CFG桩在中业大厦复合地基中的应用

CFG复合地基是粉煤灰碎石桩、桩间土和褥垫层形成的一个完整整体,上部建筑物的荷载整体上由其承担,这样的复合地基充分利用了桩间土的承载力,而不是像桩基那样利用桩来承担上部荷载。这样的设计思路加上CFG桩不配钢筋,所用粉煤灰价格低廉、易得,大大降低了工程造价。     

水泥粉煤灰碎石桩(CFG)在地基处理中的应用

结合水泥粉煤灰碎石桩在某工厂的应用,介绍该技术的施工工艺和设计计算.施工后CFG桩桩体质量好,承载力高,经济性优越,对指导软弱地基处理与加固具有一定的参考价值.     

夯实水泥土桩在烧结机余热发电工程中的应用

夯实水泥土桩法是在钻孔中夯填水泥土形成桩土复合地基。在邯钢烧结机余热发电扩建项目中,选用桩径300 mm夯实水泥土桩搭接小直径钢管桩,在地下水位较高的狭窄建筑场地,进行了素填土地基加固,获得了较好的经济效益。该项目投产运行两年来,构筑物基础沉降量仅为8~10 mm。     

水泥土群桩复合地基桩土应力比测试研究

桩土应力比是反映搅拌桩复合地基工作状态的一个重要参数,也是计算复合地基承载力和沉降的重要指标。通过在单桩、四桩承台下的桩周土和桩顶埋设土压力盒．获得系统的水泥土桩复合地基的桩土应力等原位试验数据。分析了群桩效应在搅拌桩复合地基中的影响,具有工程实用价值。     

CFG桩复合地基承载性状分析

结合内蒙古地区某工程实例,对CFG刚性桩复合地基在软弱砂质粉土地基中的承载机理、施工工艺的特点及影响CFG桩复合地基承载力和沉降量的因素进行了分析与探讨。     

后压浆技术在CFG桩地基处理工程中的应用

朔黄发展大厦工程采用CFG桩复合地基,并进行桩基的后压浆处理,不仅提高了复合地基的承载力,而且缩短了桩长,加快了施工进度.经试验及时间检验,该工程地基处理效果良好,技术经济效益显著.结合该工程实例,介绍了CFG桩后压浆施工技术.     

动力法计算振冲碎石桩桩体压缩模量

振冲碎石桩为散体结构,所形成的复合地基受垂直荷载后的变形较刚性桩复合地基要大,为研究振冲碎石桩复合地基的沉降规律,建立振冲器动力性能指标与碎石桩桩体强度的关系.根据压缩模量概念,结合振冲桩制桩工艺,建立桩体压缩模量与振冲器激振力、施工中的加密段长、加密电流及留振时间的关系式--动力法.采用动力法和载荷试验法所得的压缩模量在沉降量计算中进行对比,结果与按实际观测推算值的偏差均小于10%.     

浅谈石灰桩挤密法

1概述 挤密桩是属于柔性桩加固地基的范畴，它主要靠桩管打入地基时对地基土的横向挤密作用，在一定的挤密功能作用下土粒彼此移动，小颗粒填入大颗粒的空隙，颗粒间彼此靠紧，空隙减小，此时土的骨架作用随之增强，从而使土的压缩性减小和抗剪强度提高。由于桩体本身有较高的承载能力和较大的变形模量，且桩体断面较大，约占松软土加固面积的20％左右，故在粘性土地基中加固时，桩体与土组成复合地基，可共同承担建筑物的荷重。挤密桩属于柔性桩，而木桩、钢筋混凝土桩和钢桩等则属于刚性桩，两者有如下区别。     

CFG桩复合地基在工程中的应用

在青钢活性白灰工程地基处理中，采用CFG桩复合地基技术，通过对处理后的地基试验与检测，验证了CFG桩复合地基承载力及沉降达到设计要求。同时指出了使用CFG桩时普遍存在的问题，并给出了预防措施。     

水泥粉喷桩复合地基的研究综述

水泥粉喷桩作为一种加固地基的有效方法 ,可以较大地提高地基承载力 ,改善地基的变形特性 ,减小在荷载作用下可能发生的沉降和不均匀沉降。从水泥土的工程性质、复合地基承载力的计算以及复合地基的沉降计算几个方面分析了水泥粉喷桩的研究现状 ,并对以后研究的方向提出了见解     

