Numerical analysis of pile response to open face tunnelling in stiff clay

Three-dimensional (3D) finite element analyses have been performed to study the behaviour of a single pile and 3 × 3 and 5 × 5 pile groups during open face tunnelling in stiff clay. Several governing factors, such as tunnelling-induced ground and pile settlement, axial pile force changes and shear transfer mechanism at the pile–soil interface, have been studied in detail. Tunnelling resulted in the development of pile head settlement larger than the free-field soil surface settlement. In addition, axial force distributions along the pile change substantially due to changes in the shear transfer between the pile and the soil next to the pile, which triggers tunnelling-induced tensile forces in the piles with tunnel advancement. It was found that the relative displacements and the normal stresses at the pile–soil interface drastically affected shear transfer. The extent of slip length along a pile increased as the tunnelling proceeded. The apparent allowable pile capacity was reduced by up to approximately 42% due to the development of tunnelling-induced pile head settlement. Shear stress on the pile was increased for most of the pile depth with tunnel advancement, which was associated with changes in soil stresses and ground deformation, and hence, the axial pile force was gradually reduced with tunnel advancement, indicating the development of tunnelling-induced tensile pile force. The maximum tunnelling-induced tensile force on the pile was approximately 0.33P_a, where P_a is the allowable pile capacity applied to the pile head prior to tunnel excavation. The range affected by tunnelling in the longitudinal direction may be identified as approximately −2D ∼ +(1.5–2.0D), where D is the tunnel diameter, from the pile centre (behind and ahead of the pile axis), in terms of pile settlement and axial pile force changes based on the analysis conditions assumed in the current study. Larger pile head settlements and smaller changes in axial pile forces were computed for piles that were part of groups. It has been found that the serviceability of piles experiencing adjacent tunnelling is more affected by pile settlement than by axial pile force changes, in particular for piles inside groups. The magnitude of the tunnelling-induced excess pore pressure was small and may not substantially affect pile behaviour.     

Experimental Investigations on the Behaviour of Pile Groups in Clay Under Lateral Cyclic Loading

This paper presents the results of two-way cyclic lateral load tests carried out on model pile groups embedded in soft marine clay. The tests are conducted on 1 × 2, 2 × 2 and 3 × 3 pile groups having length to diameter ratio (L/D) of 15, 30 and 40 with the spacing to diameter ratio (S/D) of 3, 5, 7 and 9. The experimental results are presented in the form of load–deflection curves and bending moment profiles. Cyclic group efficiency, critical spacing, critical cyclic load level and cyclic p-multipliers are evaluated. It is found that the lateral capacity of the 3 × 3 group reduces by about 42% after 50 cycles of loading. The cyclic p-multipliers of 3 × 3 pile group are found to be 0.41, 0.25 and 0.29 for leading, intermediate and rear rows respectively. The test results are compared with the numerical analysis carried out by p–y method using GROUP program. The analysis carried out with experimentally evaluated p-multipliers predicts load—deflection and bending profiles of pile groups reasonably well, but underestimates the depth to maximum bending moment by about 15%.     

Uplift Capacity of Single Bent Pile and Pile Group Considering Arching Effects in Sand

In this paper an analytical method has been proposed to predict the net ultimate uplift capacity of the single bent pile and pile group with a bent embedded in sand considering arching effects. Arching develops due to relative compressibility of sand relative to pile which activates the soil-pile friction. The method takes into consideration the embedded length (L), diameter of the pile (d), bent angle, surface characteristics of pile, group configuration, spacing of the pile group and the soil properties. Log spiral failure surface with parabolic arch shape was assumed in the analysis. Theoretical investigation for uplift capacity was been carried out for the single bent pile and group of pile (2 × 1, 2 × 2) embedded in sand. The variable used in the analysis were embedded length to pile diameter (L/d = 15, 20 and 25), spacing in the group (3d, 4d and 6d) and angle of bent (6°, 14° and 20°). Typical charts for evaluation of net ultimate uplift capacity for pile groups are presented through the figures. Comparison of theoretical results shows good agreement with established experimental results.     

