Compression load tests on large open-ended pipe piles in dense North sea sand

Larger-diameter open-ended pipe piles are widely used as foundation for offshore structures. The lack of high-quality inner wall axial stress measurement results in difficulties in loading analysis of open-ended pipe piles during penetration and service period due to the effect of soil plugging. In this paper, two groups of model tests to assess the feasibility of bare fiber Bragg grating (FBG) sensing technology in monitoring the inner wall axial stress profile of open-ended aluminous pipe piles during penetration and horizontal loading are presented. One open-ended aluminous pipe pile was instrumented with bare FBG sensors and then penetrated into the sand bed with the rate of 20 and 30 mm/min, respectively. Two horizontal static loading tests were also conducted after 24-h penetration. The installation process of bare FBG sensors along open-ended aluminous pipe pile is introduced. The distribution profiles of axial stress and unit shaft resistance on inner wall and outer wall along model pile is discussed in detail. Model test results indicated that the bare FBG sensing technology was feasible to measure the axial stress of inner wall along open-ended pipe piles during penetration and horizontal loading in sand. During horizontal loading, the average values of qso/qsi are 0.717 and 0.727, respectively, for model pile with penetration rate of 20 and 30 mm/min. The axial stress difference of inner wall and outer wall along model pile in penetration and horizontal loading is the result of a combination of the inside and outside soil friction as well as of the pipe in-wall shear stiffness.

Investigations into pile behaviour in dense marine sand have been performed by lFP and [C at Dunkirk, North France. In the most recent series of tests, strain-gauged, open-ended pipe piles, driven and statically load tested in 1989, were retested in 1994. Surprisingly large increases in capacity were measured. The possible causes are evaluated in relation to previous case histories, laboratory soil tests, pile corrosion and new effective stress analyses developed using smaller, more intensively instrumented piles. The shaft capacities predicted by existing design methods are also assessed. Introduction The effects of time on the capacity of displacement piles in clay are well known and are usually explained through consolidation theory.1,2 In granular materials complete Pore pressure equalisation is expected within a few hours of driving, after which load tests are often assumed to measure long-term pile capacity. However, a growing number of reports of longer-term changes in capacity are emerging. Much uncertainty surrounds the factors responsible for these changes. This paper describes static and dynamic load tests on four open-ended piles, 0.324m in diameter, 11 and 22m long, driven in dense marine sand at Dunkirk, North France. The piles were initially installed and tested in 1989 by Brucy et al.3,4,5 as part of the 'CLAROM' research programme. In 1994 the site was revisited by imperial College, becoming the sixth location for tests using the highly instrumented Imperial College Pile (ICP)2,6,7,8 as shown on Fig. 1. In addition to single lCP test9,10 and interaction experiments,11 1 two of the original CLAROM piles were loaded statically in tension. The shaft capacity of an 11m pile was found to have increased by up to 85% during the intervening five years. Strain-gauge measurements and corrosion inspection of the extracted piles provide information on the possible causes. The paper is divided into three parts:a review of previous studiesa description of the Dunkirk testsa discussion which includes a critical appraisal of the possible causes of set-up in sands. The Appendix examines shafl capacity predictions made for Dunkirk using the existing API recommendations and other recently proposed approaches, Part 1: Previous Studies A small handful of cases have been reported where the capacities of piles in sand reduced with time. These are limited to (a) short-term relaxation due to the recovery of pore pressures following dilation during driving,12,13,14(b) piles founded on rock15,16 or (c) closely spaced pile groups.17,18 Long-term reductions would not be expected on most offshore piles in sand under normal loading conditions. Short-term(<24 hours) increases in pile capacity after or during intermissions in driving due to pore pressure dissipation have been widely documented.19,20 The first account of medium-term (1 to = 100 day) increases in sand was given by Tavenas & Audy21 who measured an average =70% rise in capacity between static load tests conducted 12 hours and 20 days after driving, with no further gains appearing over the following 30 days.

