Analysis and modification of the British Standard BS8006 for the design of piled embankments

The piled embankment is an increasingly popular construction method. The Dutch Design Guideline for piled embankments (CUR 226) was published in the first half of 2010. Several existing models have been analysed to determine the calculation rules used in the Dutch Guideline. The British Standard BS8006 sometimes calculates tensile forces in the geosynthetic reinforcement that differ considerably from other models. For quite thin embankments in particular, BS8006 designs a relatively strong and thus expensive geosynthetic (basal) reinforcement in comparison with other design models. These differences are not always fully understood, leading to uncertainty. This paper analyses BS8006 and demonstrates why it behaves differently from other models. It also examines why this behaviour is different than would be expected. For example, it is shown that calculations using BS8006 are based on a higher load than the actual load.A modification to BS8006 is proposed, which is shown to give comparable results to the German Standard EBGEO for situations where there is no subsoil support.The results of BS8006, Modified BS8006, and the German/Dutch guideline are compared with finite element calculations and field measurements. It is concluded that the results given by the Modified BS8006 are more accurate to those using BS8006.     

Large-scale load capacity tests on a geosynthetic encased column

Stone columns have been used to minimize the settlement of embankments on soft soils but their use in very soft soils can become challenging, partly because of the low confinement provided by the surrounding soil. Geosynthetic encased columns (GECs) have been successfully used to enhance to reduce settlements of embankments on soft soils. This paper describes an investigation on the performance of encased columns constructed on a very soft soil using different types of encasement (three woven geotextiles with different values of tensile stiffness) and different column fill materials (sand, gravel and recycled construction and demolition waste, RCDW). The results of load capacity tests conducted on large-scale models constructed to simulate the different types of GECs indicate that the displacement method adopted during column installation can lead to an enhanced shear strength in the smear zone that develops within the very soft soil. In addition, breakage of the column fill material was found to affect the load-settlement response of gravel and RCDW columns. Furthermore, the excess pore water pressure generated in the surrounding soil during installation, was found to remain limited to radial distances smaller than three times the GEC diameter.     

桩承式路堤土拱效应计算方法比较

桩承式路堤土拱效应对路堤的荷载传递和沉降变形性状有重要影响,桩土应力比是评价土拱效应的重要参数.通过模型试验及现场测试对目前已有的土拱效应计算方法进行了比较.结果表明:对于平面土拱,Terzaghi方法计算结果与模型试验结果最大值比较接近,Low方法计算结果整体上与模型试验结果吻合较好;对于空间土拱,Hewlett方法及陈云敏方法计算结果与现场实测结果均比较接近;英国规范BS8006推荐方法的计算结果与模型试验及现场实测结果的差异均较大.该研究成果可为桩承式路堤设计提供有益的参考.     

A simplified method for analysis of a piled embankment reinforced with geosynthetics

Piled embankments provide an economic solution to the problem of constructing embankments over soft soils. The piles and geosynthetic combination can alleviate the uneven surface settlements that sometimes occur in embankments supported by piles without reinforcement. The main focus of this paper is to present a new method for analysis of an embankment of granular fill on soft ground supported by a rectangular grid of piles and geosynthetic. This method is based on consideration of the arching effect in granular soil and similar to the method proposed by Low, B.K., Tang, S.K., Choa, V. [1994. Arching in piled embankments. Journal of Geotechnical Engineering 120 (11), 1917–1938]. The main refinements are: inclusion of a uniform surcharge load on the embankment fill, individual square caps were used, and taking into account the skin friction mechanism, which contributes to soil–geosynthetic interface resistance. Using this method, the influence of embankment height, soft ground depth, soft ground elastic modulus, and geosynthetic tensile stiffness on efficiency, stress concentration ratio, settlement ratio, tension of geosynthetic, and axial strain of geosynthetic are investigated. The results show that inclusion of a geosynthetic membrane can increase the fill load carried by piles. As a result, both the total and differential settlements of the embankment can be reduced. The new design method was verified against several current design methods. Theoretical solution showed that BS8006 [1995. Code of Practice for Strengthened/Reinforced Soils and other Fills. British Standards Institution, London, p. 162] and Guido, V.A., Kneuppel, J.D., Sweeny, M.A. [1987. Plate loading tests on geogrid-reinforced earth slabs. In: Proceedings of the Geosynthetics '87, New Orleans, USA, IFAI, pp. 216–225] methods overpredict the vertical stress acting on the geosynthetic due to that the reaction of the soft ground on the geosynthetic is not considered in their methods. It also showed that the present method is in good agreement with Low, B.K., Tang, S.K., Choa, V. [1994. Arching in piled embankments. Journal of Geotechnical Engineering 120 (11), 1917–1938] method.     

