Outer Piles Research Articles

Local scour effects on seismic behavior of pile-group foundations in cohesionless soils: Insights from quasi-static tests

Local scour has been reported as a common phenomenon for pile-group foundations in marine, coastal, and riverine sites, while its effects on the seismic behavior of pile-group foundations are yet to be well documented. This paper reported a pair of quasi-static cyclic loading tests on 2 × 3 scoured pile-group foundation specimens, one in global scour and the other in local scour, to understand the influence of local scour on the seismic behavior, particularly in terms of soil-pile interaction features and failure mechanisms. Test results indicate a flexural failure mode for both specimens. Local scour does not change the order of limit states but postpones the occurrence of concrete cover cracking and rebar yielding due to the reduced lateral stiffness of the locally scoured piles. Moreover, local scour rarely changes the aboveground damage regions at the top of outer piles (one time the side length of squared pile section, D), but significantly aggravates and deepens the underground damage regions at outer piles (from 4 ∼ 5D for global scour to 3.7 ∼ 6D for local scour). Besides, local scour reduces the displacement ductility factor from 2.50 to 1.25 for the Easy-to-Repair limit state, resulting in adverse impacts on pile-group foundations in general.

Study on the Stability of Accumulation Using a Slope Shaking Table Test during Earthquake Action

To examine the effects of different peak accelerations on the stability of the accumulation slope and the effectiveness of anti-slide piles under seismic loads, this paper used the Fanlingqian landslide as the main research object and combined it with digital image correlation (DIC) technology in order to carry out a shaking table test. Then, the acceleration response, displacement field, strain field, the bending moment distribution of the 0.05–0.3 g ground motion accumulation slope, and the anti-slide pile reinforcement were studied. The results of the test show the following: the amplification coefficient of the measuring points A1–A6 of the accumulation slope reaches the maximum at a peak acceleration of 0.2 g, and its values are between 1.25 and 1.3, respectively. Finally, it shows a decreasing trend at a peak acceleration of 0.3 g, and its corresponding values are, respectively, between 1.1 and 1.2. In the anti-slip pile reinforcement test, due to the obstruction of the anti-slip pile, the damping of the soil around the pile increases. As the peak value of the seismic wave input increases, the amplification factor shows an overall decreasing trend. A1–A6 correspond to a peak acceleration of 0.3 g. The amplification factors are all close to 1. During different peak accelerations, the accumulation slope mainly experienced the earthquake-induced stage, tensile failure stage, creeping deformation stage, and overall instability stage. In the anti-slide pile reinforcement test, under the same conditions, the slope mainly experienced the earthquake-induced stage, tensile failure stage, lower sliding surface formation stage, and soil shedding stage in front of the pile. At the same time, the displacement and strain fields of each stage of the two groups of tests are compared, and it is found that the displacement and strain values of the accumulation slope test are greater than those of the anti-slide pile reinforcement test, and the horizontal displacement difference at the top of the slope is the most significant, reaching 2.3 times at the maximum. The bending moment of the anti-slide pile first increases and then decreases with the increase in acceleration, the reverse bending point of the pile appears at 5 times the pile diameter below the soil surface, and the maximum bending moment of the middle pile, corresponding to a peak acceleration of 0.05–0.3 g, is between 7.5 N·m and 47 N·m, respectively, while the maximum bending moment of the outer pile is between 6.5 N·m and 52 N·m, respectively. It is important to apply DIC image processing technology to the monitoring of landslide structure and the evaluation of slope stability in practical engineering.

Use of digital correlation techniques for investigating behaviour of geothermal driven piles

Geothermal energy has established its huge potential as renewable source of energy for general building establishments. A key feature here is the use of energy piles which serve as major energy exchange system which is sustainable and low-cost. Energy piles are basically driven precast piles employed to provide structural support to structures and act as heat exchange units to provide heating / cooling to buildings. The behaviour of energy piles is influenced by temperature, physical and engineering properties of soil. However, uncertainty still exists at pile soil interface irrespective of its heating or cooling conditions. The present study examines the suitability of using digital correlation technique (DIC) namely GeoPIV-RG software to study the soil deformation at pile soil interface in sandy soil. A rectangular test tank made of steel with a size of 60 × 20 × 45 cm is used for experimental studies. Hollow half section aluminum piles closed at the ends with two different pile length to outer pile diameter ratios viz., 10 and 15 of same flexural rigidity are used for the experimental studies. The piles are subjected to sustained loading simulated through Universal testing machine and movements of soil around the pile are captured using a digital camera. The image analysis software named GeoPIV-RG is used to determine the shear strains and soil displacements. Analysis of results indicated that this technique could capture deformation characteristics and surface heave formation in the surrounding infill medium.

