不同加筋方式下碎石桩复合地基承载性能试验研究

制作一套碎石桩复合地基室内模型试验装置,开展11组加筋碎石桩复合地基室内模型试验和2组未加筋对比试验,以研究不同包裹长度、加筋间距及加筋组合方式下复合地基承载能力、桩土应力比、承载能力改善率和桩体鼓胀变形情况。试验结果表明：相比较传统碎石桩,筋材的加入对复合地基承载力有明显提升,全长包裹的垂直加筋提升效果最为显著;复合地基承载力提升较大的加筋方式对应的承载能力改善率和桩土应力比值较大,承载能力改善率随加筋长度和加筋间距的增加而增大;复合地基的桩土应力比值随沉降增加呈现出一定幅度的波动,并随加筋间距缩小和加筋长度增加而增大;不同加筋方式下桩身变形规律存在一定差异,采用全长包裹的垂直加筋方式下,桩体鼓胀变形较为均匀且变形量小,对复合地基承载能力的提升以及桩体鼓胀变形的抑制效果更佳。     

上部轮胎箍碎石桩承载特性数值模拟研究

为研究上部轮胎箍碎石桩的承载特性,通过ABAQUS数值模拟方法,分析了该处置措施下碎石桩的单桩极限承载力和桩体的侧向变形规律。研究结果表明:轮胎箍可有效约束碎石桩的侧向变形,可使其单桩承载力提高约50%。该结论的得出可为碎石桩的设计与施工提供参考。     

基于模型试验的竹筋格栅加筋碎石桩承载特性研究

为适应我国“双碳”发展战略需求,拟采用绿色环保竹筋格栅代替传统土工格栅加固碎石桩。基于相似理论,通过室内模型试验,对竹筋格栅加筋碎石桩的承载力特性、桩土应力比及桩体应力传递规律展开了研究。结果表明：相比普通碎石桩,土工格栅加筋碎石桩承载力提高了52.6%,竹筋格栅加筋碎石桩承载力提高了247.4%;同时竹筋格栅套筒可使桩土应力比显著增大,并改善普通碎石桩荷载传递能力较差的缺陷,进而使竹筋格栅套筒碎石桩可将上部荷载传递到更深、更广的地基土体之中,相比于土工格栅套筒,竹筋格栅套筒对碎石桩的加固效果更为理想。这可为竹筋格栅加筋碎石桩施工提供指导,为类似工程提供参考。     

不同型式复合地基试验对比分析

基于相似理论,设计并完成了土工格室加筋垫层、砂井、散体材料桩、柔性桩等九组复合地基模型试验,并对其加固效果进行对比分析,分析结果表明：水平加筋垫层的设置可扩散上部荷载,提高复合地基承载力,土工格室的加强效果优于土工格栅;桩体复合地基须考虑群桩效应的影响,其承载能力明显好于加筋垫层复合地基;不同加载范围下桩和土体的承载能力发挥程度不同,单桩加载下桩体承载能力发挥较三桩、七桩加载时大,碎石桩桩顶桩土应力比>碎石桩+土工格栅>碎石桩+土工格室,而柔性桩+土工格室的桩顶桩土应力比>柔性桩+土工格栅>柔性桩;砂井和各种型式的碎石桩复合地基桩底桩土应力比在1左右,各种型式的柔性桩桩底桩土应力比较大,最大达24;软基浅部较深部孔隙水压力大、消散速度快;桩体复合地基孔隙水压力较土工格室复合地基和软土地基小,砂井和碎石桩复合地基的排水速度明显快于柔性桩复合地基。     

基于圆孔扩张理论的筋箍碎石桩承载力计算方法研究

碎石桩沿桩身围裹一圈土工格栅后成为筋箍碎石桩,其受力机理变得更加复杂,筋材特性必定会对桩体承载力产生影响。基于圆孔扩张理论,合理假设单桩有效加固范围边缘土压力为静止土压力,获得了可考虑筋箍与桩、土协调变形的筋箍碎石桩复合地基极限承载力计算方法,并结合工程实例与现有研究成果对该计算方法进行对比验证,结果表明该方法与实际工程吻合性更高,最后,在该方法的基础上,分析了各参数对筋箍碎石桩复合地基承载力的影响,分析结果表明:筋箍碎石桩的最优加筋深度不是一个定值,而是随着筋材性能和桩周土体条件的变化而变化。     

