An experimental investigation of the time dependence of shaft friction for displacement piles in lightly over-consolidated clay

Closure to “Pile Setup in Cohesive Soil. I: Experimental Investigation” by Kam W. Ng, Matthew Roling, Sherif S. AbdelSalam, Muhannad T. Suleiman, and Sri Sritharan

Load tests on a model pile installed in Speswhite kaolin are described. The soil surrounding the pile was consolidated under a range of stress ratios K, and the pile was loaded under drained conditions in such a way that only shaft friction was generated. Pile installation techniques minimized soil disturbance so that the failure criterion for pile shaft friction could be investigated. The results, based on tests on nine normally consolidated and one overconsolidated sample, show that the angle of shaft friction is independent of the stress ratio in the soil before loading (0·7 <K <1·5), and is only justless than φ′triaxial for normally and overconsolidated kaolin. When loaded axially the lateral stresses on the pile shaft decrease with increasing shaft load for normally consolidated soil, and increase on loading in overconsolidated soil. Differences between adhesion factors α; back-calculated from the model tests and those normally encountered in practice highlight the effects of disturbance caused by most pile installation techniques. L'article décrit des essais de chargement d'un pieu-modéle mis en place dans le kaolin de Speswhite. Le sol entourant le pieu a été consolidé. successivement à différents rappart des contraintes principles K et le pieu a été chargé dans des conditions drainées de telle fa&cetil;on que seulement du frottement latéral soit mobilisé. La technique d'installation du pieu a minimiser le remaniement du sol de sorte que le critére de rupture par frottement latkral puisse être étudié. Les résultats, basés sur des essais effectés sur onze échantillons normalement consolidés et un échantillon surconsolidé, indiquent que l'angle de frottement sur le pieu est indépendant des rapports de contraintes dans le sol avant chargement (0·7 <K <l·5), et que sa valeur n'est que légérement inférieure à φ′ triaxial dans le cas du kaolin normalement consolidé et surconsolidé. Lors du chargement axial les contraintes latérals sur le fut du pieu décroissent au fur et à mesure que la charge de puits augmente dans du sol normalement consolidé, tandis qu'elles s'accroissent quand ladire charge croît dans du sol surconsolidé. Les différences entre les coefficients de frottement x calcués à parti du modéle et ceux habituellement rencontrts en practique font apparaitre clairement les effets du remaniement causé par laplupart des proédés de mise en oeuvre des pieux.

Several studies on axial pile capacity in clays have shown that the average frictional resistance, expressed as a fraction of the average undrained shear strength or average effective overburden pressure, decreases with increasing pile penetration. Procedures to compute shaft friction are reviewed, and the effect of pile length on the development of shaft friction on piles in clay is examined in terms of the relative pile-soil stiffness and lateral pile movements during installation. Correlations are developed to relate shaft friction coefficients α,β,λ, to pile length, relative pile-soil stiffness, and soil stress history. Procedures are recommended to compute the friction capacity of piles in clay.

The paper describes key results from a series of new load tests to verify the impact of time or ageing on the axial bearing capacity of driven tubular steel piles. Load tests in two different sand deposits (a silty fine loose sand deposit and a medium dense, medium sand deposit), and four different clay deposits (low and medium plastic normally consolidated clay, over consolidated glacial clay, and a very highly plastic and over consolidated clay) were undertaken. Six tubular test piles were driven at each test site, with diameter 400 to 500 mm and length around 20 m. The piles were loaded to failure typically 1, 3, 6, 12 and 24 months after pile driving. At both sand sites, the shaft friction approximately doubled from the 1 month to the 12 months tests, but tended to level off after that. The ageing effect for piles in clay vary with the clay type, and comes in addition to the normal set-up due to dissipation of excess pore pressures. After 2 years, the gain in capacity range from a factor of about 1.1 to 2.0 compared to the capacity after full re-consolidation. The tests in the low-plastic low-OCR clay deposit showed the largest gain, and the tests in the highly plastic high OCR clay deposit the smallest. The study also revealed that repeated load testing on the same pile to failure can give both lower and higher capacity than first time testing, and therefore, can lead to misleading interpretation of ageing effects. One pile at each test site, was subjected to sustained loading at 60 % of the assumed failure load before being loaded to failure after 2 years. The impact of such sustained loading was generally to increase ageing effects for piles in clay, and reduce ageing effects for piles in sand. The overall results confirms that ageing effects are a significant positive factor to account for in future pile design practice.

