The Research on Shear Properties of Deformable Ground Soil under High-speed Driving Conditions

The classical ground soil shear mechanics model is difficult to predict the maneuverability of vehicles under high-speed driving conditions. The shear mechanical properties of soil under dynamic loading are the key factor to research on the ground attachment characteristics of vehicles under high-speed driving conditions. The relationship between the shear properties of the ground soil and the loading rate was analyzed by numerical simulation method. Based on the Janosi shear model of the ground soil, the loading rate and shear rate were supplemented to establish the applicable driving conditions for high-speed and heavy-duty vehicles. The ground soil shearing characteristics model provides a theoretical basis for researching on the vehicle's driving maneuverability under high-speed driving conditions. 1 Janosi shear model for classic ground soil There are two kinds of shear stress-deformation characteristic curves for common ground soil at low shear rate. As shown in Fig. 1, the abscissa is the shear displacement and the ordinate is the soil shear stress. One is to describe the brittle ground soil, such as compacted sand, frozen snow, and other undisturbed hard ground. The shear stress-strain curve of this kind of soil will have a maximum value, and the shear stress of the soil will be smooth after reaching the yield limit. The shear models describing the brittle soil are: Bekker shear model, JYWong shear model and so on. The other is plastic soil, such as loose sand, dry soil,and other disturbed ground soil, the shear stress-strain curve of this kind of soil has no peak of protrusion, and it always changes smoothly. The shear models describing plastic soils are: Janosi shear model, hyperbolic shear model, pure exponential shear model, etc.[1-5]. Figure 1. Two common soil shear Figure 2. Simulation model of sheared stress-strain curves soil test for track shoes a Corresponding author : lvweiwei@bit.edu.cn © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). MATEC Web of Conferences 253, 01006 (2019) https://doi.org/10.1051/matecconf/201925301006 MSME 2018


Janosi shear model for classic ground soil
There are two kinds of shear stress-deformation characteristic curves for common ground soil at low shear rate. As shown in Fig. 1, the abscissa is the shear displacement and the ordinate is the soil shear stress. One is to describe the brittle ground soil, such as compacted sand, frozen snow, and other undisturbed hard ground. The shear stress-strain curve of this kind of soil will have a maximum value, and the shear stress of the soil will be smooth after reaching the yield limit. The shear models describing the brittle soil are: Bekker shear model, JYWong shear model and so on. The other is plastic soil, such as loose sand, dry soil,and other disturbed ground soil, the shear stress-strain curve of this kind of soil has no peak of protrusion, and it always changes smoothly. The shear models describing plastic soils are: Janosi shear model, hyperbolic shear model, pure exponential shear model, etc. [1][2][3][4][5]. For the plastic shear properties of deformable ground soil, there is no hump in the shear stress-strain curve, so the Janosi shear model is adopted [3]to describe the shear properties of ground soil at low shear rates, as follows: Where, j is shear displacement;k m is shear modulus; 0 max  is soil shear strength at low shear rates;  is shear stress; c is soil cohesion;  is the internal friction angle of the soil; p is soil normal pressure. (2)Selection of model materials and unit types. The ABAQUS finite element software was used to establish the crawler plate and deformable ground soil simulation model. The track shoes, pressure plate and deformable ground unit model were selected as C3D4 solid elements. The material of track shoes and pressure plate was defined as rigid body, and the soil material was selected as elastoplastic Drucker-Prager constitutive material. The parameters defined for the track shoes, the press plates and the Kangzhuang soil material model with a moisture content of 5% are shown in Tables 1 and 2 [6][7].     Fig. 3 and Fig. 4, and all the degrees of freedom of the pressure plate except the Z direction are constrained.  shear strength and shear rate with water content of 0, 5% and 10% respectively. The three curves show the variation of soil shear strength and shear rate of three different water contents. The soil with higher water content increases with the increase of shear rate, and it is easier to reach a stable state. The green curve in Figure 6 indicates the water content of 10 % soil, as the shear rate increases, its shear strength changes quickly to level and increases no longer [8][9][10][11].

Shear rate amplification factor
The variation law of shear rate of ground soil under high-speed conditions was studied. A parameter reflecting the influence of shear rate on shear strength was proposed, that is shear magnification factor  . is the ratio of shear strength at different shear rates to soil shear strength at low shear rates, with magnification factor is for the ordinate, the shear rate is for the abscissa.
Therefore, the relationship between the amplification factor and the shear rate can be expressed as: where,v 2 is the rate of movement of the track shoe relative to the ground, ie the shear rate; is magnification factor, dimensionless; 2 a and 2 b are the intercept and slope of the extension line and the ordinate axis of the fitted straight line,respectively; 0 max  is soil shear strength at low shear rates; max  is soil shear strength at high shear rates.
(1) The variation of shear strength and shear rate under different normal pressures. It can be seen from Fig. 7 that the soil shear strength amplification factor and the shear rate change trend are similarunder different normal pressures. The larger the normal pressure, the soil is compacted, the internal pores are further reduced, and the soil shear amplification factor changes. The smaller the difference, the lower the normal pressure amplification factor and the shear rate change are rather the higher the normal pressure.
The shear rate is always greater than 0, the ratio of the shear rate to the magnification factor is the ordinate, the shear rate is the abscissa, and the magnification coefficient and the shear rate curve are transformed into coordinates to obtain three nearly coincident lines (as shown in Figure 7). When the shear rate is 0.5m/s, the soil shear strength amplification coefficient is 1under different normal pressures, and 3 straight lines intersect with one point. Different normal pressures influence the intercept on the straight line and the ordinate axis a 2 , and the slope of the lineb 2 . The greater the normal pressure, the slope of the lineb 2 is larger, the intercept on the line and the ordinate axisa 2 is smaller.  The research found that the soil strength amplification factor is directly related to soil water content and normal pressure. The parameters obtained by analyzing the three water contents, three normal pressure shear rates and the shear rate curve are shown in Fig. 9 and Fig. 10 The relationship between rate and normal pressure and parameter sum.
Soil strength factorIt is directly related to soil water content and normal pressure. The parameters obtained by analyzing three kinds of water content, three kinds of normal pressure shear rate and shear rate curve are analyzed.a 2 withb 2 Figure 9 and Figure 10 show soil moisture content and normal pressure and parameters.

shear correction model
The shear deformation of deformable ground soil is affected by the shear rate of the track shoes and the vehicle's load on the ground. Under the same load, the shear rate can increase the strength of the soil. Based on the Coulomb model [12][13][14][15], shear is introduced. Intensity amplification factor, establish a soil strength formula related to driving speed.  Using the parameters in Table 3, using the modified shear model for calculation, comparing the calculation results before the correction with the numerical simulation results, which shows that the corrected model has a better prediction effect, and the error with the numerical calculation result is up to 25%.