柔性基础下碎石桩复合地基桩土应力比及沉降计算

深入研究柔性基础下碎石桩复合地基受力变形机理,考虑到碎石桩复合地基桩体侧向鼓胀及桩体的整体性,在径向位移模式分析中引入横截面剪应力的影响,并由此建立了碎石桩鼓胀段荷载传递模式。然后,结合桩身荷载传递规律,导出了柔性基础下碎石桩鼓胀变形的控制微分方程,获得了柔性基础下碎石桩复合地基桩土应力比及沉降。最后通过实例和数值模拟分析,表明该方法具有较好的合理性与可行性,参数分析表明,增大桩土模量比和减少置换率会提高桩土应力比,荷载水平则会影响径向鼓胀变形的程度。     

筋箍长度及刚度对加筋碎石桩复合地基承载力影响分析

针对加筋碎石桩复合地基中桩体性能,通过有限元数值模拟与模型试验对比分析,验证了数值模型的可靠性,进而变换加筋长度,研究分析了复合基础下端承加筋单桩与群桩的极限承载能力和破坏模式。研究结果表明:筋材强度较低时,加筋长度不会对桩体破坏模式产生影响,对极限承载能力提高有限;随着筋材强度不断提高,碎石桩在加筋体以下区域发生剪切破坏,并且随着加筋长度的增加向更深土层发展,基础的极限承载能力线性增长。加筋长度对群桩复合地基不同位置处桩体的破坏模式影响不同。相较于边桩,中心桩在桩身较深位置处发生剪切破坏,筋材需达到较深的长度才发挥约束效果。     

基于改进应变楔模型的固体-散体串联组合桩鼓胀变形及沉降分析

针对传统碎石桩易在桩顶1~3倍桩径的深度范围内发生鼓胀破坏的现象,首先,在现有工艺的基础上提出一种固体（混凝土）-散体（碎石散体材料）串联组合桩,并简要阐述其承载特性、施工工艺及破坏模式;其次,建立了串联组合桩鼓胀变形的计算模型,探讨串联组合桩的鼓胀变形机理;然后,对应变楔模型进行修正,并结合广义胡克定律,计算固体-散体串联组合桩的鼓胀变形值及桩身沉降值;最后,通过传统碎石桩及筋箍碎石桩鼓胀变形及沉降试验验证所提理论方法的合理性,结果表明：理论计算值与试验结果吻合较好,可为碎石桩复合地基设计提供一定的参考,具有理论及工程应用价值。     

冻融条件下加筋碎石桩复合地基路堤性状研究

制作了一套加筋碎石桩复合地基冷冻试验系统,开展了1组加筋碎石桩复合地基路堤冻融离心模型试验和1组未冻融的对比试验,以研究加筋碎石桩复合地基经历季节性冻土后填筑的路堤在冻融条件下的性状。研究结果表明:冻融条件下加筋碎石桩复合地基在地基土未融化前,其桩顶和桩间土沉降基本一致,而在地基土全部融化后,桩间土沉降显著增大;冻融条件下路堤边坡基本保持初始坡度,路堤下地基沉降比较均匀,而未冻融组路堤边坡明显变缓,路堤下地基不均匀沉降明显;在复合地基和桩体均处于冰冻状态时,其桩顶和桩间土应力一致,当桩体先于桩间土融化后,桩顶应力减小而桩间土应力增大,而当地基土开始全部融化后,桩间土应力快速下降而桩顶应力快速增大,冻融条件下复合地基沉降稳定时的桩土应力比是未冻融条件下桩土应力比的2/3左右;冻融条件下由于路堤加载过程中桩顶周围土体处于冰冻状态,限制了桩顶侧向位移,而冻土层以下土体推动下部桩体向外位移,使得靠近路堤边坡下的桩体向路堤内弯曲,但弯曲变形量较小,而未冻融条件下的桩体则向路堤外弯曲且弯曲变形量较大;加筋碎石桩适合用于季节性冻土区湿地软土地基处理,其复合地基经历季节性冻土后填筑的路堤整体性能较好。     

基于改进应变楔模型的固体–散体串联组合桩鼓胀变形及沉降分析

针对传统碎石桩易在桩顶1～3倍桩径的深度范围内发生鼓胀破坏的现象,首先,在现有工艺的基础上提出一种固体(混凝土)–散体(碎石散体材料)串联组合桩,并简要阐述其承载特性、施工工艺及破坏模式;其次,建立了串联组合桩鼓胀变形的计算模型,探讨串联组合桩的鼓胀变形机理;然后,对应变楔模型进行修正,并结合广义胡克定律,计算固体–散体串联组合桩的鼓胀变形值及桩身沉降值;最后,通过传统碎石桩及筋箍碎石桩鼓胀变形及沉降试验验证所提理论方法的合理性,结果表明:理论计算值与试验结果吻合较好,可为碎石桩复合地基设计提供一定的参考,具有理论及工程应用价值。     

