METHODOLOGY FOR DETERMINING RATIONAL TECHNOLOGICAL SCHEMES FOR EXCAVATING FROZEN SOIL USING BAR-TYPE EARTH-CUTTING MACHINES

I. G. Basov

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

By the technology of work one usually understands a definite scope and sequence of various kinds of operations performed according to a specified cycle and ultimately yielding a finished product. This concept may also be used to characterize the technology of conducting earthworks in winter.

The technology of excavating frozen soil includes the scope and sequence of operations for the preliminary loosening of the frozen soil layer, as well as the scope and sequence of operations for its removal. The measure of the finished product in this case will be the number of cubic meters of frozen soil removed per unit time.

The technology of winter earthworks is usually developed for a known set of machines that is used for the given type of work. If it ensures the maximum possible productivity of the set of earthmoving machines, it may be called a rational technology for excavating frozen soil for the given set of machines engaged in performing a particular type of work (cutting trenches, excavating pits, etc.). However, a number of sets of machines may be used to perform one and the same type of work, and each of them will have its own rational technological scheme. The best of these schemes may be called the rational technological scheme for the given type of winter earthworks performed under known conditions.

Thus, in order to identify the most rational technology for a particular type of work (for example, excavation of a pit), it is first necessary to identify the rational technological schemes for excavating frozen soil for each set of machines that may expediently be used in this case.

The rationality of technological schemes for excavating soil in winter should be evaluated on the basis of ensuring:

1) the maximum productivity of the machines intended for loosening and removing frozen soil;

2) the minimum possible energy consumption for loosening and removing frozen soil;

3) the minimum possible consumption of tools for loosening frozen soil;

4) the minimum cost of loosening and excavating frozen ground.

It should be noted that the identification of a rational technology for a given type of work must be carried out for the case of operating different sets of machines under equal conditions, with the same category, moisture content, depth of cutting of the ground, etc.

The methods for studying rational technological schemes for developing frozen ground should be taken as experimental-theoretical. Thus, experimentally it is necessary to identify rational technological schemes for individual sets of machines. In particular, it is necessary to find rational schemes for cutting slots in frozen ground when performing various types of earthwork (trench driving, excavation of pits, etc.) under various conditions (category, moisture content, and depth of ground freezing).

The duration of preparing a given volume of frozen ground for excavation depends both on the technical parameters of the bar-type earth-cutting machine and on the scheme for cutting the slots. The smaller the cells adopted in the grid of slots being cut, the more slots must be cut to prepare \(1 \, m^3\) of frozen ground for excavation, and the greater will be the unproductive time losses associated with the maneuvering operations of the bar machine. The sizes of the cells must be increased within reasonable limits and in such a way that the excavating machine does not experience significant overloads. It follows from this that the selection of the parameters of the technological scheme must be made with allowance for ensuring the best performance indicators of the bar machine and the technical capabilities of the excavating machine.

The duration of slot cutting can be divided into two parts

\[ T = T_1 + T_2, \tag{1} \]

where \(T_1\) is the duration of cutting the longitudinal and transverse slots, including the time for technical servicing of the machine (replacement of the cutting tool, tightening the chain, etc.);

\(T_2\) is the time spent on maneuvering operations.

The latter quantity, as was noted above, depends on the adopted scheme for cutting the slots, and its influence on the productivity of the earth-cutting machine can be taken into account by the coefficient

\[ k_{\mathrm{m}} = 1 - \frac{T_2}{T}. \tag{2} \]

Thus, the productivity of the bar-type earth-cutting machine in preparing frozen ground for excavation depends on the adopted slot-cutting scheme and can be found from the formula

\[ Q = \frac{v_{\mathrm{p}} T H_{\mathrm{sh}} k_{\mathrm{m}}}{s}, \tag{3} \]

where \(v_{\mathrm{p}}\) is the slot-cutting speed, \(m/\text{h}\);

\(H_{\mathrm{sh}}\) is the slot depth, \(m\);

\(s\) is the area of slot that must be cut in order to prepare \(1 \, m^3\) of frozen ground for excavation, \(m^2/m^3\).

The quantity \(s\) can be found from the expression

\[ s = \frac{\left(L_{\mathrm{prod}} n_1 + L_{\mathrm{pop}} n_2\right) H_{\mathrm{sh}}}{V}, \tag{4} \]

where \(L_{\mathrm{prod}}, L_{\mathrm{pop}}\) are the lengths of the slots, respectively longitudinal and transverse;

\(n_1, n_2\) — the number of longitudinal and transverse slots, respectively, cut in parallel over the area;

\(V=L_{\text{long}}L_{\text{trans}}H_{\text{sl}}\) — the volume of frozen soil being prepared for excavation, \(m^3\).

Let us replace, in expression (4), \(n_1\) and \(n_2\) by \(L_{\text{long}}\) and \(L_{\text{trans}}\) and by the distances between the longitudinal \(l_1\) and transverse \(l_2\) slots. Then

\[ s= \frac{ L_{\text{long}}\left(\frac{L_{\text{trans}}}{l_1}+1\right) + L_{\text{trans}}\left(\frac{L_{\text{long}}}{l_2}+1\right) }{ L_{\text{long}}L_{\text{trans}} }. \]

After the corresponding transformations, the last expression takes the form

\[ s=\frac{1}{l_1}+\frac{1}{l_2}+\frac{1}{L_{\text{long}}}+\frac{1}{L_{\text{trans}}}. \tag{5} \]

The productivity of the excavating machine (excavator, bulldozer) also depends on the value \(s\), since the latter determines the lumpiness of the frozen soil prepared for excavation and, consequently, the loading of the working units of the excavating machine and the fill factor of the excavator bucket or bulldozer blade. In this connection, for the most common excavating machines, when developing soils of various categories and freezing depths, it is necessary to determine the optimal values of \(s\) experimentally, by determining the forces arising in the working units of the excavating machine, as well as the unproductive time expenditures for excavating frozen soil.

