中国在桩基础分析中的最新进展(全英)Recent advances in the analysis of pile foundation.docx

Advances in Deep Foundations Kikuchi, Otani, Kimura & Morikawa (eds) 2007 Taylor & Francis Group, London, ISBN 978-0-415-43629-8Recent advances in the analysis of pile foundation in ChinaM. Huang, F. Liang & Z. LiDepartment of Geotechnical Engineering, Tongji University, Shanghai, ChinaABSTRACT: The purpose of this paper is to give a brief review of recent advances in the analysis of file foundation practiced in China. Emphasis is given to the analysis methods for composite piled raft with cushion and pile responses due to tunneling. In particular, an integral equation approach is discussed first to analyze the problems of composite piled raft with cushion, followed by an introduction of a simple analytical two-stage method for evaluating the pile responses due to tunneling. Numerical examples have demonstrated the per- formance of the proposed analytical methods. Such analytical approaches are quite efficient for the engineering computation, and provide a useful tool for engineers due to its simplicity.1151 INTRODUCTIONSince the reform and open policy was carried out in the late 1970s, China has been in strong economic growth continuously. Especially, in the 21st century, based on the rapid economic growth for the past twenty years, Beijing will host the next Olympic Games in 2008 and Shanghai will host the World Expo in 2010. Recently, rapid construction in China has provided great oppor- tunities for geotechnical engineers to use their knowl- edge and talents to solve many challenging problems involving foundations, ground improvement, excava- tions, and tunnels with innovative technologies.As the buildings become more and more high, con- sequently, load transmitted to the foundation also become heavier. The design of foundation becomes more complex and important. To bear the heavy load or control the settlement of foundation, it is customary to consider first the use of shallow foundation such as a strip foundation or a raft foundation, which was pos- sibly combined with ground improvements. If it is not adequate, deep foundation such as a fully piled foun- dation is used instead. Universal practices promote the design level and construction technology of pile foun- dations. Some innovative types of pile are adopted in practice and new analysis methods are proposed.The type of piles is often chosen dependent of the type of subsoil. Most type of piles, such as cast-in- place concrete piles, precast concrete square or pipe pile, H-type steel pipe piles, mini-piles and others, are already used in China. Table 1 gives the maximum scale of the main type of piles applied in China (Shi& Hong 2005). Since the rapid development of pilemethod of the new types of piles and the testing method for the bearing capacity of super-length piles, are faced in China.Another important problem involving the analysis of pile foundations is pile response due to deep exca- vation or tunneling. With the development of under- ground construction due to lack of available space in Shanghai, the influence of tunneling or deep excava- tion on adjacent building becomes an important prob- lem. Various analysis methods of pile responses caused by tunneling or deep excavation have alsoTable 1. Maximum scale of piles practiced in China. Max.Widthlength of(diameter)Applied Type of pilepiles /mof piles /minstanceBored cast-in-place1044.0Bridge concrete pilesManual excavated534.0Building pilePrecast concrete750.6Building square pileSteel pipe pile 83 1.2 BuildingPrecast prestressed650.8Building concrete pipe piles(d 三 1.0 m)Precast prestressed 40 1.2 Bridgeconcrete pipe piles (d 1.0 m)Driven cast-in-place350.7Building concrete pilesDeep soil-cement300.7Oil tank mixing columnsengineering, some new problems, such as the design been developed, over recent years in particular, but there is currently almost no simple analytical method available for a preliminary routine design in the engineering.This paper will present a brief review of some recent advances in the analysis of pile foundations in China. Emphasis has been placed on the analysis methods for composite piled raft with cushion and pile responses due to tunnelling.2 RECENT ADVANCES IN THE ANALYSIS FOR PILE FOUNDATIONS PRACTICED IN CHINA2.1 Piled raft foundationIn a traditional design of piled foundations, it is cus- tomary to assume that all design loads are carried by the piles beneath the raft, even though competent soil strata may exist immediately beneath the raft. This conservative approach appears to be due to a limited understanding of the raft-piles-soil interaction in such a complex three-dimensional problem. In the past few years, there has been an increasing recognition that the raft or pile cap has significant contribution to the performance of piled foundation. In situations where a shallow foundation alone does not satisfy the design requirements, it may be possible to enhance perform- ance of the raft by the addition of piles. The use of a limited number of piles, strategically located, may improve the ultimate load capacity and reduce the set- tlement of the raft foundations. It is called piled raft foundation or settlement reducing piled foundation (Poulos 2001). The conception of designing founda- tion based on the principle of controlling settlement was proposed by Burland et al. in 1977, who stated that fewer piles can be used to reduce the settlement of foundation when the bearing capacity is sufficient and the deformation is intolerable. Significant devel- opments in this area have been made by a number of researchers in the last three decades. It has been rec- ognized that the piled raft foundation can be regarded as an alternative intermediate between shallow and deep foundations (Cooke 1986, Horikoshi & Randolph 1996; et al.). The concept of piled raft foundation has been used