SOME RESULTS OF A STUDY OF SOIL CUTTING RESISTANCE ACROSS THE FROST-DEPTH LAYER

M. G. Chasovskikh, I. G. Basov

(Presented by the Department of Mining Machines)

The calculation of loads on the working member when cutting rock is usually carried out by using, in the calculation relationships, indicators of the mechanical constants of the mass being destroyed, or empirical coefficients correlated with them, or specific cutting resistances found empirically.

The calculation of loads on an executive member that cuts soil across the frost-depth layer is no exception in this respect. In this case, each cutter of the group cutting member (bar, disk cutter, rotor), along its path, overcomes a cutting resistance that varies over a wide range (Fig. 1). This is due to the fact that, over the depth of freezing, the soil has different temperature and moisture content and, consequently, different mechanical characteristics (strength limits for all types of deformation, angle of internal friction, cohesion, etc.). This situation requires that the indices characterizing the mechanical strength of the frozen soil layer represent their integral values over the entire face of destruction, since discrete values of the soil strength indices obtained by rapid methods involving trial destruction on the surface of the mass may produce significant deviations of the calculated loads from the actual ones. The point is that, with a small snow cover, changes in atmospheric temperature even over a relatively short period of time substantially affect the temperature and strength of the surface (down to 0.5 m) soil layer (Fig. 1). This is especially characteristic of the spring period, when thaws begin.

A sharp change in air temperature strongly affects the tangential cutting resistance of the upper layer of frozen soil, whereas at depths greater than 0.5 m this influence is practically absent (Fig. 1). Consequently, trial destruction of frozen soil under field conditions must be carried out at a depth of not less than 0.5 m from the surface. Only in this case can the rapid indicator be correlated with sufficient reliability with the distribution of the mechanical characteristics of the soil over the frost-depth layer.

Fig. 1. Variation of temperature \(t\), cutting force \(P\), and specific cutting force \(K_p\) over the frozen layer of loam \(H\) when cutting with a single cut at \(\alpha = 75^\circ\), \(v = 10\) mm, \(h = 25\) mm: — 2/IV 1968 (number of blows of the DorNII impact device \(C = 58\), air temperature \(15^\circ\mathrm{C}\)); - - - 4/IV 1968 (\(C = 25\), \(t_{\text{air}} = -1.5^\circ\mathrm{C}\)).

Fig. 2. Dependence of the cutting force \(P_p\) for frozen loam at a depth of 1 m \((t = -3.3^\circ\mathrm{C})\) on chip thickness \(h\) and cutting angle \(\alpha\): 1—\(45^\circ\); 2—\(60^\circ\); 3—\(75^\circ\); 4—\(90^\circ\).

To a lesser extent, changes in air temperature are reflected in the specific cutting force \(K_{\mathrm{p}}=\dfrac{R_{\mathrm{p}}}{F}\), since as \(P_{\mathrm{p}}\) increases due to a decrease in soil temperature, the area \(F\) of the cross section of the furrow formed after the cutter passes increases almost proportionally (owing to the decrease in soil brittleness), with the same chip thickness over the entire freezing depth.

The cutting angle \(\alpha\) (Fig. 2) and the chip thickness \(h\) have a substantial influence on the cutting force. Increasing the thickness of the cut chip from 14 to 40 mm when cutting from an open surface leads to a proportional increase in the cutting force \(P_{\mathrm{p}}\). Increasing the cutting angle from 45 to 90° leads to an increase in the force \(P_{\mathrm{p}}\). Moreover, this dependence is nonlinear in character (Fig. 2) and is the same for any point as the cutter moves through the frozen soil layer.