Investigation of monotonic behavior of different-pattern helical pile groups in sandy soils

The disc overhang diameter can significantly affect the uplift bearing capacity of new concrete expanded-plate pile groups, affecting their design and practical applications. Accordingly, this effect was investigated considering the failure laws of the soil surrounding various pile types and groups. Based on the uplift bearing capacities of single and double piles, a finite element simulation was adopted to establish models for the four-, six-, and nine-pile groups. The relationship between the disc overhang diameter and uplift-bearing capacity of each pile group was explored: as the disk overhang diameter increased, the uplift-bearing capacities of the pile groups increased; however, this relationship is nonlinear. The optimal disc overhang diameter was determined as 1.5–1.75 times the pile diameter. For a constant disc overhang diameter, corner piles have a greater uplift bearing capacity than side piles in the six-pile group, and a greater uplift bearing capacity than the side and center piles in the nine-pile group. Thus, the pile-group effect depends on the pile position. The uplift bearing capacity did not increase linearly with the number of piles, and the average uplift bearing capacity of a pile in a pile group was less than that of a single pile. Therefore, the uplift bearing capacity of the pile groups decreased as the number of piles increased. The reliability of the simulation was verified via visual testing of a small-scale half-cut pile model.

Laboratory model tests were carried out to study the behaviour of vertical piles and batter pile groups under vertical and lateral load. The model pile groups were made up of mild steel rod of 8-mm diameter. Parameters such as degree of batter and different length to diameter ratios of 7.50, 15.00 and 22.50 were considered in this study. The size of the model tank was 1000 mm × 1000mm × 800 mm. Experiments were performed on 3 × 3 model pile groups with a row of batter piles both positive and negative in addition to vertical pile groups with batter angle 0° in sandy soil subject to vertical and lateral loads. It was observed that the behaviour of vertical pile groups and group of piles with batter piles were similar but it showed substantial variation in the capacity of pile groups. Results indicated that the load–settlement relationships were non-linear for all model pile groups both under vertical and lateral loading. Numerical FEM analysis using ABAQUS/CAE 6.11 was also used to compare and validate the load carrying capacity of pile groups obtained from the experimental model tests.

In recent years, the use of helical piles as a deep foundation option for structures has increased dramatically because they offer definite advantages over other solutions. The present study undertook a comprehensive investigation of the axial compressive behavior of helical piles in both sandy and clayey types of soil using finite element (FE) modeling. Since the helices of helical piles are more likely to disturb the soil adjacent to the pile, first some approaches are proposed to consider the effect of soil disturbance during a helical pile installation in numerical models. The numerical models have been validated and calibrated through full-scale compressive loading results using the ABAQUS software. The validated numerical models were used to investigate the load transfer mechanism of a typical helical pile in different types of sandy and clayey soil, and the numerically obtained ultimate bearing capacities were compared with the theoretical ones. Based on the comparison, it was found that the theoretical procedure overpredicts the ultimate capacity of helical piles in medium and dense sand. Additionally, different geometrical aspects of a typical helical pile were investigated using FE modeling in order to employ perfect geometry for studying the compressive behavior of a helical pile group with a square grid arrangement. The results revealed that the shape of the block failure mechanism in clayey soil differs from that in sandy soil. The acquired shapes are used to suggest a theoretical method for calculating the ultimate bearing capacity of a helical pile group based on the block failure mechanism.

This paper is a numerical study based on a number of numerical model tests of pile groups, subjected to static lateral loading. Tests were conducted on single, (2x1), and (2x3) pile groups at three-diameter (3D) spacing. The model piles studied is concrete piles with 25 cm diameter and 10m length founded in loose sand (Dr=25%). The model is developed to study the effect of the number of piles in group on its lateral capacity on level ground and adjacent to a slope of inclination (3H:2V). The behavior of the piles was analyzed using a 3-D elasto-plastic finite element method (FEM) computer program Plaxis 3D ver. 2020, which represents a realistic model to simulate the problem. The response of the piles in the group was compared with the response of the single pile. It was found that the behavior of laterally loaded pile group depends largely on the arrangement of piles in group with respect to direction of load. In case of a slope it was found that the behavior of pile groups depends on the distance of lead row of pile groups from the slope crest (B/D) and arrangement of piles in group corresponding to load direction. &nbsp