Experimental Study on Bending Moment of Double‐Row Steel Pipe Piles in Foundation Excavation

Double‐row steel pipe piles have been widely used in retaining and protection of foundation excavation because of the advantages of high bearing capacity, high flexural rigidity, fast construction speed, and so on. This study presents a field test to assess the feasibility of strain gauges in monitoring the strain of double‐row steel pipe piles during foundation excavation. Two steel pipe piles were instrumented with strain gauges and then installed into the drilling holes. The installation method of strain gauges is introduced first. Then, the bending moment of the test piles during the foundation excavation was analyzed. The field test results indicate that the survival rate of strain gauges was 100%, and the monitoring method used in the test was feasible to measure the bending moment of double‐row steel pipe piles. Moreover, with the increase in foundation excavation depth, the bending moment of the test piles all increased, and the bending moment of the inner pile was obviously higher than that of the outer pile. The bending moment distribution of the whole support system accords with the conventional pile‐anchor mode. The test results can provide reference and basis for the design and construction of double‐row steel pipe piles.

Nonlinear Response of Piled Gravity Base Foundations Subjected to Combined Loading

This study investigates the soil-structure interaction of a piled Gravity Base Foundation (piled GBF) supporting an offshore wind turbine tower. The piled GBF is a gravity base foundation supported by five piles with one center and four outer piles. A series of three-dimensional finite element analyses were performed to investigate p-y characteristics in the piled GBF. The results were validated against centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of foundation type, vertical load due to the gravity base self-weight, loading height, undrained shear strength, and the pile bearing condition. Overall, the center pile of the piled GBF showed the highest ultimate soil reaction force, while the leading pile indicated the lowest value. Due to the rotation of mat and self-weight of GBF, significant shear failure was mobilized near the leading pile, which reduced the ultimate soil reaction force. This was also confirmed through the comparison studies with group pile (no mat), piled raft (low self-weight of mat) and rock-socketed piled GBF (no rotation of mat).

Soil-pile-structure kinematic and inertial interaction observed in geotechnical centrifuge experiments

This paper reports the results of a series of centrifuge tests examining dynamic responses of single and 3×3 grouped piles embedded in sand and supporting SDOF and 2DOF structures. A total of 7 model tests were conducted with the centrifugal acceleration of 40g. Each model was subjected to 12 sinusoidal waves with constant acceleration amplitude and varying frequencies. The results of the tests indicate that pile-head motion is dominated by two sequential frequencies: a lower frequency (fSSI) where pile-head motion is substantially maximized and a higher one (fpSSI) where the response is minimized with respect to free field surface motion. These results confirm recent published numerical results on single piles supporting SDOF structures and generalize their findings to grouped piles supporting SDOF and 2DOF structures. The results show also strong mobilized kinematic interaction effect generating significant pile bending when the ground is excited at its resonant frequency. On the other hand, structural vibrations tend to impose large bending moments as the excitation frequency approaches the natural frequency of the coupled soil-pile-structure system. Distribution of pile bending moments in the group is found to be a function of the pile position and the excitation frequency. In contrast to inner piles having the greatest kinematic bending moments, outer piles have a more pronounced inertial ones.

Effect of crack opening and recovery on chloride penetration into reinforced concrete hollow piles

This study explores the chloride penetration characteristics of reinforced concrete (RC) hollow piles. Special splitting tests were designed to produce different crack widths in the inner and outer surfaces of RC hollow piles. The cracked specimens were then immersed in 5 % NaCl solution for 180 days. The chloride concentration profiles in the cracked zones were obtained by potentiometric titration. This study indicates that the rate of chloride accumulation in the inner pile surface is faster than that in the outer surface for the same exposure time. This may be attributed to differences in the material compositions of the inner and outer surfaces, which are caused by centrifugal processing during specimen preparation. The critical value for V-shaped cracks on the outer surface of RC hollow piles was found to be around 70 µm; however, a critical value was absent for cracks on the inner surface because these present parallel-wall characteristic. In addition, the characteristics of crack development and recovery in the inner and outer pile surfaces were evaluated. The analysis indicates that the depth at which chloride affects the concrete significantly increases with the crack-recovery rate for the apparent crack widths around 130 µm on the outer surface of the concrete layer.

