Open-ended Piles Research Articles

Jacking penetration resistance and mechanical characteristics of bucket foundations in sand

Model tests have been performed to investigate the penetration and tip resistance and inner and outer earth pressure of bucket foundations embedded in sand with the jacking installation method. The soil plug, the soil-skirt interaction, and the penetration resistance of the bucket are determined by these tests. Based on the soil arch theory, analytical expressions of the inner soil pressure and the penetration resistance for the bucket are presented and verified by the test results. The results show that the soil plug begins to form when the penetration displacement reaches 1 times the width of the bucket, and the incremental filling ratios of each bucket are 0.65 to 0.7 and 0.8 at the end of penetration. The inner earth pressure of the bucket is in the form of a double broken line, which is same as the open-ended piles. The theoretical formula of the penetration force considering the soil arching effect can predict the penetration force well, and the ratio of the soil arch resistance to the total resistance during the penetration is up to 85%, which indicates that the soil arch resistance plays an important role in the total penetration resistance during the jacking installation process.

DEM investigation of installation responses of jacked open-ended piles

This paper presents a Discrete Element modelling of the installation responses of rigid open-ended piles with different diameters. A two-dimensional model of granular soil is generated using the Particle Flow Code (PFC). The model is generated under 100g gravity to simulate deep foundations. The simulation results show good agreement with the published experimental measurements. Particle-scale information is retrieved to reveal the micromechanics underlying these dynamic behaviours. It is found that relative particle displacement field provides clear micro-scale evidence to the development of shear zones. The instantly fully plugged mode and the partially plugged mode continuously exchange during pile installation, which is more apparent in slimmer pile. Pile base resistance (plug resistance plus annulus resistance) contributes to the majority of pile resistance. The plug resistance is only concentrated within the zone 2-3 times of pile diameter above the pile base. The unit lateral resistance at a specific depth decreases in pile installation. Pile installation causes increases in the average stresses surrounding the pile, with the major influence zone concentrated within six times of pile diameter.

FE-based identification of pile–soil interactions from dynamic load tests to predict the axial bearing capacity

The static axial pile bearing capacity depends mainly on the developed pile–soil interaction. Hence, to determine a realistic and reliable ultimate pile bearing capacity, the pile–soil interaction needs to be identified and described precisely. One relevant field for this application is dynamic load tests (DLTs). During a DLT, the pile bearing capacity is deduced from measurements at the pile head. Current procedures of deriving the bearing capacity of open-ended piles from these measurements are lagging behind regarding the description of the pile–soil interaction for open-ended piles and the influence of the pore fluid response. Hence, a new technique based on the finite element method is developed, which captures the key aspects for DLT results correctly and is capable of deriving the static pile bearing capacity. The method is validated against dynamic and static load test data obtained from centrifuge tests on large diameter monopiles. An application to field test data of tubular steel piles is shown. The developed approach seems suitable to assess the static pile bearing capacity based on DLT measurements.

Dynamic Response of Offshore Open-Ended Pile under Lateral Cyclic Loadings

Foundations for offshore wind turbines (OWTs) are mainly open-ended piles that are subjected to cyclic loadings caused by winds, waves and currents. This study aims to investigate the dynamic responses of open-ended pipe pile under lateral cyclic loadings, as well as the characteristics of the soil plug and surrounding soil. Both large-scale indoor model test and discrete element simulation were adopted in this study. The test results show that the resistance of each part of the pipe pile increases linearly with depth during the process of pile driving. The pile side resistance degradation effect was also observed along with the friction fatigue. The soil plug formation rate decreases gradually with an increase in the pile depth. The influence range in the surrounding soil is about 5~6 times of the pile diameter. The cumulative displacement of the pile head increases with the number of cycles. Lateral tangential stiffness and lateral ultimate bearing capacity decreases with an increase in number of cycles. The severe disturbance range of soil around the pile is 2~3 times of the pile diameter. The center of rotation of the pile body is about 0.8 times of the pile body depth. The side frictional resistance and lateral pressure of the pile body is found to fluctuate along the pile body. Additionally, the lateral pressure and side friction resistance decreases gradually with decreasing tendency of the former more than the latter.

