has to be established first. Due to the extensive monitoring data of the NEL C704, a 3-D finite element model was set-up to back-analyse the case study. The aim of the back-analysis is also to verify the reliability of the field monitoring data. Subsequently, a simplified 3-D model was used for parametric studies to identify the critical and non-critical aspects in the analysis of pile responses due to tunnelling. Although 3-D finite element model is a powerful tool for analysing such a complex interaction problem, 3-D software is usually not adopted in practice for design due to the limitation of computational resources available and also the time consuming effort. Therefore, the problem is usually idealised in plane strain condition in 2-D analysis. The plane strain idealisation is just an approximation to the actual 3-D nature of the problem. The idealisation techniques available are not well understood and could be misused since it is difficult to select adequate parameters to represent the 3-D effect. Besides, there are very few studies being carried out on 2-D idealisation of tunnel-pile interaction. Therefore, the current research would also provide greater insight into the use of 2-D finite element model in analysing the problem. 1.3 Organisation of thesis Firstly, a literature review on the current research topic is presented in Chapter 2. The review consists of past observations from case histories, laboratory tests and field studies. This is followed by a review on the various prediction and design methods available in practice. The current understanding and outstanding issues were also discussed. Chapter presents a unique case history in Singapore on the monitoring of the effects of tunnel construction adjacent to full-scale working piles. The field instrumentation data were studied and presented together with the project overview, ground condition and construction details. Also presented is the initial analysis carried out using existing design charts. Chapter presents the development of a three-dimensional finite element model to back-analyse the case history reported in Chapter 3. Full tunnel construction process was simulated. The analysis results for single tunnel advancement, twin tunnel advancement, pile response to single tunnel and twin tunnels were discussed. Sensitivity studies were also carried out, investigating various factors affecting the 3-D model results. Chapter gives a general overview on some of the considerations required in finite element analysis of piles due to tunnelling. Parametric studies were carried out to cover a wider range of tunnel-pile configurations beyond the case study examined in Chapter 4. Chapter presents the study of tunnel-pile interaction problem using plane strain idealisation technique. Some of the techniques available for modelling pile foundation and tunnelling were reviewed followed by thorough comparative study between 2-D and 3-D models. Sensitivity studies were carried out and calibration charts were produced to serve as a guide for analysing the tunnel-pile problem in plane strain model. Three case studies were analysed using 2-D FE model, engaging the technique and charts presented earlier. Chapter lists the summary of findings and conclusions from the research work carried out in this thesis. Recommendations for future research areas are identified. CHAPTER LITERATURE REVIEW 2.1 Introduction Research on the effects of tunnel construction on nearby pile foundation began since the 1970s notably by Morton & King (1979). Since then, research on this topic has been discontinued all the way until the 90s. More recently, the number of researchers carrying out such study has increase tremendously owing to the demand such as the construction of Channel Tunnel Rail Link (CTRL2) in UK, Circle Line (CCL) in Singapore and North-South Line (NSL) in Amsterdam. In these projects, tunnel alignment was designed inevitably close to pile foundation and some even have to cut through piles. Physical observations from case histories as well as laboratory tests and full scale pile tests are presented in Section 2.2. Section 2.3 discusses the design and prediction methods that have been used by engineers to analyse the problem. The advantages and shortcomings of each method are highlighted. Based on the limited publications available, the current understanding and outstanding issues are identified and discussed in Section 2.4. 