Uplift capacity of pile group anchors

Experimental investigations on model single pile anchor and pile group anchors of configuration 2 × 2 subjected to uplift loads were conducted on dry Ennore sand, obtained from Madras, India. The embedment length to shaft width ratios, L/d = 20 and L/d = 30, and enlarged base width to shaft width ratios, B/d = 1,2,3, center to center spacing of pile anchors in the groups, 3d, 4d, 6d and 8d were used. The load displacement response, ultimate resistance and variation of group efficiency with L/d, B/d and spacing have been studied quantitatively and qualitatively. For short pile group anchors (L/d = 20), the isolation spacing appears to be at a spacing of about 4d to 6d and 8d for B/d = 1 and B/d = 2 and 3, respectively. For long pile group anchors (L/d = 30), the isolation spacing appears to be at a spacing of about 4d, 6d and 8d for B/d = 1, 2 and 3, respectively. The analytical model of limit equilibrium method has been proposed to predict the net uplift capacity of pile group anchors. The predicted results compare reasonably well with the experimental results.     

Uplift capacity of single piles: predictions and performance

The paper pertains to the development of a simple semi-empirical model for predicting the uplift capacity of piles embedded in sand. Various pile and soil parameters such as length (L), diameter (d) of the pile and angle of friction (ϕ), soil–pile friction angle (δ) and unit weight (γ) of the soil which have direct influence on the uplift capacity of the pile are incorporated in the analysis. A comparative assessment of the ultimate uplift capacity of piles predicted by using the proposed theory and some of the available theories are made with respect to each other and with reference to the measured values obtained from model tests in the laboratory. For this purpose experimental data have been collected from the literature and also from model tests conducted as a part of the present investigation. The study shows the proposed model has an excellent potential in predicting the uplift capacity of piles embedded in sand that are consistent with model pile test results.     

Model Pile Groups Under Oblique Pullout Loads – an Investigation

Experimental investigations on model pile groups of configuration, 1 × 1(single pile), 3 × 1, 2 × 2 for embedment length to diameter ratio, L/d = 38, were conducted in uniform dry medium dense Ennore sand. The spacing of piles in the groups varied from 3 to 6 pile diameter. Soil–pile friction angles were δ = 16° and 28°. The pile groups were subjected to oblique pulling loads at angles θ = 0°, 30°, 60° and 90° with the vertical central axis of the groups. The load–displacement response, oblique ultimate pulling resistances have been qualitatively and quantitatively studied. The inclinations of the load, at which maximum oblique resistance for the groups were observed, have been reported. Predictions of ultimate resistance of pile groups under uplift, lateral and oblique pulling loads have been carried out respectively by the methods of Patra and Pise (2002) (Electronic Journal of Geotechnical Engineering, 8, Bundle B), Patra and Pise (2001) (Journal of Geotechnical and Geoenvironmental Engineering ASCE, 127(6), 481–487) and Chattopadhyay and Pise (1986c) (Proceedings of IST East Asian Conference on Struct Engineering and Const., Vol. 1, pp. 1632–1641). A comparison of the measured values of the Writers and others with the predicted values showed reasonable agreement.     

Effect of Arching on Uplift Capacity of Single Piles

An analytical method has been proposed to predict the ultimate uplift capacity of single vertical piles embedded in sand considering arching effect. The present analysis takes into consideration of various pile and soil parameters such as length (L), diameter (d) of the pile, angle of internal friction of soil (ϕ), soil pile friction angle (δ) and unit weight of soil (γ). A modified value of coefficient of lateral earth pressure in uplift has been developed considering the arching effect of soil. A comparative assessment of the uplift capacity of piles predicted by using proposed theory and the existing available theories is made with the existing field and model test results. It has been observed that the present model considering the arching effect predicts the results closer.     