Quasi-Static Cyclic Testing of Elevated RC Pile-Cap Foundation for Bridge Structures

In current practices, the elastic design principle for the seismic design of pile foundations is an overly conservative strategy. In this regard, cyclic lateral loading tests were conducted to investigate the seismic failure mechanism and ductile behaviors of elevated RC pile-cap foundations. Two six-pile specimens with different aboveground heights were constructed and embedded in a single layer of cohesionless soil. Hysteretic characteristics and observed damages of the pile groups were presented, together with the corresponding soil pressure and pile curvature responses. From the test results, the sequence and positions of the pile plastic hinges were established. Ultimate limit states of the pile groups were determined according to test data, as well as numerical analysis results. Test results revealed a ductile flexural failure mode for both specimens. Plastic hinges were detected first at the top of outer piles, then underground at a depth of 4–6 times the section width. A displacement ductility capacity of 3.5 was observed for the elevated pile-cap foundation. Test results also indicated a linear relation between the displacement ductility factor and the cap rotation, which was important for determining the displacement at the top of piers.

Reply to the discussion by Fellenius on “Plugging effect of open-ended piles in sandy soil”

The authors would like to thank Bengt H. Fellenius for his discussion of our paper (Fellenius 2015). The Discussion incorrectly states that no such fixed connection was made at the pile head. The gap at the pile toe was welded to prevent any intrusion of soil during the tests, and the gap at the pile head was also welded to fix the two pipe piles. The fixed connection between the two piles ensured that the two piles moved in unison. As stated by the discusser, dynamic test gauges were installed on the outer piles, and the cross-sectional area for dynamic test analysis applied to the sum of the two pile annulus areas. Due to the setup effect, the ultimate bearing load derived from static load test is generally larger than the one from the “end of initial driving” (EOID). However, in this study, geometric characteristics of the double-walled pile system and the assumed sum of the two pile annulus areas allowed the ultimate bearing capacity of EOID to be greater than the single pipe pile (not double-walled pile). This value is approximately the same with the static load test. Figure 2 in Ko and Jeong (2015) represents the general bearing capacity mechanism for open-ended piles. This figure is not illustrating the force direction of the core, but rather the shaft resistance acting between the core and inside of the pile. Nevertheless, inner shaft resistance, Q in, is the frictional force generated between the core and inside of the pile that has displayed upward direction. The values of the outer shaft resistance, Q out, also signify that the frictional force generated between the soil and the outside of the pile are all on the outside of the pile in an upward direction. As mentioned in Ko and Jeong (2015), Q plug is the smaller value of either the inner shaft resistance, Q in, or the bearing capacity of the soil beneath the plug base, Q base. Thus, if Q in is greater than Q b, the pile will fail in the pluggedmode. In pile driving, the inner shaft resistance, Rin, can exceed the end-bearing resistance of the soil beneath the plug, Rb, because the inertia force of the soil plug is mobilized. As the discusser states, it has been noted that the core is compressed as spring action. It is similar to the concept of the soil plugging index (SPI) in this study. In Ko and Jeong (2015), the authors state that the inner shaft resistance was mostly mobilized at the location between the pile tip and 18%–34% of the total plug length. As shown in fig. D2 in Fellenius (2015), there is misleading information.When the analysis was rerun, the increase of axial load for outer pile TP-2 was extrapolated, and is estimated to be 120 kN not 80 kN. Figure R1 shows the load–movement curves resulting from applying the data from fig. 12 in Ko and Jeong (2015). The measured data of the inner pile at the first gauge level is not 610 kN, but is 814 kN due to an error that is illustrated in fig. D2 by the discusser. Thus, the difference between the “head” and “outer plus inner piles” is not approximately 400 kN, but about 200 kN as shown in Fig. R1. In addition, the axial load at the inner pile head does not change, and the axial load at the outer pile head is predicted to increase to a value of 120 kN by extrapolating the axial load at the pile head. Therefore, it is assumed that the axial load at the inner pile head is 814 kN and is 1100 kN at the outer pile head in the final loading stage. Figure R2 shows the axial load distribution of the outer pile of pile 2 as determined by extrapolation. Figure R3 shows the load–movement curves determined by extrapolation. The difference between the “head” and “outer plus inner piles” is about 75 kN. When comparing this with the ultimate bearing capacity, the difference between the two is within the range 0%–10%. This difference may have occurred due to the double-walled system. As mentioned by the discusser, the difference is determined to occur when using the welding method rather than the sealing method. However, in this study, the weights of test piles TP-1, TP-2, and TP-3, were 1.71, 2.91, and 5.24 t, respectively. These are heavier than the test piles studied in the literature. Thus, the welding rather than sealing method was chosen, as a solution to the problem of heavy pile weight.

