ON THE EXCAVATION OF FROZEN GROUND IN NORTHERN TOMSK OBLAST

I. G. Basov, B. I. Yuzhakov

(Presented by the Department of Mining Machines)

The development of the natural resources of northern Tomsk Oblast will be connected with the construction of oil and gas pipelines, the development of oil and gas fields, the construction of housing and other structures. Moreover, the planned pace of construction requires year-round, nonseasonal performance of work, in connection with which, under our conditions, the problem arises of excavating frozen soils in the winter period.

For digging trenches for main pipelines, our industry at present produces sufficiently powerful trench excavators capable of excavating permafrost as well. But for carrying out relatively small winter earthworks (excavating foundation pits for residential buildings and other structures, digging trenches for utilities, constructing supports for power transmission lines, etc.), these machines cannot be used. For such purposes other, more mobile units are needed, which, unfortunately, our industry still does not mass-produce. In this connection, in order to excavate soil in winter it is necessary either to protect it from freezing, or to thaw frozen soil, or to loosen it by drilling-and-blasting or by a mechanical method.

Protecting soil from freezing is desirable, but it is not always possible to implement because of the absence of insulating materials, the tying of the construction site to the locality, etc.

Thawing frozen soil is a lengthy (20–25 hours), energy-intensive (25–65 kWh/m³), expensive (1.2–2.2 rubles/m³) and, in this connection, undesirable process.

Less energy-intensive and, consequently, more economical are methods of preparing frozen soil for excavation by loosening (Table 1).

The drilling-and-blasting method of loosening frozen soil, despite its relative simplicity, cannot be recommended for wide application because of the comparatively high labor input and cost of the work, caused by the high cost of explosives, the involvement of highly qualified workers (blasters), and the need to have special drilling mechanisms available. In addition, a limiting factor in the application of this method is the danger of carrying out blasting operations near existing structures.

Among the mechanical methods, the method of preparing frozen soils for excavation by means of mounted rippers based on powerful DET-250 tractors has the best technical-and-economic indicators. The advantages

Table 1

Techno-economic indicators of methods for excavating frozen ground

The advantages of this method are high productivity, low energy intensity and cost of soil loosening, and the simplicity and reliability of the mounted equipment. These merits place this method among the advanced ones. However, the need to use tractors with a power of 300 hp and more, which are not yet produced in sufficient quantity by domestic industry, hinders the development and application of this method of loosening frozen ground.

In addition, the effectiveness of using mounted tractor rippers for destroying frozen ground at great freezing depths (2 m and more) and on small excavation areas (small pits, trenches) is substantially reduced because of the increase in the reworking coefficient, and in a number of cases (in trenches) becomes impractical.

For excavating frozen ground on small areas, it is more expedient to use impact, vibration-impact, and slot-cutting bar and disk-milling machines.

According to data available in the literature, the cost of excavating frozen ground with these machines is approximately the same (see Table 1); therefore the expediency of using one or another type of machine must be sought in the simplicity of design and mobility of the machines, and in their most effective use under the given soil conditions.

The advantage of impact and vibration-impact machines over bar-type earth-cutting machines is the possibility of using them in excavating frozen soils containing inclusions of rock and construction debris. However, under the conditions of the sparsely populated Tomsk north, with soils that do not contain inclusions of rock, bar-type earth-cutting machines have no obstacles to their use.

In addition, an advantage of bar-type earth-cutting machines is their higher reliability in comparison with existing impact rippers, whose assemblies and parts are subject to rapid wear as a result of the action of large dynamic loads on them.

The sufficiently high reliability of bar-type earth-cutting machines is explained by the very simple design of the mounted working equipment, which can be mounted fairly quickly on any T-100 tractor. In the summer period the mounted equipment is easily dismantled, and the tractor is used for its direct purpose. In addition, mounted bar equipment can easily be installed, for example, on a pipe-layer; that is, in this way a multipurpose machine can be obtained.

The productivity of bar-type earth-cutting machines is determined not only by their technical perfection, but to a greater degree by the technology of preparing the frozen soil layer for subsequent excavation. Excavation of frozen ground by slot-cutting machines is carried out by cutting a grid of mutually perpendicular slots. Thus, about 25%, depending on the density of the slot grid, of the soil volume is destroyed by cutting, while the remaining part is left in the form of prismatic blocks, which are subsequently destroyed by other means. For the conditions of the central and southern districts of Tomsk Oblast, with soil freezing to 1.3–1.5 m, the distance between slots is taken as no more than 80 cm, and excavation of the blocks is performed by excavators with a bucket capacity of 0.5–0.65 m³. When the frozen soil layer has considerable strength, excavator operation is possible only after the blocks have been separated from the mass, or else after increasing the density of the grid of cut slots. But the denser the slot grid, the lower the productivity and the higher the cost of preparing frozen ground for excavation.

To increase productivity and reduce the cost of preparing froz-

...of the soil for excavation can be accomplished by applying the combined method, proposed by the authors, for breaking up the frozen soil layer. Its essence is that, simultaneously with the cutting of a slot by a trenching machine, the web between slots is automatically sheared toward the previously cut slot by means of an additional hydraulic device. The new method of soil preparation makes it possible to reduce by half the number of slots to be cut and to reduce by almost a factor of 2 the cost of preparing the frozen soil layer.

In addition, with this method of preparation, when the volume of the detached blocks of the web between slots reaches \(1.3\ \text{m}^3\) and does not exceed \(3\ \text{t}\), they can be removed by a truck crane equipped with a special grab. In this case the excavator will be excavating only thawed soil. Use of this set of machines will make it possible to shorten the time required for soil excavation under winter conditions both by increasing the productivity of preparing the frozen soil layer for removal and by increasing the productivity of the excavator, since the productivity of the latter, even when working on loosened frozen soil, decreases by almost \(40\%\) in comparison with work on thawed soils.

On the basis of the foregoing, the following conclusions may be drawn:

Under the conditions of northern Tomsk Oblast, tractor rippers should be recommended for excavating frozen soil over large areas.

For excavating frozen soils over small areas, the use of trenching earth-cutting machines is the most expedient. The mounted equipment for creating these machines can be manufactured at any industrial enterprise in the city of Tomsk.