2.2 Pile responses caused by tunnelling: Physical observations 2.2.1 Case histories Engineers have reported evidence of case histories where tunnels were constructed close to pile foundation. Table 2.1 summarises some of the case histories and its details including the distance between tunnel axis and pile centre (Xpile), tunnel depth (Htun), tunnel diameter (Dtun), pile diameter (Dpile), pile length (Lp) and volume loss (VL). In Singapore, construction of the MRT North-East Line (NEL) C704 was a unique case for studying the responses of pile subjected to tunnelling (Coutts & Wang, 2000). As part of the contract C704, a viaduct bridge was planned in conjunction with the tunnel advancement. The bridge which consists of abutments and 39 piers was constructed in parallel alignment with the new twin tunnels configuration (Figure 2.1a). The piers were supported by groups of four to six 1.2m and 1.8m diameter bored piles. Along the alignment, 6.5m diameter tunnels were located very closely with 1.6m clear distance to the pile foundation. The piles were founded at relatively greater depth to tunnel level with Lp/Htun ratio of (i.e. long pile). A total of twelve piles were installed with strain gauges at various levels where the axial force and bending moment were obtained. Only a summary of the monitoring results for six piles were reported. Substantial dragload and bending moment were observed in the piles, as high as 91% and 59% of the design working load and bending moment respectively. Unfortunately, many important details such as tunnel volume loss, pile-tunnel configurations, construction sequence and soil data were not mentioned by the authors, which lead to limited interpretation. Despite that, further collection of data from the contractor and the authority was carried out as part of the works in this thesis and formed the study as presented in Chapter 3. Tham & Deutscher (2000) reported another stretch of the MRT NEL tunnel (Contract C705) passing by a 4-storey workers’ quarters supported on 0.45m diameter bored piles with tunnel-pile clear spacing of approximately 1.85m (Figure 2.1b). The tunnel was located at a depth of 14m.b.g.l. which correspond to the same length of the piles (i.e. Lp/Htun of 1). Comprehensive monitoring by building settlement markers showed that the building did not suffer detrimental effect. Settlement of up to 7mm was measured at the edge of the building. The success of the operation was due to the good tunnelling control where volume loss was only 0.4%. In Hong Kong, construction of the 7.9m diameter twin tunnels for Mass Transit Railway (MTR) Island Line posed the same concern to adjacent piled-building which was located approximately 3m to the tunnel extrados (Forth & Thorley, 1996). The 31-storey building was supported on 2m diameter bored pile with pile group consisting of up to piles. The Lp/Htun ratio was between 1.6 and 2.5 which were considered as long pile (Figure 2.2). Again, the performance of piles could only be judged indirectly from building movement at ground surface. Settlement of only 5mm was observed on the near side of the building. In London, Mair (1993) and Lee et al. (1994) reported the construction of a hand-dug escalator tunnel at the Angel Underground Station where the tunnel was constructed very close to pile foundation (i.e. 1m clear distance between tunnel and pile). The new 7-storey building was supported by 1.2m diameter under-reamed bored piles (Figure 2.3) which were installed below the tunnel depth hence, a long pile condition. These piles were de-bonded down to 4m above the pile tip to reduce negative skin friction above the tunnel level. Owing to the early planning and cooperation between tunnel owner and building developer, both in-ground and in-pile instrumentations were possible prior to tunnelling. Measurements of the in-pile inclinometers showed that the nearest pile was only subjected to maximum lateral deflection of 8mm for volume loss up to 2%. Besides, both the in-ground and in-pile inclinometers results were very similar. The 10 • Pile stiffness, Epile = 28GPa • Tunnel diameter, Dtun = 6.58m (twin tunnels) • Tunnel volume loss, VL = 0.35% (assumed) • Normalised soil shear stiffness, Gmax/p’ = 500 According to the above parameters, the problem falls into Condition (pile group with pretunnelling loading). As described in Section 6.6.4, Condition coupled with Lp/Htun 1.0). Furthermore, prediction of axial force was not obtainable by this technique. Other type of technique using elements such as link element (Naylor, 1982), spring and membrane elements (Potts & Zdravkovic, 2001) can be explored. 163 [...]... pile foundations to restrict settlement of piles More recently, Takahashi et al (20 04) reported the construction of Rinkai Line in Tokyo where twin tunnels were driven underneath pile foundations with minimum clearance of 3.4m Figure 2. 