考虑蠕变效应的钢柱高温承载力计算方法

高温蠕变对火灾下钢构件的内力和变形影响较大,现行《建筑钢结构防火技术规范》(GB51249—2017)中未考虑蠕变对钢柱高温承载力的影响。采用ANSYS软件分析考虑蠕变后钢柱在高温下的受力性能,并与钢柱的抗火试验进行对比,发现考虑蠕变的钢柱有限元模拟结果与试验数据吻合更好。利用验证的有限元模型进行参数分析,结果表明:考虑蠕变效应后,钢柱的高温承载力受初始缺陷(残余应力、初弯曲、初偏心)、弯曲方向、荷载比、长细比、升温速率的影响较大,受截面形式和钢材屈服强度的影响较小。给出了考虑蠕变效应后计算钢柱高温承载力的简化方法。     

Load transfer mechanism and deformation of reinforced piled embankments

A series of twenty-eight centrifuge tests was performed on piled embankments with basal geosynthetic reinforcement to assess the influence of pile spacing, embankment height, pile cap size and geosynthetic stiffness on the load transfer mechanism and surface settlements. The measurements of the forces on the piles made it possible to assess the load transfer mechanisms, and 100% efficiency was achieved for all tests performed. The results showed that for the thicker mattress and/or closer piles, the surface settlements were smaller or negligible. Geosynthetic maximum deflections were also examined experimentally and analytically, the latter based on BS8006 (2010) and its further corrigendum in 2012. Close agreement in the predictions of the maximum reinforcement deflection was reached with BS8006 (2012) by adopting a slight modification in the ratio of diagonal and orthogonal maximum deflection (y_d/y = √2).     

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.     

Displacements of column-supported embankments over soft clay after widening considering soil consolidation and column layout: Numerical analysis

The common challenges for constructing embankments on soft clay include low bearing capacity, large total and differential settlements, and slope instability. Different techniques have been adopted to improve soft clay, such as the use of foundation columns including stone columns, deep mixed columns, and vibro-concrete columns, etc. Due to increased traffic volume, column-supported embankments may be widened to accommodate the traffic capacity need. Adding a new embankment to an existing embankment generates additional stresses and deformations under not only the widened portion but also the existing embankment. Differential settlements between and within the existing embankment and the widened portion may cause pavement distresses. Limited research has been conducted so far to investigate widening of column-supported embankments. In this study, a two-dimensional finite difference numerical method was adopted. This numerical method was first verified against field data and then used for the analysis of widened column-supported embankments over soft clay. The modified Cam-Clay model was used to model the soil under the existing embankment and the widened portion. Mechanically and hydraulically coupled numerical models were created to consider the consolidation of the foundation soil under the existing embankment and the widened portion. Different layouts of foundation columns under the existing embankment and the widened portion were investigated. The numerical results presented in this paper include the vertical and horizontal displacements, the maximum settlements, the transverse gradient changes, and the stress concentration ratios, which depended on column spacing. The columns installed under the connection side slope were most effective in reducing the total and differential settlements, horizontal displacement, and transverse gradient change of the widened embankment.     