The growing rate of occurrence of earthquakes in Iraq and the whole region has driven to conduct this experimental research aiming at evaluating the uplift capacity of piles under seismic influence. Shaking table was especially manufactured to simulate the seismic loading resulting from Halabjah earthquake with high accuracy. Pullout tests were carried out on piles embedded in dry sand to determine the ultimate uplift load capacity before and after applying the seismic loading for different length to diameter ratios (L/D) of pile model 20, 25 and 30 and for loose and dense dry sand. It has been found that the maximum pull-out load of the pile after applying only Halabjah seismic loading was reduced between (8.16%) and (10.46%) in loose dry sand. However, there is no clear effect on tension pile capacity in dense dry sand except in pile of L/D=30, that bearing capacity increased by about 15.94%. And when apply combined loading of Halabjah seismic loading and uplift loading equal to one-third of maximum tension capacity the reduction of maximum pull-out capacity was between (55.02% to 73.22%) in loose sand while in dense sand there is a reduction about 50% for pile with L/D=20 and 25% and a low reduction of 8.06% for pile with L/D=30.

Experimental Investigation for Group Efficiency of Driven Piles Embedded in Cohesionless Soil

Scour effects on lateral behavior of pile groups in sands

Pile spacing is an important factor affecting the bearing capacity of concrete expansion pile (CEP) groups. In this study, a pile group was simulated and analyzed using ANSYS software R19.0. The influence of pile spacing on the bearing capacity of the pile group under a vertical load was determined using three sets of four-, six-, and nine-pile models with different pile spacings. The grid division of the pile soil model adopts a mapping method, using the contact types of rigid and flexible bodies and applying surface loads to the model piles step-by-step. After vertical pressure was applied to the model pile, in-depth analysis was conducted on the displacement cloud map, pile top displacement, and other data. The different stress conditions of corner, edge, and center piles in each model group were compared and analyzed, revealing the relationship between the stress mechanism and failure law of the soil around the pile and the pile spacing. It was found that the soil displacement range of edge piles is slightly larger than that of corner piles. This phenomenon gradually decreases with increasing pile spacing. When the pile spacing increases to four times the cantilever diameter, the difference in soil displacement at different pile positions is small, and the pile spacing has little effect on the compressive bearing capacity of the pile group. Thus, it is reasonable to control the pile spacing at three to four times the cantilever diameter. In the nine-pile model, when the load is loaded to the 20-step level, the displacement value of the central pile is −72.278 mm, while the displacement values of the edge pile and corner pile are −69.012 mm and −66.806 mm. It is shown that increasing the pile spacing can effectively reduce the pile group effect and improve the bearing capacity of the pile foundation. At present, CEP pile groups are gradually being applied in practical engineering, but research on the influence of pile spacing on the compressive bearing performance of CEP pile groups is still at a very early stage. This article reinforces the influence of pile spacing on the compressive bearing performance of CEP pile groups. It provides theoretical support for its application in practical engineering.

Evaluation of horizontal and vertical bearing capacities of offshore bucket work platforms in sand