In 1995 an extensive load testing program was conducted on 30' pipe piles in dense silica sands similar to those found in the southern North Sea. A highly instrumented pile was driven at one location, extracted and redriven at a second location. A total of twelve static compression and tension load tests were performed at three penetration depths between 30 and 47 meters. Test objectives included acquiring reliable data on capacity of offshore type piles in sands which hitherto were hardly available and to understand the failure mechanisms in order to improve current foundation design. The paper describes the design of the testing programme and project organisation and funding. Details of the lest pile and instrumentation to measure the loads in the pile and the pile-sand interaction are highlighted with an emphasis on quality of the measurements. The field-work and innovative testing facilities are also described. Introduction Estimates of axial capacity of open-ended pipe piles in sand are an area of great uncertainty. Various authors (e.g. Pelletier et al., 1993) have argued that the world-wide used API recommended practice (RP2A, 1982-1991) does not properly account for fundamental parameters (pile length, plug behaviour, horizontal stresses, driving history, etc.) and underestimates the capacity of driven piles in very dense sands. On the other hand no other widely recognised design method is currently available. The base for a design method should be an understanding of the pile-sand load transfer in parallel with representative full scale pile load tests to confirm this (Hobbs, 1992). Some new promising design methods have been published in recent years (Randolph et nf., 1992, Lehane et al., 1992), but the existing data base on pile tests in sand is limited to piles of relatively small size and/or low capacity. Moreover few tests provide instrumented data of enough quality and resolution to confirm the load transfer in detail. While this unsatisfactory situation has been recognised by most parties involved in foundation design it appeared very difficult to resolve because high capacity pile tests in sand are relatively costly. In 1985 Heerema and Fugro developed a plan to test three 42' piles in very dense sands nearshore Eemshaven in The Netherlands (Fig. 1). On each pile compression and tension tests were foreseen at four penetration depths with a potential ultimate compression load of 90 MegaNewton (MN). The programme costs were estimated at US$ 12 million. Even though a 40% European subsidy was available not enough support from industry could be generated. This contrasts with the fact that about US$ 50 million is spent annually on pile foundations in the southern North Sea and a 30% increase in allowable load could lead to a saving of about US$ 60,000 per pile installed (van Zandwijk, 1986).

The present study investigates the behaviour of plug on pile load capacity and effect of plug removal. The sand used as a foundation soil is poorly graded clean sand. It was prepared at different densities using a raining technique. To simulate the pile load test in the field, a new apparatus was manufactured. A driving–pressing system for pile installation was manufactured. The soil plug is removed by a device manufactured to remove the soil column entrapped inside the pipe piles during installation by driving and pressing devices. The present study focuses on the determination of effect of soil plug on the ultimate compression capacity of single open-ended steel pipe pile, and makes a comparison with closed-ended pipe pile. A new type of pipe piles is suggested; it is closed–open-ended pipe piles driven and pressed into sandy soil of different densities (medium and dense) in which axial compression load tests were performed on model piles. The pile end will be open to a predetermined depth in order to make pile penetration easy and closed at a distance in order to increase the pile base resistance. Twenty-four models of open-ended piles have been modified by closing the pile ends by a plate welded at a distance of 2D, 3D and 4D (where D is the diameter of the pile) from tip of the pile. These piles have been installed in sand by two types of installation, driving or pressing. It was concluded that the pile load carrying capacity in dense sand is several times greater than those in loose and medium sands, especially in the case of closed-ended or open-ended piles, since the pipe pile can produce external and internal skin friction in addition to end-bearing resistance that makes the total pile capacity close to that of closed-ended pile. On the other hand, the removal of soil plug decreases the pile load capacity. This decrease becomes apparent in dense sand. The decrease in load capacity is about 45%–63%, 55%–63% and 51%–79% in loose, medium and dense sands, respectively. Open-ended pipe piles behave as closed-ended, if the soil plug formed inside piles in a state of partial plug or full plug. The length of soil plug depends on the type of installation and relative density.For the type of pile proposed in this study, open-ended piles are closed with a plate welded at a distance of 2D, 3D and 4D from the tip of the pile, and the open part of the pipe pile has a limited length, which was found to be 3D. This length revealed the maximum pile capacity due to the development of both interior and exterior skin friction in addition to end resistance. In addition, at this length, the soil column is pressed inside the pipe and hence the soil was densified leading to increase the skin resistance.

