基于统一强度理论抗滑桩桩间距的计算

在已有抗滑桩桩间距研究的基础上,对桩间土拱进行受力分析.从侧阻力条件和土拱强度条件两个方面对抗滑桩桩间距进行了计算,将统一强度理论引入土拱强度的分析,藉此分别判断拱顶前缘、后缘及拱脚处土体是否处于临界状态,可得3个桩间距值,取相应的最小桩间距作为设计桩间距.此方法对于滑坡推力的矩形、三角形和梯形分布形式均适用,并可考虑土体自重应力的影响,同时可推及锚索抗滑桩桩间距的计算.对两个计算实例进行了分析,本方法的计算值与已有计算值或设计值的比较表明本方法效果良好.具体计算中统一强度参数b取0.2～0.7较为合适,滑坡推力为矩形分布时b值约为0.3,滑坡推力为三角形分布时b值约为0.6,滑坡推力为梯形分布时b值介于两者之间.     

关于抗滑桩内力不同计算方法的探讨

分别用m法与有限元法对某抗滑桩支护工程中抗滑桩内力进行求解,结果表明沉埋法求得的抗滑阻力较小,需要进行修正。修正后,m法与有限元法计算的桩身剪力大小十分接近。但由于m法要事先假设滑坡推力分布形式,故桩身弯矩的计算结果与有限元法差异较大。建议在工程中将两种方法结合使用,可使抗滑桩设计更加安全、有效。     

抗滑桩及预应力锚索应用于边坡治理的研究

近年来,随着边坡工程理论及技术的日益完善,边坡治理工程获得了迅速的发展,抗滑桩和预应力锚索应用效果的好坏在一定程度上将直接决定着边坡工程治理水平的高低,在施工过程中通过强化对二者的应用,有助于提高边坡工程的治理成效。基于我国的地质构造情况,伴随着社会经济的发展与基础建设的开展,每年都会增加大量的工程,诸如公路、房地产开发与城市建设等,由此极大的增加了地质灾害出现的频率,其中边坡失稳问题最为严重,不仅范围大、危害广,极易引起各种地质灾害,如崩塌和滑坡等,造成巨大的经济损失,并且严重威胁着人民的生命安全。在治理边坡工程的过程中,应用抗滑桩与预应力锚索可以最大限度的保障边坡及其周边的安全,有效改善边坡土体的受力状况,治理效果十分显著。     

抗滑桩在矿山边坡滑坡整治工程中的应用

随着我国经济的快速发展,国家越来越重视矿山边坡滑坡整治工程。为了进一步的了解抗滑桩的应用治理效果,必须要根据现阶段的矿山开采情况,结合抗滑桩的特点开展施工工作。矿山边坡在地质学上本身就具备不稳定性,由于受到地质学上的重力影响,各种自然因素以及人为因素的影响,比较弱的表面(带),就会出现缓慢以及周期性的滑动变形,有时候甚至会出现剧烈的下降,即属于滑坡地质灾害,这种现象不仅对建筑工作本身造成影响,也会影响人们的生活以及财产安全,因此评估以及预测斜坡的具体稳定性,防止以及管理它们是非常重要的。本文主要针对现阶段的抗滑桩在矿山边坡滑坡整治工程中的应用进行简要分析,并提出合理化建议。     

双排抗滑桩滑坡推力分配影响因素分析

以作用形式较为简单的双排悬臂抗滑桩(未带连梁)为模型并基于结构力学位移法得出的双排桩滑坡推力传递分配计算公式为基础,结合推力较大的红石包滑坡作为工程实例,分别分析了双排桩净排距,前后排桩的截面尺寸和桩间土弹性模量对推力传递分配的影响,得出了以下结论:(1)当增大后排桩的刚度或减小前排桩的刚度时,桩土相互挤压作用越小,A2/A1随之减小,反之A2/A1增大.(2)增大排距b时,A1减小,A2在b=3～10m时逐渐减小,但是变化幅度不大;在b=10～21m时,不断增大.(3)当增大桩间土弹性模量时,A2增大.(4)随着前排桩截面高度的增大,A1、A2逐渐增大;随着后排桩截面高度的增大,A1、A2逐渐减小.     

竖向预应力锚索抗滑桩的优化研究

竖向预应力锚索抗滑桩是一种锚索从桩顶竖向穿过抗滑桩,直接嵌入基岩以下的新型抗滑桩.本文以某滑坡治理工程为例,采用数值模拟方法,研究竖向预应力锚索抗滑桩的内力,从桩长、锚固力、偏心距几个方面对竖向预应力锚索抗滑桩的桩身结构进行优化研究.模拟研究结果表明,其他条件不变时,竖向预应力锚索抗滑桩随着桩长的减小,桩身弯矩最大值变化不大,剪力最大值呈增大趋势;随锚固力增加,桩身内力最大值均呈先减小后增大的趋势,表明选取合适的锚固力对桩身内力影响很大;随着锚索偏心距的增大,桩身内力最大值均逐渐减小.优化结果表明,竖向预应力锚索抗滑桩锚固段比悬臂桩减少16.7%,桩身剪力最大值减少31.7%,桩身弯矩最大值减少41.1%.竖向预应力锚索抗滑桩是一种有应用前景的抗滑桩.     

