Visualization and quantification of soil laboratory impact compaction

As important methods to guide the field soil compaction, the standard and modified Proctor tests for laboratory compaction have remained unchanged for decades, which should be improved to better understand the compaction process and the properties of soils. In this study, an accelerometer was installed on a Marshall impact compactor to capture the dynamic response of three types of soils during compaction. The experimental test results indicated that the acceleration curve for each blow gradually evolved to a stable pattern following the progress of compaction, and the impact and gyratory locking points were linearly related with coefficient of determination R² equal to 0.59. The impact compaction curve could be further constructed by filtering the structural resonance, which can be used to quantify the compactability of soil materials. Although each type of soil had a unique set of compaction curves, the slope and value of compaction curve altered accordingly as the moisture content changed for the same soil. In addition, the average acceleration value at the final compaction stage could serve as the target value of soil stiffness.     

A dynamic gradient ratio test apparatus

The soil-geotextile filtration mechanism is a complex process which depends on physical compatibility between the geotextile and the soil to be retained. Several methods have been proposed by researchers for assessing the filtration behaviour of soil-geotextile composite systems under steady state conditions. The Gradient Ratio (GR) test is the most commonly used method for measuring filtration compatibility of soil-geotextile systems. This paper describes the design of a modified GR permeability test apparatus to overcome disadvantages associated with traditional GR test devices. The apparatus can perform filtration tests under static and dynamic conditions and can be used to evaluate the filtration compatibility of fine-grained soils with geotextiles. The apparatus is incorporated within a triaxial testing system, hence representative field stress conditions can be applied to test specimens. Some exemplar GR tests performed on coarse and fine-grained soils with a non-woven geotextile are presented in this paper. Unidirectional dynamic loads are applied within the filtration tests to replicate highway traffic loading. Test results show that dynamic loading affects the filtration behaviour at the soil-geotextile interface by increasing the fine particles migration towards the geotextile, but that, for the soil evaluated here, this effect was small.     

浚填砂土水位对动力夯实成效之实验研究

以自行研发室内控制水位之单击试验系统与室内自动圆锥贯入仪,探讨浚填砂土在不同水位受到夯击时,下列各项物理量之差异:①内部激发之动态孔隙水压增量;②有效动态土压力增量;③夯击后之土壤贯入阻抗等。并且引入改良率与贯入功之观念,藉以评估整体之改良效果。结果显示:无水位试体夯击时,在同一深度之有效动态土压力增量远比有水位者为高,且夯击后之土壤贯入阻抗、改良率与贯入功又比有水位者高。同时,低水位试体之夯击成效又比高水位者为佳。     

Soil stiffness as a function of dry density and the degree of saturation for compaction control

A data set of the coefficient of vertical subgrade reaction for a plate diameter of 30 cm, K_P.FWD, measured by Portable Falling Weight Deflectometer tests in full-scale compaction tests on sandy soil using various types of compaction machines was analysed. Based on this analysis and the previous analysis of a CBR data set from another full-scale compaction test series, a set of conclusions could be drawn. The soil stiffness indexes (SSIs), CBR and K_P.FWD, can be expressed by, respectively, an empirical equation multiplying a function of ρ_d and another of S_r, where CBR and K_P.FWD decrease significantly as S_r increases. As such, ρ_d values cannot be estimated from SSI alone. By fixing the field ρ_d – w compaction curve by keeping the soil type and the field compaction energy level, CEL_f, to those in the field compaction tests in which the SSI – ρ_d – S_r correlation has been calibrated, field compacted ρ_d & w states can be estimated by measuring the SSI values. Then, by the upper-bound and lower-bound control of SSI, the field compacted states approach the specified target, where, typically, ρ_d = the maximum dry density for CEL_f and S_r = the optimum degree of saturation, (S_r)_opt. A method to incorporate this indirect but fast SSI-based compaction control into the conventional ρ_d & w-based compaction control is proposed.     