Centrifuge Model Study of High Strength Piles Composite Foundation Settlement & Instability under Embankment in Different Kind Soils by Different Pile Spacing

A series of centrifuge model tests has been conduced to examine the behavior of high strength piles composite foundation stability under railway embankment in silt or clay or sand different kind of soils. The high strength composite foundation has a symmetrical plan layout consisting of 3×4, 4×6, and 5×8 piles with a center-to-center spacing of 6 or 4 or 3 times pile width. The piles are under the same height of railway embankment, with the same length, and on the same lying soil layer. The high strength composite foundation stability test results are expressed in terms of soils kinds-pile displacement response of the composite foundation, embankment load experienced by soil between piles and piles in the composite foundation, and bending moment profile along individual pile. It is established that the high strength composite foundation stability efficiency reduces significantly with decreasing of the strength of soil between piles in composite foundation. The tests reveal the shadowing effect phenomenon in which the marginal piles experienced larger laterally and bending moment than that of the central piles. The shadowing effect is most significant for the lead row piles and considerably less significant for subsequent rows of middle piles. The approach adopted by many researchers of taking the average performance of piles in the same row is found to be inappropriate for the middle rows, of piles for composite foundation as the outer piles in the row carrying significantly more laterally load and experience considerably higher bending moment than those of the inner piles.

Weave and Process Design of Monolayer Two-Sided Velvet Fabric

Weft pulling weave structure on double segmentation method of two-sided velvet fabric is studied based on single-shuttle loom. Through reasonable weave design, raw materials of warp and weft yarn selection and finishing process, adopting inner pile formed technology of double-cutting process and outer pile formed technology of weft pulling, the monolayer two-sided velvet fabric is developed. That product with comfortable and novel style will broad the application of silk velvet fabric on home textile and clothing and provide certain guiding significant on velvet product development.

Numerical modelling of perimeter pile groups in clay

The load distribution among piles in a group varies such that the inner piles often carry a smaller share of the total load compared to the outer piles, which is a result of increased soil–pile interaction. The main objective of this paper is to establish the relative effectiveness of pile groups with no inner piles (perimeter group), when compared to the more common grid configuration. The numerical investigation utilized the finite element programme ABAQUS and considered a range of variables that affect pile group behaviour including number of piles, pile spacing, length/diameter ratio, and soil strength. It was demonstrated that a complete grid group is less efficient than a perimeter group, where efficiency is defined as the load capacity of the whole group expressed as a ratio of the number of piles in the group multiplied by the load capacity of a single isolated pile. Efficiencies close to unity were observed for some perimeter groups. Perimeter groups also showed that a “block” type group failure could occur, where piles were placed at a spacing of less than 2.0 pile diameters,d, centre-to-centre. This often, but not always, led to a reduction in the efficiency of the pile group.