This paper presents a case study of large-diameter open-end steel pipe piles for the Ebey Slough Bridge Replacement Project. The new Ebey Slough Bridge was an approximately 700-ft-long, four-lane bridge over Ebey Slough in Marysville, Washington. The project site was underlain by more than 300 ft of very loose to medium dense silt, silty sand, and silty clay. New bridge foundations consisted of 4- and 6-ft-diameter open-end steel pipe piles driven to depths ranging from approximately 150 to 240 ft. Geotechnical design of the open-end pipe piles was performed by Shannon & Wilson, Inc. Total pile compressive resistances were confirmed through a dynamic test pile program during pile installation. The static design methods provided in AASHTO are compared here to results of the dynamic test pile program. Results of the dynamic test program suggest that static design methods overestimate nominal side resistance of 4- and 6-ft-diameter open-end pipe piles. Comparison of factored side resistances using the static design methods and AASHTO resistance factors suggest potential limitations of AASHTO resistance factors for large-diameter piles.

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.

Dynamic response of open pipe pile under vertical cyclic loading in sand and clay

The behavior of open-ended pipe piles is different from that of closed-ended pipe piles due to the soil plugging effect. In this study, a series of field tests were conducted to investigate the behavior of open-ended prestressed high-strength concrete (PHC) pipe piles installed into clay. Two open-ended PHC pipe piles were instrumented with Fiber Bragg Grating (FBG) sensors and jacked into clay for subsequent static loading tests. Soil plug length of the test piles was continuously measured during installation, allowing for calculation of the incremental filling ratio. The recorded data in static loading test were reported and analyzed. The distribution of residual forces after installation and the effect on the bearing capacity were also discussed in detail. The test piles were observed to be in partially plugged condition during installation. The measured ultimate shaft resistance and total resistance of the test piles were 639 and 1180 kN, respectively. The residual forces locked in the test piles after installation significantly affected the evaluation of the axial forces, and thus the shaft and end resistances. It tended to underestimate the end resistances and overestimate the shaft resistances if the residual forces were not considered in the loading test. However, the residual forces did not affect the total bearing capacity of open-ended PHC pipe piles in this study.

This paper describes results from an experimental programme that investigated factors affecting the shaft capacity of open-ended (pipe) piles in sand. A number of jacked pile installations in a test chamber filled with loose sand were performed using both open- and closed-ended, 114 mm diameter piles. The test series was designed to investigate the effects of in situ stress level, pile end condition, and degree of plugging on the development of pile shaft resistance. The results indicate that the maximum local shaft resistance that can develop at a given location on a pipe pile may be expressed as a function of the incremental filling ratio of the soil plug during installation, the cone penetration test (CPT) qc value, and the relative position of the pile toe. The experimental results allowed a simple expression to be developed for the plug resistance during pile installation, and this is used in conjunction with a popular design method for closed-ended piles to provide a means of estimating the shaft capacity of open-ended piles. The new approach is shown to provide good estimates of overall shaft capacity and skin friction distribution.Key words: shaft capacity, pipe piles, sand.

Although many studies have been done to investigate the axial behaviors of open-ended piles in sands, few studies have been reported for weak clayey silts. To develop reliable models for the design of open-ended steel-pipe piles driven into 29-m-thick varved clayey silt deposits, a series of full-scale field load tests including large-strain dynamic tests and static cyclic axial-compression-load tests was conducted on two groups of instrumented piles. Through analysis of the test data, soil parameters were back-calculated for estimation of pile capacities using the static-bearing-capacity formulas and cone-resistance-based methods. The comparisons between the calculated results and the field load test data demonstrated that the following considerations can be adopted in the design of static compression capacities of an open-ended pipe pile penetrating through thick varved clayey silts to end-bearing in dense cohesionless soils: (1) a fully plugged condition can be assumed, (2) cone resistance with an upper limit of 4,788 kPa (100 ksf) can be used for unit base resistance on the soil plug, and (3) exterior unit shaft resistance can be estimated using two-thirds of the total unit shaft resistance.

Open-ended steel pipe piles are used to support jacket structures in the offshore wind sector. These piles experience significant tensile axial loading in-service and the tensile shaft resistance thus governs their design. Although pile ageing (increased shaft resistance with time) has been noted by a number of workers and incorporation in design could lead to significant efficiencies, there is a dearth of high-quality field test data that measure its effects. This paper presents the results from an experimental investigation designed to examine the effect of ageing on the tension shaft resistance developed by open-ended piles in sand. As part of this investigation, four 340 mm diameter open-ended steel piles are driven 7 m into a dense sand deposit in Blessington, Ireland. Each pile is subjected to a series of static axial tension load tests at different time intervals after driving and the effects of ageing are assessed. The tension capacity of the piles is seen to increase by as much as 185% over a period of 7 months after driving.