Genetic programming for predictions of effectiveness of rolling dynamic compaction with dynamic cone penetrometer test results

Rolling dynamic compaction(RDC),which employs non-circular module towed behind a tractor,is an innovative soil compaction method that has proven to be successful in many ground improvement applications.RDC involves repeatedly delivering high-energy impact blows onto the ground surface,which improves soil density and thus soil strength and stiffness.However,there exists a lack of methods to predict the effectiveness of RDC in different ground conditions,which has become a major obstacle to its adoption.For this,in this context,a prediction model is developed based on linear genetic programming(LGP),which is one of the common approaches in application of artificial intelligence for nonlinear forecasting.The model is based on in situ density-related data in terms of dynamic cone penetrometer(DCP)results obtained from several projects that have employed the 4-sided,8-t impact roller(BH-1300).It is shown that the model is accurate and reliable over a range of soil types.Furthermore,a series of parametric studies confirms its robustness in generalizing data.In addition,the results of the comparative study indicate that the optimal LGP model has a better predictive performance than the existing artificial neural network(ANN)model developed earlier by the authors.     

Application of artificial neural networks for predicting the impact of rolling dynamic compaction using dynamic cone penetrometer test results

Rolling dynamic compaction (RDC), which involves the towing of a noncircular module, is now widespread and accepted among many other soil compaction methods. However, to date, there is no accurate method for reliable prediction of the densification of soil and the extent of ground improvement by means of RDC. This study presents the application of artificial neural networks (ANNs) for a priori prediction of the effectiveness of RDC. The models are trained with in situ dynamic cone penetration (DCP) test data obtained from previous civil projects associated with the 4-sided impact roller. The predictions from the ANN models are in good agreement with the measured field data, as indicated by the model correlation coefficient of approximately 0.8. It is concluded that the ANN models developed in this study can be successfully employed to provide more accurate prediction of the performance of the RDC on a range of soil types.     

A new model for evaluating the dynamic shear strength of rocks based on laboratory test data for earthquake-resistant design

The dynamic shear strength of rocks is required for the earthquake-resistant design of nuclear power plants in Japan.This research aims to propose a mathematical model for estimating the dynamic strength and to validate the model.Two different types of specimens were prepared for the model validation,and the monotonic and cyclic loading tests were conducted to obtain the mathematical model parameters.Subsequently,multistep cyclic loading tests were performed,followed by simulations using the mathematical model.The test results demonstrated that the dynamic shear strength exceeded the static shear strength,which agreed with previous researches.Furthermore,the dynamic shear strength calculated using the mathematical model was generally consistent with that obtained from the experimental data.     

一种新型非饱和土γ射线土工三轴仪

分析了传统非饱和土三轴仪的特点及其存在问题,提出了非饱和土三轴仪的研制目标,研制出一种新型的与γ射线双源双能法同时测定土样密度和含水率技术相结合的非饱和土三轴仪。介绍了此仪器与一般非饱和土三轴仪的改进及功能实现,并用试验验证了该仪器的适用性。     

A miniature triaxial apparatus for investigating the micromechanics of granular soils with in situ X-ray micro-tomography scanning

The development of a miniature triaxial apparatus is presented. In conjunction with an X-ray micro-tomography (termed as X-ray μCT hereafter) facility and advanced image processing techniques, this apparatus can be used for in situ investigation of the micro-scale mechanical behavior of granular soils under shear. The apparatus allows for triaxial testing of a miniature dry sample with a size of \mathrm{8}\hspace{0.17em}\mathrm{mm}\times \mathrm{16}\hspace{0.17em}\mathrm{mm} (diameter \times height). In situ triaxial testing of a 0.4–0.8 mm Leighton Buzzard sand (LBS) under a constant confining pressure of 500 kPa is presented. The evolutions of local porosities (i.e., the porosities of regions associated with individual particles), particle kinematics (i.e., particle translation and particle rotation) of the sample during the shear are quantitatively studied using image processing and analysis techniques. Meanwhile, a novel method is presented to quantify the volumetric strain distribution of the sample based on the results of local porosities and particle tracking. It is found that the sample, with nearly homogenous initial local porosities, starts to exhibit obvious inhomogeneity of local porosities and localization of particle kinematics and volumetric strain around the peak of deviatoric stress. In the post-peak shear stage, large local porosities and volumetric dilation mainly occur in a localized band. The developed triaxial apparatus, in its combined use of X-ray μCT imaging techniques, is a powerful tool to investigate the micro-scale mechanical behavior of granular soils.