On the Methodology for Studying the Resistance of a Frozen Soil Layer to Destruction

I. G. Basov, M. P. Chasovskikh, A. N. Shchipunov

(Presented by the Department of Mining Machines, Mine Transport, and Mining Mechanics)

The absence of quantitative indicators on the change in the mechanical characteristics of soil over the depth of its freezing does not make it possible to develop a methodology for sufficiently accurate calculation of the load on the working members of machines that cut through a frozen soil layer over its height (disk, milling, and bar machines; trenching and rotary excavators). Hence there arises the need for a comprehensive study of the resistance of a frozen soil layer to destruction. Along with determining the mechanical characteristics of soil taken from different height sections of the frozen layer, for the given conditions it is also necessary to determine the resistance of the soil to static and dynamic indentation by a punch, the coefficient of the soil’s resistance to cutting, and the coefficient of friction of cutters against the soil.

To determine the resistance of a frozen soil layer to cutting, a special installation was created (Fig. 1), the operating principle of which is as follows. A guide frame 1 is lowered into a slot-trench cut by an earth-cutting machine. The cutter-holder carriage 2 is lowered along the guide frame to the lowest position. When the electric motor 3 is switched on (a manual drive may also be installed) and the cam clutch 4 is engaged, the drum 5 is actuated. A cord is wound onto it, which at one end moves carriage 2 with cutter 6. At the initial moment the latter turns relative to the fastening axis and penetrates the soil, and then cuts a chip of a certain thickness. At the same time, the reactive force \(P_{\mathrm{p}}\), arising on crown 8 of the reduction dynamometer, is recorded on a paper tape as a function of travel.

The moment arising on the drum shaft can be calculated by the formula

\[ M_2 = \frac{P_{\mathrm{p}} R i \eta}{\eta_{\mathrm{p}}(1 - i\eta)} = P_{\mathrm{p}} R k, \]

where
\(R\) — the radius at which the spring acts, preventing rotation of the crown of the planetary reducer;
\(i\) — the transmission ratio of the reducer;
\(\eta\) — the efficiency of the reducer;
\(\eta_{\mathrm{p}}\) — the efficiency of the suspension of the reducer crown.

The cutting force is determined from the expression

\[ P_{\text{cut}}=\frac{2M_2}{D_b}\eta_b, \]

where
\(D_b\) is the drum diameter;
\(\eta_b\) is the efficiency of the pulley block and the carriage bearings.

The speed of motion of the cutter-holder carriage (the cutting speed) can be varied by installing drums of 5 different diameters. The range of possible variation of the cutting speed (with a motor power of 1.0 kW) lies within \(0.45\text{–}1.1\ \text{m/s}\).

Fig. 1. Schematic diagram of an installation for determining the resistance of soil to cutting by a single cutter along the freezing layer

The cutter-holder carriage is designed in such a way that it can also serve as a strain-gauge trolley, with the aid of which an oscillographic record can be made of the tangential and normal components of the cutting force. The use, for measuring the cutting force, of a less accurate but simpler reduction dynamometer makes it possible to collect a large amount of statistical material on the cutting resistance of soils with various physicomechanical properties. Strain gauging, in turn, establishes a more accurate qualitative and quantitative assessment of the process of soil cutting along the freezing layer.

In addition, the installation described above can also be used to evaluate the coefficient of friction of steel on frozen soil. For this purpose, instead of carriage 2 (Fig. 1), special sleds are fastened to the end of the cable. The guide frame is lifted out of the slot. The sleds are lowered to the full depth into the slot, and when the previously compressed spri-

Fig. 2. Schematic diagram of the apparatus for determining soil resistance to static indentation of a punch

…are pressed against its walls with a known force. When the runners are pulled from bottom to top, the expended force is recorded; knowing this force, one can determine the change in the coefficient of friction along the depth of the slot.

To determine the resistance of frozen soil to static indentation of punches, a special apparatus was created. Its operating principle is as follows. An embedded hydraulic jack 1 (Fig. 2), on the rod of which the indented punch 2 is fastened, is installed at the required height in a slot cut by an earth-cutting machine. The punch is driven in by operating the manual hydraulic press A. The pressure in the rod-side cavity of the embedded hydraulic jack 1 is monitored by pressure gauge 3 and recorded by recorder 4 on the chart 6, drawn by hydraulic jack 5, with a tape fastened to it. The diagram…

gives the dependence of the force on the path of penetration of the punch. The depth of penetration is 8 cm. The punch is driven in by means of hydraulic press Б through hydraulic jack 5. The cross-sectional area of the punches varied within the range 0.5–2.0 cm².

With minor modification, the apparatus was used to determine the ultimate strength of frozen soil under uniaxial compression in field conditions.