Bi-directional Load Test Research Articles

Interpretation of bi-directional tests on piles with the evaluation of stress relief at the pile toe

This paper presents the interpretation of bi-directional load tests performed on three auger piles, in the city of São Paulo, Brazil, using a method based on transfer functions for the shaft and toe. Elastic shortenings of the shaft were directly measured through a displacement indicator at the pile top and two telltales at the upper and bottom plates of the expansive cell. The equivalent top-down load-settlement curves were estimated and compared with two other methods from the literature, one which considers the pile infinitely rigid; and the other, which takes the pile elastic shortening into account. The curves resulted in good agreement considering the pile compressibility. Yet for the infinitely rigid pile, the settlements resulted in up to 75% smaller. Furthermore, the influence of stress relief on the toe behavior due to shaft lifting was investigated. For the cases studied, involving bored and auger piles with the slenderness ratio (Ls/r) greater than 20, the percentage of this effect was generally small, up to 5% of the toe load, being negligible for practical uses.

Geotechnical Evaluation of Multi-Layered Simsima Limestone using Bi-Directional Static Load Test (BDSLT)

Abstract Static load tests are vital to assess the genuine behaviour of piles to substantiate the geotechnical design parameters and construction approaches. A load testing program was undertaken to determine the working capacity, skin friction, and settlement of drilled shafts in Simsima limestone, the predominant foundation stratum in Doha, Qatar. The weathering profile of the Simsima limestone is complex with the degree of weathering likely to increase with depth. This paper presents the performance of three preliminary instrumented bored cast-in-situ test piles (Type 1, Type 2 and Type 3) by Bi-directional static load test (BDSLT) using sacrificial hydraulic jacks, socketed in the weathering zones of Simsima limestone formation in Lusail FIFA World Cup 2022 stadium project, Qatar. The test piles were instrumented with vibrating wire strain gauge technology and were load tested using the BDSLT loading method to determine the pile-rock interactions within the various weathering zones of Simsima limestone. The unit shaft friction values obtained from the load test are compared with the initial design values. Results indicate that the skin friction was increased from values reported before pile load testing and was used for the optimization of pile design. BDSLT method with strain gauge instrumentation is a modern tool in value engineering of pile design to provide accurate information to the Qatar infrastructure foundation industry.

Bi-Directional Static Load Tests of Pile Models

This work examined a new method of bi-directional static load testing for piles, referencing the Osterberg test. Measurements were taken, on a laboratory scale, using six models of piles driven into a box filled with sand. This method allowed for separate measurements of pile base and pile shaft bearing capacities. Based on the results, the total pile bearing capacity and equivalent Q–s diagrams were estimated. The results obtained show that the structure of the equivalent curve according to Osterberg is a good approximation of the standard Q–s curve obtained from load tests, except for loads close to the limit of bearing capacity (those estimates are also complicated by the inapplicability and ambiguity of a definition of the notion of limit bearing capacity); the equivalent pile capacity in the Osterberg method represents, on average, about 80% of the capacity from standard tests.

Assessment of Pile Capacity Using Bidirectional Static Load Test (BDSLT)

Bidirectional Static Load Test (BDSLT), an innovative load test method, two static load tests performed simultaneously in opposite directions, adopting sacrificial hydraulic cells was globally used in the deep foundation industry. This paper presents the results of value engineering and optimization of the foundation design for the proposed pile tests from the Dubai Creek Harbour Project on plot no. A004, Dubai, UAE. The main objective of this load test was to proof-load the test pile to its ultimate capacity using the BDSLT method. For this purpose, 4 × 900 tons of capacity hydraulic cell/jacks was utilized. Two test piles of 1500 mm diameter were used, namely PTP1 and PTP2. Five levels of Geokon USA model 4200 vibrating wire-type strain gauges comprising four units at each level were also installed on the test pile to measure strain at nominated locations, and from that, load and unit skin frictions were derived. Results indicate that the skin friction was increased from values reported before pile testing. Pile capacities were optimized using the skin friction values obtained from the load test results.

Prediction of compression and deformation behaviours of gangue backfill materials under multi-factor coupling effects for strata control and pollution reduction.

