A comparison of ground vehicle mobility analysis based on soil moisture time series datasets from WindSat,LIS, and in situ sensors

Soil moisture is a key terrain variable in ground vehicle off-road mobility. Historically, models of the land water balance have been used to estimate soil moisture. Recently, satellites have provided another source of soil moisture estimates that can be used to estimate soil-limited vehicle mobility. In this study, we compared the off-road vehicle mobility estimates based on three soil moisture sources: WindSat (a satellite source), LIS (a computer model source), and in situ ground sensors (to represent ground truth). Mobility of six vehicles, each with different ranges of sensitivity to soil moisture, was examined in three test sites. The results demonstrated that the effect of the soil moisture error on mobility predictions is complex and may produce very significant errors in off-road mobility analysis for certain combinations of vehicles, seasons, and climates. This is because soil moisture biases vary in both direction and magnitude with season and location. Furthermore, vehicles are sensitive to different ranges of soil moistures. Modeled vehicle speeds in the dry time periods were limited by the interaction between soil traction and the vehicles’ powertrain characteristics. In the wet season, differences in soil strength resulted in more significant differences in mobility predictions.     

攀枝花地区冰期膨胀土的工程特性研究

攀枝花地区冰期膨胀土生成于二冰期晚期中更新统冰期,通过室内和现场物理力学试验和分析,揭示攀枝花地区冰期膨胀土的物理力学性质具有很大的离散性,其性质受土体的矿物成分、含水率、裂隙面、埋藏深度、级配、试验条件等因素、条件的影响。试验研究表明:膨胀土中的裂隙面按光滑程度不相同,可分为两种类型,即光滑裂隙面和光滑擦痕裂隙面; 粗粒矿物对胀缩性影响不大,而细粒矿物对胀缩性影响较大; 裸露或近地表,土层风化剧烈,裂隙较发育,膨胀力度就稍大; 同一地区的膨胀土,蒙脱石含量愈高,自由膨胀率愈大,其膨胀等级就愈高; 不同场地内的膨胀土其物理力学性质指标存在一些差异,但差异性较小; 攀枝花冰期膨胀土层的黏粒含量并不大; 不同场地内的膨胀土抗剪强度值变化较大,同一场地内的膨胀土抗剪强度值有起伏; 攀枝花冰期膨胀土多数呈硬塑-坚硬状,自然状态下其抗压变形能力较好,在浸水饱和状态下,膨胀土的承载力几乎成倍数下降。     

Determination of the soil pressure distribution around a cone penetrometer

The objective of this paper was to investigate the pressure distribution around a cone penetrometer using a pressure sensing mat under laboratory conditions. The investigation was conducted under (1) constrained conditions using cylindrical split pipe molds and (2) unconstrained conditions using a soil box. These tests were conducted in Capay clay and Yolo loam soil containing two different moisture conditions and two compaction levels.In the constrained tests, a maximum radial pressure of 111 kPa was observed in the Capay clay soil with 3.4–4.3% d.b. moisture content and three blows of compaction (cone index value of 2040 kPa) when using the 41 mm diameter split pipe mold. These pressure levels decreased to 82 and 22 kPa, respectively, when 65 and 88 mm diameter molds were used. In both the Capay clay and Yolo loam tests, the average radial pressure and average cone index values showed similar trends.In the unconstrained tests, a maximum pressure of 9.0 kPa was observed in the Capay clay with 4.5% d.b. moisture content and three blows of compaction (cone index value of 550 kPa) at a horizontal distance of 25.4 mm from the vertical axis of the cone penetrometer and minimum pressure levels in the range of 0.2–0.3 kPa when the horizontal distance of the penetrometer was in the range of 56.8–66 mm. The pressure levels are much smaller than the ones obtained in the constrained tests and may suggest that the pressure distribution under field conditions is small at a distance of 25.4 mm or higher from the tip of the cone.The experimental data were statistically analyzed to identify significant factors. The results of the analysis for the constrained test indicated that the mold diameter and number of blows significantly increased the pressure readings within the soil mass. Increasing the mold diameter led to a decrease in the average radial pressure and increasing the number of blows contributed to an increase in the average radial pressure. In the unconstrained test, the average radial pressure distribution at a given point were significantly influenced by the horizontal distance of the point from the vertical axis passing through the center of the penetrometer shaft, soil type, and soil moisture content. Higher pressure values were obtained in the Capay clay tests compared to the Yolo loam tests. In all cases, the pressure levels were greater for the drier soil than for the moist soil.     