According to a new concept proposed for correlating results of pile load tests with soil parameters, the total frictional capacity of pipe piles embedded entirely in clay is expressed as Qs = ?(_?m + 2 cm)As, where _?m is the mean effective vertical stress between the ground surface and the pile tip, cm is the mean undrained shear strength along the pile As is the pile surface area, and ? is a dimensionless coefficient. Using this concept, values of ? were determined for 43 previously reported load tests on steel pipe piles in clay and 4 additional tests not previously reported. For these tests including piles 8 to 333 ft long with capacities of 6 to 1760 kips, the frictional capacity coefficient ? was found to have a close correlation with embedded pile length. Application of the new criteria to typical clay profiles indicates lesser capacities for very stiff to hard clays the new criteria will predict capacities greater than the API procedure. Prediction of pile capacity by this new procedure based on effective vertical stress as well as undrained shear strength is shown to have greater reliability than by precious methods based solely on shear strength. INTRODUCTION The static method of predetermining the bearing capacity of piles is based on empirical data gathered from model studies and field load tests and interpreted in accordance with accepted soil mechanics theories. The ultimate pile capacity, Q, at a given penetration is the sum of Qs, the skin frictional capacity, and Qp, the end bearing or point capacity, so that(mathematical equation)(available in full paper) Where dAs and Ap represent, the embedded elemental surface area at any depth and the cross sectional area at the pile tip, respectively; f and q represent the unit skin friction and the unit end bearing, respectively. Various methods have been suggested previously for the prediction of f based on the shear strength of the clay. Mansur (10), Seed(18) and Tomlinson(19) have suggested that the unit skin friction, f, along the pile shaft in soft to firm clays is approximately equal to the undrained shear strength of the clay. From a comprehensive study of comparative behavior of friction piles, Peck (16) concluded that friction piles in stiff clays do not develop the full strength of the clay. Based on large number of pile load tests, Tomlinson(19) suggested that skin friction may be a function of pile material and therefore should be limited by adhesion between pile and soil; the adhesion values suggested were considerably lower than the shear strength of the clay. Woodward (23) reported that skin friction observed during a series of pile load tests was greater than the adhesion values suggested by Tomlinson, though it was less than the shear strength of the clay. In a later paper(20), Tomlinson concluded that the mobilized adhesion is also influenced by the sequence of soil strata into which the pile is driven. McCleland, Focht and Emrich(13) suggested a general predictive criteria giving depth some consideration in addition to shear strength. For stiff over-consolidated clay, the unit skin friction by this procedure is computed as 100 psf or 33 percent of the effective overburden pressure.

A test apparatus to measure the load transfer along the shaft of a model pile inserted in a specimen of clay is described. This apparatus allows independent control of the boundary stresses of a cylindrical clay specimen in the vertical and horizontal directions. The top and lateral surfaces of the clay specimen can deform freely and the load transfer zone on the pile is separated from the rigid base. The load transfer along the shafts of smooth and rough surface steel model piles in a kaolinitic clay under varying vertical and horizontal effective stress combinations was measured. The pile installation technique used was designed to minimize soil disturbance so that the failure criterion for pile shaft friction could be investigated. Normally consolidated and overconsolidated conditions, both in the vertical and horizontal directions, were considered in tests using eleven clay specimens with three different thicknesses and steel model piles of three different diameters. The results show that the angle of pile‐clay friction is independent of the vertical consolidation pressure in the clay, the overconsolidation ratio (both in the vertical and horizontal directions), and the length of the pile‐soil contact. The influence of pile diameter is not clearly established. The surface roughness of the pile clearly affects the pile‐clay friction angle, which is approximately equal for smooth surface piles to the Hvorslev true friction angle and for rough piles to the effective friction angle of the clay. There is no reasonable correlation with the undrained strength. The results support the effective stress approach.