Coal mining causes serious ecological and environmental damage. The crushed gangue is backfilled into underground goaf, which not only inhibits mining-induced subsidence but also reduces accumulation of waste on the ground: however, the effects of backfilling with gangue backfill materials in goaf are affected by a combination of multiple factors. To predict compression-induced deformation (CID) of gangue backfill materials, key factors influencing compression and deformation characteristics of gangue backfill materials in an underground confined space (lithology, particle size distribution, lateral stress and lateral loading times) were determined. Moreover, two key factors, namely, lithology and particle size distribution of gangue backfill materials, were quantified. Based on orthogonal test design, the compression characteristics of gangue backfill materials were measured under different stress levels and coupling effects of the four key factors by utilising a self-made bidirectional loading test system for bulk materials. Furthermore, through regression, the relationships of the four key factors and axial strain were determined as well as undetermined parameters in axial stress-axial strain equations and axial stress. Based on this, an equation for predicting stress-strain relationship during compression-induced deformation of gangue backfill materials under multi-factor coupling effects was established. Comparison with the orthogonal test results shows that this equation can predict compression-induced deformation of gangue backfill materials in goaf. This is beneficial to providing a basis for predicting strata movement and surface subsidence and guidance for designing backfilling process, thus protecting the surface environment.

Value engineering using load-cell test data of barrette foundations – La Maison, Dubai

This paper presents a case history of the design, Osterberg load-cell testing, value engineering and revised design by back-analysis of reinforced concrete barrette foundations, resulting in a reduction of barrette length by about 11%, for the 100-storey La Maison tower in Dubai. Rectangular barrettes were selected as an efficient foundation system to transfer 55 MN load per barrette after comparing the design with equivalent-sized circular piles. Bidirectional static load test results were back-analysed using a finite-element model to modify the barrette length. The stiffness parameters were modified after calibrating against the load-cell results, which was matched with a 20% modulus of elasticity from geophysics tests. The relation between unconfined compressive strength and skin friction was back-calculated and compared with commonly used methods in the UAE. Finite-element analysis of a group of barrettes with revised length showed settlement within the limit values. Revising the barrette length by about 11% resulted in significant savings in both cost and construction time.

An experimental study of the influence of lithology on compaction behaviour of broken waste rock in coal mine backfill.

The research aims to explore the influences of lithology on the compaction behaviours of broken waste rocks. For this purpose, a WAW-1000D servo test machine and a self-made bidirectional loading test system for granular materials were used to conduct axial and lateral compaction tests on four typical types of broken waste rocks: sandstone, mudstone, limestone and shale. On this basis, we analysed the relationships between lateral and axial stress with the strain in, and porosity of, the four types of broken waste rocks. In addition, the relationship of axial stress with lateral stress and lateral pressure coefficient, and the changes in the particle size distribution of broken waste rocks before and after compaction were discussed. The test results demonstrated that the samples of higher strength were found to have low lateral and axial strains as well as a lower porosity in axial and lateral loading tests, while samples of lower strength showed low lateral stress and lateral pressure coefficient under axial load. After being compacted, the samples of the four types of broken waste rocks were found to have a higher proportion of small particles, indicating some particle crushing. Moreover, the samples of lower strength were broken to a greater extent.

A Numerical Study on the Estimation Method of the Results of Static Pile Load Test Using the Results of Bi-directional Pile Load Test of Barrette Piles

A Numerical Study on the Estimation Method of the Results of Static Pile Load Test Using the Results of Bi-directional Pile Load Test of Barrette Piles

Recycling of crushed waste rock as backfilling material in coal mine: effects of particle size on compaction behaviours.

Crushed waste rocks can be used as materials for backfilling goafs, so as to achieve the simultaneous goals of processing solid waste and controlling surface subsidence; however, particle size distribution directly affects the compaction of crushed waste rocks. Therefore, by employing a self-designed bidirectional loading test system for granular materials, this study tested compaction characteristics of crushed waste rocks with four different particle size distributions. Moreover, this research tested the changes of parameters in lateral and axial loading of crushed waste rocks and analysed the influence of particle size distribution on lateral strain, axial strain, porosity, lateral stress, and lateral pressure coefficient during compaction. The test results show that (1) particle size distribution affects porosity, strain, and lateral pressure coefficient of crushed waste rocks under lateral and axial loading. (2) For the samples under particle size distribution ranging from 0 to 10mm, the initial porosity is low and deformations are small under axial loading, so that particles can make contact and bear effective stress in grain-grain contact. Therefore, more stress is transferred to the lateral direction. (3) After compaction, the curves of the samples of crushed waste rocks under four particle size distributions all shift upwards in comparison with those before compaction, indicating that particles are crushed and the proportion of small particles constantly increases. (4) A reasonable particle size distribution can significantly improve stress characteristics, reduce crushing of particles in the samples, and increase the stiffness of the samples, so as to achieve better compaction effects.