This paper presents a new method for estimating the base capacity of open-ended steel pipe piles in sand, a difficult problem involving great uncertainty in pile foundation design. The method, referred to as the Hong Kong University (HKU) method, is based on the cone penetration test (CPT), and takes into consideration the mechanisms of annulus and plug resistance mobilization. In this method the annulus resistance is properly linked to the ratio of the pile length to the diameter—a key factor reflecting the influence of pile embedment—whereas the plug resistance is related to the plug length ratio, which reflects the degree of soil plugging in a practical yet rational way. The cone tip resistance is averaged over a zone in the vicinity of the pile base by taking into account the failure mechanism of the piles in sand, the condition of pile embedment (i.e., full or partial embedment), and the effect of soil compressibility. The predictive performance of the new method is assessed against a number of well-documented field tests including two fully instrumented large-diameter offshore piles, and through comparisons with major CPT-based methods in current engineering practice. The assessment indicates that the HKU method has attractive capabilities and advantages that render it a promising option.

Open ended pipe (OEP) piles are frequently used as a deep foundation solution in the Atlantic Coastal Plain (ACP) physiographic province. Typically, these piles act as low displacement piles during installation as the pile advances through the soil. However, if the soil plug becomes stationary within the pile, the pile acts as a full displacement pile. Plugging of an OEP pile can have significant effects on pile design and installation. Typically, measurements of pile plugging are conducted after pile installation is completed by measuring the depth to the soil plug and the embedment depth of the pile. The following paper presents the results of eight case histories of open ended pipe piles with pile plugging measurements in the Atlantic Coastal Plain. For each case history, the individual pile plug length ratio (PLR) and incremental fill ratio (IFR) was determined and the plug behavior during static and/or dynamic load testing (if available) was examined. Examination of the case history data showed that the piles experienced little plugging during installation, although pile plugging did increase with depth. In addition, the increase in measured tip capacity with time from pile installation suggests that OEP piles driven in ACP soils act as plugged with time.

Large-diameter open-ended pipe piles typically refer to pipe piles with diameters larger than 0.91 m. These piles, when properly designed and constructed, could provide high load-carrying capacity against axial loading, uplift, and overturning in the offshore environment. The current American Association of State Highway and Transportation Officials (AASHTO) specification for pipe piles is based on the database collected from piles with diameters less than 0.61 m and is, therefore, insufficient for the design of large-diameter open-ended pipe piles. This paper introduces an experimental program to investigate the static load-carrying behavior of open-ended pipe piles. Two types of restriction plates, i.e., one-hole plate and four-hole plate, were designed and installed inside the pipe to study their influence on the loading bearing behavior. Beside the laboratory-scale pile experiments, the testing program was augmented with geotechnical centrifuge experiments, which upscale the dimensions of model piles close to the size used at the field scale. The combined laboratory and geotechnical centrifuge experiments help to analyze the load-carrying behavior of pipe piles with different pile diameters, types of restriction plate, and embedment depths. The experimental results indicate that the formation of a soil plug is crucial to mobilize the bearing capacity of pipe piles. The restriction plate helps to form the soil plugging, which leads to a higher end bearing capacity. The selection of types of restriction plate should be determined on a case-by-case basis with considerations of mechanical performance and pile drivability.

Open-ended pipe piles with diameters of 36 in. (914 mm) or larger and wall thicknesses in excess of 0.5 in. (13 mm) are increasingly being used for deep foundation support of large bridge projects in the United States. Over the last several years, initial driving dynamic test data from these large-diameter open-ended pipe piles on some bridge projects as well as on offshore platforms have indicated overall wave speeds for the measured length in excess of the commonly used value of 16,800 ft/s (5,122 m/s) for steel piles. Measurements were undertaken on a recent Minnesota bridge project to directly quantify the pile wave speed. Two pairs of accelerometers were positioned 64.2 ft (19.6 m) apart during the initial driving of a test pile. These dynamic test data indicated a most likely wave speed for the pile of 17,000 ft/s (5,182 m/s). Steel coupon testing also indicated a higher elastic modulus than commonly assumed. This paper presents the field dynamic test data and laboratory test results that indicate a faster wave speed and higher elastic modulus, with respect to standard values, observed on this project. Other recent cases supporting the use of a faster steel wave speed and higher elastic modulus are included and discussed. Recommendations are provided for dynamic testing on large-diameter open-ended steel pipe piles.