Bearing capacity of horizontally layered geosynthetic reinforced stone columns

In very soft soils, the bearing capacity of stone columns may not improve significantly due to very low confinement of the surrounding soil. Therefore, they may be reinforced with geosynthetics by using vertical encasement or horizontal layers. Very limited studies exist on horizontally reinforced stone columns (HRSCs). In this research, some large body laboratory tests have been performed on horizontally reinforced stone columns with diameters of 60, 80, and 100?mm and groups of stone columns with 60?mm diameter. Results show that the bearing capacity of stone columns increases by using horizontally reinforcing layers. Also, they reduce lateral bulging of stone columns by their frictional and interlocking effects with stone column aggregates. Finally, numerical analyses were carried out to study main affecting parameters on the bearing capacity of HRSCs. Numerical analysis results show that the bearing capacity increases considerably with increasing the number of horizontal layers and decreasing space between layers.     

Geosynthetic-encased stone columns: Numerical evaluation

Stone columns (or granular piles) are increasingly being used for ground improvement, particularly for flexible structures such as road embankments, oil storage tanks, etc. When the stone columns are installed in extremely soft soils, the lateral confinement offered by the surrounding soil may not be adequate to form the stone column. Consequently, the stone columns installed in such soils will not be able to develop the required load-bearing capacity. In such soils, the required lateral confinement can be induced by encasing the stone columns with a suitable geosynthetic. The encasement, besides increasing the strength and stiffness of the stone column, prevents the lateral squeezing of stones when the column is installed even in extremely soft soils, thus enabling quicker and more economical installation. This paper investigates the qualitative and quantitative improvement in load capacity of the stone column by encasement through a comprehensive parametric study using the finite element analysis. It is found from the analyses that the encased stone columns have much higher load carrying capacities and undergo lesser compressions and lesser lateral bulging as compared to conventional stone columns. The results have shown that the lateral confining stresses developed in the stone columns are higher with encasement. The encasement at the top portion of the stone column up to twice the diameter of the column is found to be adequate in improving its load carrying capacity. As the stiffness of the encasement increases, the lateral stresses transferred to the surrounding soil are found to decrease. This phenomenon makes the load capacity of encased columns less dependent on the strength of the surrounding soil as compared to the ordinary stone columns.     

Bearing capacity of geogrid reinforced sand over encased stone columns in soft clay

Stone columns develop their load carrying capacity from the circumferential confinement provided by the surrounding soils. In very soft soils, the circumferential confinement offered by the surrounding soft soil may not be sufficient to develop the required load carrying capacity. Hence a vertical confinement would yield a better result. The load carrying capacity is further increased with the addition of a sand bed over the stone columns. In the present study, a series of laboratory model tests on an unreinforced sand bed (USB) and a geogrid-reinforced sand bed (GRSB) placed over a group of vertically encased stone columns (VESC) floating in soft clay and their numerical simulations were conducted. Three-dimensional numerical simulations were performed using a finite element package ABAQUS 6.12. In the finite element analysis, geogrid and geotextile were modeled as an elasto-plastic material. As compared to unreinforced clay bed, an 8.45 fold increase in bearing capacity was observed with the provision of a GRSB over VESC. The optimum thickness of USB and GRSB was found to be 0.2 times and 0.15 times the diameter of the footing. A considerable decrease in bulging of columns was also noticed with the provision of a GRSB over VESC. Both the improvement factor and stress concentration ratio of VESC with GRSB showed an increasing trend with an increase in the settlement. It was observed that the optimum length of stone columns and the optimum depth of encasement of the group of floating VESC with GRSB are 6 times and about 3 times the diameter of the column respectively.     

Uniaxial compression behavior of geotextile encased stone columns

The bearing capacity and failure mechanism of encased stone columns are affected by many factors such as encasement length, relative density, strength and stiffness of the encasement material. In soft soils where surrounding soil pressure is low, especially in the top section, the stone columns may be close to a uniaxial compression state, where the uniaxial compression strength controls the bearing capacity of the stone columns. A series of large-scale triaxial tests on ordinary stone columns and uniaxial tests on geotextile encased stone columns have been performed. The stone columns were 300?mm in diameter and 600?mm in height. Samples of four different relative densities, and five types of geotextiles were used in the tests to study the effect of initial void ratio and encasing materials on the uniaxial compression behavior of the stone columns. The results show the uniaxial compressive strength of the encased stone columns is not affected by the initial void ratio but mainly by the tensile strength of the encasing geotextiles. The stress strain curves of the encased stone columns under uniaxial loading condition are nearly liner before failure, which is similar to the tensile behavior of the geotextiles.     