Development and laboratory evaluation of a rheometer for soil visco-plastic parameters

A motorized rheometer was developed for determining soil visco-plastic parameters that works on the principle of torsional shear applied to a standard vane with controlled strain rate. Rheological measurements were carried at different soil moisture contents (10%, 13%, 17% and 20% dry basis (gravimetric)) and soil compaction levels (100, 150, 200, 300 and 400 kPa) to find their effects on soil viscosity and yield strength. The values of viscosity of the clay loam soil (29% clay, 24% silt and 47% sand) were found to spread in the range of 53–283 kPa s, and yield stress variation had a span of 4–28 kPa. Increase in soil compaction was accompanied by a sharp increase in soil viscosity, while moisture content affected soil viscosity negatively. Effect of both these parameters was statistically significant (95% confidence interval). Yield stress was positively related to soil compaction and the effect was statistically significant. However, it was negatively related to moisture content and the effect was not statistically significant for the levels of moisture content tested.     

Mechanical properties in relation to vehicle mobility of Sepang peat terrain in Malaysia

An analytical framework for determining the mechanical properties of peat and predicting the tractive performance of tracked vehicle is presented. It takes into account the load-sinkage and shearing characteristics of peat as well as all major design parameters of tracked vehicle. An experimental study on the mechanical properties of peat soil was conducted at Sepang area, Selangor, Malaysia. The stiffness values of surface mat and underlying weak peat deposit from load-sinkage test were determined by specially made bearing capacity apparatus. The mean values of surface mat stiffness before and after drainage were found to be 31 and 45.62 kN/m³, respectively and the mean values of underlying peat stiffness before and after drainage were found to be 252 and 380.20 kN/m³, respectively. The mean value of the internal frictional angle, cohesiveness and shear deformation modulus of the peat soil sample were determined using a direct shear box apparatus in the laboratory. The mean values of internal friction angle, cohesiveness and shear deformation modulus before and after drainage were found to be 22.80° and 24.31°, 2.63 and 2.89 kN/m², and 1.21 and 1.37 cm, respectively.     

Simulating shear behavior of a sandy soil under different soil conditions

Understanding of soil shear behavior is very important in the field of agricultural machinery and soil dynamics. In this study, a discrete element model was developed using a simulation tool, Particle Flow Code in Three Dimensions (PFC^3D). The model simulates direct shear tests of soil and predicts soil shear behavior, in terms of shear forces and displacements. To determine and calibrate model parameters (stiffness of particles, strength and stiffness of bond between particles), laboratory direct shear tests were conducted to examine effects of soil moisture content and bulk density on shear behaviors of a sandy soil. Three soil moisture levels (0.02%, 13.0%, and 21.5%) and four bulk density levels (0.99, 1.28, 1.36, and 1.50 Mg/m³) were used in the tests. The test results showed that in general drier and denser soil conditions produced higher shear forces. Based on the test results, the bond strengths of the model particles were determined from soil cohesion and internal friction angle. The model particle stiffness was calibrated based on the yield forces from the tests. The calibrated particle stiffness varied from 1.0 × 10³ to 8.2 × 10³ N/m, depending on soil moisture and density levels. The bond stiffness calibrated was 1.0 × 10⁷ Pa/m for all soil conditions.