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In this research, a numerical study through three-dimensional finite element method is carried out to investigate the behaviours of pile foundations including piled raft and pile group subjected to...

As the demand for using piled raft foundations (PRFs) on clayey grounds increases, so the need arises to understand their time-dependent behavior. This is because the stresses and strains in clayey grounds change for a long time after the completion of the construction work due to ground consolidation. For a better understanding of this behavior, the present paper used small-scale physical modeling to investigate the long-term behavior of two vertically loaded PRF models on saturated clay. In the experiments, the model ground was prepared by consolidating a slurry clayey mixture. Each of the model foundations comprised a square raft and four piles with different pile spacings. The applied load, PRF deformation, axial forces along the piles, pore water pressure (PWP), and earth pressure beneath the raft base were measured. The results show that the piles were effective at suppressing the PRF settlement when the applied load was less than 70% of the piles' ultimate estimated capacity. In the primary consolidation stages, the pile resistance increased with time especially at the top sections because of the PWP dissipation and the corresponding increase in the effective soil stresses below the raft base. In the secondary consolidation stages, both the raft and the pile resistances were stable even though creep settlement had continued. When the applied load was larger than 50% of the PRF predicted capacity, the PRF with the narrower pile spacing suppressed the settlement better than the PRF with the wider pile spacing. The study involved small-scale experiments; however, it emphasized the importance of considering the interaction among the piles, the raft, the ground, and the PWP.

Landslide dams (LDs) usually form from natural debris materials and exhibit heterogeneous strata along both the depth and run-out directions. In addition, an LD usually has a weaker structure than that of undisturbed ground and is more vulnerable to seepage loading. Considering that the surface layer of naturally packed LD materials is generally in an unsaturated state, it is undoubtedly important to investigate the stability of the unsaturated debris materials in the heterogeneous strata of LDs. In this paper, a systematic flume test program was first conducted, in which the Tangjiashan LD was carefully referenced for model design. Three water level rising rates and two stratal arrangements were considered in the flume tests. Then, soil-water-air coupled finite element analyses were conducted to simulate the flume tests, and all the material parameters of the LD materials were carefully determined based on the results of the element tests. A comparison of the test and calculated results shows the possibility of using the proposed numerical method to estimate the occurrence of dam breaching and the risk of LD failure. Moreover, the hydraulic/mechanical behaviors of the LD materials and the heterogeneous strata of the LD were very important to the stability of the Tangjiashan LD. Finally, from an engineering viewpoint, the possibility of utilizing a naturally formed LD and thus not destroying it when it forms is also discussed, e.g., dam breaching risk can be reduced by excavation of a drainage tunnel, and the dam stability can be carefully estimated based on accurate geological data.

Friction fatigue is an important factor to consider for pile drivability during the installation of piles. Only a few studies have focused on the friction fatigue of vibratory driven piles. However, the friction fatigue of jack-in piles has been investigated extensively. Therefore, this study focuses on the friction fatigue of vibratory driven piles. A total of six in situ vibratory pile driving tests were conducted with different frequencies and different vibratory forces. Both the acceleration at the pile head and the axial strain of the pile were monitored during vibratory driving. The shaft friction, τSij, was analyzed at each section of the pile during vibratory driving, and the decrease in friction (i.e., friction fatigue) was observed. The concept of accumulated shear work, W, was introduced during vibratory driving, and a unified relationship was established between the decrease in shaft friction normalized by the cone tip resistance, τSij/qc, and W. From a practical viewpoint, this concept will be helpful for the construction management of vibratory pile driving and for determining the vibratory hammer specifications.

Behaviours and resistance mechanisms of pile groups and piled rafts with or without batter piles were investigated through a series of load tests on foundation models at 1g field. Three-pile pile foundation models were used in the experiments. Dry silica sand having a relative density, Dr, of ∼82% was used for the model ground throughout the experiments. A series of triaxial CD tests of the sand were carried out to characterise the sand behaviour. Close-ended model pipe piles having a length of 255 mm and an outer diameter of 20 mm were used in the foundation models. The results of the load tests are presented and discussed in detail, focusing mainly on the horizontal load tests. Numerical analyses were conducted for cases of the piled rafts subjected to horizontal load to confirm the experimental results and to get a deeper insight into the resistance mechanisms of the piled raft foundations. It is shown from the results that the foundations with batter piles have advantages over the foundations with only vertical piles in aspects of resistance and settlement reduction.