Experimental investigation of the behavior of pile groups in sand under different loading rates

The behavior of pile groups in sand under different loading rates is investigated. A total of 60 tests were conducted in the laboratory using model steel piles embedded in a medium dense sand. The model piles have an outside diameter of 25 mm and embedment length of 500 mm. Five different configurations of pile groups (2 × 1, 3 × 1, 2 × 2, 2 × 3, 3 × 3) with center to center spacing between the piles of 3d, 6d and 9d (d is the pile diameter) were tested. The piles were subjected to axial compressive loads under four different loading rates: 1.0, 0.5, 0.1 and 0.05 mm/min. Test results indicated that the axial compressive capacity of pile group increases with the loading rate such that the pile capacity versus logarithm of loading rate data plot approximately along a straight line. The slope of this line increases as the number of piles in a group increases and it decreases by increasing the spacing between piles in a group.     

Experimental Investigations on the Behaviour of Pile Groups in Clay Under Lateral Cyclic Loading

This paper presents the results of two-way cyclic lateral load tests carried out on model pile groups embedded in soft marine clay. The tests are conducted on 1 × 2, 2 × 2 and 3 × 3 pile groups having length to diameter ratio (L/D) of 15, 30 and 40 with the spacing to diameter ratio (S/D) of 3, 5, 7 and 9. The experimental results are presented in the form of load–deflection curves and bending moment profiles. Cyclic group efficiency, critical spacing, critical cyclic load level and cyclic p-multipliers are evaluated. It is found that the lateral capacity of the 3 × 3 group reduces by about 42% after 50 cycles of loading. The cyclic p-multipliers of 3 × 3 pile group are found to be 0.41, 0.25 and 0.29 for leading, intermediate and rear rows respectively. The test results are compared with the numerical analysis carried out by p–y method using GROUP program. The analysis carried out with experimentally evaluated p-multipliers predicts load—deflection and bending profiles of pile groups reasonably well, but underestimates the depth to maximum bending moment by about 15%.     

Effect of Compressive Load on Uplift Capacity of Cast-Insitu Bored Piles

Field tests were conducted to study the effect of compressive loading on the uplift capacity of single piles embedded in silty sand. The test program consists of four instrumented cast in situ axial pile load tests in compression, pure tension and tension with 25 and 50% of compressive load of ultimate capacity in compression. The experimental results indicate that the net ultimate uplift capacity of single pile decreases with increase in compressive load. The shaft friction is non linear in nature. It observed that as the compressive load increases the shaft friction along the length of pile decreases.     

Response of Pile Groups Driven in Sand Subjected to Combined Loads

Although the loads applied on piles are usually a combination of both vertical and lateral loads, very limited experimental research has been done on the response of pile groups subjected to combined loads. Due to pile–soil–pile interaction in pile groups, the response of a pile group may differ substantially from that of a single pile. This difference depends on soil state and pile spacing. This paper presents results of experiments designed to investigate pile interaction effects on the response of pile groups subjected to both axial and lateral loads. The experiments were load tests performed on model pile groups (2 × 2 pile groups) in calibration chamber sand samples. The model piles were driven into the sand samples prepared with different relative densities using a sand pluviator. The combined load tests were performed on the model pile groups subjected to different axial load levels, i.e., 0 (pure lateral loading), 25, 50, and 75% of the ultimate axial load capacity of the pile groups, defined as the load corresponding to a settlement of 10% of the model pile diameter. The combined load test results showed that the bending moment and lateral deflection at the head of the piles increased substantially for tests performed in the presence of axial loads, suggesting that the presence of axial loads on groups of piles driven in sand is detrimental to their lateral capacity.     