원심모형 실험을 이용한 건조토 지반에서의 군말뚝 효과 분석

본 연구에서는 조밀한 건조 사질토 지반에서 단말뚝 및 <TEX>$3{\times}3$</TEX> 군말뚝에 대해 정현파를 이용하여 동적 원심모형실험을 수행하였으며, 군말뚝의 경우 말뚝 중심간격을 지름의 3배, 5배, 7배로 변화시켜 실험을 실시하였다. 실험 결과로 얻은 단말뚝과 군말뚝의 동적 p-y 곡선들을 비교하여, 말뚝 중심 간격 및 군말뚝 말뚝 위치에 따른 말뚝의 동적 군말뚝 효과를 분석하였다. 분석 결과, 첫 번째 열과 세 번째 열의 말뚝인 측면 말뚝에서 유사한 동적 p-y 거동이 나타났으며, 두 번째 열 말뚝들은 측면 말뚝들에 비해 지반반력 감소 효과가 작게 나타났다. 또한, 두 번째 열 내에서 말뚝 위치에 따른 동적 p-y 거동을 비교한 결과, 두 번째 열의 바깥 말뚝에서 중앙 말뚝보다 지반반력 감소 효과가 크게 나타났다. 실험 결과를 바탕으로 제안된 말뚝 중심 간격에 따른 p-승수 값은 측면 말뚝에서 0.28 ~ 0.77, 중앙 말뚝에서 0.55 ~ 1.0, 바깥 말뚝에서 0.39 ~ 0.87로 나타났다. In this study, a series of centrifuge shaking-table tests for a <TEX>$3{\times}3$</TEX> group pile and a single pile applied by sinusoidal wave was performed in dry sand for various pile spacings, ranging from three to seven times the pile diameter. A comparison of centrifuge tests of both single pile and group pile showed that the lateral ground response of the group pile was smaller than that of the single pile. In addition, the reduction in subgrade reaction for the group pile increased with decreasing pile spacing. The side piles, that is, the 1st row and 3rd row piles showed identical dynamic p-y behavior and the center pile in the 2nd row caused a lower reduction effect compared with the 1st and 3rd row piles. From the comparison between the p-y curves of the 2nd row piles, it was found that the lateral ground response of the outer pile in the 2nd row was less than that of the center pile in the 2nd row. The p-multipliers for the side piles, for the center pile and for the outer pile ranged from 0.28 to 0.77, from 0.55 to 1.0 and from 0.39 to 0.87, respectively.

The influence of soil slip on negative skin friction in pile groups connected to a cap

INTRODUCTION There have been several investigations into the behaviour of isolated piles in groups subject to negative skin friction (NSF), from experimental observations and theoretical studies (Shibata et al., 1982; Jeong et al., 2004; Lee & Ng, 2004). However, to date, the behaviour of piles in groups connected to a cap in consolidating soil has attracted less attention among engineers than the study of isolated piles in groups. There is only a limited number of relevant case histories on the behaviour of piles connected to a cap (Chellis, 1961; Garlanger & Lambe, 1973). These authors have reported the development of tensile forces in the outer piles, leading to pulling of the outer piles from the pile cap in some cases. Kuwabara & Poulos (1989) and Chow et al. (1996) reported distributions of axial forces along piles obtained from elastic solutions using Mindlin’s elastic theory. They showed the development of tensile forces in outer piles near the pile head. It is well known, however, that by ignoring soil slip at the pile/soil interface the elastic solution resulted in an unrealistic estimation of pile behaviour under the influence of NSF (Jeong et al., 2004; Lee & Ng, 2004). Therefore, in this study, a series of three-dimensional (3D) numerical analyses, including soil slip, was conducted to clarify the behaviour of piles in groups connected to a cap.

Development of Downdrag on Piles and Pile Groups in Consolidating Soil

Development of pile settlement (downdrag) of piles constructed in consolidating soil may lead to serious pile foundation design problems. The investigation of downdrag has attracted far less attention than the study of dragload over the years. In this paper, several series of two-dimensional axisymmetric and three-dimensional numerical parametric analyses were conducted to study the behavior of single piles and piles in 3×3 and 5×5 pile groups in consolidating soil. Both elastic no-slip and elasto-plastic slip at the pile–soil interface were considered. For a single pile, the downdrag computed from the no-slip elastic analysis and from the analytical elastic solution was about 8–14 times larger than that computed from the elasto-plastic slip analysis. The softer the consolidating clay, the greater the difference between the no-slip elastic and the elasto-plastic slip analyses. For the 5×5 pile group at 2.5 diameter spacing, the maximum downdrag of the center, inner, and corner piles was, respectively, 63, 68, and 79% of the maximum downdrag of the single pile. The reduction of downdrag inside the pile group is attributed to the shielding effects on the inner piles by the outer piles. The relative reduction in downdrag (Wr) in the 5×5 pile group increases with an increase in the relative bearing stiffness ratio (Eb/Ec), depending on the pile location in the group. Compared with the relative reduction in dragload (Pr),Wr at the corner pile is less affected by the group interaction for a given surcharge load. This suggests that the use of sacrificing piles outside the pile group will be more effective on Pr than on Wr. Based on the three cases studied, the larger the number of piles in a group, the greater the shielding effects on Wr. Relatively speaking, Wr is more sensitive to the total number of piles than to the pile spacing within a pile group.