弹性抗滑桩内力计算的有限差分法

在传统抗滑桩内力计算的悬臂桩法的基础上，将滑动面以上的部分视为定向铰支的悬臂梁，以使滑动面上下位移符合连续性条件，滑动面以下采用地基系数“m”法计算桩身内力，并在此基础上推导出有限差分计算公式。用MATLAB编制了弹性桩全桩内力计算程序，用于滑坡推力和桩前剩余抗滑力为梯形、三角形和矩形的情况，可得到较好的可视化计算结果。     

矿山边坡支挡结构方式选择与设计

为保证矿山安全高效生产,对开采过程中边坡的支挡结构方式选择与设计,桩基托梁锚索挡土墙支挡结构在矿山边坡建设及开采中使用效果较好,本文通过针对矿用挡土墙的墙体及抗滑桩相关参数进行计算,设计抗滑桩桩径0.8m,长5~10m,抗倾覆稳定系数与抗滑动稳定系数均大于挡土墙建设的安全标准,能够满足矿山护坡的设计要求。     

锚拉抗滑桩在某矿山滑坡治理中的设计应用

以某矿山厂区内道路滑坡为例,在收集了大量水文地质资料的基础下,对滑坡在两种不同工况下进行了下滑力计算。经过方案必选,确定了锚拉抗滑桩治理滑坡的方案,并叙述了设计的主要思路和具体设计方案。监测结果表明,本工程锚拉抗滑桩设计有效控制了滑坡变形,可供同类工程借鉴。     

河北省宽城县遵(化)—小(寺沟)铁路某滑坡治理措施研究

河北省宽城县遵(化)-小(寺沟)铁路穿越一个体积约77×104m3的古滑坡体,因在滑坡前缘以路堑方式穿过,开挖施工导致古滑坡复活.为保障未来铁路的运营安全,需要对其进行彻底治理.在现场地质调查及试验,以及分别对沿复活滑坡体新滑面和古滑面在不同工况下的稳定性计算的基础上,结合复活滑坡体的具体特征,提出2种治理方案,即"抗滑桩"方案和"削方+抗滑桩+桩间挡土墙"方案.通过综合比选确定后者为采用方案,并具体进行了削方、抗滑桩、截排水系统和坡面防护等设计.     

Slope Stabilizing Piles and Pile-Groups: Parametric Study and Design Insights

This paper uses a hybrid method for analysis and design of slope stabilizing piles that was developed in a preceding paper by the writers. The aim of this paper is to derive insights about the factors influencing the response of piles and pile-groups. Axis-to-axis pile spacing (S), thickness of stable soil mass (Hu), depth (Le) of pile embedment, pile diameter (D), and pile group configuration are the parameters addressed in the study. It is shown that S = 4D is the most cost-effective pile spacing, because it is the largest spacing that can still generate soil arching between the piles. Soil inhomogeneity (in terms of shear stiffness) was found to be unimportant, because the response is primarily affected by the strength of the unstable soil layer. For relatively small pile embedments, pile response is dominated by rigid-body rotation without substantial flexural distortion: the short pile mode of failure. In these cases, the structural capacity of the pile cannot be exploited, and the design will not be economical. The critical embedment depth to achieve fixity conditions at the base of the pile is found to range from 0.7Hu to 1.5Hu, depending on the relative strength of the unstable ground compared to that of the stable ground (i.e., the soil below the sliding plane). An example of dimensionless design charts is presented for piles embedded in rock. Results are presented for two characteristic slenderness ratios and several pile spacings. Single piles are concluded to be generally inadequate for stabilizing deep landslides, although capped pile-groups invoking framing action may offer an efficient solution.     

