Static Pile Load Tests Research Articles

A Correlation Between Dynamic and Static Pile Load Test Results

A Correlation Between Dynamic and Static Pile Load Test Results

Reduction Factor for the Unloading Point Method at Clay Soil Sites

Full-scale testing can be an integral component of quality control/quality assurance for projects involving construction of deep foundations. Rapid load tests are being used in the deep foundation industry as a method for assessing the axial static behavior of deep foundations. Since rapid load tests involve dynamics, inertial and damping forces must be considered in analyzing measured pile response to estimate the static pile response. The unloading point method (UPM) is typically used for this purpose. Generally considered a consequence of load rate effects in clays, results from the UPM must be further modified by a reduction factor to obtain a reasonable estimate of the static pile response. A reduction factor of 0.65 applied to the UPM for clay soil sites has been recommended by others. However, a review and analysis of readily available literature reporting static and rapid pile load test results at sites predominantly consisting of clay soils indicate that an average reduction factor of 0.47 is more appropriate. Rapid load testing should be used judiciously. When using the UPM to estimate static pile capacity from rapid load tests in clay, static load tests should be performed to validate the reduction factor used to interpret rapid load tests.

Pile foundation analysis and design using experimental data and 3-D numerical analysis

Capacity based design of pile foundations limits the soil-structure interaction mechanism to group bearing capacity estimation, neglecting, in most cases, the contribution of the raft. On the other hand, a straightforward, nonlinear, 3-D analysis, accounting for soil and structural nonlinearities and the effects arising from pile–soil–pile interaction, would be extremely high CPU-time demanding and will necessitate the use of exceptionally powerful numerical tools. With the aim of investigating the most efficient, precise, and economical design for a bridge foundation, a hybrid method, compatible with the notion of sub-structuring is proposed. It is based on both experimental data and nonlinear 3-D analysis. The first step to achieve these targets is a back-analysis of a static pile load test, fitting values for soil shear strength, deformation modulus, and shear strength mobilization at the soil–pile interface. Subsequently, the response of 2 × 2 and 3 × 3 pile group configurations is numerically established and the distribution of the applied load to the raft and the characteristic piles is discussed. Finally, a design strategy for an optimized design of pile raft foundations subjected to non-uniform vertical loading is proposed.

High-Capacity Piles at the Stony Creek Bridge Project

High-capacity piles (HCPs) offer a highly attractive foundation option to geotechnical designers, but regular acceptance and wider implementation remain elusive. Until recently, large-diameter high-capacity piling was mainly used in the offshore oil industry; there the unique environment and design requirements led to piling where capacity did not need to be verified. When larger-diameter piling began to emerge as a cost–benefit option onshore, little was known about the unique behavior. Although there were no scalability problems in dynamic stress measurement and energy calculations, engineers found dynamically estimated pile capacity to be unreliable. To provide the required capacity information for HCPs, the California Department of Transportation developed static axial pile load tests for piles up to 8,000 kips (35 MN). Through a case study of the Stony Creek Bridge Project, the California Department of Transportation presents its experience with the advantages and limitations of HCPs. The 8-ft (2.44-m) diameter and 170-ft (51.8-m) long steel piles at the Stony Creek Bridge are reputed to be the largest piles ever driven on land.

Pile Instrumentation Using Retrievable Sensors

Strain gauges are normally used to monitor the shortening or compression of pile during static pile load test. For concrete spun pile, the technique used either by incorporating high temperature-resistant strain gauges into the heat-cured production process of the spun piles or by installing an instrumented steel pipe into the hollow core of the spun piles followed by cement grout infilling. The former is extremely unpopular due to high cost of these gauges and the uncertainty over their ability to survive the pile production and driving processes. The shortcoming of the other technique is the infilling of cement grout substantially alters the structural properties of the piles, thus rendering their load-response behaviour significantly different from that of the actual working piles. To address the difficulties of the above techniques a new method was recently developed by the authors, which uses retrieval sensors instead of strain gauges (which have to be sacrificed in every test). The method also has the ability to monitor loads and displacements at various levels along the pile shaft and toe of instrumented piles. Results of field tests show high quality, reliable and consistent data, clearly far exceeding the capability of both conventional and approximate methods of using strain gauges.