9 shows the relative location of tunnels and pile foundations With the use of mitigation measure such as grout injection and good control of tunnel construction which limit... concluded that tunnel could be constructed very close to pile foundations in London Clay and would only cause small horizontal deflection Powderham et al (1999) reported construction for the Jubilee Line Extension (JLE) station beneath piled-supported buildings Figure 2. 4a shows sectional view of the tunnels and buildings Generally, the tunnels were driven below the 20 m long bored piles Prior to tunnelling,... of the main MRT network in 1990, the 22 km long track is fully underground and consisted of 16 stations (Figure 3.1) Contract C704 of NEL involved the construction of two cut and cover stations i.e Woodleigh Station and Serangoon Station, 992m twin tunnels between these stations (Ser-Wdlh) and 1 522 m twin tunnels from Serangoon Station to Kovan Station (Ser-Kov) The tunnels were bored with two Earth Pressure... concluded that no adverse effects were encountered on the piled-structure Recent tunnelling works for the CTRL2 in London also exposed piled-structures to potential damage Jacobsz et al (20 05) reported the monitoring of three piled-bridge foundations with one on end bearing piles (Figure 2. 5a) and two on friction piles (Figures 2. 5b and c) The authors recommended that re-assessment of pile capacity to be... (19 92) and Inose et al (19 92) described the planning of tunnels below piled bridges (Figure 2. 7) and a large piles supported dome stadium (Figure 2. 8) respectively Pile settlement and bearing capacity were of major concern during tunnel construction due to its relative position directly below tunnels In similar condition, Ikeda et al (1996) made use of compensation grouting between tunnels and pile foundations. .. railway transportation are usually constructed in a pair or more to cater for the different bounding lines So far, there is no reported data looking into the effect of multiple -tunnel advancement on nearby pile foundation 2. 5 Concluding remarks A review of the published literature on the effects of tunnel construction on nearby pile foundation has been reported in this chapter A series of case histories,... general agreement among all the investigations could not yet be obtained Furthermore, contradictory observations were found in the study 26 CHAPTER 3 CASE STUDY: TUNNELLING ADJACENT TO PILE FOUNDATION FOR THE CONTRACT C704 - GROUND CONDITION AND FIELD MONITORING 3.1 Introduction This chapter presents a unique case history in Singapore on the monitoring of the effects of tunnel construction adjacent to full-scale... Besides the tunnels and stations, the contract also included the construction of a 1.9km long dual-lane viaduct bridge The viaduct bridge consisted of 2 abutments and 39 piers and was constructed in parallel alignment with the new twin tunnels configuration along the road Figure 3 .2 shows the relative position of the tunnels alignments and bridge viaducts The piers are supported by pile groups of four... (20 01) In the front pile of the pile group, reduction of 12% , 29 %, 6% and 15% were observed in pile head settlement, axial force, lateral deflection and bending moment respectively when compared to a single pile of the same distance to tunnel Similarly to the rear pile in the same pile group, reduction of 10%, 43%, 0% and 21 % were observed respectively To be noted, lateral deflection for the rear pile. .. followed by the tunnels The key concern of Option 1 was the effects of bored pile installation on tunnel lining Little research has been reported on these effects and this option was ruled out to avoid additional loading on the lining Option 2 was chosen as the understanding on this interaction is better compared to Option 1 Studies have previously been carried out using centrifuge test and finite 28 element . particularly on the Jubilee Line Extension (Burland et al., 20 02) on the effects of tunnelling on nearby structures. However, most of the structures in the studies are supported on shallow foundations. authors concluded that tunnel could be constructed very close to pile foundations in London Clay and would only cause small horizontal deflection. Powderham et al. (1999) reported construction. damage. Jacobsz et al. (20 05) reported the monitoring of three piled-bridge foundations with one on end bearing piles (Figure 2. 5a) and two on friction piles (Figures 2. 5b and c). The authors