In this research, pull-out experiments of model flip-type ground anchors (flip anchor, hereafter) were conducted in two-dimensional (plane strain) condition to investigate the fundamental pullout mechanisms of flip-type ground anchor. A transparent acrylic soil box was used as a soil box. Rectangular model flip anchors had the depth of 98 mm, and breadths, B, of 48 mm or 80 mm, so that the depths of the soil box and the model anchors were the same. The anchor was buried in the dry sand model ground and pulled-out. The experiments were carried out by changing embedment depth, h, initial positions of the anchor head (open or closed), and width of anchor head, B. Pull-out force, F, and pull-out displacement, w, were measured. The ground movements were observed using a PIV analysis. Based on the image analyses, ground failure pattern was simply modelled. The model simulated the tendency of the experimental values well. It would be expanded to three-dimension to design actual flip-type anchors.

The effect of vertical load on the horizontal response of pile foundations including pile group and piled raft are investigated through a three-dimensional finite element method. In this study, horizontal load tests on pile foundation models, working in different supporting conditions of pile group or piled raft, in a dry silica sand ground are simulated. The hypoplastic model, an incrementally nonlinear constitutive model, is used to model the ground. The foundations including raft and piles are modelled as linear elastic. The analysed results of a typical case are compared with the results of the corresponding model load test, which was carried out by the authors, to evaluate the applicability of the numerical model. After that, a series of numerical analyses are conducted to investigate the influence of vertical load on the horizontal response of the foundations. The analysed results indicate that the increase of vertical load leads to the increase of the horizontal resistance and the internal forces of the piles are influenced by the vertical load level.

In a series of laboratory experiments, pile penetration resistance and the bearing capacity of a model pile installed by three methods (push-in, surging, and vibratory pile driving) in both dry and saturated sand ground were compared. It was observed that the pile penetration resistance decreased due to soil contraction caused by the cyclic pile behaviour of surging and vibration. Furthermore, the bearing capacity in the static load test (SLT) became higher than the pile penetration resistance at the end of the pile installation phase. This is because the contraction behaviour of soil became soil dilation under monotonic pile movement in the SLT. It was found that the pile penetration resistance and bearing capacity were related to the soil deformation behaviour, namely contraction under cyclic pile movement and dilation under monotonic pile movement.

This paper aims to investigate long-term behavior of a piled raft (PR) through small-scale physical modelling. In the experiment, the model ground was prepared by consolidating a slurry mixture of Kasaoka clay and Silica sand. The model foundation consisted of a square raft and 9 piles with a pile spacing of 3D (D: pile diameter). For the loading test of PR, vertical load was increased by multiple steps and each load step was maintained for a sufficient duration to observe the long-term behavior. The results show that, when small loads were applied, the piles carried a major part of the load. For larger applied loads, the increment of the load was mainly supported by the raft. In primary consolidation period, the pile resistance increases with elapsed time. The rate of settlement was basically proportional to the dissipation rate of pore pressure beneath the raft base. In secondary consolidation period, the pile load was slightly reduced under smaller applied load, and stable under larger applied load.

In Japan and Turkey, predominantly traditional design concept of pile group is still adopted even though most pile foundations are piled raft foundations in reality. This situation may be attributed to the still limited knowledge about behaviour of piled rafts subjected to the combination of static and dynamic loads. Hence, in this study, a series of shaking table tests on 3-pile piled raft and 3-pile pile group models were carried out in saturated sand at 1-g field under the combinations of static vertical and dynamic horizontal loading, where the input motion was sinusoidal wave having a frequency of 20 Hz and 1.50 and 6.00 m/s(2) amplitudes. The results showed that, raft-model ground contact in piled raft is very effective to reduce settlement compared to pile group even in liquefied soil. Moreover, even in piled raft, the horizontal resistance reduces if liquefaction of the ground surface occurs, due to the loss of the raft base resistance.