Several rigorous, theoretical methods, using effective stress concepts, have been developed in the last five years to predict the ultimate shaft friction on piles supported in clays. The latest generation of these methods, which were developed as part of an industry sponsored project administered by Amoco, is described and critiqued. Computed results for four effective stress models and two conventional methods are compared with values measured on 10 piles at two field and one laboratory test sites. Predicted and measured values are in good agreement. The development of effective stress approaches to compute shaft friction provides for an improved understanding of factors that affect shaft friction and should lead in the future to improved techniques.

The ultimate bearing capacity of instrumented vertical single rigid model piles in homogeneous loose sand and soft clay under vertical eccentric and central inclined loads has been investigated. The results of these load tests provide a more realistic lateral soil pressure distribution on the pile shaft and better theoretical estimates of pile capacity under pure moment and under horizontal load. For intermediate eccentricities and inclinations of the load, the bearing capacity can be obtained from simple interaction relationships between the axial load and moment capacities and between the axial and horizontal load capacities, respectively. The influence of lateral soil pressures due to installation of displacement piles in clay is examined in relation to the ultimate load of the pile. The analyses are compared with the results of model tests and some field case records. Key words: bearing capacity, clay, eccentric loading, horizontal load, instrumentation, model test, pile, sand.

Long-term gains in the shaft capacity of displacement piles in clay are an important consideration for end-of-life decommissioning and platform life extension assessments. However, predicting ageing effects is a challenge as the underlying mechanisms are not fully understood, and there is clear variability in ageing rates observed at different sites. This paper presents results from a series of model-scale first-time axial pile tests performed at various times after installation in reconstituted medium plasticity clay. It is shown that, after consolidation due to pile installation is completed, peak frictions continue to increase and vary approximately with the logarithm of time. The post-consolidation gains in capacity are shown to be comparable to gains in shaft friction with time observed in a parallel field study involving driven piles in a natural deposit of the same clay. The agreement between the model scale and field scale tests suggests that ageing of pile shaft friction in clays can be estimated in model-scale investigations.

Previous studies on laterally loaded piles in clay have mainly focused on flexible and rigid piles. Little attention has been paid to semi-rigid piles (whose pile–soil stiffness lies somewhere between those of rigid and flexible piles), which may behave as either flexible piles or rigid piles, depending on the change in soil stiffness during cycling. This study aims to understand the cyclic lateral response of a repeatedly loaded semi-rigid pile in soft clay and the failure mechanisms of the soil around the pile, through a series of centrifuge model tests and three-dimensional finite element analyses using an advanced hypoplastic clay model. Numerical parametric studies were also performed to investigate the evolution of soil flow mechanisms with increasing pile rigidity. It is revealed that the semi-rigid pile behaved as if it were a flexible pile (i.e., flexural deformation dominated) during the first few cycles, but tended to behave like a rigid pile (i.e., rotational movement prevailed) during subsequent cycles, which progressively softened the surrounding soil. As a result, the mechanisms of soil flow around the semi-rigid pile exhibited an intermediate behaviour combining the mechanisms of both flexible and rigid piles. Three distinctive mechanisms were identified: a wedge-type mechanism near the surface, a full-flow mechanism (within the transverse sections) near the middle of the pile, and a rotational soil flow mechanism (in the vertical symmetrical plane of the pile) near the lower half of the pile. By ignoring the rotational soil flow mechanism, which has a much lower resistance than the full-flow mechanism, the American Petroleum Institute code (published in 2007) underestimated the cyclic bending moment and the lateral pile displacement by 10% and 69%, respectively. Application of jet grouting around the semi-rigid pile at shallow depth significantly altered the soil flow mechanism (i.e., it was a solely wedge-type mechanism around the grouted zone).