Centrifuge Model Study of Laterally Loaded Pile Groups in Clay

A series of centrifuge model tests has been conducted to examine the behavior of laterally loaded pile groups in normally consolidated and overconsolidated kaolin clay. The pile groups have a symmetrical plan layout consisting of 2, 2 32, 233, 333, and 434 piles with a center-to-center spacing of three or five times the pile width. The piles are connected by a solid aluminum pile cap placed just above the ground level. The pile load test results are expressed in terms of lateral load-pile head displacement response of the pile group, load experienced by individual piles in the group, and bending moment profile along individual pile shafts. It is established that the pile group efficiency reduces significantly with increasing number of piles in a group. The tests also reveal the shadowing effect phenomenon in which the front piles experience larger load and bending moment than that of the trailing piles. The shadowing effect is most significant for the lead row piles and considerably less significant for subsequent rows of trailing piles. The approach adopted by many researchers of taking the average performance of piles in the same row is found to be inappropriate for the middle rows, of piles for large pile groups as the outer piles in the row carry significantly more load and experience considerably higher bending moment than those of the inner piles.

外管杭を有する耐震杭の水平挙動 : その2 PHC杭と鋼管杭から構成される耐震杭の水平耐力

This paper continues from the previous thesis (Part 1). It presented experimental result of model earthquake-proof piles guarded with short outer sheath pile and also a calculation method for the pile system of this type. In this study, we calculate the horizontal behavior of earthquake-proof pile composed with a PHC-pile (main pile) and a steel pile (outer pile). It takes into account the non-linear property of bending rigidity of both piles subjected to the vertical load in an earthquake. From these results, it was found that the seismic horizontal capacity of the earthquake-proof pile in both short time and ultimate state could increase remarkably in comparison with an isolated main pile, even if the outer sheath pile length is set up comparatively short.

Analysis of Scour around a Group of Vertical Piles in the Field

A study of the scour around a group of piles in the field exposed to oscillatory waves was carried out. The main purpose was to derive simple predictive formulas for the geometric properties of the scour hole in terms of nondimensional parameters describing the flow, sediment, and pile characteristics. A comprehensive data set from the Pacific coast of Japan was employed where the beach profile along a pier experiencing scour around its piles was surveyed every week. Detailed wave measurements were available, and only the outer piles exposed to nonbreaking (oscillatory) waves were investigated. The maximum scour depth showed a distinct correlation with the Keulegan-Carpenter number, and an empirical relationship was derived that agreed with some earlier laboratory experiments, if an equivalent diameter was defined for the pile groups. The width of the scour hole displayed little correlation with the Keulegan-Carpenter or other nondimensional numbers employed, although it should be noted that only one pile configuration was investigated in the present study.

Downdrag Forces in Group of Piles

This paper describes a boundary element analysis for analyzing the behavior of a group of end‐bearing piles subjected to negative friction. It is found that group effects lead to a reduction in downdrag force in a pile, as compared with a single isolated pile, and that inner piles in a group develop less downdrag force than the outer piles. Among the factors influencing the distribution of downdrag force in the group are the number of piles, the relative stiffness of the piles, the distribution of soil modulus with depth, and the presence of pile‐soil slip. A significantly larger soil surface settlement is required to mobilize full pile‐soil slip along all piles in the group than along a single pile. When applied to a published field test, the theory shows reasonable agreement with the measured behavior.

Finite element analysis of mechanisms of pile group behaviour

AbstractSolutions are presented for the non‐linear load‐settlement behaviour of square‐configuration pile groups. The groups are represented by an equivalent axially symmetric model, and a non‐linear finite element is used to examine the mechanisms of group behaviour and their variation with pile spacing. It is shown that, at close spacings, the block failure mechanism occurs, with significant plastic zones being developed below the group and full pile‐soil slip only being developed along the outer piles. As the pile spacing increases, the failure mechanism gradually changes to the ‘single‐pile’ mode, whereby full pile‐soil slip occurs along all piles. Within the limitations of accuracy of the finite element solution, the values of group settlement ratio and efficiency are in reasonable agreement with values derived from existing theories.