DEM analysis of the sand plug behavior during the installation process of open-ended pile

A DEM simulation of the installation process of open-ended pile is conducted by means of the particle flow code PFC 2D. Focus is placed on the investigation of the soil plug behavior, from both macroscopic and microscopic perspectives. A soil assembly with natural initial stress state is first generated and the validity of the numerical installation process is further checked. Afterwards, a macroscopic analysis is performed based on the porosity and stress state. Numerical results indicate that a dense zone, with the length about half the pile diameter, has formed at the pile tip. A dramatic load transfer is observed at the bottom of the soil plug, where the lateral pressure coefficient peaks at around 2.4. From a microscopic perspective, two modes of soil layer deflection exist for soil mass inside and under the pile, respectively. Different particle displacement patterns indicate that with increasing penetration depth and resistance accumulation, soil mass flows into the pile with a lower rate. Further analysis based on the distribution of contact force chain and principal stress rotation show that soil particles, when subjected to external disturbance, tend to rearrange to the most stable structure, which is in the arch shape. Finally, an improved arch model based on the numerical results is proposed to facilitate the understanding of the plug behavior.

Characterization of the Blessington sand geotechnical test site

The paper describes the site characterization of a dense sand test site developed by the authors in an active quarry located near the village of Blessington, Ireland. The site has been used to investigate the field response of a number of foundation systems including jacked closed and open-ended piles, driven concrete and steel open-ended piles, shallow footings, bored axially loaded piles and laterally loaded steel piles. The soil at the location is an over-consolidated, glacially deposited, dense to very dense fine sand with a CPT q c resistance, that increases from ≈10 MPa near the ground surface to 15–20 MPa over a depth of 10 m. The paper describes the geological history of the deposit, composition and mineralogy analyzed through Scanning Electron Microscopy. The results of in-situ tests including CPT, DMT and MASW are presented. Material properties including strength and stiffness derived from laboratory tests are compared to correlation made with in-situ testing. The results of previously unpublished laboratory interface shear box tests to investigate pile ageing effects are presented.

Direct interface shear tests on Dunkirk sand

After a field test campaign conducted in Dunkirk (north of France) on open-ended steel piles aiming to study the ageing phenomenon, laboratory scale shear tests were designed to study the behaviour of the sand-steel interface. In order to carry out this laboratory investigation, the direct interface shear apparatus was used for characterizing Dunkirk sand (in dry or unsaturated conditions with about 6% water content as in the field) consolidated on initially smooth mild steel plates at different consolidation time intervals (0, 1 and 7 days) and different consolidation stresses (50, 100, 200 and 300 kPa). The test program also included two normal boundary conditions (Constant normal load CNL and constant volume CV) so that they can be compared to the field results and determine the most approaching configuration. More, the unsaturated condition induced a corrosion of the mild steel plates, causing a layer of sand remaining glued to the plate after removing the shear box. Traces of corrosion were also observed on the lower part of the sand samples (in contact with the plate). These observations lead to the interpretation of an increase of the mechanical properties (local cohesion and increase of the friction angle). In order to follow the evolution of the corrosion for each plate, thickness measurements of the sand layer stuck on the plates were carried out.

A Load-Testing Program on Large-Diameter (66-Inch) Open-Ended and PSC-Instrumented Test Piles to Evaluate Design Parameters and Pile Setup

This paper presents the results from a pile load testing program for a bridge construction project at Chalmette, Louisiana. The load testing includes three 66-in. spun-cast post-tensioned open-ended cylinder piles and one 30-in. square prestressed concrete (PSC) pile driven at four different locations along the bridge site in clayey-dominant soil. Both cone penetration tests and soil borings/laboratory testing were used to characterize the subsurface soil conditions. All test piles (TP) were instrumented with strain gauges to measure the load distribution along the length of the TPs and to measure the side and tip resistances, separately. Dynamic load tests (DLT) were performed on all TPs at different waiting periods after pile installations to quantify the amount of setup (i.e., increase in pile resistance with time). Case Pile Wave Analysis Program (CAPWAP®) analyses were performed on the DLT data to calculate the resistance distributions along the TPs. A static load test was performed only on the PSC pile and statnamic load tests (SNLT) were conducted on both pile types. Design parameters such as the total stress adhesion factor, α, and the effective stress coefficient, β, were back-calculated. The α values ranged from 0.41 to 0.86, and the β values ranged from 0.13 to 0.29. The load test results showed that SNLT overestimated the tip resistance as compared with dynamic and static load tests. Moreover, the pile tip resistance was almost constant during the testing period, and setup was mainly attributed to increase in pile side resistance with time.

Pile load test of jacked open-ended prestressed high-strength concrete pipe pile in clay

The performance of open-ended piles can be different from that of closed piles due to the effect of soil plugging. This paper presents the results of a large-scale field pile load test conducted on an open-ended pile in clay under static compressive loads. A prestressed high-strength concrete pipe pile with an outer diameter of 400 mm and a wall thickness of 75 mm was instrumented with strain sensors and installed in a marine deposit for static load testing. The measured load-transfer data during pile installation and the pile load test results are reported and analysed. The data show that the pile was in a partially plugged condition during jacking and behaved in a fully plugged mode in the static load test. The measured ultimate shaft resistance and total resistance of the test pile were 659 and 1180 kN, respectively, which were smaller than the values calculated using a previously proposed method. The back-calculated α and β values (as used in the α and β methods) for axial pile capacity in marine deposits were 0·42 and 0·56, respectively.