Lateral Behavior of Pile Groups in Layered Soils

Assessment of the response of a laterally loaded pile group based on soil–pile interaction is presented in this paper. The behavior of a pile group in uniform and layered soil (sand and/or clay) is evaluated based on the strain wedge model approach that was developed to analyze the response of a long flexible pile under lateral loading. Accordingly, the pile’s response is characterized in terms of three-dimensional soil–pile interaction which is then transformed into its one-dimensional beam on elastic foundation equivalent and the associated parameter (modulus of subgrade reaction Es) variation along pile length. The interaction among the piles in a group is determined based on the geometry and interaction of the mobilized passive wedges of soil in front of the piles in association with the pile spacing. The overlap of shear zones among the piles in the group varies along the length of the pile and changes from one soil layer to another in the soil profile. Also, the interaction among the piles grows with the increase in lateral loading, and the increasing depth and fan angles of the developing wedges. The value of Es so determined accounts for the additional strains (i.e., stresses) in the adjacent soil due to pile interaction within the group. Based on the approach presented, the p–y curve for different piles in the pile group can be determined. The reduction in the resistance of the individual piles in the group compared to the isolated pile is governed by soil and pile properties, level of loading, and pile spacing.     

Development of Downdrag on Piles and Pile Groups in Consolidating Soil

Development of pile settlement (downdrag) of piles constructed in consolidating soil may lead to serious pile foundation design problems. The investigation of downdrag has attracted far less attention than the study of dragload over the years. In this paper, several series of two-dimensional axisymmetric and three-dimensional numerical parametric analyses were conducted to study the behavior of single piles and piles in 3×3 and 5×5 pile groups in consolidating soil. Both elastic no-slip and elasto-plastic slip at the pile–soil interface were considered. For a single pile, the downdrag computed from the no-slip elastic analysis and from the analytical elastic solution was about 8–14 times larger than that computed from the elasto-plastic slip analysis. The softer the consolidating clay, the greater the difference between the no-slip elastic and the elasto-plastic slip analyses. For the 5×5 pile group at 2.5 diameter spacing, the maximum downdrag of the center, inner, and corner piles was, respectively, 63, 68, and 79% of the maximum downdrag of the single pile. The reduction of downdrag inside the pile group is attributed to the shielding effects on the inner piles by the outer piles. The relative reduction in downdrag (Wr) in the 5×5 pile group increases with an increase in the relative bearing stiffness ratio (Eb/Ec), depending on the pile location in the group. Compared with the relative reduction in dragload (Pr), Wr at the corner pile is less affected by the group interaction for a given surcharge load. This suggests that the use of sacrificing piles outside the pile group will be more effective on Pr than on Wr. Based on the three cases studied, the larger the number of piles in a group, the greater the shielding effects on Wr. Relatively speaking, Wr is more sensitive to the total number of piles than to the pile spacing within a pile group.     

Note on the Interaction Factor for Two Laterally Loaded Piles

This technical note revisits the interaction factors for two piles under lateral loading by means of a rigorous analytical method. The basic idea of the approach presented is to decompose the problem into an extended elastic soil and two fictitious piles having Young’s modulus equal to the difference between the modulus of the real pile and the surrounding soil. By considering the displacement compatibility condition, the pile–soil interaction problem is found to be governed by a Fredholm equation of the second kind. The displacement and bending moment distribution along the fictitious piles, and consequently, the desired interaction factor at the pile head are obtained. Comparison with existing solutions validates the accuracy of the present formulation and confirms that the conventional interaction factor approach would exaggerate the interaction effect for long flexible piles. Some numerical examples are presented to illustrate the influences of the pile spacing, pile–soil stiffness ratio, pile slenderness ratio, and departure angle of the loading direction on the calculated results. A set of interaction factor charts is also provided.     

Effects of Tip Location and Shielding on Piles in Consolidating Ground

Pile foundations located within consolidating ground are commonly subjected to negative skin friction (NSF) and failures of pile foundations related to dragload (compressive force) and downdrag (pile settlement) have been reported in the literature. This paper reports the results of four centrifuge model tests, which were undertaken to achieve two objectives: first, to investigate the response of a single pile subjected to NSF with different pile tip location with respect to the end-bearing stratum layer; and second, to study the behavior of floating piles subjected to NSF with and without shielding by sacrificing piles. In addition, three-dimensional numerical analyses of the centrifuge model tests were carried out with elastoplastic slip considered at the pile-soil interface. The measured maximum β value at unprotected single end-bearing and floating pile was similar and slightly smaller than 0.3. On the contrary, smaller β values of 0.1 and 0.2 were mobilized at the shielded center piles for pile spacings of 5.0 d and 6.0 d, respectively. The measured maximum dragload of the center pile in the group at 5.0 d and 6.0 d spacing was only 53% and 75% of the measured maximum dragload of an isolated single pile, respectively. Correspondingly, the measured downdrag of the center pile was reduced to about 57% and 80% of the isolated single pile. Based on the numerical analyses, it is revealed that sacrificing piles “hang up” the soil between the piles in the group and, thus, the vertical effective stress in the soil so reduced, as is the horizontal effective stress acting on the center pile. This “hang-up” effect reduces with an increase in pile spacing. For a given pile spacing, shielding effect on dragload is larger than that on downdrag.     

