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Volume 7, Number 5, November 2014

This study examines the behaviour of single micropiles subjected to axial tension or compression load in collapsible loess under in-situ moisture content and saturated condition. Five tension loading tests and five compression loading tests on single micropiles were carried out at a typical loess site of the Loess Plateau in Northwest China. A series of laboratory tests, including grain size distribution, specific gravity, moisture content, Atterberg limits, density, granular components, shear strength, and collapse index, were carried out during the micropile loading tests to determine the values of soil parameters. The loess at the test site poses a severe collapse risk upon wetting. The tension or compression load-displacement curves of the micropiles in loess, under in-situ moisture content or saturated condition, can generally be simplified into three distinct regions: an initial linear, a curvilinear transition, and a final linear region, and the bearing capacity or failure load can be interpreted by the L1-L2 method as done in other studies. Micropiles in loess should be considered as frictional pile foundations though the tip resistances are about 10%-15% of the applied loads. Both the tension and compression capacities increase linearly with the ratio of the pile length to the shaft diameter, L/d. For micropiles in loess under in-situ moisture content, the interpreted failure loads or capacities under tension are 66%-87% of those under compression. However, the prewetting of the loess can lead to the reductions of 50% in the tensile bearing capacity and 70% in the compressive bearing capacity.

Key Words
loess; micropile; load test; ultimate load; skin friction

(1) Zeng-zhen Qian:
School of Engineering and Technology, China University of Geosciences, No. 29 Xueyuan Road, Haidian District, Beijing, 100083, China;
(2) Xian-long Lu, Wen-zhi Yang, Qiang Cui:
China Electric Power Research Institute, No. 15, Xiaoying East Road, Haidian District, Beijing, 100192, China.

To investigate the soil-pile interactive performance under lateral loads, a set of laboratory model tests was conducted on remoulded test bed of soft clay and medium dense sand. Then, a simplified boundary element analysis had been carried out assuming floating pile. In case of soft clay, it has been observed that lateral loads on piles can initiate the formation of a gap, soil heave and the tension crack in the vicinity of the soil surface and the interface, whereas in medium dense sand, a semi-elliptical depression zone can develop. Comparison of test and boundary element results indicates the accuracy of the solution developed. However, in the boundary element analysis, the possible shear stresses likely to be developed at the interface are ignored in order to simplify the existing complex equations. Moreover, it is unable to capture the influence of base restraint in case of a socketed pile. To bridge up this gap and to study the influence of the initial stress state and interface parameters, a field based case-study of laterally-loaded pile in layered soil with socketed tip is explored and modelled using the finite element method. The results of the model have been verified against known field measurements from a case-study. Parametric studies have been conducted to investigate the influence of the coefficient of lateral earth pressure and the interface strength reduction factor on the results of the model.

Key Words
single pile; boundary element; finite element; soil-pile interface

(1) Behzad Fatahi, Patrick Ryan, Hadi Khabbaz:
School of Civil and Environmental Engineering, University of Technology Sydney (UTS), Sydney, Australia;
(2) Sudip Basack:
School of Civil, Mining and Environmental Engineering, University of Wollongong, New South Wales, Australia;
(3) Wan-Huan Zhou:
Department of Civil and Environmental Engineering, University of Macau, Macau, China.

The typical design of ground improvement with prefabricated vertical drains (PVD) and surcharge preloading involves a series of deterministic analyses using averaged or mean soil properties for the various combination of the PVD spacing and surcharge preloading height that would meet the criteria for minimum consolidation time and required degree of consolidation. The optimum design combination is then selected in which the total cost of ground improvement is a minimum. Considering the variability and uncertainties of the soil consolidation parameters, as well as considering the effects of soil disturbance (smear zone) and drain resistance in the analysis, this study presents a stochastic cost optimization of ground improvement with PVD and surcharge preloading. Direct Monte Carlo (MC) simulation and importance sampling (IS) technique is used in the stochastic analysis by limiting the sampled random soil parameters within the range from a minimum to maximum value while considering their statistical distribution. The method has been verified in a case study of PVD improved ground with preloading, in which average results of the stochastic analysis showed a good agreement with field monitoring data.