Response of Pile Groups Driven in Sand Subjected to Combined Loads

Although the loads applied on piles are usually a combination of both vertical and lateral loads, very limited experimental research has been done on the response of pile groups subjected to combined loads. Due to pile–soil–pile interaction in pile groups, the response of a pile group may differ substantially from that of a single pile. This difference depends on soil state and pile spacing. This paper presents results of experiments designed to investigate pile interaction effects on the response of pile groups subjected to both axial and lateral loads. The experiments were load tests performed on model pile groups (2 × 2 pile groups) in calibration chamber sand samples. The model piles were driven into the sand samples prepared with different relative densities using a sand pluviator. The combined load tests were performed on the model pile groups subjected to different axial load levels, i.e., 0 (pure lateral loading), 25, 50, and 75% of the ultimate axial load capacity of the pile groups, defined as the load corresponding to a settlement of 10% of the model pile diameter. The combined load test results showed that the bending moment and lateral deflection at the head of the piles increased substantially for tests performed in the presence of axial loads, suggesting that the presence of axial loads on groups of piles driven in sand is detrimental to their lateral capacity.     

路堤荷载下碎石桩处理软基沉降计算

以沉降控制为目的的碎石桩复合地基,其沉降计算在设计、施工中具有重要地位。本文基于典型的桩土单元体模型,建立了复合地基体积压缩系数与复合地基弹性模量的关系,考虑施工中的时间因素及相应的固结度对沉降的影响,提出相应的沉降计算公式,并结合四川省遂-资高速公路软基变形监测的数据进一步验证其可行性。研究表明本算法计算值比实测沉降约大10% ～20%。填筑过程中,计算沉降历时曲线与实测曲线拟合较好,更接近实测值;在此基础上,初步探讨了影响路堤荷载下碎石桩处理软基沉降变化的因素,得出桩径、桩间距对其影响较为明显。同时结合本例,建议布桩时桩间距与桩径比值最好控制在2～4之间。     

Field Test Research on Embankment Supported by Plastic Tube Cast-in-place Concrete Piles

The plastic tube cast-in-place concrete pile (TC pile) with a small diameter consists of pre-driven plastic tube filled with concrete. Based on the case of TC pile-reinforced embankment on soft ground, and according to the monitoring data of the TC pile-reinforced embankment system, the treatment effect and reinforcement characteristics for this system were analyzed. The field monitoring results indicates that the critical height of embankment is about 1.1 times the pile net spacing, and the small-spacing arranged TC piles can be applied to low embankment engineering; the load share rate can reflect the degree of soil arching more better and steadily and exceeds 70 % at the end of monitoring period; the settlements of pile cap and soil between piles mainly occurs in the embankment construction period; the different settlement between pile cap and soil approaches the maximum and then reduces gradually when the embankment height is about 2.2 times the pile net spacing. The variation of layered settlement and pore water pressure illustrates that the embankment settlement is mainly caused by the compression of soils within pile length, which is about 90 % of the total settlement; the influence depth of pore water pressure is about 1/3 pile length.     

Visco‐elastic load transfer models for axially loaded piles

Viscoelastic or creep behaviour can have a significant influence on the load transfer (t–z) response at the pile–soil interface, and thus on the pile load settlement relationship. Many experimental and theoretical models for pile load transfer behaviour have been presented. However, none of these has led to a closed‐form expression which captures both non‐linearity and viscoelastic behaviour of the soil. In this paper, non‐linear viscoelastic shaft and base load transfer (t–z) models are presented, based on integration of a generalized viscoelastic stress–strain model for the soil. The resulting shaft model is verified through published field and laboratory test data. With these models, the previous closed‐form solutions evolved for a pile in a non‐homogeneous media have been readily extended to account for visco‐elastic response. For 1‐step loading case, the closed‐form predictions have been verified extensively with previous more rigorous numerical analysis, and with the new GASPILE program analysis. Parametric studies on two kinds of commonly encountered loading: step loading, ramp (linear increase followed by sustained) loading have been performed. Two examples of the prediction of the effects of creep on the load settlement relationship by the solutions and the program GASPILE, have been presented. Copyright © 2000 John Wiley & Sons, Ltd.     