Axial Load Tests on Bored Piles and Pile Groups in Cemented Sands

The behavior of bored pile groups in cemented sands was examined by a field testing program at a site in South Surra, Kuwait. The program consisted of axial load tests on single bored piles in tension and compression and compression tests on two pile groups each consisting of five piles. The spacing of the piles in the groups was two- and three-pile diameters. Soil exploration included standard penetration tests, dynamic cone tests, and pressure meter tests. Laboratory tests included basic properties and drained triaxial compression tests. Test results on single piles indicated that 70% of the ultimate load was transmitted in side friction that was uniform along the pile shafts. The calculated pile group efficiencies were 1.22 and 1.93 for a pile spacing of two- and three-pile diameters, respectively. Since settlement usually controls the design of pile groups in sand, the group factor defined herein as the ratio of the settlement of the group to the settlement of a single pile at comparable loads in the elastic range was determined from test results. A comparison between the measured values and calculated values based on a simplified formula was made.     

Ultimate Lateral Resistance of Pile Groups in Sand

Experimental investigations on model pile groups of configuration 1 × 1, 2 × 1, 3 × 1, 2 × 2, and 3 × 2 for embedment length-to-diameter ratios L∕d = 12 and 38, spacing from 3 to 6 pile diameter, and pile friction angles δ = 20° and 31°, subjected to lateral loads, were conducted in dry Ennore sand obtained from Chennai, India. The load-displacement response, ultimate resistance, and group efficiency with spacing and number of piles in a group have been qualitatively and quantitatively investigated. Analytical methods have been proposed to predict the ultimate lateral capacity of single pile and pile groups. The proposed methods account for pile friction angle, embedment length-to-diameter ratio, the spacing of piles in a group, pile group configuration, and soil properties. These methods are capable of predicting the lateral capacity of piles and pile groups reasonably well as noted and substantiated by the comparison with the experimental results of the writers and other researchers.     

Pile Spacing Effects on Lateral Pile Group Behavior: Analysis

Using the results from three full-scale lateral pile group load tests in stiff clay with spacing ranging from 3.3 to 5.65, computer analyses were performed to back-calculate p multipliers. The p multipliers, which account for reduced resistance due to pile–soil–pile interaction, increased as pile spacing increased from 3.3 to 5.65 diameters. Extrapolation of the test results suggests that group reduction effects can be neglected for spacings greater than about 6.5 for leading row piles and 7–8 diameters for trailing row piles. Based on analysis of the full-scale test results, pile behavior can be grouped into three general categories, namely: (1) first or front row piles; (2) second row piles; and (3) third and higher row piles. p multiplier versus normalized pile spacing curves were developed for each category. The proposed curves yield p multipliers which are higher than those previously recommended by AASHTO in 2000, the US Army in 1993, and the US Navy in 1982 based on limited test data, but lower values than those proposed by Reese et al. in 1996 and Reese and Van Impe in 2001. The response (load versus deflection, maximum moment versus load, and bending moment versus depth) for each row of the pile groups computed using GROUP and Florida Pier generally correlated very well with measurements from the full-scale tests when the p multipliers developed from this test program were employed.     

Design Charts for Piles Supporting Embankments on Soft Clay

This paper describes the development and application of design charts for piled embankment designs. It outlines the computational approach adopted, the geotechnical profiles used, and the application of the design procedure using the charts. The soil profile used for the charts is representative of a Malaysian soft clay profile, involving a more or less normally consolidated soil, with a strength and stiffness that varies linearly with depth. Such a profile is typical of the ground conditions in a variety of countries in the Southeast Asian region. The design charts address the issues of pile capacity, settlement due to embankment load, settlement due to a temporary piling construction platform, and lateral response of piles near the edge of the embankment. The charts consider variations in ground conditions, embankment height, pile length, and pile spacing. An illustrative example is given to demonstrate the use of the charts.     

Group Interaction Effects on Laterally Loaded Piles in Clay

This paper presents the results of static lateral load tests carried out on 1×2, 2×2, 1×4, and 3×3 model pile groups embedded in soft clay. Tests were carried out on piles with length to diameter ratios of 15, 30, and 40 and three to nine pile diameter spacing. The effects of pile spacing, number of piles, embedment length, and configuration on pile-group interaction were investigated. Group efficiency, critical spacing, and p multipliers were evaluated from the experimental study. The experimental results have been compared with those obtained from the program GROUP. It has been found that the lateral capacity of piles in 3×3 group at three diameter spacing is about 40% less than that of the single pile. Group interaction causes 20% increase in the maximum bending moment in piles of the groups with three diameter spacing in comparison to the single pile. Results indicate substantial difference in p multipliers of the corresponding rows of the linear and square pile groups. The predicted field group behavior is in good agreement with the actual field test results reported in the literature.