This study is to investigate behaviours of winged monopiles subjected cyclic loading in a calcareous ground through numerical analyses. In the analyses, axial load tests (push-in or pull-out) of monopiles with and without wings in a calcareous ground are simulated using a three-dimensional finite element program, PLAXIS 3D. Triaxial tests on a calcareous ground sampled in Vietnam are carried out prior to the analyses of the load tests to investigate the mechanism behaviours of the calcareous ground and to select an appropriate soil model as well as to evaluate the corresponding soil parameters. In this study, the hypoplastic model is used to model the ground. Interface elements are employed to simulate the slippage between the pile shaft and the soil. The analysed results show that the resistance of the winged pile is considerably higher than that of the pile without wings.

In this study, a series of vertical and cyclic horizontal load tests on 3-pile piled raft model were carried out in dry sand to investigate the piled raft behaviour under combined loads. Also, numerical modelling of experiments were carried out by using FEM software, PLAXIS 3D. Hardening soil model was employed to model the sand, and elastic model was used for the pile and the raft. Soil parameters were estimated from consolidated drained triaxial tests carried on the sand samples and pile parameters were estimated from simple bending tests. It is seen that the behaviour of piled raft is considerably influenced by the interaction of the raft and the piles, and that it is not possible to estimate piled raft behaviour by examining its components individually.

An estimation method of retaining structures used at deep excavation sites, such as an open-cut tunneling site, is investigated in this article. A frame analysis is widely used in Japanese practices. However, based on the field measurements, this frame analysis sometimes leads to conservative design, i.e., calculated deflections of retaining walls are greater than those measured in situ. To achieve more rational design, the authors have reviewed the frame analysis critically and proposed alternative method utilizing a simple soil-retaining wall interaction model using elastic-perfectly plastic spring to estimate the behavior of retaining walls. Some case studies of real open-cut tunneling works were carried out to check the validity of the proposed method. Calculated deflections and their modes by the proposed method can fit the measured deflections better than or similar to those by frame analysis, especially for a case of clayey soils.

This paper focuses on the bearing capacity of soil plugs (internal shaft resistance) through fundamental research on the bearing mechanism of dry silica sand plugs. Push-up load tests on the dry silica sand plugs inside a model pipe pile and DEM simulations were carried out to investigate the plugging behaviour. The influences of the packing state of the soil plugs (the relative density), and the height of the plugs on the bearing capacity were investigated. Prior to the push-up load tests, element tests on the silica sand and DEM analyses were performed to characterise the silica sand and to determine suitable DEM analysis parameters. The experimental and DEM results clearly show that the push-up force increases significantly with the increase in the aspect ratio of the soil plug, H/D, and with the relative density of the soil plug. The DEM analyses show a good agreement with the experimental results when the push-up force is small. Furthermore, the DEM results reveal that only the density of the soil plug in the lower portion, adjacent to the pile tip, increases gradually with the increase in the push-up displacement as well as the increase in H/D. Hence, it is the lower portion of the soil plug that mainly controls the capacity of the soil plug.

An approximate method of analysis has been developed to estimate the settlement and load distribution of large piled raft foundations. In the method the raft is modelled as a thin plate, and the piles and the soil are treated as interactive springs. Both the resistances of the piles as well as the raft base are incorporated into the model. Pile-soil-pile interaction, pile-soil-raft interaction and raft-soil-raft interaction are taken into account based on Mindlin's solutions. The proposed method makes it possible to solve problems of large non-uniformly arranged piled rafts in a time-saving way using a PC. The method can also be used for the deformation analysis of pile groups by setting the soil resistance at the raft base equal to zero. The validity of the proposed method is verified through comparisons with existing solutions. Two case studies on settlement analyses of a free-standing pile group and a large piled raft are presented. In the analyses, applicability of the equivalent pier concept is also examined and discussed. The computed settlements compare favourably with the field measurements.