Formation of soil plug in open-ended pipe piles in sandy soils

Open-ended pipe piles are commonly used for the foundations of offshore jacket platforms, mooring piles, offshore wind turbines and, in general, for pile foundations like micro piles, etc. Driveability of open-ended piles is greatly affected by the formation of soil plug during the driving, particularly in sandy soils. In order to study the behaviour of formation of soil plug during driving, open ended model piles were driven into uniformly graded sand in a calibration chamber in the laboratory scale as well as in the field. The influence of the pile diameter, hammer energy and pile penetration depth were studied. The results of the model pile tests showed that the incremental filling ratio (IFR, which indicates the degree of plugging) during driving decreased with pile penetration and increased with increase in hammer energy. IFR has also increased with increase in pile diameter. A correlation between plug length ratio (PLR, percentage plug height with respect to total penetration) and IFR is proposed based on the laboratory experiments.

Incremental filling ratio of pipe pile groups in sandy soil

Formation of a soil plug in an open-ended pile is a very important factor in determining the pile behavior both during driving and during static loading. The degree of soil plugging can be represented by the incremental filling ratio (IFR) which is defined as the change in the plug length to the change of the pile embedment length. The experimental tests carried out in this research contain 138 tests that are divided as follows: 36 tests for single pile, 36 tests for pile group (2x1), 36 tests for pile group (2x2) and 30 pile group (2x3). All tubular piles were tested using the poorly graded sand from the city of Karbala in Iraq. The sand was prepared at three different densities using a raining technique. Different parameters are considered such as method of installation, relative density, removal of soil plug with respect to length of plug and pile length to diameter ratio. The soil plug is removed using a new device which is manufactured to remove the soil column inside open pipe piles group installed using driving and pressing device. The principle of soil plug removal depends on suction of sand inside the pile. It was concluded that the incremental filling ratio (IFR) is changed with the changing of soil state and method of installation. For driven pipe pile group, the average IFR for piles in loose is 18% and 19.5% for L/D=12 and 15, respectively, while the average of IFR for driven piles in dense sand is 30% and 20% for L/D=12 and L/D=15 respectively. For pressed method of pile installation, the average IFR for group is zero for loose and medium sand and about 5% for dense sand. The group capacity increases with the increase of IFR. For driven pile with length of 450 mm, the average IFR % is about 30.3% in dense sand, 14% in medium and 18.3% for loose sand while when the length of pile is 300 mm, the percentage equals to 20%, 17% and 19.5%, respectively.

Analysis of Bearing Capacity Improvement Effect of Inner Cone Penetration Equiped Open-Ended Steel Pipe Pile

Analysis of Bearing Capacity Improvement Effect of Inner Cone Penetration Equiped Open-Ended Steel Pipe Pile

Effects of inner sleeves on the inner frictional resistance of open-ended piles driven into sand

In open-ended piles, inner friction is developed between inner pile shaft and the inner soil. Inner frictional resistance depends largely on the degree of soil plugging, which is influenced by many factors including pile diameter, relative density and end conditions of piles. In this paper, effects of inner sleeves on inner frictional resistance are discussed. The experiments were conducted on a medium-dense sandy ground using laboratory-scale piles. It was observed that the piles penetrated under partially-plugged or unplugged state. The results suggest that inner frictional resistance, Qin increases with sleeve height, l linearly and requires 2D (D is pile outer diameter) of l to produce a large as 50% of Qt by Qin (Qt is total resistance). The results also indicate that bearing capacity increases with wall thickness at the pile tip, which can be attributed to the increase in annular area. The results also indicate that soil plug height is independent of sleeve height. The results also reveal that the penetration of straight piles is closer to unplugged state than the sleeved piles. The results of incremental filling ratio and plug length ratio also indicate that the degree of soil plugging is affected by the sleeve height.