Equivalent pile load–head settlement curve using a bi-directional pile load test

An equivalent pile load–head settlement curve to predict pile capacity has been obtained by using test results from the Osterberg Cell (O-cell). The method assumes that the pile is a rigid body and predicts very stiff pile behavior. This study describes a new method for predicting an equivalent pile load–head settlement curve considering the elastic shortening of piles. The static pile load test is compared to the O-cell test using two piles constructed 10 m apart (center-to-center distance). A measured O-cell test settlement consists of an elastic shortening under the load applied inside the pile and a net settlement. The measured net settlement is added to the elastic shortening, which has a different unit skin friction distribution under the top-down load on the pile head, to predict the equivalent pile load–head settlement curve. The new equivalent pile load–head settlement curve, considering the elastic shortening under top-down load, is similar to the pile load–head settlement curve obtained by the static pile load test before the pile yields. The new method can be used to effectively estimate the pile head settlement as well as pile capacity by using the bi-directional load test results.

Comparing dynamic and static test results of bored piles

Owing to increasing time, cost and transportation difficulties, Irish contractors are seeking an alternative to conventional static pile load tests. As a result several firms have adopted dynamic testing techniques to supplement and in some cases replace conventional static tests. In order to assess the reliability of the systems and to address the concerns of owners and consulting engineers, a database comprising 43 pairs of static and dynamic tests on piles from 24 sites around Ireland has been assembled. The database was limited to medium- to large-diameter continuous flight auger (CFA) piles, typically 450 mm to 600 mm in diameter. Comparisons between actual measured settlement in static tests and predicted settlement for dynamic tests showed a clear pattern of underestimation of settlement by the dynamic tests, with the ratio between the two values typically varying between 0·4 and 1·2, with an average of about 0·8. This is due to the limited energy being imparted to the pile in the dynamic test. The results were best for piles in rock and worst for piles in clay. Scatter in the results increased with increasing pile load. However, as all of the measured settlements were modest, the absolute numerical difference between the actual and predicted values was typically less than 3 mm, which may not be significant in many practical cases. For piles supporting non-sensitive structures, in areas where the ground conditions are well known, and provided the ratio of the drop weight to the pile weight and to the pile specified working load is at least 50% and 1·5% respectively, it seems possible that static tests can be replaced by dynamic ones. However, for structures sensitive to settlement or for those in areas where ground conditions are uncertain, then it is suggested that the dynamic test should be calibrated by at least one static test.

Performance of bored piles with different construction methods

The design of bored piles in residual soil is usually based on empirical correlation with the results of the standard penetration test, regardless of the construction method of the bored pile. This paper presents the pile load test results of two bored piles with different construction methods. A preliminary bored pile of 1000 mm diameter was constructed using a wet method with bentonite as the stabilising fluid. The static pile load test result shows that the shaft friction could be overestimated by the empirical method. A 1200 mm diameter bored pile was constructed about 5 m away from the preliminary bored pile but using the dry method. The static pile load test result shows that higher shaft friction can be obtained. The empirical method could underestimate the shaft friction in this case.

Application of an effective stress design method to concrete piles driven in Mexico City clay

Piled foundations are used widely in the lake clay zones of Mexico City, with “friction” piles being driven into soft diatomaceous clay deposits that show an extraordinary range of mechanical properties. The paper assesses the applicability of the Imperial College pile (ICP) design method to the concrete piles driven in these deposits. After the ICP methodology and Mexico City ground conditions are reviewed, an experimental study is reported of the key soil properties required for the ICP procedures: yield stress ratio, unit weight, interface friction angle (δ), sensitivity (St), and intrinsic properties. The new data are then applied to a specially assembled database of static pile load tests from different Mexico City clay locations, with allowance being made for site-specific details and local installation procedures. A quantitative assessment shows that the ICP methodology is free from significant bias and leads to more reliable estimates of static capacity than the total stress (α) design method routinely used in Mexico City.Key words: validation, database, static axial pile capacity, driven friction pile, effective stress design, silty clay.