A series of experimental and analytical studies on the behaviours of model pile groups and model piled rafts in dry sand subjected to static vertical loading and static cyclic horizontal loading were carried out in order to investigate the influence of various pile head connection conditions between the raft and the piles on the behaviours of the foundations models and to examine the applicability of an simplified analytical method to simulate the load tests. In the load tests, the behaviours of the model foundations were investigated in detail, with particular focus on cyclic horizontal loading, and behaviour such as horizontal stiffness and the rotation of the foundation, the load proportions between the raft and the piles, and the bending moments and shear forces generated in the piles. A simplified three-dimensional deformation analysis method was used to simulate the experiments.

This paper reports the results of confirmation tests aiming at understanding the strength characteristics of ground improved by fracture grouting. It was found that the improvement effect of the fracture grouting could be obtained with less quantity of grout materials as compared to the conventional grouting method, although the test cases are limited. In addition, the paper describes the applicability of the grouting method to housing sites.

杭の動的な鉛直載荷試験法は地盤工学会において基準化されているが，動的な水平載荷試験法は現在のところ基準化に至っていない。杭の動的な載荷試験法は，静的な載荷試験法に比べて低コストでかつ工期が短く済み，試験を容易に実施できる利点がある。そこで筆者らは鋼管杭を対象に動的水平載荷試験シグナルの波形マッチング解析を実施し，杭の静的な水平荷重－水平変位関係を推定する手法を開発してきた。この手法の妥当性および適用範囲を検証するために，筆者らは主に実際に橋脚を支持する用途に供する実大鋼管杭を用いて，静的および動的水平載荷試験を継続的に実施してきた。測定した動的試験シグナルに対する波形マッチング解析を行うことにより地盤物性値を同定し，この地盤物性値を用いて静的水平載荷試験における杭の荷重－変位量関係等をシミュレートしたところ，今後まだ改善の余地が残されているものの実用的な範囲で試験結果をよく再現できた。

Analyses of a single pile, a pile group and a piled raft in a homogenous elastic ground subjected to earthquake are carried out using FEM and a simplified analysis method, In the FEM analyses, the raft and the piles are modelled by solid elements, or the raft is modelled by the thin plate elements and the piles are modelled by beam elements. As a simplified analysis method, a three-dimensional dynamic analysis of piled raft foundations subjected to earthquake using a hybrid model is proposed in this paper. In the analysis, the flexible raft and the pile are modelled as thin plates and beam elements, respectively. The soil is treated as springs and dashpots. The results from different modelling are compared, in order to investigate adequate modelling of raft, pile and ground and to investigate dynamic responses of the pile foundations. It is shown from the analyses that horizontal displacements and horizontal accelerations are not influenced by methods of modelling. In contrast, vertical responses such as vertical displacements hence the rocking motion of the pile, axial forces of the pile are influenced by methods of modelling, although the vertical responses are much smaller than the horizontal responses. Details of the results of the analyses are presented, and suggestions for the improvement of the simplified analysis method are discussed.

The Jinnosuke-dani landslide is a giant landslide 2,000 m long and 500 m wide in the Haku-san Mountain area, Japan. It was also the first landslide to be designated as a “Landslide Prevention Area” according to the “Japan Landslide Prevention Law.” This landslide consists of alternating layers of sandstone and shale in the Tedori Formation, which was deposited from the Jurassic period to the Early Cretaceous. Based on deformation monitoring results for more than 7 years, the landslide is divided into upper and lower blocks. The upper block has moved at a speed of 80 to 170 mm/year, while the lower block has moved more slowly (3 to 15 mm/year). Monitoring data show that the variation of the groundwater level has a great influence on the landslide movement. The deteriorating effect of the weathering of the alternating layers of sandstone and shale on the landslide deformation has been confirmed by borehole exploration and monitoring.

A simplified method of numerical analysis has been developed to estimate the deformation and load distribution of piled raft foundations subjected to ground movements induced by tunnelling and incorporated into a computer program ‘PRAB’. In this method, a hybrid model is employed in which the flexible raft is modelled as thin plates, the piles as elastic beams, and the soil is treated as interactive springs. The interactions between structural members, pile–soil–pile, pile–soil–raft and raft–soil–raft interactions, are modelled based on Mindlin's solutions for both vertical and lateral forces. The validity of the proposed method is verified through comparisons with some published solutions for single piles and pile groups subjected to ground movements induced by tunnelling. Thereafter, the solutions from this approach for the analysis of a pile group and a piled raft subjected to ground movements induced by tunnelling are compared with those from three-dimensional finite difference program. Good agreements between these solutions are demonstrated. The method is then used for a parametric study of single piles, pile groups and piled rafts subjected to ground movements induced by tunnelling. Copyright © 2005 John Wiley & Sons, Ltd.