Shaft Capacity of Open-Ended Piles in Sand

This paper presents the results from an experimental investigation designed to examine the effect of soil-core development and cyclic loading on the shaft resistance developed by open-ended piles in sand. An instrumented open-ended model pile was installed either by driving or jacking into an artificially-created loose sand deposit in Blessington, Ireland. The tests provided continuous measurements of the soil-core development and the radial effective stresses during installation and subsequent load tests. The equalized radial effective stresses developed at the pile-soil interface were seen to be dependent on the degree of soil displacement (plugging) experienced during installation, the distance from the pile toe, and the number of load cycles experienced by a soil element adjacent to the pile shaft. A new design method for estimating the shaft capacity of piles in sand is proposed and compared with measurements made on prototype field-scale piles.     

Horizontal Bearing Capacity of Piles in a Lateritic Soil

The behavior of pile foundations subjected to horizontal loading is typically evaluated using horizontal load tests. Although load tests are valuable to understand site-specific soil-structure interaction phenomena, validated predictive methods are also useful during the design phase. In this study, the results from horizontal load tests are compared with methods which predict the horizontal bearing capacity of piles using in situ measurements of soil behavior. Specifically, several horizontal load tests were performed in order to evaluate the behavior of two 12-m long Strauss piles and four bored piles with similar length, all installed in a lateritic soil profile. Two prediction methods were evaluated using p-y curves computed from the results of Marchetti’s dilatometer test (DMT) results. The predictive methods using the p-y curves from the DMT showed good agreement with the behavior observed in the pile loading test.     

Nonlinear Analysis of Torsionally Loaded Piles in a Two-Layer Soil Profile

This paper presents a method for predicting the nonlinear response of torsionally loaded piles in a two-layer soil profile, such as a clay or sand layer underlain by rock. The shear modulus of the upper soil is assumed to vary linearly with depth and the shear modulus of the lower soil is assumed to vary linearly with depth and then stay constant below the pile tip. The method uses the variational principle to derive the governing differential equations of a pile in a two-layer continuum and the elastic response of the pile is then determined by solving the derived differential equations. To consider the effect of soil yielding on the behavior of piles, the soil is assumed to behave linearly elastically at small strain levels and yield when the shear stress on the pile-soil interface exceeds the corresponding maximum shear resistance. To determine the maximum pile-soil interface shear resistance, methods that are available in the literature can be used. The proposed method is verified by comparing its results with existing elastic solutions and published small-scale model pile test results. Finally, the proposed method is used to analyze two full-scale field test piles and the predictions are in reasonable agreement with the measurements.     

Behavior of Open- and Closed-Ended Piles Driven Into Sands

Both the driving response and static bearing capacity of open-ended piles are affected by the soil plug that forms inside the pile during pile driving. In order to investigate the effect of the soil plug on the static and dynamic response of an open-ended pile and the load capacity of pipe piles in general, field pile load tests were performed on instrumented open- and closed-ended piles driven into sand. For the open-ended pile, the soil plug length was continuously measured during pile driving, allowing calculation of the incremental filling ratio for the pile. The cumulative hammer blow count for the open-ended pile was 16% lower than for the closed-ended pile. The limit unit shaft resistance and the limit unit base resistance of the open-ended pile were 51 and 32% lower than the corresponding values for the closed-ended pile. It was also observed, for the open-ended pile, that the unit soil plug resistance was only about 28% of the unit annulus resistance, and that the average unit frictional resistance between the soil plug and the inner surface of the open-ended pile was 36% higher than its unit outside shaft resistance.     

海上风电复合基础承载性能对比研究

受到上部结构自重以及海洋环境荷载的影响,海上风电基础设计时应考虑竖向荷载、水平荷载以及弯矩荷载作用下基础的承载性能。本文通过有限元软件ABAQUS,对比研究了饱和黏土场地中大直径单桩基础、桩?平台复合基础以及桩?筒复合基础在竖向荷载V、水平荷载H、弯矩荷载M作用下的承载性能。研究结果表明两种复合基础较单桩基础呈现出显著的承载性能优势。桩?平台复合基础的竖向承载力、水平承载力以及抗弯承载力随着附加平台直径的增大呈指数型增加;桩?筒复合基础的竖向承载力以及抗弯承载力随着筒结构入土深度的增加先增大然后趋于稳定,桩?筒复合基础的水平承载力与筒直径以及筒入土深度为双参数线性增加关系。V?H以及V?M复合荷载加载条件下,两种复合基础比单桩基础的破坏包络线空间大,两种复合基础的稳定性相对单桩基础有显著提升。在一定承载范围内,附加平台结构或筒型结构可以减小桩的直径或入土深度。      