长期荷载作用下带脱空缺陷钢管混凝土柱的受力性能研究

为研究长期荷载作用对带脱空缺陷钢管混凝土柱受力性能的影响,以脱空类型、脱空率和长期荷载比为主要参数,对长期荷载作用下带脱空缺陷钢管混凝土短柱的变形特性和承载力进行了试验研究。试验结果表明：在长期荷载作用下,与普通无脱空缺陷钢管混凝土柱类似,脱空缺陷钢管混凝土柱徐变系数终值的上限值为0.9;长期荷载比大的脱空缺陷钢管混凝土柱较长期荷载比小的脱空缺陷钢管混凝土柱的承载力略大。采用有限元分析软件ABAQUS,对考虑长期荷载作用影响时脱空构件的受力性能进行分析,有限元计算结果和试验结果吻合良好。利用验证过的有限元模型,系统地分析了长细比、含钢率、钢管屈服强度、混凝土强度、长期荷载比、脱空率和荷载偏心率等参数对长期荷载作用下带脱空缺陷钢管混凝土柱的承载力影响规律。结果表明,混凝土脱空缺陷对考虑长期荷载作用的承载力影响系数的不利影响小于1.5%;现有的考虑长期荷载影响的普通钢管混凝土柱的设计方法可用于预测带脱空缺陷钢管混凝土柱的承载力。     

Bearing capacity and failure behaviors of floating stiffened deep cement mixing columns under axial load

This research aims to clarify and gain an insight into the impact of the length of the stiffened core and the strength of the deep cement mixing (DCM) socket on the behaviors of floating stiffened deep cement mixing (SDCM) columns. The observed behaviors include the axial ultimate bearing capacity, settlement and failure mode. The study begins by conducting a series of physical model tests as a preliminary investigation. The results reveal that the strength of the DCM socket can be reduced to a certain value by inserting a sufficiently long reinforced core to achieve the highest possible load-carrying capacity, indicating an optimum length of the stiffened core for a specific DCM socket strength. For a parametric study on the actual scale condition, full-scale load tests on a floating DCM and an SDCM column with eucalyptus wood as a core in the thick soft clay layer area were carried out to provide a reference case. The extended numerical analysis results suggest that the modes of failure depend on the length of the stiffened core and the strength of the DCM socket. The results from the numerical parametric study were used to establish a guideline chart for suggesting the appropriate length of the core in accordance with the strength of the DCM socket of the floating SDCM columns. The field pile load test results also confirm that core materials with a lower strength and stiffness, such as eucalyptus wood, could potentially be used as a reinforced core.     

Two-dimensional soil arching evolution in geosynthetic-reinforced pile-supported embankments over voids

Soil arching and tensioned membrane effects are two main load transfer mechanisms for geosynthetic-reinforced pile-supported (GRPS) embankments over soft soils or voids. Evidences show that the tensioned membrane effect interacts with the soil arching effect. To investigate the soil arching evolution under different geosynthetic reinforcement stiffness and embankment height, a series of discrete element method (DEM) simulations of GRPS embankments were carried out based on physical model tests. The results indicate that the deformation pattern in the GRPS embankments changed from a concentric ellipse arch pattern to an equal settlement pattern with the increase of the embankment height. High stiffness geosynthetic hindered the development of soil arching and required more subsoil settlement to enable the development of maximum soil arching. However, soil arching in the GRPS embankments with low stiffness reinforcement degraded after reaching maximum soil arching. Appropriate stiffness reinforcement ensured the development and stability of maximum soil arching. According to the stress states on the pile top, a concentric ellipse soil arch model is proposed in this paper to describe the soil arching behavior in the GRPS embankments over voids. The predicted heights of soil arches and load efficacies on the piles agreed well with the DEM simulations and the test results from the literature.     

考虑蠕变和残余应力释放的Q460钢柱抗火设计方法

承受荷载的钢结构在火灾下可发生明显的蠕变变形,钢结构中的焊接残余应力在火灾下也会一定程度地释放,因而高温蠕变变形和残余应力会对钢柱的耐火性能产生影响.为了准确地对高强度Q460钢柱进行抗火设计,有必要定量分析高温蠕变和残余应力释放对钢柱承载力的影响.采用电炉对2根焊接H形Q460钢柱进行耐火试验,得到无保护Q460钢柱...     