2002年11月8日に金沢市山科町で長雨を誘因とする流動性地すべりが発生した。発生直後の地すべり土塊は完全に飽和されており.竹林の分布している範囲では地すべり土塊が厚く堆積していた。地すべり地は第三紀泥岩層 (犀川層) からなり.現地調査より泥岩は下位から新鮮な泥岩, 亀裂の発達した風化泥岩, 粘土化した強風化泥岩の三層に分類され, 風化泥岩と新鮮部との境界で地すべりが発生したことがわかった。また, 運動機構を明らかにするために行った運動シミュレーションより, すべり面の定常状態でのせん断抵抗が強風化泥岩層では10kPa, 竹林部では50kPa (竹林の土塊のせん断抵抗値ではなく, 地すべり土塊中に含まれる竹の地すべり運動による衝突が地すべりを減速させる効果を表現した値) であることがわかり, 竹林が分布している範囲に, 地すべり運動を減速させる何らかの要因があった。山科地すべりの流動化した原因の一つとして, 長雨によって飽和された低透水性の強風化泥岩が, 地すべり土塊の衝撃載荷により, 非排水せん断され, そのせん断抵抗が低くなり流動化したと考えられる。

A series of static horizontal load tests were carried out on model piled rafts at 1-g gravitational field by using dry Toyoura sand. In order to examine the effects of confined stress on soil modulus and confirm the similitude for the present 1-g models, triaxial consolidated drained shear tests of the sand were conducted. Two experimental variables were considered, i.e. the height of the acting force, and the rigidity of the pile head connection. Also presented in this paper are the results of the simplified three-dimensional deformation analyses of the model tests, which shows the reasonable agreement with the experimental results in the initial elastic region. From the model tests and the analyses, it is shown that horizontal displacement and inclination of the raft, and bending moments of piles for a given horizontal load are increased as the height of horizontal loading point (i.e. moment load) is increased. It is also shown that horizontal displacement and inclination of the raft and bending moments of piles are reduced in the piled raft with hinged pile head connection compared to the piled raft with rigid pile head connection when subjected to horizontal load and the associated moment load.

A series of shaking table tests were carried out on model piled rafts in dry Toyoura sand using a shaking table at 1-g field. Two experimental variables were considered, i.e. frequency of input acceleration and height of the centre of gravity of superstructure, while keeping the mass of superstructure constant. This paper provides experimental results of piled rafts having a rigid connection condition between the raft and the pile heads, and compares them with the results obtained from the static horizontal load tests of the same models, which have been carried out by Matsumoto et al. (2004). Principal findings from the present study are 1) Resonant frequency of superstructure decreases as height of its centre of gravity increases, as expected; 2) At input frequency lower than the resonant frequency, the behaviours of the piled rafts subjected to shaking are close to those obtained from static horizontal loading tests; 3) Even if the horizontal acceleration responses at the centres of gravity are the same, inclination of the raft, shear forces and bending moments of the piles increase as height of the centre of gravity of the superstructure increases.

This paper employs a simplified analytical method to investigate the influence of reaction piles on the load–displacement behaviour of a test pile in static load testing. A parametric study is conducted to examine the effects of factors such as pile spacing ratio, pile slenderness ratio, and pile soil stiffness ratio. Several soil profiles are considered in the study. The parametric study also includes the cases of static load testing in the lateral direction. The correction factors for the initial pile head stiffness obtained from static pile load tests with the use of reaction piles are given in charts. Furthermore, analyses of centrifuge modelling of axially loaded piles are carried out using the simplified analytical method. Good agreements between the test results and the analysis results are demonstrated, and the applicability of the correction factors is verified.Key words: reaction piles, interaction, initial pile head stiffness, static vertical load testing, static lateral load testing, simplified analytical method.