Field-Measured Response of an Integral Abutment Bridge with Short Steel H-Piles

Integral abutment bridges (IABs) with short steel H-pile (HP) supported foundations ( ? 4?m of pile depth) are economical for many environmentally sensitive sites with shallow bedrock. However, such short piles may not develop an assumed, fixed-end support condition at some depth below the pile cap, which is inconsistent with traditional pile design assumptions involving an equivalent length for bending behavior of the pile. In this study, the response of an IAB with short HP-supported foundations and no special pile tip details such as drilling and socketing is investigated. Instrumentation of a single-span IAB with 4-m-long piles at one abutment and 6.2- to 8.7-m-long piles at the second abutment is described. Instrumentation includes pile strain gauging, pile inclinometers, extensometers to measure abutment movement, earth pressure cells, and thermistors. Pile and bridge response during construction, under controlled live load testing, and due to seasonal movements are presented and discussed. Abutment and pile head rotations due to self-weight, live load, and seasonal movements were all found to be significant. Measured abutment movements were likely affected by both temperature changes and deck creep and shrinkage. Based on the field study results presented here, moderately short HPs driven to bedrock without special tip details appear to perform well in IABs and do not experience stresses larger than those seen by longer piles.     

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.     

Experimental Study of Eccentrically Loaded Raft with Connected and Unconnected Short Piles

Piled raft foundations are often used when the supporting soil has adequate bearing capacity but the raft settlements exceed allowable values. In traditional practice, long piles with high load capacity are usually used that may lead to two structural problems: the structural collapse of the pile and large strains mobilized in the raft leading to an uneconomic design. This paper presents an experimental study of the effectiveness of using short piles either connected or unconnected to the raft (instead of long piles) on the behavior of an eccentrically loaded raft. The load configuration was designed to simulate rafts under vertical loads and overturning moment. Several arrangements of piles with different lengths and numbers along with the effect of the relative density of the soil and the load eccentricity were studied. Test results indicate that the inclusion of short piles adjacent to the raft edges not only significantly improves the raft bearing pressures but also leads to a reduction in raft settlements and tilts leading to an economical design of the raft. However, the efficiency of the short piles-raft system is dependent on the load eccentricity ratio and pile arrangement. Also, connecting short piles to the raft gives greater improvement in the raft behavior than unconnected piles. Based on test results, the effects of different parameters are presented and discussed.     

Behavior of Slender Piles Subject to Free-Field Lateral Soil Movement

Soil movements associated with slope instability induce shear forces and bending moments in stabilizing piles that vary with the buildup of passive pile resistance. For such free-field lateral soil movements, stress development along the pile element is a function of the relative displacement between the soil and the pile. To investigate the effects of relative soil-pile displacement on pile response, large-scale load tests were performed on relatively slender, drilled, composite pile elements (cementitious grout with centered steel reinforcing bar). The piles were installed through a shear box into stable soil and then loaded by lateral translation of the shear box. The load tests included two pile diameters (nominal 115 and 178?mm) and three cohesive soil types (loess, glacial till, and weathered shale). Instrumentation indicated the relative soil-pile displacements and the pile response to the loads that developed along the piles. Using the experimental results, an analysis approach was evaluated using soil p-y curves derived from laboratory undrained shear strength tests. The test piles and analyses helped characterize behavioral stages of the composite pile elements at loads up to pile section failure and also provided a unique dataset to evaluate the lateral response analysis method for its applicability to slender piles.     

Reliability of Axially Loaded Driven Pile Groups

A method was presented to evaluate the reliability of axially loaded pile groups designed using the traditional concept of group efficiency along the lines of load and resistance factor design. Group effects and system effects were identified as the major causes that led to a significantly greater observed reliability of pile foundations than calculated reliability of single piles. Statistical analyses were conducted to evaluate these effects based on observed pile performance. A database of pile group load tests was collected and interpreted for this purpose. Subsequently, the reliability of pile groups associated with the allowable stress design practice was calculated using the suggested method. The calculated probability of failure of pile groups was found to be one to four orders of magnitude smaller than that of single piles, depending on the significance of group effects and system effects. Finally, values of the target reliability index βTS for single piles required to achieve a specified target reliability of pile group foundations were calculated for several design methods. Due to group effects and system effects, the values of βTS should be different for single piles, a pile group, and a pile system of several groups.