SOME RESULTS OF A STUDY OF THE FAILURE OF AN INTER-SLOT PILLAR BY AN EMBEDDED HYDRAULIC JACK

I. G. BASOV, A. A. SDOBNIKOV

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

In [1] we described the principle of loosening frozen ground by cutting slots with a slot-cutting machine, with simultaneous destruction of the inter-slot pillar by tearing off and breaking away its elements by means of an embedded hydraulic jack. The present article gives some results of experiments carried out under laboratory and production conditions in order to establish the necessary forces and the expedient ways of applying them for the destruction of the inter-slot pillar into large blocks (pieces).

Fig. 1. Diagram of destruction of an inter-slot pillar by an embedded hydraulic jack

Experiments on separating large blocks of frozen ground from an inter-slot pillar under laboratory conditions were carried out on a stand specially designed for these purposes. The inter-slot pillar was simulated by a T-shaped model (Fig. 1) made of soil. The models were manufac-

Fig. 2. Change in the force \(P\), necessary for separating a block from the inter-slot pillar, as a function of the place of its application at \(H_{\mathrm{sh}}=200\ \mathrm{mm}\); \(m=100\ \mathrm{mm}\); \(L_p=50\ \mathrm{mm}\) \((a, v, d)\) and \(L_p=100\ \mathrm{mm}\) \((b, g, e)\); \(h=50\ \mathrm{mm}\) \((zh)\), \(h=100\ \mathrm{mm}\) \((i)\), and temperature of the frozen mixture (moisture content \(W=19\text{--}22\%\)) in °K: \(1\)—268°; \(2\)—263°; \(3\)—258°; \(4\)—253°.

were made at \(1/10\) of full scale by forming them from soils of natural structure. During forming, the soil was compacted to a density approximately equal to its density under natural conditions, which was monitored with a DorNII penetrometer. The specimens were held at a specified temperature in a thermal chamber for not less than 72 hours. The gravimetric moisture content of the specimens was kept constant. During the experiment, the thickened part of the model (Fig. 1) was clamped between the jaws of the stand, and the destructive force \(P\) was produced by a manual hydraulic press. Failure of models of inter-slot pillars of a prescribed thickness \(m\) (Fig. 1) was carried out while simulating a specified slot depth \(H_{\mathrm{sh}}\), the depth \(h\) of embedment of the hydraulic jack, and the lever arm \(L_p\) of application of the force \(P\) in the horizontal plane.

The experiments showed that, with changes in \(h\) and \(L_p\), failure of the frozen soil of the inter-slot pillar occurs through the development of cracks forming, in the principal planes, angles varying within the limits \(\alpha=90\text{--}120^\circ\), \(\beta=100\text{--}110^\circ\), and \(\gamma=100\text{--}140^\circ\). Such values of the angles indicate that failure of the inter-slot strip by an embedded hydraulic jack occurs in the presence of both shearing de-

formations as well as tensile deformations. With constant \(H_{\text{sh}}, h, L_p, m\) and other equal conditions, the volumes of the separated blocks are approximately the same.

With an increase in \(h\) on the model from 30 mm to 150 mm, the force required to separate a block of soil from the inter-slot pillar increases monotonically almost threefold (Fig. 2, \(a\)–\(e\)). In this case the angle \(\beta\) usually decreases to its lower limit. The increase in the force \(P\) and the decrease in the angle \(\beta\) with a shortening of the arm \((H_{\text{sh}} - h)\) can be explained by the fact that in this case failure occurs mainly as a result of shear deformations.

Fig. 3. Dependence of the force of failure of the inter-slot pillar of frozen sandy loam (\(W = 10\%\), \(t = 267^\circ\mathrm{K}\), \(C = 340\)) on the point of its application at \(H_{\text{sh}} = 1600\) mm; arm length \(L_p\): 1—500 mm; 2—1000 mm; depth \(h\) of installation of the hydraulic jack: 3—300 mm, 4—500 mm, 5—1000 mm; and thickness \(m\) of the inter-slot pillar: \(a\)—400 mm, \(b\)—1200 mm, \(c\)—750 mm.

An increase in \(L_p\) from 30 mm to 150 mm leads to a decrease in the force \(P\) by approximately 30% (Fig. 2, \(zh\), 3). In this case the angles \(\alpha\) and \(\gamma\) approach their upper limits. This indicates that increasing the arm leads to the predominance of tensile deformations occurring during the separation of blocks. With a decrease in the temperature of the specimens from \(268^\circ\mathrm{K}\) to \(253^\circ\mathrm{K}\), the force \(P\) increases approximately threefold (Fig. 2).

The experiments showed that models of inter-slot pillars made of sandy loam resist failure significantly better than models made of loamy soils and clay. This once again confirms that frozen sandy loams in natural occurrence resist any types of deformation significantly better (in comparison with other soils).

To verify the results of the laboratory studies, experiments were carried out according to an analogous scheme under production conditions, in the failure of inter-slot pillars of sandy loam with a freezing depth of up to 2 m. The character of the change in the force \(P\) as a function of the point of its application, obtained under production conditions (Fig. 3), as well as of the angles \(\alpha, \beta, \gamma\), differs, though only slightly, from the analogous dependences revealed under laboratory conditions (Fig. 2). This could have been expected, since the models of the inter-slot pillars were not...

it was possible to carry out with variable strength over the depth of the slot, which occurs under field conditions. If it is taken into account that the forces \(P\) obtained in the failure of the models must be increased by a factor of 100 for conversion to field conditions, then the data of the laboratory studies may be regarded as commensurable with production data.

On the basis of the investigations carried out, the following conclusions may be drawn:

1. The possibility has been established of regulating the size (volume) of the blocks separated from the inter-slot pillar.

2. With a decrease in the temperature of frozen soils, their resistance to failure by an embedded hydraulic jack increases. The greatest resistance to failure, other conditions being equal, is exhibited by loamy-sand soils.

3. A decrease in the arms \((H_{\text{sh}} - h)\) and \(L_{\text{p}}\) of application of the force \(P\) leads to an increase in the latter. In this case the angles \(\alpha\), \(\beta\), \(\gamma\) decrease, indicating the predominance of shear failure deformations.

4. To reduce the force \(P\), the depth \(h\) of embedment of the hydraulic jack should not exceed 500 mm. In order to regulate the degree of loosening of the inter-slot pillar, the actuation step of an automatically operating embedded hydraulic jack may be taken within the range of 300–1000 mm.

5. For the failure of inter-slot pillars of soils with a freezing depth of up to 2 m and more, with variation of \(m\) and \(L_{\text{p}}\) within the ranges, respectively, of 0.5–1.2 m and 0.3–1.5 m, the embedded hydraulic jack must develop a force of not less than 350–400 kN.

REFERENCES

1. O. D. Alimov, I. G. Basov, A. A. Slobnikov, F. F. Zelinger. Cutting frozen ground with simultaneous splitting. Mekhanizatsiya stroitel’stva, No. 1, 1966.