Response evaluation of axially loaded fixed‐head pile groups in clayey soils

The aim of this paper is to investigate the interaction between the piles in a group with a rigid head and correlate the response of a group of piles to that of a single pile. For this purpose, a computationally intensive study using 3‐D nonlinear numerical analysis was carried out for different pile group arrangements in clayey soils. The responses of the groups of piles were compared with that of a single pile and the variation of the settlement amplification factor R_a was then quantified. The influence of the number of piles, the spacing, and the settlement level on the group response is discussed. A previously proposed relationship for predicting the response of a pile group, based on its configuration and the response of a single pile, has been modified to extend its applicability for any pile spacing. The modified relationship provides a reasonable prediction for various group configurations in clayey soils. Copyright © 2009 John Wiley & Sons, Ltd.     

Expanded superposition method for impedance functions of inclined‐pile groups

This paper presents a superposition method expanded for computing impedance functions (IFs) of inclined‐pile groups. Closed‐form solutions for obtaining horizontal, vertical, and rocking IFs, estimated by using pile‐to‐pile interaction factors, are proposed. IFs of solitary inclined piles, crossed IFs, and explicit incorporation of compatibility conditions for pile‐head movements are also appropriately taken into consideration. All of these factors should be known in advance and will be computed and shown for the most relevant cases. The accuracy of the proposed closed‐form solutions is verified for 2 × 2 and 3 × 3 square inclined‐pile groups embedded in an isotropic viscoelastic homogeneous half‐space soil medium, with hysteretic damping. The pile‐to‐pile interaction factors are computed by means of a three‐dimensional time‐harmonic boundary elements–finite elements coupling formulation. The results indicate that the IFs obtained from the proposed method are in good agreement with those obtained from the coupling formulation. Furthermore, crossed vertical‐rocking IFs of solitary piles need to be appropriately considered for obtaining rocking IFs when the number of piles is small. Copyright © 2015 John Wiley & Sons, Ltd.     

Settlement‐time behaviour of granular embankments

Creep settlements are the main cause of deterioration of road pavement and impervious elements of dams, and therefore a method to calculate them is needed. Viscoelastic models (e.g. the standard linear solid) have been chosen to represent the creep of granular materials (Figure 1). Finite element calculations show that quasi‐oedometric conditions exist near the centre of embankments. Explicit expressions for one‐dimensional viscoelastic settlements of an embankment during and after construction have been obtained for any loading law and drawn for a linear load. The three viscoelastic parameters, E_o, R_c and T_r can be determined through laboratory or field testing, and the results can be adjusted by using settlement records. Good agreement has been found between measured and calculated settlements at several dams. Copyright © 2000 John Wiley & Sons, Ltd.     

大规模群桩基础相互作用近似解耦方法

针对传统弹性理论法过高地估计了桩-桩相互作用效应,基于杆件有限单元法建立了半无限土体中群桩基础的桩侧剪应力求解方程,通过简化桩-桩的相互作用效应,将群桩基础桩侧摩阻力的求解方程近似解耦,实现了群桩基础桩身剪应力和位移的快速求解目标。通过两桩相互作用系数以及柔性承台下群桩基础差异沉降的对比分析,验证了该简化方法的合理性。参数分析结果表明,该简化方法计算得到的相互作用系数与严格的边界积分方程法解答较为接近,稍小于Poulos弹性理论法的计算结果;柔性承台下群桩的差异沉降在桩间距较小时与经典解答较为接近,而在桩间距较大时则存在一定的差别。该简化方法大幅减少了群桩计算工作量,适用于大规模群桩基础的快速计算要求。     

Numerical modelling of the improvements to primary and creep settlements offered by granular columns