Effect of Cyclic Lateral Loading on the Compaction Behaviour of Waste Rock Backfill Materials in Coal Mines

Crushed waste rock (CWR) can be used as backfill for goafs allowing disposal of solid waste and control of surface subsidence. Waste rocks for backfilling (WRBs) have a certain density before use if cyclic lateral loads are applied to CWRs, therefore, by employing a self-designed bidirectional loading test system for granular materials, the influence of the number of lateral loading cycles on the compaction characteristics of WRBs was explored. Through testing, changes in mechanical parameters of WRBs during lateral and axial loading were attained to analyse the influence of lateral loading on lateral strain, axial strain, porosity, and lateral pressure coefficient during their compaction. The test results showed that: (1) the lateral loading exerted a significant influence on the porosity, strain, and lateral pressure coefficient of crushed WRBs during lateral and axial loading; (2) under lateral load, the more cycles of lateral loading applied, the greater the lateral strain and the reduction in lateral porosity of samples; (3) during axial loading, for samples subjected to multiple cycles of lateral loading, owing to the porosity of WRBs having been decreased in advance to improve their density, the final axial strain was low; (4) after compaction, the particle size distributions of CWR samples after different numbers of cycles (1, 3, 5 and 7) of lateral loading all shifted upwards compared with those obtained before compaction, implying that rock particles were crushed. However, the number of cycles of lateral load did not affect crushing of particles before, and after, compaction.

Influence of Lateral Loading on Compaction Characteristics of Crushed Waste Rock Used for Backfilling

Crushed waste rock can be used as backfill in goafs to allow re-use of otherwise solid waste and to control surface subsidence. If a certain lateral stress is applied to crushed waste rocks beforehand, they are densified. Therefore, this research investigated the effects of lateral stress on compaction characteristics of waste rocks for backfilling by utilising a self-designed bidirectional loading test system for granular materials. Furthermore, this study tested the changes in the mechanical parameters on lateral and axial loading of waste rocks for backfilling and measured the influence of lateral stress on lateral strain, axial strain, porosity, and lateral pressure coefficient during compaction. The test results demonstrate that (1) lateral stress affects porosity, strain, and the lateral pressure coefficient of crushed waste rocks for backfilling in lateral and axial loading. (2) In lateral loading, the greater the lateral stress, the larger the lateral strain and the reduction in lateral porosity. (3) Under axial loading, for the samples on which a high lateral stress is applied, because the porosity of waste rocks is decreased in advance, the density increases, thus finally resulting in a lower axial strain. (4) After compaction, the particle size distributions of the samples of the crushed waste rocks under four lateral stresses all shift upwards compared with those before compaction, implying that particles are crushed. However, lateral stress does not reach the crushing strength of waste rock particles, which exerts only a small influence on the crushing of particles before and after compaction.

On the Interpretation of the Bidirectional Static Load Test

. The paper deals with the bidirectional test, aiming at interpreting its results, showing the factor that governs the upward movement and presenting approximate formulas to find the equivalent load-settlement curve of a conventional compressive test. The factor is the product c’.k = (1-c).k, where c’ and c are the Leonards-Lovell coefficients, related to the elastic shortening of the shaft under bidirectional and conventional static loading tests, respectively, and k is the relative stiffness of the pile-soil (shaft) system. The paper shows that when this product is constant the properly normalized upward curve is invariant, independently on shaft resistance distribution. Nomograms are presented to quickly determine c’ for two patterns of shaft friction associated to weak upper layers over deep more resistant soils. Under this condition, it is shown that c > c’, i.e., the load causing the downward movement in the conventional test induces greater elastic shortening than in the bidirectional test. Therefore, additional movement must be added to the measured displacement in the bidirectional test, which constitutes the basis of the proposed approximate formulas to find the equivalent curve of the conventional test. To validate these findings a mathematical model is used together with five case histories, comprising short to long piles. In one case a conventional compressive loading test was also available.