An Experimental Study on Bearing Capacity of Steel Open Ended Pipe Pile with Exterior Wings Under Compression Load

The present study investigates the increasing in ultimate pile capacity and studied the soil plugging phenomenon and the incremental filing ratio for a modified type of open-ended pipe pile. The modification performed by adding steel plates as wings with special dimensions and fixed on the exterior face of the pipe pile wall at a location near the pile tip with specified dimensions. Five wings have used for each new model of pipe pile. These wings distributed in equal spacing along with the circumstances of the exterior wall of the open-ended pipe piles. The efficiency of the proposed type studied by modelling and manufacturing twelve piles (40 mm diameter, L/D = 15 and L/D = 20). Complete setup manufactured for installing and loading the piles in a constant rate of penetration. The model piles installed in poorly graded loose dry sand. The obtained results show that the proposed type has a higher ultimate bearing capacity. The percentage of increase reaches more than 50%. The development of the load capacity is due to the three effects. The first is increases of the exterior shaft friction, and the second effect creates a new end-bearing capacity under the constrained soil between the exterior wings. And the third effect is developing the end-bearing capacity under the soil plug inside open-ended pipe pile due to the first and the second effects.

     

Effect of Geotextile Ties on Uplift Capacity of Anchors Embedded in Sand

This paper presents the results of experimental investigation on the effect of geotextile ties on uplift capacity of anchors embedded in sand. Uplift capacity of anchor increases with increase in embedment depth to base diameter (H/D) ratio irrespective of type of anchor. With the introduction of tie to anchors, uplift capacity of anchors increases and optimum number of layers of ties is found to be 2. A non linear power model has been developed to predict the uplift capacity at any settlement (Q _R) of anchors with tie in terms of uplift capacity at any settlement (Q _URs) of anchor without tie, H/D ratio, number of layers of tie and displacement to base diameter ratio (Δ/D). The model is applicable for predicting Q _R having the values of Q _RS, H/D, N and Δ/D in the range of 0.257 ≤ Q _URs ≤ 1.420, 1.5 ≤ H/D ≤ 3.0, 1 ≤ N ≤ 4, 0.8 ≤ Δ/D ≤ 8.     

Ultimate uplift capacity of model rigid metal piles in clay

Summary Laboratory model test results for estimation of the ultimate uplift capacity of rigid metal piles embedded in a compacted near-saturated clayey soil are presented. The length-diameter ratio of the piles was varied from 10 to 15. The direction of the uplift load on the pile was varied from 0 to 50°. Based on the present test results and the results of existing model studies, an empirical relationship for estimating the pile uplift capacity has been presented.     

Palaeomagnetic and rock magnetic study of charnockites from Tamil Nadu,India, and the ‘Ur’ protocontinent in Early Palaeoproterozoic times

Palaeomagnetic and magnetomineralogical results are reported from charnockites in basement terrane at the eastern sector of the WSW–ENE granulite belt of South India. Magnetite is the dominant ferromagnet identified by rock magnetic and optical study; it is present in several phases including large homogeneous titanomagnetites and disseminated magnetite in microfractures linked to growth stages ranging from primary charnockite formation to uplift decompression and exhumation within the interval ～2500–2100 Ma. Several components of magnetization are resolved by thermal demagnetization and summarized by four pole positions; in the northern (Pallavaram) sector these are P1 (33°N, 99°E, dp/dm = 8/9°) and P2 (79°N, 170°E, dp/dm = 3/6°), and in the southern (Vandallur) sector they are V1 (23°N, 116°E, dp/dm = 8/9°) and V2 (26°S, 136°E, dp/dm = 5/10°). These magnetizations are linked to uplift cooling of the basement and unblocking temperature spectra suggest acquisition sequences P1 → P2 and V1 → V2 in each case implying movement of the shield from higher to lower palaeolatitudes sometime between 2500 and 2100 Ma. Palaeomagnetic poles from the cratonic nuclei of Africa, Australia and India all identify motion from higher to lower palaeolatitudes in Early Palaeoproterozoic times, and this is dated ～2400 and ～2200 Ma in the former two shields. The corresponding apparent polar wander (APW) segments match the magnetization record within the charnockite basement terranes of southern India to yield a preliminary reconstruction of the ‘Ur’ protocontinent, the oldest surviving continental protolith with origins prior to 3000 Ma. Although subject to later relative movements these nuclei seem to have remained in proximity until the Mesozoic break-up of Gondwana.     