The energy consumption for loosening frozen soil by a bar machine depends both on its technical characteristics and on the parameters of the grid of slots being cut:

\[ q_{\text{r}}= \frac{0.736\cdot N_{\text{eng}}\cdot s}{H_{\text{sl}}\cdot v_{\text{p}}}, \ \text{kW}\cdot\text{h}/m^3 \tag{6} \]

The energy consumption for excavating frozen soil by an excavating machine is

\[ q_{\text{e}}= \frac{0.736\cdot N_{\text{eng}}\cdot T_{\text{sh}}}{V}, \ \text{kW}\cdot\text{h}/m^3, \tag{7} \]

where \(N_{\text{eng}}\) is the engine power, hp;

\(T_{\text{sh}}\) is the duration of operation of the machine during a shift, h;

\(V\) is the volume of soil excavated by the machine per shift.

The assessment of technological schemes for preparing frozen soil with the aid of bar machines must also be carried out in terms of the specific consumption of the cutting tool (teeth, chains, guide frames of the bars), i.e., the consumption of the tool per \(1\ m^3\) of frozen soil prepared for excavation:

\[ \omega=\omega' \cdot s,\ 1/m^3, \tag{8} \]

where \(\omega'\) is the number of teeth (or chains, or guide frames of the bars) consumed in cutting \(1\ m^2\) of slot in frozen soil.

Thus, the value \(s\) is a general indicator characterizing the parameters of the adopted slot-cutting scheme, on which depend the productivities of the earth-cutting and excavating machines, the energy consumption for loosening and excavating frozen soil, and the consumption of the cutting tool.

In addition, the economic efficiency of using an earth-cutting machine will also depend on the parameters of the adopted scheme for cutting slots in frozen soil; in determining this efficiency, operating costs are taken into account: the cost of the work, overhead costs dependent on the amount of expenditure on the basic wages of workers, and overhead costs whose magnitude is determined by the labor intensity of the work, as well as capital expenditures for construction machines and mechanisms involved in carrying out earthworks in winter.

Since, for each technological scheme, it is necessary to compare the indicators of several machines, it is expedient to determine efficiency on the basis of reduced costs. The reduction of capital expenditures to the level of operating costs is carried out by using the normative efficiency coefficient, whose value for construction is established at 0.17.

The sum of reduced costs for each technological scheme carried out by a particular set of machines can be found from the formula

\[ Z_{\mathrm{p}} = A\left[(C_{\mathrm{pr}} + 0.15 Z_{\mathrm{or}} + 0.4 T_{\mathrm{r}} + K_{\mathrm{udr}} E_{\mathrm{n}}) +\right. \]

\[ \left. + (C_{\mathrm{pv}} + 0.15 Z_{\mathrm{ov}} + 0.4 T_{\mathrm{v}} + K_{\mathrm{udv}} E_{\mathrm{n}})\right], \]

where \(A\) is the comparable volume of work;

\(C_{\mathrm{pr}}\) and \(C_{\mathrm{pv}}\) are the direct cost, taking into account indirect expenses but excluding overhead costs, per unit of work, rubles;

\(Z_{\mathrm{or}}\) and \(Z_{\mathrm{ov}}\) are the basic wages of workers per unit of work, respectively for loosening and excavating soil, rubles;

\(T_{\mathrm{r}}\) and \(T_{\mathrm{v}}\) are the labor intensity per unit of work, respectively for loosening and excavating soil, man-days;

\(K_{\mathrm{udr}}\) and \(K_{\mathrm{udv}}\) are the specific capital investments per unit of work, respectively for loosening and excavating soil, rubles;

\(E_{\mathrm{n}}\) is the normative coefficient of efficiency of capital investments.

The most effective methods of comprehensively mechanized excavation of frozen soil and the set of machines must be selected depending on the specific conditions and types of work and on the geological data.

The selection of the set of machines is carried out in the following sequence:

a) the leading excavation machines that can be used are selected;

b) their maximum loading is calculated on the basis of the volumes of work, the technical parameters of the buildings and structures being erected, the geological conditions of the construction site, and the performance of the work within the specified time at minimum cost;

c) the nomenclature and number of machines to be included in the set are established, ensuring the maximum productivity of the leading machine and the comprehensive mechanization of the entire cycle of work provided for by the technological chart.

To achieve the greatest productivity of the leading machine, the productivity of the auxiliary machines must be 5–10% higher.

The technological chart must provide the most rational scheme for preparing and excavating frozen soil; calendar schedules for production and for combining technological processes; instructions for carrying out the work; and the principal technical-and-economic indicators for the adopted...

interchangeable mechanisms; labor inputs per unit volume and for the entire volume of work.

The selection and application of technological charts for soil excavation under winter conditions for a specifically assigned construction project should be carried out in two stages:

1. From the set of available technological charts for the production of analogous types of work, the most effective ones are selected on the basis of the best technical-and-economic indicators.

2. The final selection of a technological chart is made from among those previously selected, with preference given to the one that best meets the specified deadlines for carrying out the work. If several charts meet the specified deadlines and the available production capabilities and mechanisms, then the criterion for the final selection of the technological chart is the lowest cost of its implementation.

After selection, the chart is tied to the construction site.