Pre-bored precast piles with an enlarged base (PPEB piles) have been increasingly used in China over the past 5 years. However, the distributions of the shaft and toe resistance of the pile are still unclear. In order to reveal the ultimate bearing capacity and load transfer mechanism of PPEB piles in clay, three full-scale ultimate static loading tests were carried out in Shanghai. The test piles were instrumented with vibrating wire strain gauges to enable accurate measurement of the local shaft resistance along the pile shaft during the loading tests. The tests results revealed that about 75% of the pile bearing capacity was provided by shaft resistance when the ultimate load was applied and the majority of the shaft resistance was mobilised along the lower part of the test piles. After load testing, the soils around the test piles were excavated to evaluate the reliability and constructability of PPEB piles. The applicability of conventional methods (α-method and β-method) in estimating shaft resistance was assessed. In comparison with instrumented bored piles, the local shaft resistance of the PPEB piles in the lower stiff clays was significantly higher.

The beneficial effects of suction on the break-out resistance of short, hollow piles (I.e. suction piles) in clays as well as sands have been verified by model tests. For sands increased break-out resistance is proportional to the developing suction inside the pile. For clays the increase in break-out resistance is due to a suction induced transition from local failure along the shaft to a reversed bearing capacity failure. INTRODUCTION As a part of a research programme on the utilization of suction piles as foundation for small light-weight platforms of mono tower type model tests were carried out in connection with the development of a design procedure. This paper presents model test results and their preliminary interpretation. Steel tubes closed at the top and installed in the seafloor by the differential water pressure obtained when pumping from the pile interior is termed suction piles. The literature on suction piles deals primarily with the conditions for installing and anchoring by the differential water pressure created from continued pumping. The possibility of utilizing the differential pressure created by the piles themselves, when subjected to uplift is investigated in this paper. TEST PROGRAMME The total test programme analyzed includes tests on Kaolin l, Nivaa Clay and G12 sand2 . Variable test parameters comprise:Pile diameterEmbedment/diameter ratioSoil typeUndrained shear strength of claysLoad type (monotonic, sustained)Regeneration time prior to loadingRate of monotonic breakoutMagnitude of sustained breakout load THEORETICAL CONSIDERATIONS Breakout from clays The short term breakout capacity of embedded objects has been suggested determined by the use of plasticity theory on general shear failure mechanisms. Model tests at embedment ratios less than 1 support this3, 4. At larger embedment ratios 3 different failure mechanisms, (i)-(iii), have been recognized 1,2 . as illustrated in Fig. 1:General shear failure in terms of a reversed bearing capacity failure is formulated [1]: (available in full paper) The value of N which is determined at the time of maximum breakout force F, and inserting a as the ultimate value in [II, is termed N1 The value of N that fulfils the vertical equilibrium equation [2] for the clay plug at the time of maximum breakout force, (inserting a as the ultimate value), is termed N2 (available in full paper)Local tension failure occurs when the inside soil-wall friction and suction exceed the tensile strength of the soil [3]. A clay plug inside the pile is then torn apart from the seafloor leaving a hole: (available in full paper) (iii) If no suction is allowed to develop inside the pile, the soil-wall friction cannot exceed the tensile strength of the soil and failure develops as a local shear failure along the pile shaft [4]:(available in full paper) Notations used in [1],[2],(3) and (4) are outlined in Fig.2.