Inclined single piles under vertical loadings in cohesionless soil

Physical-scaled model testing under 1 g conditions is carried out in obtaining the vertical response of fixed head floating-inclined single piles embedded in dry sand. Practical pile inclinations of 5° and 10° besides a vertical pile (0°) subjected to static and dynamic vertical pile head loadings are considered. To account for the effects of soil nonlinearity as well as the soil–pile interface nonlinearity on the response of piles, a range of low-to-high magnitude of pile head displacements is considered for the static case while a varying amplitude of harmonic accelerations for a wide range of frequencies is considered for the dynamic case. Experimental results are obtained in the form of pile head stiffnesses and strains generated in the pile under both the static and dynamic loadings. Results suggest that the nonlinear behavior of soil as well as the nonlinearity generated at the interface between the soil and the pile as the result of applied loading considerably affect the response of piles. The soil–pile interface nonlinearity that governs the slippage of pile shows a clear influence on the pile head stiffnesses by providing two distinct values of stiffnesses corresponding to the push and the pull directional movement of piles; the two values are significantly different. Axial and bending strains generated in the piles show expected dependency on the amplitude of applied loading; the pile head-level bending strain increases almost linearly with the increase in the angle of pile inclination.

     

Effect of Compressive Load on Oblique Pull-out Capacity of Model Piles in Sand

Model tests on steel piles embedded in sand were carried out in the laboratory to study the effects of compressive load (i.e. 0, 25, 50, 75, and 100% of their ultimate capacity in compression) on oblique pull-out capacity of piles. The model piles were of 20 mm × 20 mm cross section, which had an embedded length of 400 and 600 mm. The pull was applied at an inclination of 0°, 30°, 60° and 90° with vertical axis of the piles. The experimental results indicate that the net oblique pull-out capacity of piles decreases with increase in % of compressive load and the decrease depends on the magnitude of the compressive load. Semi-empirical methods, based on experimental results, have been suggested to determine the oblique pull-out resistance of piles subjected to static compressive loads. A comparison of predicted values of the ultimate oblique resistance by proposed methods of analysis with experimental values, and also with those reported by others, showed reasonably good agreement.     

Three-dimensional numerical and analytical study of horizontal group of square anchor plates in sand

In this paper, numerical and analytical methods are used to evaluate the ultimate pullout capacity of a group of square anchor plates in row or square configurations, installed horizontally in dense sand. The elasto-plastic numerical study of square anchor plates is carried out using three-dimensional finite element analysis. The soil is modeled by an elasto-plastic model with a Mohr–Coulomb yield criterion. An analytical method based on a simplified three-dimensional failure mechanism is developed in this study. The interference effect is evaluated by group efficiency η, defined as the ratio of the ultimate pullout capacity of group of N anchor plates to that of a single isolated plate multiplied by number of plates. The variation of the group efficiency η was computed with respect to change in the spacing between plates. Results of the analyses show that the spacing between the plates, the internal friction angle of soil and the installation depth are the most important parameters influencing the group efficiency. New equations are developed in this study to evaluate the group efficiency of square anchor plates embedded horizontally in sand at shallow depth (H = 4B). The results obtained by numerical and analytical solutions are in excellent agreement.     

A New Computer Program for Pile Capacity Prediction using CPT Data

An interactive computer program “GLAMCPT” is developed for application in soil profiling and prediction of pile load capacity using cone penetration test (CPT) and laboratory soil test results. GLAMCPT calculates pile capacity according to 10 selected methods from European design codes, refereed international publications and recommendations of professional institutions. To demonstrate the capabilities of the program, a database of comprehensive ground investigation and full-scale pile tests in sand, at a Belgian site, is analysed using GLAMCPT. The database comprises 11 static tests and 12 dynamic tests on piles of different construction techniques, including driven pre-cast concrete piles and screwed cast in-situ piles, installed using 5 different procedures. Prior to pile installation, CPTs were carried out at each proposed pile location. Comparison of GLAMCPT predictions with the observed pile capacities reveals that the most accurate of the existing methods yields an average, μ, of predicted to observed pile head capacity [P_uh(p)/P_uh(m)] equal to 0.94. The most consistent method produces a coeffcient of variation (COV) of [P_uh(p)/P_uh(m)] equal to 0.1 and ranking index (RI) of 0.08. Parametric studies have been carried out using GLAMCPT to formulate an improved predictive method, which yielded: μ = 0.99, COV = 0.07 and RI = 0.04.     