Effects of pile geometry on bearing capacity of open-ended piles driven into sands

Bearing capacity of open-ended piles depends largely on inner frictional resistance, which is influenced by the degree of soil plugging. While a fully-plugged open-ended pile produces a bearing capacity similar to a closed-ended pile, fully coring (or unplugged) pile produces a much smaller bearing capacity. In general, open-ended piles are driven under partially-plugged mode. The formation of soil plug may depend on many factors, including wall thickness at the pile tip (or inner pile diameter), sleeve height of the thickened wall at the pile tip and relative density. In this paper, we studied the effects of wall thickness at the pile base and sleeve height of the thickened wall at the pile tip on bearing capacity using laboratory model tests. The tests were conducted on a medium dense sandy ground. The model piles with different tip thicknesses and sleeve heights of thickened wall at the pile tip were tested. The results were also discussed using the incremental filling ratio and plug length ratio, which are generally used to describe the degree of soil plugging. The results showed that the bearing capacity increases with tip thickness. The bearing capacity of piles of smaller sleeve length (e.g., ≥ 1D; D is pile outer diameter) was found to be dependent on the sleeve length, while it is independent on the sleeve length of greater than a 1D length. We also found that the soil plug height is dependent on wall thickness at the pile base. The results on the incremental filling ratio revealed that the thinner walled piles produce higher degree of soil plugging at greater penetration depths. The results also revealed that the soil plug height is dependent on sleeve length of up to 2D length and independent beyond a 2D length. The piles of a smaller sleeve length (e.g., ≥ 1D) produce higher degree of soil plugging at shallow penetration depths while the piles of a larger sleeve length (e.g., ≦ 2D) produce higher degree of soil plugging at greater penetration depths.

개단말뚝의 폐색효과 영향인자 분석

개단말뚝은 폐색정도에 따라 지지력이 다르게 나타나므로 개단말뚝 설계시 폐색효과를 고려하여야 한다. 이에 본 연구에서는 대변형 수치해석 기법 중 하나인 Coupled Eulerian-Lagrangian(CEL) 기법을 이용하여 말뚝의 항타 관입을 모사하여 사질토, 점성토 지반조건, 단일 지반의 탄성계수, 말뚝 선단지지 조건, 다층지반 조건에 대한 영향 분석을 수행하였다. 해석 결과 사질토 지반에서 점성토 지반보다 폐색정도가 큰 것으로 확인되었고, 사질토의 경우 지반의 강성이 클수록 폐색정도가 증가하는 것으로 나타났다. 말뚝 선단이 지지층에 근입된 경우, 그렇지 않은 경우보다 정지토압계수가 크게 나타나 폐색정도가 큰 것으로 확인되었다. 또한, 지반의 배열이 이질층인 경우 지반강성에 따라 정지토압계수가 다르게 결정됨을 알 수 있었다. 따라서 지반조건, 지층 등에 따라 정지토압계수를 다르게 적용하여 관내 내부마찰력을 산정할 필요가 있으며 이를 반영한 합리적인 지지력 산정을 할 필요가 있음을 알 수 있었다. This paper presents the investigation of the major influence factors on the degree of soil plugging for open-ended piles based on the Coupled Eulerian-Lagrangian (CEL) numerical technique. The main objective of this study was to investigate the effect of soil plugging on the response of piles in various conditions. Through comparison of the results of field load tests, the CEL methodology was found to be in good agreement with the general trend observed by in situ measurement. Additionally, the parametric studies were performed by controlling the soil conditions, soil elastic moduli, end-bearing conditions and multi layers. It was found that the degree of soil plugging for sand layers was greater than that of clay layers. Also, the degree of soil plugging increased with an increase in both the soil stiffness and length of pile embedded in the bearing layer.

Estimation of bearing capacity of open-ended model piles in sand

The plugging of pipe piles is an important phenomenon, which is not adequately accounted for in the current design recommendations. An open-ended pipe pile is said to be plugged when the soil inside the pile moves down with the pile, resulting in the pile becoming effectively closed-ended. Plugging is believed to result in an increase in the horizontal stresses between the pile and the surrounding soil, which results in an increase in skin friction. A total number of 60 model pile tests are carried out to investigate the behavior of plugs on the pile load capacity and the effects of plug removal. Different parameters are considered, such as pile diameter–to–length ratio, types of installation in sands of different densities, and removal of the plug in three stages (50, 75, and 100 %) with respect to the length of plug. The changes in the soil plug length and incremental filling ratio (IFR) with the penetration depth during pile driving show that the open-ended piles are partially plugged from the outset of the pile driving. The pile reached a fully plugged state for pressed piles in loose and medium sand and partially plugged (IFR = 10 %) in dense sand. For driven piles, the IFR is about 30 % in loose sand, 20 % in medium sand, and 30 % in dense sand. The pile load capacity increases with increases in the length of the plug length ratio (PLR). The rate of increase in the value of the pile load capacity with PLR is greater in dense sand than in medium and loose sand. Based on test results, new empirical relation for the estimation of the load carrying capacity of open-ended piles based on the IFR is proposed.