The ultimate capacity of a novel type of piled foundation called an ‘impression’ pile has been investigated using centrifuge modelling techniques. The name derives from the small discrete impressions created in the side walls of a bored cast in situ pile to increase the soil/pile friction such that the impressions form nodules on the shaft of the concreted pile. The technology is suitable for bored piles in overconsolidated clay, such as London Clay. The experiments explored the influence of geometrical parameters such as the vertical spacing of the impressions, their number at each level and their shape. The data show a consistent increase in pile capacity of 40% when the impressions extend over 85% of the pile length. The ultimate capacity of the pile is primarily affected by the length of the pile which is impressed, the number of nodules at a given cross-section and the spacing of the nodules in the vertical direction, as long as this is greater than a threshold value. According to the experimental evidence, a block failure occurs for a spacing lower than this threshold value. Plastic failure mechanisms for the impression pile were established. These were used successfully to calculate the ultimate capacity of the impression piles tested with an error of less than 10%.

The existing foundations of a bridge can sometimes be reused. This may occur when a completely new bridge is built but also in bridge or superstructure widening projects. Reuse of existing foundations not only eliminates the costs associated with demolishing and disposing of old foundations, but also reduces the costs of the design and construction of new foundation elements. However, several challenges exist, including assessing the structural integrity, estimating the current capacity, estimating the remaining service life, and considering current design codes and specifications of the existing foundations, and clear guidelines for foundation reuse. The absence of foundation reuse guidelines by INDOT hinders the reuse of bridge foundations and prevents design consultants from designing new structures using existing foundations. In this project, comprehensive foundation reuse guidelines were developed in the form of flow charts based on a literature review on bridge foundation reuse design—including technical publications and existing standards and codes—and a set of analyses. The proposed guidelines include detailed guidance on inspection of the structural integrity of existing foundations, determination of as-built geometry of existing foundations, capacity estimation of existing foundations, minimum requirements for foundation reuse, and selection of foundation reuse solutions. The proposed guidelines for foundation reuse design were tested in an ongoing INDOT project. From the implementation project, we found that a complete site investigation that produces reliable estimation of soil profile and properties, is essential to determine whether there is reserve capacity in existing foundations. Additional site investigation is generally worth doing, not only to account for any strengthening of the soil that may have occurred over time, but also because of the greater accuracy in interpretation and analysis that results. We also found that use of the most current, cutting-edge methods can be useful in estimating the reserve capacity of existing foundations, and that design checks using different design codes can produce contrasting results.

A vertically loaded floating pile in clay affects a neighbouring pile by increasing the latter's displacement due to its own load. As a result, a group of rigidly capped piles exhibits a force/settlement ratio (‘vertical stiffness’) that is smaller than the sum of the individual stiffnesses of each pile – ‘efficiency’ in static stiffness less than 1. However, under dynamic steady-state loading the response of the pile group is an oscillatory function of frequency, and at certain frequencies a complete reversal of the static trend occurs, with the elastic dynamic group ‘efficiency’ exceeding not only the static ‘efficiency’, but also unity. To assess the realism of such behaviour, finite-element inelastic soil models were utilised to explore the influence of soil non-linearity on pile-to-pile interaction factors, under both static and dynamic loading. It is found that, with realistically inelastic undrained clay behaviour, the influence of a loaded pile on its neighbour diminishes radically with increasing amplitude of imposed displacement. The presence of a number of in-between piles, as well as the neighbouring pile's own rigidity, has no substantial effect on the interaction. The observed trends are explained by recourse to simple physical arguments. The diagrams provided for the pile-to-pile interaction factor are utilised to obtain the vertical dynamic impedance (i.e. stiffness and damping) of a 2 × 2 and a 3 × 3 rigidly capped pile group. It is found that these impedances are in accord with those resulting from three-dimensional analysis of the complete pile group. The difference between elastic and inelastic efficiency factors is shown to be substantial. The validity of the numerical results is strictly limited to piles in soft clays, whose resisting stress on the pile shaft equals their undrained shear strength.