路堤荷载下砼芯水泥土搅拌桩复合地基现场试验研究

针对砼芯水泥土搅拌桩复合地基处理高速公路深厚软土段的工程实例,通过地表沉降、分层沉降、深层水平位移、砼芯荷载以及桩土应力比测试,讨论了砼芯水泥土搅拌桩处理深厚软土地基的加固效果,荷载分布和传递规律。测试结果表明：砼芯水泥土搅拌桩对于路基沉降和水平位移控制效果优于水泥土搅拌桩,且横断面差异沉降较小。复合地基的主要压缩量发生在桩顶至砼芯底端一定范围的土体内,沉降发生深度由砼芯控制。砼芯水泥土搅拌桩上部出现负摩阻力,中性点位于砼芯1/3长度处。桩土应力比为水泥土搅拌的2～3倍,与刚性桩相近,桩体承担大部分路堤荷载。砼芯水泥土搅拌桩复合地基排水通畅,超静孔隙水压力消散迅速。路堤荷载下砼芯水泥土搅拌桩工作特性与载荷板试验下的测试结果有所不同。最后从沉降和承载力控制角度给出了砼芯水泥土搅拌桩复合地基的设计方法     

公路软基蠕变—固结耦合有限元分析

软土的蠕变特性常常导致路堤出现沉降过大、甚至失稳等现象。本文采用同时考虑蠕变和固结效应的修正的广义Kelvin蠕变—固结模型,对公路软基的时效性变形进行了有限元分析。在某软基上路堤填筑工程的变形分析中,该方法的计算结果和监测数据基本吻合,由此验证了该模型的有效性。本文针对该工程进行了一系列的参数分析,讨论了软土的蠕变效应、塑料排水板布置方式和堆载速率等因素对该路堤变形发展和路堤稳定性的影响。     

桩承式加筋路堤的改进设计方法研究

具有深厚软土层的路堤若采用桩承加筋式复合地基,可提高地基承载力,减少路堤不均匀沉降,也可布置成疏桩,降低工程成本,在国内外得到越来越广泛的应用,但还没有可靠实用的设计计算方法,且现有的设计均忽略了桩间土的承载作用,这与工程实际有很大差别。基于三维土拱效应,改进Hewlett土拱效应算法,得到桩承式路堤的桩土荷载分担比,进而考虑加筋体影响以及桩间土承载作用,推导桩土应力比计算式,并将此式应用于路堤的设计。     

砂填料桩承式路堤土拱效应模型试验

现有的桩承式路堤荷载传递计算方法主要依据3类土拱效应力学计算模型。由于宏观土拱形态观察的难度较大,现有计算方法普遍缺乏对不同填料与参数下拱效应传力机制以及宏观土拱拱形参数的深入探讨。采用自制的试验装置对砂填料桩承式路堤土拱效应模型进行探讨,进行了3组不同桩距比下3种填土高度的模型试验。模型试验装置配备了位移控制装置模拟与精确控制桩间土下沉,在下沉过程中连续、同步的采集土压力以及砂箱内部填料的照片,并通过摄影测量技术获取全场位移数据。通过对桩土应力比曲线特征以及曲线特征点所对应的填料颗粒位移图的综合分析,探讨了砂填料桩承式路堤拱效应传力机制,揭示了填料内部存在的初始三角形松动滑移面。基于此提出了初始三角拱力学计算模型,分析得到了滑移面角度随桩距比变化的规律,并利用滑移面夹角统计数据确定了拟合计算公式,通过力学推导建立了适用于砂填料桩承式路堤的桩土应力比计算方法。通过与Rogbeck法、BS8006法、Terzaghi法以及模型试验实测数据的对比,验证了计算方法的合理性。     