Analytical vibro-replacement design approaches typically quantify the settlement reduction using a dimensionless settlement improvement factor, defined as the ratio of the settlements without and with treatment. Most approaches do not explicitly consider the improvement to creep settlement. This is unfortunate because the ‘stone column’ technique is being increasingly used to reduce settlement and improve bearing capacity in soft normally consolidated and lightly overconsolidated cohesive soils (in which creep settlement tends to account for a significant proportion of the total settlement). Analytical design approaches typically consider primary settlement only. In this study, two-dimensional axisymmetric analyses have been carried out using PLAXIS 2D to establish the variation of improvement factor with time using different soil models, one of which incorporates creep behaviour. Two different approaches have been used to establish the influence of creep on predicted settlement improvement factors. The first approach is based on a direct comparison of two different soil models (one of which incorporates creep) whereas the second approach is based solely on the model incorporating creep. The settlement improvement factors have been evaluated for different area-replacement ratios, modular ratios and column lengths. The primary settlement improvement factors are in good agreement with some of the more popular analytical design methods while the creep settlement improvement factors are either equivalent or lower (depending on the approach used). The primary settlement improvement factors show a dependence on the modular ratio whereas it appears that the corresponding creep settlement improvement factors are relatively independent of it.     

现浇X形混凝土桩竖向承载性状模型试验研究

为研究现浇X形混凝土桩的荷载沉降特性及其荷载传递机制,利用河海大学的大型试验模型槽开展X形桩的足尺模型静载荷试验。通过在桩身设置的钢筋测力计及在桩底埋设的土压力盒,实测了在竖向荷载作用下X形桩与土相互作用时的工作性状。试验结果表明：X形桩的荷载沉降曲线为缓变型;桩侧摩阻力与桩端阻力随桩顶沉降的增加均呈递增性;但随着桩顶荷载的增加,桩侧摩阻力分担桩顶荷载比例却呈递减性,其分担桩顶荷载比例由初期的约90%下降到极限状态时的70%左右,而桩端阻力分担桩顶荷载的比例呈递增性,其分担桩顶荷载比例由初期的约10%上升到极限状态时30%左右。研究成果可为今后竖向受荷X形桩的设计与研究提供重要参考。     

超长大直径桩的变形特性研究

应用有限元方法,分析了不同几何参数情况下超长大直径桩的荷载-沉降关系、桩顶沉降量的构成及其比例。根据计算结果得出以下规律：超长桩极限承载力的发挥需要较大的桩顶位移,且随着长径比的增大而增大;超长桩的变形主要由桩身压缩量控制,桩端沉降所占比例较小,且各沉降分项所占比例与长径比存在明显的函数关系;对长径比为50～83的超长大直径桩,极限状态下的桩身压缩量所占比例为65.2 %～71.83 %,桩端荷载所引起的沉降量所占比例为15.7 %～9.72 %;工作荷载作用下的桩身压缩量所占比例为66.1 %～72.5 %,桩端荷载所引起的沉降量所占比例为10.8 %～3.9 %。     

Group interaction on vertically loaded piles in saturated poroelastic soil

The pile-to-pile interaction was obtained for vertically loaded piles embedded in homogeneous poroelastic saturated soil. Deduced from Biot’s theory, the fundamental functions of the quasi-static development for the force, displacement and pore pressure were acquired in cylindrical coordinates. The pile–soil system was decomposed into extended soil and fictitious piles, and the compatibility condition was set up between the axial strain of the fictitious piles and the corresponding average strain over the extended soil. This approach results in the governing equations, which consist of the Fredholm integral equations of the second kind and the basic unknowns of the axial forces along the fictitious pile shaft. The axial force and settlement along the pile shaft were calculated based on the axial forces of the fictitious piles. The interaction between the piles was investigated under different consolidation conditions through a two-pile model, and two pile interaction factors were obtained. Stemming from the two-pile analysis, numerical analyses on the settlement of the pile groups were conducted to probe pile interaction with consolidation. The conventional solutions for the single-phase soil-pile problem seem to underestimate the interaction factor if the consolidation effect is taken into account as pile settlement continues. The pile-to-pile interaction can also aggravate the percentage of consolidation settlement (PCS), and as the pile number increases, the value of the PCS will also increase. Several key factors, such as the pile stiffness, pile slenderness ratio and pile spacing, are investigated to better understand the impact of consolidation on pile analysis.     