Lapisan lensa pasir memiliki karakteristik kuat geser yang lebih besar jika dibandingkan dengan lapisan tanah di atas maupun di bawahnya. Hal ini lah yang menyebabkan lapisan ini seringkali dikira sebagai lapisan tanah keras diakibatkan pembacaan dari hasil sondir menunjukkan nilai tahanan ujung yang hampir mendekati dengan nilai tahanan ujung lapisan tanah keras. Memiliki nilai c yang hampir sama dengan lapisan tanah keras membawa peneliti berasumsi bahwa lapisan ini sebenarnya memiliki potensi untuk dijadikan sebagai lapisan penahan pondasi untuk group tiang. Penelitian ini merupakan penelitian dengan skala kecil dengan membuat model group tiang dalam finite element dengan variabel pengaruh ketebalan lensa pasir (tlensa pasir), diameter tiang (Btiang), dan jarak tiang (Stiang) terhadap daya dukung. Permodelan lapisan tanah berlensa pasir dianalisa dengan menggunakan permodelan linear elastis yang terdapat di dalam program Plaxis. Data Tanah Permodelan diambil dari data tanah proyek flyover Gatot Subroto Banjarmasin. Kemudian permodelan group tiang dibuat dengan memodelkan baris plane dari tiang disederhanakan sebagai wall elements atau disebut plane strain tiang. Wall element didefinisikan per meter; kekakuan normal, kekakuan lentur dan berat dari tiang di luar baris plane dari tiang adalah dianggap per meter. Untuk mengekivalensikan wall element kedalaman kondisi yang sebenarnya di lapangan dimana group tiang berbaris kebelakang, maka digunakan rumus ekivalen terhadap kekakuan seperti yang dipaparkan oleh Andre Rytenius dalam master disertasinya. Pada tesis ini, daya dukung pondasi tiang tunggal dianalisis menggunakan metode Meyerhof dan Hanna’s. Hasil perhitungan daya dukung ultimit pondasi tiang tunggal (40 m) berdasarkan perhitungan manual diperoleh hasil sebesar 10438,1477 kN/m2. Dengan Plaxis, daya dukung yang didapat adalah 9000 kN/m2. Daya dukung tiang tunggal pada lapisan lensa hasil PLAXIS lebih kecil dari hasil perhitungan daya dukung secara teoritis. Daya dukung kelompok tiang pada lapisan lensa hasil PLAXIS didapatkan untuk diameter yang sama didapatkan kenaikan daya dukung berbanding lurus dengan kenaikan ketebalan lensa pasir. Kenaikan daya dukung tersebut dikarenakan karakteristik kuat geser dari lapisan lensa pasir yang memberikan pengaruh besar dan bahwa lensa yang memiliki tebal lebih dari 4 meter sudah memiliki karakteristik seperti tanah keras. Sedangkan untuk ketebalan lensa pasir yang sama maka didapatkan penurunan daya dukung berbanding lurus dengan kenaikan diameter tiang. Penurunan daya dukung tersebut dikarenakan area tekan pada lapisan lensa pasir memberikan pengaruh yang signifikan. Pada konfigurasi 4 tiang, jarak antar tiang yang paling berperan dalam memberikan daya dukung adalah di jarak antar tiang 3D, dimana hasil daya dukung yang dihasilkan pada jarak ini lebih tinggi. Sedangkan hasil daya dukung pada jarak antar tiang 4D dan 5D mengalami penurunan, dimungkinkan karena cara penarikan garis pada grafik daya dukung dengan penurunan. Pada konfigurasi 9 tiang, hasil didapatkan hampir serupa dengan hasil yang didapatkan pada konfigurasi 4 tiang. Dimana jarak antar tiang yang paling berperan dalam memberikan daya dukung adalah di jarak antar tiang 3D dan 5D, dimana hasil daya dukung yang dihasilkan pada jarak ini lebih tinggi.Sedangkan hasil daya dukung pada jarak antar tiang 4D mengalami penurunan, dimungkinkan karena cara penarikan garis pada grafik daya dukung dengan penurunan. Konfigurasi tiang yang lebih baik untuk diterapkan pada tanah berlensa pasir adalah konfigurasi 4 tiang simetris, dimana hasil daya dukung yang dihasilkan pada konfigurasi ini lebih tinggi dibandingkan konfigurasi 9 tiang simetris.

Design of foundations in earthquake prone areas needs special considerations. Shallow foundations may experience a reduction in bearing capacity and increase in settlement and tilt due to seismic loading. The reduction in bearing capacity depends on the nature and type of soil and ground acceleration parameters. In the case of piles, the soil-pile behavior under earthquake loading is generally non-linear. The nonlinearity must be accounted for by defining soil-pile stiffness in terms of strain dependent soil modulus. Several behavioral and design aspects of shallow foundations and pile groups subjected to earthquakes have been critically reviewed.

Assessment of the response of a laterally loaded pile group based on soil–pile interaction is presented in this paper. The behavior of a pile group in uniform and layered soil (sand and/or clay) is evaluated based on the strain wedge model approach that was developed to analyze the response of a long flexible pile under lateral loading. Accordingly, the pile’s response is characterized in terms of three-dimensional soil–pile interaction which is then transformed into its one-dimensional beam on elastic foundation equivalent and the associated parameter (modulus of subgrade reaction Es) variation along pile length. The interaction among the piles in a group is determined based on the geometry and interaction of the mobilized passive wedges of soil in front of the piles in association with the pile spacing. The overlap of shear zones among the piles in the group varies along the length of the pile and changes from one soil layer to another in the soil profile. Also, the interaction among the piles grows with the increase in lateral loading, and the increasing depth and fan angles of the developing wedges. The value of Es so determined accounts for the additional strains (i.e., stresses) in the adjacent soil due to pile interaction within the group. Based on the approach presented, the p-y curve for different piles in the pile group can be determined. The reduction in the resistance of the individual piles in the group compared to the isolated pile is governed by soil and pile properties, level of loading, and pile spacing.