Coupled deformation–seepage analysis of dynamic capacity tests on open-ended piles in saturated sand

Open-ended piles such as tubular piles or I-beams are used as foundations for offshore and nearshore construction. After the pile installation a load test to estimate the bearing capacity of these open-ended piles is necessary. Due to the offshore conditions and the high bearing capacity of the installed piles a static load test is not normally feasible. Therefore, dynamic load tests are carried out where the wave propagation due to an dynamic impact at the pile head is measured. The methods to estimate the bearing capacity from the measured signal of the dynamic tests were derived for solid pile profiles. It is questionable whether these evaluation techniques are applicable for open-ended piles. Hence, the influence of various important system parameters as well as the differences between static and dynamic load tests on open-ended piles is investigated in this paper.

Effects of the lateral stress on the inner frictional resistance of pipe piles driven into sand

The inner frictional resistance of pipe piles depends on the degree of soil plugging. Many factors including pile diameter, lateral stress at the pile tip and geometrical conditions of piles could influence the soil plugging. In this paper, the effects of inner sleeves attached at the pile base on the inner frictional resistance are discussed, particularly highlighting the lateral stresses at the sleeve using small-scale steel pipe piles penetrated into a medium dense sandy ground. A closed-ended pile of the same diameter was also tested to compare it with similar open-ended piles. The results of incremental filling ratio (IFR) and plug length ratio (PLR) were also discussed. A simple method was also proposed to evaluate IFR and PLR for the sleeved piles since they have originally been defined for non-sleeved piles. The results of the IFR indicate that all the piles penetrated under partially-plugged or unplugged state producing smaller penetration resistance than a similar closed-ended pile. The results of the corrected IFR give a better indication of the soil plugging of the sleeved piles, particularly at shallow penetration depths. The results also suggest that the inner frictional resistance increase with the sleeve height. The results of the coefficient of lateral earth pressure, K h also indicate that K h increases with the sleeve height. The effects however become less significant at higher sleeve heights. Therefore, we can recommend the use of the inner sleeve as an improvement method to increase the bearing capacity of open-ended piles installed in sandy grounds.

BASE RESISTANCE OF OPEN-ENDED PILES EVALUATED BY VARIOUS DESIGN METHODS

The design bearing capacity of an open-ended pile depends largely on the accuracy of the design method. Although the knowledge of shaft resistance has been understood quite well, the base resistance has not yet been completely understood due to the effects of soil plugging. The mechanisms of soil plugging is yet to be fully understood particularly for large diameter and long length piles installed in large construction projects. This paper compares the base resistance of two open-ended field piles constructed in the Tokyo Bay project evaluated by various design methods including cone penetration test (CPT)and standard penetration test (SPT)-based methods. In total, five design methods including the conventional American Petroleum Institute (API) approach were included. In Japan, SPT-based design methods are used in practice. The CPTbased design methods, which are not popular in Japan were also included to evaluate their effectiveness. The CPT-design methods discussed in this paper classify open-ended piles into plugged or unplugged modes. The results reveal that the Fugro (i.e., CPT-based) and the API design methods overestimate the base resistance. In contrast, the ICP (i.e., CPT-based) and Port and Airport Research Institute (PARI) (i.e., SPT-based) design methods underestimate the base resistance. Based on the results, we can recommend the University of Western Australia (UWA) design method (i.e., CPT-based) as it produces the nearest results to the actual base resistance measured from the field load tests.

Understanding inner friction mechanism of open-ended piles - an experimental study

An open-ended pile can produce a bearing capacity similar to a closed-ended pile depending on inner frictional resistance, Qin, which depends on the degree of soil plugging. The degree of soil plugging may depend on many factors including pile outer diameter (D), tip thickness (t), sleeve height (l) and relative density (Dr). In this research, we studied the effects of t, D and l on bearing capacity using laboratory scale model piles. The tests were conducted on a medium dense sandy ground using model piles with different values of D, t and l. The results showed that bearing capacity increases with t, which can be attributed to increase in annular area. They also showed that soil plug height, h is dependent of l for larger diameter piles. The results of incremental filling ratio (IFR) showed that penetration of straight piles (i.e., no sleeve) is closer to unplugged state than the piles with a sleeve. While Qin is independent of l for smaller diameter piles (D=30 mm), it is dependent of l for larger diameter piles (D=50 mm). The results of Qin suggested that as large as 50% of Qt (Qt is total resistance) is contributed by Qin even with a small l (e.g., 10 mm) for smaller diameter piles, while Qin in larger diameter piles increases with l and requires 2D of l to produce a large as 50% of Qt by Qin