Model Pile Groups Under Oblique Pullout Loads – an Investigation

Experimental investigations on model pile groups of configuration, 1 × 1(single pile), 3 × 1, 2 × 2 for embedment length to diameter ratio, L/d = 38, were conducted in uniform dry medium dense Ennore sand. The spacing of piles in the groups varied from 3 to 6 pile diameter. Soil–pile friction angles were δ = 16° and 28°. The pile groups were subjected to oblique pulling loads at angles θ = 0°, 30°, 60° and 90° with the vertical central axis of the groups. The load–displacement response, oblique ultimate pulling resistances have been qualitatively and quantitatively studied. The inclinations of the load, at which maximum oblique resistance for the groups were observed, have been reported. Predictions of ultimate resistance of pile groups under uplift, lateral and oblique pulling loads have been carried out respectively by the methods of Patra and Pise (2002) (Electronic Journal of Geotechnical Engineering, 8, Bundle B), Patra and Pise (2001) (Journal of Geotechnical and Geoenvironmental Engineering ASCE, 127(6), 481–487) and Chattopadhyay and Pise (1986c) (Proceedings of IST East Asian Conference on Struct Engineering and Const., Vol. 1, pp. 1632–1641). A comparison of the measured values of the Writers and others with the predicted values showed reasonable agreement.     

考虑加筋与遮帘效应计算群桩沉降的相互作用系数法

群桩中各基桩在地基土中的加筋与遮帘效应是客观存在的,然而,在目前的桩基沉降理论与实践中,相关的研究仍显不足。基于剪切变形法理论,考虑桩的加筋与遮帘效应,求得各基桩在自身桩顶荷载作用下产生的沉降以及其引起相邻桩的附加沉降量,由此提出群桩中任两桩的相互作用系数简化公式,同时,也得到各基桩桩侧及桩端桩-土接触等效弹簧刚度,并基于荷载传递法原理,建立了成层地基条件下各基桩在自身桩顶荷载作用下的桩身位移平衡方程,推导出各土层层顶处桩身沉降、轴力与层底处桩身沉降、轴力之间的递推关系,进而将公式推广到高、低承台群桩基础计算中。工程算例分析表明,用该方法计算有较高的精度,求得的荷载-沉降曲线及两桩相互作用系数与实测值吻合较好;相互作用系数要明显小于弹性理论计算结果。     

桩端后注浆对单桩承载性状的作用效应研究

以桩的长径比l/d、桩间距与桩径之比s/d、注浆加固体厚度h为因素,以单桩极限承载力Qu与某一荷载下的桩顶沉降Sp为评价指标,设计L9(34)正交试验方案,采用有限元法进行了桩端后注浆对单桩承载性状的作用效应研究,得出各因素作用效应的重要性次序和作用规律。研究表明,无论对桩基承载力还是对桩基沉降,桩间距与桩径之比都是最重要的影响因素,s/d较大时采用桩端后注浆来提高桩基承载力,减小桩基沉降,效果更显著;桩端后注浆使桩的端阻增大,承载性状改变,由摩擦桩逐渐过度为端承摩擦桩、摩擦端承桩或端承桩。     

Negative skin friction on pile groups

A numerical procedure is presented for the downdrag analysis of group piles which penetrate a consolidating upper soil layer to socket into a firm bearing stratum of finite stiffness. The settlement of the consolidating upper soil layer under a surcharge load is estimated using Terzaghi's one-dimensional consolidation theory. Parametric solutions are presented to show the influence of various parameters on the performance of the socketed pile groups in terms of the development of the induced downdrag forces and associated pile head settlements. In general, pile–soil–pile interaction has the beneficial effect of reducing the downdrag forces and settlements of the group piles when compared to the corresponding single pile values, provided that the soil settlements are not so large as to cause full slippage at the interface in all the piles. Reasonable agreement is obtained between the theoretical and experimental results for pile groups subjected to negative skin friction.     

桥头粉喷桩复合地基工后沉降原因分析

为了研究引起搅拌桩复合地基工后沉降的原因,针对粉喷桩加固桥头软基沉降进行Biot固结有限元正反分析。有限元分析时的土体本构模型采用Duncan-Chang非线性弹性模型,反分析时的土体本构模型分别采用Duncan-Chang非线性 弹性模型和Merchant黏弹性模型。计算分析表明,引起粉喷桩加固地基工后沉降的主要因素：早期时是交通荷载引,而后期土体蠕变的影响则占主导地位。