Large-diameter rock-socketed shafts have been widely used in large-scale bridge constructions due to its higher bearing capacity and less settlement. Difficulties in pore-forming and uncertainty of pile bearing capacity have brought challenges to the construction and design of shafts in karst areas. Based on the engineering geological investigation, only parts of caves can be found in boring, however, the spanning and height of caves is not accurately identified if the comprehensive investigation was not implemented. After detailed investigations, characteristics of cave can be explored at the site of formerly-designed piles, and possibly the design scenario will be changed in usage of pile foundations. Under consideration of economy and effectiveness in the piles design, a new method, named as post-grouting pile groups was proposed to solve this problem in karst area. The post-grouting treatment can also improve the bearing capacity of pile groups. Numerical modeling for multiple small-diameter pile groups after post grouting was carried out by FLAC3D, and the results show that the bearing capacity and settlement of the pile groups can satisfy the standards of design. It was also noted that the bearing capacity increases significantly with the depth of reinforced soils above the pile tip. However, it was found that if the depth of reinforced soils under the pile tip reaches two times larger than the pile diameter, bearing capacity of pile groups almost becomes unchanged.

This study investigated the thermomechanical behavior of thermal pile groups in sand. To analyze the influence of cyclic thermal loading on the stress–strain behavior of piles in a group, pile groups containing four thermal piles installed in dense Toyoura sand were numerically investigated. A concrete pile cap was considered on the piles. Axial mechanical load was applied instantaneously on the pile cap and thermal load was applied along the pile length for six thermal cycles consisting of alternate heating and cooling of the piles. The concrete piles and pile cap were considered to behave in a linear-elastic manner under the mechanical load and thermal load. The soil was assumed to possess elastoplastic behavior and was simulated using the Mohr–Coulomb plasticity model. The effect of mechanical and cyclic thermal load on the pile groups was studied by varying the pile group parameters such as pile spacing (sp), pile diameter (Dp), and thickness of the pile cap (tpc). In all the analyses, the effect of cyclic thermal load was compared in terms of development of axial displacement, axial strains, and axial stresses in the piles. Thermomechanically loaded piles exhibited higher displacements, strains and stresses than did mechanically loaded piles. The results also indicate that the axial load on the piles under cyclic thermal loading was larger for the piles with higher aspect ratio than for the piles with lower aspect ratio. The effect of pile spacing was more pronounced in case of piles with higher aspect ratio. An increased thickness of the pile cap resulted in higher axial load and displacement in the piles. The cyclic thermal load resulted in residual or plastic strains in the piles. The thermomechanical behavior of a pile group is influenced by parameters such as pile spacing, aspect ratio, and pile cap thickness, which should be considered in thermal pile group design for structural stability performance of the pile group.

Model study was carried out in the laboratory to know the effect of uplift loads on vertical piles, batter piles and pile groups. Mild steel piles having 22 mm outer diameter and 1 mm thickness were considered for the study. Different slenderness ratios of 18, 28, and 38 and various angles such as 0°, 15°, 25°, 35°, and 45° were considered for the study. Sand was poured at a fixed relative density in the mild steel tank. The experimental study shows that the pile and pile group attain a maximum value of uplift load capacity when the batter angle reaches to 35°. Further, increase in batter angle causes a reduction in the uplift capacity of the pile. It is also noticed from the experiment that the uplift capacity increases with the slenderness ratio of the pile but its variation is not linear. Pile group of four piles with two vertical piles having L/D ratio 38 and two batter piles of L/D ratio 28 offers same resistance as of the same pile group with all the piles having L/D ratio 38. Numerical modeling was performed using Plaxis-3D to analyse the behavior of vertical piles, batter piles and pile groups under uplift loads. A good correlation between Plaxis-3D results and experimental results was observed.