刚性基础下碎石桩复合地基临塑荷载的确定

为探讨碎石桩复合地基的承载力,从碎石桩的荷载传递和破坏性状出发,建立碎石桩复合地基的计算模型。基于半空间轴对称弹性理论及基本假定,考虑散体材料桩在荷载传递过程中的径向膨胀变形,根据桩土变形协调及桩土界面上径向应力平衡条件,得到桩土应力比的表达式。应用莫尔-库仑破坏准则,给出了碎石桩复合地基临塑荷载和临界荷载的计算公式。最后,与碎石桩复合地基极限承载力经典方法的计算结果进行对比,结果表明,计算所得的临界荷载十分接近。     

Improvement of soft soils using geogrid encased stone columns

In recent years, geotextile encasement has been used to extend the use of stone columns to extremely soft soils. Although the technique is now well established, little research has been undertaken on the use of other encasement materials such as geogrid. This paper discusses the results of a series of small-scale model column tests that were undertaken to investigate the behaviour of geogrid encased columns. The tests focused on studying the effect of varying the length of encasement and investigating whether a column that was partially encased with geogrid would behave similarly to a fully-encased column. In addition, isolated column behaviour was compared to group column behaviour. The results of partially encased column tests indicated a steady reduction in vertical strain with increasing encased length for both isolated columns and group columns. Bulging of the column was observed to occur directly beneath the base of the encasement. A significant increase in column stiffness and further reduction in column strain was observed for fully-encased columns, with strain reductions in the order of 80%. This range of performance may lend the techniques of partial and full geogrid encasement to a series of potential site applications.     

Centrifuge tests on geosythetic-encased stone column supported embankment on seasonal frozen soil

Geosynthetic-encased stone columns (GESCs) have been widely applied into soft foundation. This paper aimed to evaluate the availability and efficiency of GESCs in seasonal frozen ground. Four centrifuge tests were conducted on GESCs supported embankment on seasonal frozen soil under embankment load and thawing, where the foundation was frozen before the construction of embankment. The effects of encasement stiffness and lateral reinforced cushion were investigated. Three phases could be distinguished in the tests by two timing nodes due to the complete thawing of columns and soil, and analysis were made based on the three phases. It is found that high-stiffness encasement can effectively reduce the differential settlement between soil and columns before complete thawing of soil. The GESCs appeared a deformation mode of inward bending, which is in inverse to that in common composite foundation. The reinforced cushion rearranged stress on columns and soil, and influenced the development of pore pressure. The stress concentration ratio (SCR) first decreased to less than 1 due to column thawing, and experienced a steady stage until soil was completely thawed. The SCR rapidly increased after complete thawing of soil, and decreased to a constant value due to soil consolidation.     

Shaking table tests on geosynthetic encased columns in soft clay

This paper presents the results of an experimental research on the behavior of geosynthetic encased stone columns and ordinary stone columns embedded in soft clay under dynamic base shaking. For this purpose, a novel laminar box is designed and developed to run a total of eight sets of 1-G shaking table tests on four different model soil profiles: Soft clay bed, ordinary stone column installed clay bed, and clay beds with geosynthetic encased columns with two different reinforcement stiffnesses. The geosynthetic encased columns are heavily instrumented with strain rosettes to quantify the reinforcement strains developing under the action of dynamic loads. The responses of the columns are studied through the deformation modes of the encased columns and the magnitude and distribution of reinforcement strains under dynamic loading. The response of the granular inclusion enhanced soft subsoil and embankment soil and the identification of the dynamic soil properties of the entire soil body are also discussed in this article. Finally, to determine the effect of dynamic loading on the vertical load carrying capacity, stress-controlled column load tests are undertaken both on seismically loaded and undisturbed columns.     

土工合成材料约束碎石桩承载特性研究

土工合成材料约束碎石桩作为一种新型软土地基处理技术在工程中广泛应用,其单桩承载力取决于土工合成材料抗拉强度和土的工程性质。通过对土工合成材料、碎石桩及地基土的相互作用机理进行分析,提出了考虑土工合成材料约束拉力与土体围压的桩身强度计算方法,进而推导出考虑上部荷载作用的,由桩身强度控制的单桩极限承载力计算方法,并采用MATLAB编写了计算程序,根据得出的单桩极限承载力计算了土工合成材料拉力沿深度的分布,结合一算例说明了计算所需要的参数及计算过程,成果可为土工合成材料碎石桩的设计提供计算依据。