매입말뚝의 설계식 적정성 및 지지력 상향 가능성 분석 연구

ABSTRACT To improve the pile design efficiency(design bearing capacity⁄the strength of materials) from 70 percent(160tonf) to 80 percent(190tonf), this paper analysed the existing pile loading test data and performed the precise dynamic loading test and Bi-directional loading test for the first time in Korea. Analysis result of the existing dynamic loading test data by Davisson method showed that bearing capacity of piles penetrated at weathered rock stratum(N=50/15) exceeded 190tonf. But the analysis result by CAPWAP method showed that piles less than the target bearing capacity were 40% due to the lack of impact energy. To get the target bearing capacity from the dynamic loading test, using the hammer over 6tonf to trigger the enough impact energy is necessary. Allowable bearing capacty of Bi-directional static loading test by Davisson method was 260.0～335tonf(ave. 285.3tonf) and exceeded overwhelmingly the target capacity. And this exceeded the bearing capacity of precise dynamic loading test(ave. 202.3tonf) performed on the same piles over 40%. The difference between the capacity of Bi-directional loading test and dynamic loading test was caused by the insufficient impact energy during dynamic loading test and increase by interlocking effect by near piles during Bi-directional static loading test. Key words : Dynamic Loading Test, Davisson Method, CAPWAP, Bearing Capacity, Striking Energy, Bi-directional Loading Test

헬리컬 파일의 지지력 산정을 위한 양방향 재하시험의 적용성 평가

The helical pile has become popular with some constructional advantages because relatively compact equipment is needed for installing helical piles. However, field loading tests for estimating the bearing capacity of helical piles have drawbacks that the required dead load should be as much as the operation load, and reaction piles or anchors are required. In this paper, the bi-directional load test without necessity of reaction piles and loading frames was applied to the helical pile, and the load-settlement curves of the helical piles were measured. The bi-directional load test was performed in two separate stages with the aid of a special hydraulic cylinder whose diameter is equal to that of the pile shaft. In the first stage, the hydraulic cylinder is assembled immediately above the bottom helix plate, and the end bearing capacity of the helical pile is measured. In the second stage, the hydraulic cylinder is assembled above the top helix plate, and the skin friction of the helical pile is measured. The pile loading-test program was carried out for the two different helical piles with the shaft diameter of 89 mm and 114 mm, respectively. However, the configuration of helix plates is identical with three helix plates of 450-, 350-, 200- mm diameter. Results of the bi-directional load test were verified by the conventional static pile loading test. As a result, the bearing capacity estimated by the bi-directional load test is in good agreement with the result of the conventional pile loading test.

Determination of Loading Capacities for Bi-directional Pile Load Tests Based on Actual Load Test Results

A bi-directional pile load test (PLT) is regarded as the most reliable method for verifying the design capacity of a large-diameter drilled shaft. The loading capacity is often improperly set while conducting this test, leading to inadequate verification of appropriate design capacities for large-diameter drilled shafts. This problem necessitates a new, rational method for estimating the loading capacity for bi-directional PLTs. In this study, results of numerous bi-directional PLTs conducted by different researchers were analyzed for their failure patterns, load increasing ratios, loading capacity increasing ratios, and sufficiency ratios of the design load. The results indicate that most failure patterns involved a lack of loading capacity. In our assessment, the load increasing and loading capacity increasing ratios were less than 2, confirming that the maximum equivalent test load is not always twice the total bi-directional load. Hence, it is difficult to verify the design capacity using the current planned loading capacity. An analysis of the sufficiency ratios of the design load revealed that 18.6 % of the test results did not satisfy the requirement. To eliminate the uncertainties in verifying the design load, the one-directional loading capacity should be at least 2.5 times the design load.