A series of shaking table tests were conducted on piled raft foundations on sand by using a geotechnical centrifuge. The results were compared with those obtained from the static horizontal loading tests of the same models which have been reported by Horikoshi et al. (2003). Effects of the rigidity at pile head connection on the dynamic behavior were examined. Also shown are the results of the dynamic loading test of a free-standing pile group with the same number of piles to examine the contribution of the load transferred through the raft base. Principal findings from the present study are: 1) The horizontal load-displacement relationships and the bending moment distributions agree well between the dynamic models and the static models. The proportion of the horizontal load carried by each component is highly dependent on the horizontal displacement of the piled raft system; 2) The horizontal stiffness of the piled raft with the hinged pile head connection was smaller than that of the piled raft with the rigid pile head connection; 3) The proportion of the vertical load carried by the piles remains largely unchanged during dynamic loading, while the proportion of horizontal load carried by the piles increases as the horizontal displacement increases; 4) The inclination of the piled raft during shaking is much smaller than that of the pile group due to the contribution of the soil resistance just beneath the raft.

A series of static loading tests were conducted vertically and horizontally on piled raft models and their components (single piles and rafts alone) on sand by using a geotechnical centrifuge. Much focus was placed on the load-displacement relationship and the load sharing between the piles and the raft in the piled raft system. Effects of the rigidity at pile head connection on the piled raft behavior were also examined. This paper provides basic information on the performance of piled raft foundations subjected to horizontal loads. Principal findings from the present study are: 1) The stiffness and the resistance of the single pile in piled raft foundations are different from those observed in the isolated single piles of the same size, due to the difference in the confining stress condition around the piles; 2) Piles play important roles in increasing horizontal ultimate resistance of piled raft foundations; 3) The initial horizontal stiffness of a piled raft is not always higher than that of a raft (alone) as the piles reduce the contact pressure between raft and soil, and hence the stiffness of the upper soils; 4) Higher horizontal load is transferred to the piles in the piled raft with rigid pile head connection, which leads to higher initial horizontal stiffness compared with that in the piled raft with hinged pile head connection; 5) The proportion of vertical load carried by the piles in a piled raft remains largely unchanged during horizontal loading, while the proportion of horizontal load carried by the piles increases as the horizontal displacement increases.

A simplified method of numerical analysis based on elasticity theory has been developed for the analysis of axially and laterally loaded piled raft foundations embedded in non-homogeneous soils and incorporated into a computer program “PRAB”. In this method, a hybrid model is employed in which the flexible raft is modelled as thin plates and the piles as elastic beams and the soil is treated as springs. The interactions between structural members, pile–soil–pile, pile–soil–raft and raft–soil–raft interactions, are approximated based on Mindlin's solutions for both vertical and lateral forces with consideration of non-homogeneous soils. The validity of the proposed method is verified through comparisons with some published solutions for single piles, pile groups and capped pile groups in non-homogeneous soils. Thereafter, the solutions from this approach for the analysis of axially and laterally loaded 4-pile pile groups and 4-pile piled rafts embedded in finite homogeneous and non-homogeneous soil layers are compared with those from three-dimensional finite element analysis. Good agreement between the present approach and the more rigorous finite element approach is demonstrated. Copyright © 2002 John Wiley & Sons, Ltd.

A simplified method of numerical analysis has been developed to estimate the deformation and load distribution of piled raft foundations subjected to vertical, lateral, and moment loads, using a hybrid model in which the flexible raft is modelled as thin plates and the piles as elastic beams and the soil is treated as springs. Both the vertical and lateral resistances of the piles as well as the raft base are incorporated into the model. Pile–soil–pile, pile–soil–raft and raft–soil–raft interactions are taken into account based on Mindlin's solutions for both vertical and lateral forces. The validity of the proposed method is verified through comparisons with several existing methods for single piles, pile groups and piled rafts. Workable design charts are given for the estimation of the lateral displacement and the load distribution of piled rafts from the stiffnesses of the raft alone and the pile group alone. Additionally, parametric studies were carried out concerning batter pile foundations. It was found that the use of batter piles can efficiently improve the deformation characteristics of pile foundations subjected to lateral loads. Copyright © 2002 John Wiley & Sons, Ltd.