Response evaluation for horizontally loaded fixed-head pile groups using 3-D non-linear analysis

The response of laterally loaded pile foundations may be significantly important in the design of structures for such loads. A static horizontal pile load test is able to provide a load–deflection curve for a single free-head pile, which significantly differs from that of a free- or fixed-head pile group, depending on the particular group configuration. The aim of this paper is to evaluate the influence of the interaction between the piles of a group fixed in a rigid pile cap on both the lateral load capacity and the stiffness of the group. For this purpose, a parametric three-dimensional non-linear numerical analysis was carried out for different arrangements of pile groups. The response of the pile groups is compared to that of the single pile. The influence of the number of piles, the spacing and the deflection level to the group response is discussed. Furthermore, the contribution of the piles constituting the group to the total group resistance is examined. Finally, a relationship is proposed allowing a reasonable prediction of the response of fixed-head pile groups at least for similar soil profile conditions. Copyright © 2005 John Wiley & Sons, Ltd.

Influence of reaction piles on the behaviour of a test pile in static load testing

This paper employs a simplified analytical method to investigate the influence of reaction piles on the load–displacement behaviour of a test pile in static load testing. A parametric study is conducted to examine the effects of factors such as pile spacing ratio, pile slenderness ratio, and pile soil stiffness ratio. Several soil profiles are considered in the study. The parametric study also includes the cases of static load testing in the lateral direction. The correction factors for the initial pile head stiffness obtained from static pile load tests with the use of reaction piles are given in charts. Furthermore, analyses of centrifuge modelling of axially loaded piles are carried out using the simplified analytical method. Good agreements between the test results and the analysis results are demonstrated, and the applicability of the correction factors is verified.Key words: reaction piles, interaction, initial pile head stiffness, static vertical load testing, static lateral load testing, simplified analytical method.

Load-settlement behaviour of socketed piles in sandstone

Nine Static vertical pile loading tests were carried out in Abu-Dhabi, the United Arab Emirates in order to assess the end bearing capacity and settlement of piles bored in slightly silty sands overlying soft rocky horizons like sandstone and calcarenite. The paper is aimed at presenting the results of interpretation of the pile tests in order to estimate the end bearing capacity in correlation with the unconfined compressive strength Rc and the rock quality designation (RQD). Load-settlement behaviour is also studied through an analysis by the elastic methods commonly used in pile design.

Probability Method Applied to Dynamic Pile-Driving Control

An energy approach previously developed as an alternative to the conventional dynamic pile­ driving control methods (e.g., ENR and Gates formula) has been formulated in a probabilistic framework. The Smith-model parameters incorporated in the energy approach, including the damping factor and the quake, were treated as random variables whose mean values, standard deviations, and probability distribution functions were determined from a statistical analysis of a database of 204 pile cases. The Smith parameters for each pile case were derived from the back-calCUlation process in relation to the Case Pile Wave Analysis Program (CAPWAP) signal-match procedure. The first-order second-moment (FOSM) method was used in the probabilistic energy approach to compute the mean value and standard deviation of the pile capacity. A detailed description is given of the database, the statistical analysis results, and the mathematical formulation of the probabilistic energy approach. The developed probability approach was applied to the 204 pile cases. The agreement between the computed pile-capacity and the static load test results is either slightly better than or equally as good as the comparison between the traditional CAPWAP deduced pile-capacity and the static pile-load test results. The required inputs in the probabilistic energy approach are the energy delivered to the pile head during each hammer blow, the blow count per 0.3 m of pile penetration, the maximum velocity at the pile head, and the pile dim~nsions and elastic properties. The required instrument for this method is much simpler compared to that reqUired by the Case and CAPWAP methods, leading it to be an attractive and economic method for pile-driving control in the field.

New method of static pile load test system VUIS-P : Cemak, B; Hlavacek, J; Klein, K Proc 1st International Geotechnical Seminar on Deep Foundations on Bored and Auger Piles, Ghent, 7–10 June 1988P291–302. Publ Rotterdam: A A Balkema, 1988

New method of static pile load test system VUIS-P : Cemak, B; Hlavacek, J; Klein, K Proc 1st International Geotechnical Seminar on Deep Foundations on Bored and Auger Piles, Ghent, 7–10 June 1988P291–302. Publ Rotterdam: A A Balkema, 1988