extensively in Europe and Asia, and has been proven to be an economical way to improve the serviceability of foundation performance by reducing settlements to an acceptable level.Due to the economical benefit, more attention has been paid to the piled raft foundation in China. Back to 1979, Tong (1979) discussed the possibility of applying the conception of settlement-reducing piles in Shanghai. Later, Huang et al. (1991) proposed a simple method for the design of settlement-reducing pile foundation, which was based on an analyticalmethod derived from the Geddess stress solution (Geddes 1966). In order to promote the application of settlement-reducing pile foundation, a seminar on the design philosophy based on controlling deformation was held at Tongji University in 1996 (Hou & Yang 1996). After the seminar, such a settlement-reducing pile foundation has been widely applied in Shanghai and other coastal cities in China such as Shanghai, Ningbo, Wenzhou, Tianjing, Shenzhen, and the others. The Construction Committee of Shanghai Manucipal Government demanded that all piled raft foundation of high-rise buildings in Shanghai should be designed with the settlement-controlling principle. Furthermore, the guide for the design of settlement-reducing pile foundation has been complied not only in the local code such as Design code of foundation in Shanghai but also in the National code for design of foundation in China. Yang (2000) gave a comprehensive review concerning the research and engineering practices of piled raft foundation in China.As well known, such a complex problem can be analyzed only by a few numerical methods due to the three dimensional nature of piled raft foundations. All the methods require the discretization of raft and piles along pile shaft in the analysis, which will affect their efficiency when performing analysis of large pile groups commonly used in practice.In an attempt to overcome some of the limitations of the existing methods, a variational approach was developed by Shen et al. (2000) to investigate the behaviour of piled rigid raft foundations, while an extension of the variational solutions for the analysis of piled flexible foundation was reported by Chow et al. (2001). In their researches, the free-body for analysis was usually whole piled raft or pile group- soil system isolated from piled raft foundation, the pile group-soil interaction was calculated by Mindlins solution or other complex methods, and the flexural behaviour of the raft is described by Poisson-Kirchhoff thin plate theory.Further development of the variational method was made by Liang & Chen (2004). The proposed approach was similar to that of Chow et al. (2001), while some important modifications were made to improve it. First, the free-body for the analysis is a flexible raft isolated from piled raft foundation instead of pile group-soil system or whole piled raft system, and then only the deflections of raft and the contact stress at the raft-pile and raft-soil interfaces are required to be solved. It will simplify the solution procedures. Secondly, pile groupsoil interaction is calculated with a simplified model, in which the dis- cretization of piles along pile shaft is not needed either. Compared to other rigorous approaches, the proposed method is computationally efficient and inexpensive. Hence, it is feasible to perform the analysis of large piled raft foundations.1172.2 Composite piled raft with cushionThe design concept of piled raft foundation has been proved to be an economical way to improve the per- formance of foundation by reducing settlements to acceptable levels. However, the piled raft foundation might not be applied in some circumstances. Poulos (2001) outlined the unfavourable situations that involve the presence of soft clay layer or loose sand layer near the surface, soft compressible layers at relatively shal- low depths and some others. In the former cases, the raft might not be able to provide significant loading capacity and stiffness, while in the latter case, long- term settlement of the compressible underlying layers might reduce the contribution of raft to the long-term stiffness of foundation.However, in the coastal regions of China, building construction often meets with deep soft soil deposit soft and hence the piled raft foundation is unfavourable. In order to use the piled raft foundations in such unfavourable cases, engineers have developed various practical methods in China. However, most of the engi- neering practices applied the equal length of piles in the piled raft foundation. The stiffness profile of piles does not match with the distribution of contact pressure beneath foundation. From an economical point of view, the parameters of piles should be chosen according to the soil properties and contact pressure of foundation, and etc. For this purpose, a new type of piled raft foun- dations was developed in the engineering practices. Liang et al. (2003) proposed a type of foundation as is shown schematically in Figure1. It is named as “com- posite piled raft with cushion” (for short as “CPRC”). For this new type of foundation, short piles composed of relatively flexible materials such as mixed soil- cement or sand-gravel columns were applied to improve the bearing capacity of shallow natural subsoil. Long and rigid piles were embedded in deep stiff clay or other supporting stratum to reduce the settlement, while the gravel cushion placed on top of the piles plays anFigure 1. Sketch of composite piled raft foundation.important role in mobilizing the bearing capacity of subsoil and modifying the stresses transferred to piles.CPRC is an innovative concept for the use of piles with non-uniform properties below a raft foundation in order to improve its performance. More and more buildings with such a new composite foundation were practiced in the coastal cities of China. Due