양방향 재하시험 결과를 이용한 암반소켓 현장타설말뚝의 주면 마찰력과 선단 지지력

In this paper, the empirical relations of skin friction and end bearing resistance with the results of site investigation in soft rock are proposed through the analysis of bi-directional pile load tests of rock socketed drilled shafts performed at large offshore bridge foundations and high-rise building projects (13 test piles in 4 projects). The site investigation and drilling for bi-directional pile load tests were performed at the centers of test piles, and f-w curves for skin friction and q-w curves for end bearing were plotted based on load-transfer measurements. From the above curves, the empirical relations of skin friction and end bearing resistance with the results of site investigation depending on the mobilized displacement are determined by multiple regression analysis and compared with previous studies. Since the f-w and q-w curves of rock-socketed piles in Korea show hardening behavior according to mobilized displacement, the developed empirical relations by the mobilized displacement are more reasonable than those of previous studies which could not consider the mobilized displacement and suggested the ultimate capacity with unconfined compressive strength only. Particularly, the developed equations correlated with unconfined compressive strength show the best correlations among the equations correlated with other parameters.

Local Buckling Restraining Behavior of Thin-Walled Circular CFT Columns under Seismic Loads

AbstractThe local buckling restraining behavior of thin-walled circular concrete-filled tubular (CFT) columns is examined under seismic loads by conducting a bidirectional cyclic loading and a bidirectional shaking table test. According to the responses of the deformations and strains measured in the bidirectional cyclic loading test, the axial compressive force acting on the buckled part of a steel tube is reduced with the increase in its local buckling deformation because most of the compressive axial force is transferred from the steel tube to the in-filled concrete. The reduction in the compressive axial force in the steel tube slows the progress of the buckling deformation. In addition, under a cyclic load applied after the occurrence of local buckling, the opening and closing of major horizontal cracks and dilation occur in the in-filled concrete. As a result, a predominant tensile axial force acts repeatedly on the buckled part of the outer steel tube. This tensile force restrains or restores the l...

Experimental study of the seismic behavior of partially concrete‐filled steel bridge piers under bidirectional dynamic loading

SUMMARYThe seismic behavior of steel bridge piers partially filled with concrete under actual earthquake conditions was investigated by using 20 square section specimens subjected to static cyclic loading tests and single‐directional and bidirectional hybrid loading tests. Acceleration records of two horizontal NS and EW directional components for hard (GT1), medium (GT2), and soft grounds (GT3), obtained during the 1995 Kobe earthquake, were adopted in dynamic tests. Experimental results clearly showed that maximum and residual displacements under actual earthquake conditions cannot be accurately estimated by conventional single‐directional loading tests, especially for GT2 and GT3. A modified admissible displacement was proposed on the basis of bidirectional loading test results. The concrete fill can effectively improve the seismic resistance performance if the concrete inside the steel bridge piers is sufficiently high in quantity. Copyright © 2013 John Wiley & Sons, Ltd.

Equivalent head-down load vs. Movement relationships evaluated from bi-directional pile load tests

The increasing use of bi-directional (Osterberg cell) load tests in piles necessitates a reliable evaluation of the load-movement relationship equivalent to the head-down load test. In this study, the existing evaluation methods were reviewed, and a new method for evaluating the equivalent head-down curve was proposed. This method considers possible situations, such as non-measured axial load distribution along the pile and the existence of residual load distribution. Three cases, in which bi-directional and head-down load tests were performed at the same site, were analyzed in order to validate the proposed method. The results of the case studies showed that the equivalent load-movement curves obtained either without assuming unit shaft resistance distribution or by considering the effect of residual load agreed well with the measured curves obtained from the head-down test. Therefore, the proposed method is recommended for practical use.

Application of bi-directional static loading test to deep foundations

Bi-directional static loading test adopting load cells is widely used around the world at present, with increase in diameter and length of deep foundations. In this paper, a new simple conversion method to predict the equivalent pile head load-settlement curve considering elastic shortening of deep foundation was put forward according to the load transfer mechanism. The proposed conversion method was applied to root caisson foundation in a bridge and to large diameter pipe piles in a sea wind power plant. Some new load cells, test procedure, and construction technology were adopted based on the applications to different deep foundations, which could enlarge the application scopes of bi-directional loading test. A new type of bi-directional loading test for pipe pile was conducted, in which the load cell was installed and loaded after the pipe pile with special connector has been set up. Unlike the conventional bi-directional loading test, the load cell can be reused and shows an evident economic benefit.