Key Words
ground improvement; prefabricated vertical drain (PVD); surcharge preloading; stochastic cost optimization; direct Monte Carlo (MC) simulation; importance sampling (IS)

(1) Hyeong-Joo Kim, Kwang-Hyung Lee, Jay C. Jamin:
Department of Civil and Environmental Engineering, Kunsan National University, Gunsan 573-701, Republic of Korea;
(2) Jose Leo C. Mission:
SK Engineering and Construction (SK E&C), Seoul 100-192, Republic of Korea.

A simplified probability-based design charts for stone column-improved ground have been presented based on the unit cell approach. The undrained cohesion (cu) and coefficient of radial consolidation (cr) of the soft soil are taken as the most predominant random variables. The design charts are developed to estimate the diameter of the stone column or the spacing between the stone columns by employing a factored design value of cr and cu so as to satisfy a specific probability level of the target degree of consolidation and/or a target safe load that needs to be achieved in a specified timeframe. The design charts can be used by the practicing engineers to design the stone column-improved ground by considering consolidation and /or bearing capacity of the improved ground.

Key Words
bearing capacity; consolidation; design charts; probability; stone column-improved ground; uncertainty

Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India.

Borehole instability during drilling process occurs frequently when drilling through shale formation. When a borehole is drilled in shale formation, the low permeability leads to an undrained loading condition. The pore pressure in the compressed area near the borehole may be higher than the initial pore pressure. However, the excess pore pressure caused by stress concentration was not considered in traditional borehole stability models. In this study, the calculation model of excess pore pressure induced by drilling was obtained with the introduction of Henkel\'s excess pore pressure theory. Combined with Mohr-Coulumb strength criterion, the calculation model of collapse pressure of shale in undrained condition is obtained. Furthermore, the variation of excess pore pressure and effective stress on the borehole wall is analyzed, and the influence of Skempton\'s pore pressure parameter on collapse pressure is also analyzed. The excess pore pressure decreases with the increasing of drilling fluid density; the excess pore pressure and collapse pressure both increase with the increasing of Skempton\'s pore pressure parameter. The study results provide a reference for determining drilling fluid density when drilling in shale formation.

Key Words
borehole stability; collapse pressure; Mohr-Coulumb strength criterion; undrained; drilling fluid density; excess pore pressure

(1) Jian-Guang Wei:
Institute of Petroleum Engineering of Northeast Petroleum University, Daqing City of Heilongjiang Province, 163318, China;
(2) Chuan-Liang Yan:
School of Petroleum Engineering, China University of Petroleum, Qingdao 266555, China;
(3) Chuan-Liang Yan:
State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, 102249, China.

Triaxial compression creep tests were performed on salt rock samples using cyclic confining pressure with a static axial pressure. The test results show that, up to a certain time, changes in the confining pressure have little influence on creep properties of salt rock, and the axial creep curve is smooth. After this point, the axial creep curve clearly fluctuates with the confining pressure, and is approximately a straight line both when the confining pressure decreases and when it increases within one cycle period. The slope of these lines differs: it is greater when the confining pressure decreases than when it increases. In accordance with rheology model theory, axial creep equations were deduced for Maxwell and Kelvin models under cyclic loading. These were combined to establish an axial creep equation for the Burgers model. We supposed that damage evolution follows an exponential law during creep process and replaced the apparent stress in creep equation for the Burgers model with the effective stress, the axial creep damage equation for the Burgers model was obtained. The model suitability was verified using creep test results for salt rock. The fitting curves are in excellent agreement with the test curves, so the proposed model can well reflect the creep behavior of salt rock under low-frequency cyclic loading. In particular, it reflects the fluctuations in creep deformation and creep rate as the confining pressure increasing and decreasing under different cycle periods.

Key Words
salt rock; creep properties; low-frequency cyclic loading; damage model; fluctuation

(1) Jun-Bao Wang, Xin-Rong Liu, Xiao-Jun Liu:
School of Civil Engineering, Xi'an University of Architecture and Technology, Xi'an, China;
(2) Jun-Bao Wang, Xin-Rong Liu:
School of Civil Engineering, Chongqing University, Chongqing, China;
(3) Ming Huang:
School of Civil Engineering, Fuzhou University, Fuzhou, China.

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