桩-土界面特性对砂土地基中抗拔桩承载特性影响的模型试验研究

针对砂土地基中不同L/d的抗拔桩进行了模型试验,模型桩采用了3种不同界面,结合界面剪切试验探讨了桩-土界面特性对抗拔桩荷载-位移曲线、极限承载力及残余承载力的影响,得到了以下几点结论：模型桩长径比L/d不同时,每种界面抗拔桩荷载-位移曲线是类似的,光滑钢桩上拔力达极限承载力后保持不变,粗糙界面模型桩上拔力达极限承载力后降低,直至达残余承载力;抗拔桩达极限承载力时的静止土压力系数K受桩-土界面粗糙程度的影响,界面越粗糙,K值越大;界面越粗糙,抗拔桩残余承载力与极限承载力的比值越小,该比值随L/d的变化较小;对于粗糙界面抗拔桩,残余承载力与极限承载力的差别一部分是由于桩-土界面摩擦角的降低引起的,另一部分是抗拔桩达到极限承载力后作用在桩表面上的水平土压力降低引起的。     

桩-桶基础抗拔承载力分析

根据新型基础一桩-桶基础在上拔荷载作用下的颗粒流模拟试验结果,分析了桩桶基础在上拔荷载作用下土体的破坏过程,并对颗粒流模拟试验的颗粒的破坏面进行拟合,提出桩桶基础的上拔承载力计算模式。桩-桶基础的极限上拔承载力由破坏土体侧表面抗剪强度在竖直方向投影的集合及其包围土体土重和桩-桶基础自重组成,建立了桩-桶基础的上拔承载力计算公式,分析了上拔承载力影响因素。     

软土地基中扩底抗拔中长桩的极限承载力分析

扩底抗拔桩的研究和应用以短桩居多,并且大都以砂土或软岩为持力层,对于软土地基中扩底抗拔桩的承载力研究较少。基于扩大头局部剪切滑移面假设的极限平衡法,推导出扩底抗拔中长桩在分层地基中极限承载力的简化计算公式;结合有关工程实例的原位试桩结果,对扩大头影响高度进行了确定,同时还探讨了所提出方法在超长桩中的适用性。研究结果表明,提出的简化分析方法能合理地揭示上海软土地区扩底抗拔中长桩的破坏机制,并获得承载力变化的一般规律。     

On the Study of Pile Driving Formula Based on Blow Counts

Accuracy of predicting pile capacities by pile driving formulas have been investigated. Five test piles were driven up to a depth of about 9 m of clay deposit and the penetrations due to final blows were recorded. The pile bearing capacity of each pile was predicted using 6 different pile driving formulas and the predicted pile capacity was compared with measured pile capacity from the pull up tests. Hiley formula, Modified Engineering News Record (ENR) formula, Janbu formula, Dutch formula, Danish formula, and Gates formula were used. The performance and accuracy of each formula was evaluated and the correlation coefficient of each pile driving formula was determined for a more accurate pile capacity prediction. Methods used to evaluate the performance of each formula were; (1) the best fit line for Q _p versus Q _m (2) cumulative probability for Q _p/Q _m and (3) the arithmetic mean and standard deviation for Q _p/Q _m. From the study, it was found that using Dutch formula provided the most accurate pile capacity estimate compared to the other formulas with an average of 7% deviation from value obtained from the field pull up test. It was followed by the Danish formula, Janbu formula, Hiley formula, Modified ENR formula, and Gates formula. The ability to predict the accuracy of estimating pile capacity using an appropriate method is very important and valuable to contractors, developers, geotechnical engineers, and manufacturers.