to its com- plicated constitution, it is rather difficult to perform an accurate analysis with existing analysis methods. The development in theory drops behind practices evi- dently. Liang et al. (2005a) summarized the origins, developments and practices of the new composite foundation.Numerous investigations have shown that the behaviour of group piles is not identical to that of a single pile under the average load of the piles in the group. Using different length or diameter of piles could be a beneficent decision for high-rise build- ings, since both the bearing capacity of the slab and that of the piles will be fully mobilized. Ahner & Sukhov (1996) reported several projects with the non- uniform piles in Germany. Most of them locate in Frankfurt Main, where the top soil layer consists of settlement-active clay. Lee et al. (2002) presented a 2500 ton oil storage tanks supported by 137 piles with varying length. The lengths and arrangements of piles have significant effect on the stresses and deflections distributed over the raft, and a rational design of piles may greatly improve the performance of piled raft foundations.However, the application of non-uniform piles has been restricted in China in view of the safety of foun- dation. In the Technical Code for Building Pile Foundation of China, it specifies that the non-uni- form piles should not be used in one unit of the same building. Despite of the restriction of the Technical Code, raft foundation with non-uniform piles has been practiced in China. Gong (2002) presented a simple analytical method and case histories for the non-uniform piles. Liang et al. (2003) developed a finite element model for the CPRC foundation.CPRC acts as a composite structure consisting of raft, cushion and non-uniform piles. In the new type of foundation, the cushion made of gravels on top of the piles plays an important role. Firstly, gravel cush- ion between the raft and piles can adjust the load- sharing ratios of piles and subsoil, and enhance the strength of subsoil among piles. Particularly, it could avoid the separation between raft and subsoil effec- tively, which occurred occasionally in the practices of coastal cities in China such as Shanghai (Zhao 1989). Secondly, the setting of gravel cushion makes the assumption of analysis more close to the real bound- ary conditions of piled raft foundation. In the analysis of piled raft foundation, the interface between raft and soil is commonly assumed to be smooth and continu- ous, and the connection between the raft and the pileFigure 2. Pile group adjacent to tunnel.is assumed to be a sliding ball joint for only the verti- cal forces transmitted from the raft to the head of piles (Hain & Lee 1978). The two assumptions imply that only the vertical component of the contact stress is presented. The presence of gravel cushion makes only the vertical forces be transmitted.2.3 Pile responses due to tunnelingUnavoidable ground movements due to tunneling will impose additional axial and lateral internal forces on pile which may lead to structural distress or failure of the piled foundation as shown in Figure 2. Morton & King (1979) and Loganathan & Poulos (2002) carried out the static 1 g laboratory testing and centrifuge model testing respectively to investigate the effects of tunneling on the bearing capacity and pile deforma- tion. They found that the influence of tunneling on adjacent piled foundation in weak soil may be a great concern to geotechnical engineers. Bezuijen & Schrier (1994) found that the pile settlement can be significant, if the volume loss caused by tunnelling is 1% or more and the distance between the pile and the tunnel is less than 1 tunnel diameter.3 ANALYSIS METHOD FOR CPRC AND PILE RESPONSES DUE TO TUNNELING3.1 Analysis method for CPRCCPRC is rather complex and no design method be fol- lowed so far, several investigations for the design of CPRC have been reported (Ma et al. 2001, Liu et al. 2003, Song et al. 2003, Liang et al. 2003, Liang et al. 2005b).Ma et al. (2001) discussed the basic conception of CPRC and proposed an extended composite modulusmethod to calculate the bearing capacity and deforma- tion of CPRC. Composite modulus method is a sim- plified method for the design of composite ground recommended by the China Technical Standard for Foundation Treatment. Piles are considered as equiva- lent subsoil according to the compress modulus ratio of piles to subsoil. It is only suitable for piles made of low strength materials. Long piles in CPRC are made of rigid materials, and hence the method based on composite modulus is not appropriate for CPRC.To study the variation of the pile-soil stress ratio and the failure mode for this new type of composite foundation, Liu et al. (2003) conducted a field plate load test for CPRC. Reactions on the top of piles and subsoil were measured in the test. By comparing with the in-situ test data, the paper analyzed the reduction coefficient and the influence factor for the bearing capacity of each type of pile and the subsoil.Song et al. (2003) proposed a analysis method for the settlement and internal force of rigid pile compos- ite raft foundation. In this method, the raft is modelled with Mindlin plate element, and the soil-pile-cushion system is modelled through a semi-analytical solu- tion. Besides an algorithm is proposed to consider the rigidity of the super-structure, the choices of its param- eters are discussed in details. Numerical examples are presented to compare the calculated results with field measurements and with those obtained by truly three- dimensional finite element method calculations.Analytical approach is usually not suitable for deal- ing with the problem of CPRC. Most of the studies carried out so far do not incorporate the effect of cush- ion in the analysis and all the piles have the equal length. In order to clarify be