Compaction-induced stress in geosynthetic-reinforced granular base course——A discrete element model

A discrete element method (DEM) model was used to simulate the development of compaction-induced stress in a granular base course, with and without geogrid reinforcement. The granular base course was modeled as a mixture of uniformly sized triangular particles. The geogrid was modeled as a series of equally spaced balls that interact with each other through long-range interaction contacts. The long-range interaction contact was also used to simulate a deformable subgrade. The compactor was modeled as a solid cylinder rolling at a constant speed. The DEM model shows that the geogrid-reinforced granular base course gains additional compaction-induced stress due to the residual tensile stress developed in the geogrid. The residual tensile stress in the geogrid increases with the number of compaction passes. Parametric analyses were also conducted to assess the effects of geogrid stiffness and subgrade modulus on the compaction-induced stress.     

Transition of gap-graded soil fabric – shear wave measurements and dispersion relation

The dynamic response of gap-graded soils is complex; it requires massive experimental and theoretical investigation in order for a sound understanding to be developed. The complexity of the behaviour of such soils arises partially due to the presence of many influential parameters, such as the finer sand/silt content (F_s), size ratio (R_d), and void ratio (e). In this contribution, experiments are conducted on a series of gap-graded soil mixtures, where the coarser sand is mixed with four different finer sands or non-plastic silt with different sizes, namely various R_d, and one well-graded material is also tested. The S-wave velocity (V_s) and low-pass threshold frequency (f_lp) are measured in the isotropic stress state using a triaxial apparatus equipped with planar piezoelectric transducers. The experimental results reveal that, for a given e or relative density (D_r), both f_lp and V_s decrease at a low F_s, but increase at a high F_s. The slope of the linear regression between f_lp and V_s appears to capture this transitional behaviour, depending on F_s and R_d, which can then be used to correlate the physical state of the gap-graded soil fabric. The transitional behaviour is more noticeable at a larger R_d, which is in line with the existing literature. To understand the effect of the R_d on the shear wave propagation at a low F_s, the particle-scale dispersion relation for binary chains is newly introduced and correlated with accompanying experimental observations.     

A unified approach to link small-strain shear modulus and liquefaction resistance of pumiceous sand

Natural pumiceous (NP) sands containing pumice particles, a type of volcanic soil, are commonly found in the central part of the North Island in New Zealand. The pumice particles are highly crushable, compressible, lightweight and angular, making engineering assessment of their properties problematic. In this paper, several series of bender element and undrained cyclic triaxial tests were performed on reconstituted and undisturbed NP sands to determine their small-strain shear modulus (G_max) and cyclic resistance ratio (CRR). Furthermore, similar tests were also conducted on normal hard-grained sands (e.g., Toyoura sand) for the purpose of comparison. The results showed that the NP sands have considerably lower G_max compared to normal sands, resulting in their higher deformability during the initial stages of the cyclic loading test. The high angularity of NP sands play an important role toward the end of the cyclic loading and contributed to their higher CRR. Next, the ratio of CRR/G_max for each sample was correlated to a level of strain denoted as cyclic yield strain (ε_ay), which was found to be significantly dependent on the percentages of pumice particles present in the natural soils. On the other hand, the ε_ay was found to be less sensitive to the consolidation stress (σ′_c) and the relative density (Dr) of the materials. For example, over different values of σ′_c and Dr, NP sands have substantially higher values of cyclic yield strain due to their lower G_max and higher CRR when compared with those of ordinary sands.     

Technical report on the compression,structure, and creep behaviors of lacustrine soil deposits in Bogotá, Colombia

This technical report on the compression behavior of soft soils in the Colombian capital, Bogotá, was conducted because the city’s recent construction history has led to excessive settlement of soils in the city over the last forty years. Several studies have shown the city’s soil to contain unusually high concentrations of diatoms. The stationary piston method was used to obtain samples from depths of between five and 100 m. Samples were subsequently examined in the laboratory, being subjected to triaxial and oedometer compression tests, and tested also for Atterberg limits and grain size distributions. Test results for undisturbed soil returned very high liquid limit values, compressibility indices C_C, secondary compression coefficients C_α, and soil structure effects. Some samples showed particle mixtures of different sizes, mineralogy, and diatom concentrations. Although high C_C and C_α indices explain the excessive settling of the city’s buildings, most construction projects do not currently take settlement caused by secondary compression into account. This report shows that secondary compression is an important parameter in the total settlement of buildings in Bogotá and that, furthermore, settlement is also affected by geological history. Finally, useful correlations such as the relationship between the liquid limit, C_α and geological history are presented.     