Reinforcement load in geosynthetic-reinforced pile-supported model embankments

The stress conditions of geosynthetic reinforcements (GRs) are crucial in achieving the accurate serviceability design of geosynthetic-reinforced pile-supported (GRPS) embankments. However, the sensitivity of load distribution to the settlement process has been reported in geosynthetic-reinforced embankment overlying cavities. In this study, a three-dimensional model embankment was used to perform experiments and evaluate the load acting on the GR. A flexible pressure-mapping sensor was introduced to investigate the pressure distribution for two types of supporting conditions: partitioned displacement by multiple movable trapdoors and even trapdoor settlement underneath different subsoil materials. The results showed that the load on the GR was concentrated on the strip areas between adjacent pile heads along with the settlement. The measured load on the GR strip area was related to the settlement process and finally exhibited a U-shaped distribution after detachment from the support underneath. The soil arch height in the subgrade continuously increased with the settlement; meanwhile, the pile head load increased rapidly at first and then decreased slightly or remained stable depending on the foundation support stiffness. For both types of settlement behaviours, soil arching exhibited stress history-related characteristics that influence the load transfer in GRPS embankments.     

Serviceability design for geosynthetic reinforced column supported embankments

In recent years, geosynthetic reinforced column supported embankments (GRCSEs) have become an increasingly popular design solution for road and rail infrastructure constructed over soft soil sites. However, the serviceability behaviour and deformation that often govern the suitability of their design is not well understood. This is due, in part, to the difficulties in describing the arching stress development in the load transfer platform (LTP). This paper highlights the need for coupled arching stress-deformation models to describe accurately serviceability behaviour. This approach contrasts the widely adopted two-step design approach, which uses limit-equilibrium models that de-couple the arching stress-deformation relationship to describe ultimate limit state behaviour. Using an analytical example, an arching stress/deformation model and an empirical relationship (developed by others) relating base LTP settlement to surface settlement, the relationship between serviceability behaviour and soft soil parameters is highlighted and the conditions leading to progressive collapse in GRCSEs are described. The approach presented provides a means to predict serviceability behaviour, and at the same time, raises questions about the long-term performance and the manner in which acceptable performance has been achieved in the short-term in several field case studies. In particular, those constructed at, or near, a minimum embankment height.     

Deformation and consolidation around encased stone columns

A new analytical solution is presented to study soft soil improvement by means of encased stone columns to reduce both settlement and consolidation time. The proposed solution aims to be a simple and useful tool for design. Only a unit cell, i.e. an end-bearing column and its surrounding soil, is modelled in axial symmetry under a rigid and uniform load. The soft soil is treated as an elastic material and the column as an elastic-plastic material using the Mohr-Coulomb yield criterion and a non-associated flow rule, with a constant dilatancy angle. An elasto-plastic behaviour is also considered for the encasement by means of a limit tensile strength. The solution is presented in a closed form and is directly usable in a spreadsheet. Parametric studies of the settlement reduction, stress concentration and consolidation time show the efficiency of column encasement, which is mainly ruled by the encasement stiffness compared to that of the soil. Column encasement is equally useful for common area replacement ratios but columns of smaller diameters are better confined. Furthermore, the applied load should be limited to prevent the encasement from reaching its tensile strength limit. A simplified formulation of the solution is developed assuming drained condition. The results are in agreement with numerical analyses.     

钢管混凝土柱极限承载力的统一算法研究

介绍国内外各设计规程关于钢管混凝土柱极限承载力的计算方法并进行比较分析。钢管混凝土单肢柱、哑铃形柱、格构柱的受力性能具有共性,即长细比和偏心率对柱面内整体极限承载力的影响基本上是独立的。基于上述规律,在分析评价国内外各规程计算方法的基础上,对钢管混凝土柱极限承载力的统一算法开展研究。分别进行钢管混凝土单肢柱、哑铃形柱、格构柱的算例分析,研究结果表明,计算值与试验值吻合良好。最后提出适合工程应用的钢管混凝土柱极限承载力的统一算法,为钢管混凝土计算理论的进一步完善和规程修订提供参考。