基于联合强度理论的主动土压力计算方法

考虑结构性黄土的抗拉强度和有限土体对土压力的影响,推导了抛物线加直线形式的联合强度理论。基于此强度理论,建立了土体主动极限平衡状态,推导了无限土体主动土压力的计算公式;通过滑动块体理论推导了有限土体主动土压力的计算公式。通过算例对公式进行了验证比较,研究结果表明:基于联合强度理论计算的土体开裂深度明显小于朗肯土压力理论的开裂深度,且基于联合强度理论土体应存在拉应力区,在此区域内,挡土墙与土体存在胶合力的情况下,需考虑挡土墙受到的拉力和因此导致的局部应力集中;有限土体的土压力小于朗肯主动土压力,且随着挡土墙与既有构筑物距离的增加,有限土体土压力趋近于无限土体土压力。基于联合强度理论和有限土体理论建立的主动土压力计算方法能更合理地评价黄土的主动土压力。     

丙环唑土壤降解影响因素研究

建立了高效液相色谱串联质谱(LC-MS/MS)检测土壤中丙环唑的方法。采用室内模拟实验,对不同土壤及含水率、微生物、有机质和pH等因素对丙环唑降解的影响进行了研究。结果表明:土壤中有机质和微生物不是丙环唑在土壤中降解的主要影响因子。含水率和pH对丙环唑的降解产生显著影响,土壤的含水率越高,pH越小,土壤中丙环唑的降解越快。丙环唑在土壤中的降解主要受pH的影响。     

软黏土地基沉降的时变效应对超高层建筑结构内力的影响

软黏土地基沉降具有时变性,其规律被广泛认知。随着超高层建筑的大量兴建,工程师对超高层建筑与地基基础共同作用问题越来越重视。从地基沉降的时变性出发,综合考虑地基沉降的时变效应与地基-基础-上部结构共同作用的耦合问题,以时间切片的形式进行共同作用分析,提出了考虑地基沉降时变效应的共同作用计算方法,经初步分析得到地基沉降的时变效应对超高层建筑结构关键构件内力的影响。对软土地基上高500 m的超高层结构的分析结果表明,地基沉降的时变效应对上部结构的影响仅限于中低层区;其对巨柱、核心筒轴力的影响较小,轴力最大增、减幅值仅约为5%,在工程实践中可以忽略不计;但其对伸臂构件的影响较大,尤其是斜腹杆最为明显,其内力最大增幅可达200%,在工程实践中应给予重视。     

Zum Tragverhalten der selbstregulierenden interaktiven Membrangründung

On the load bearing behavior of the self‐regulating interactive membrane foundation system The paper reports on a new foundation system, the self‐regulating membrane foundation, for embankments on deep soft soil deposits, such as stockpiles or infrastructure embankments. The load bearing behavior of the system was analyzed by means of centrifuge model tests and comprehensive numerical simulations, such as global sensitivity analyses and parametric studies. The results of those analyses and the derived design approach are presented in the paper. The paper concludes with a validation of the design approach, by comparison with results of the numerical simulation.     

填土强度对埋地钢塑复合缠绕管受力影响分析

钢塑复合缠绕管是一种用于市政工程埋地排污排水的新型管道,在诸多性能方面优于传统的混凝土排水管,目前正在我国的城市建设中进行推广应用。这种管道外层为缠绕钢带,内层为高密度聚乙烯塑胶,是一种新型结构,在埋地受力机理方面,目前的研究做得尚不多。本文采用有限元的数值分析方法,针对不同的回填土情况,对这种管道的管土共同作用受力进行了分析,得出了填土强度对钢塑复合缠绕管管土共同作用受力的影响规律,可供相关的工程设计和施工参考。