STANDARDIZING THE OPERATION O F FLOATING DREDGES B. M. Shkundin

UDC 624.132.345:621.879.45

A dredge is an earth excavating and moving machine. Its working process consists of two basic operations: the excavation of soil in an underwater cut with the simultaneous preparation of a water--soil mixture and the transport of this mixture to the point where the soil is being placed. Dredge productivity is measured as the product of the quantity of soil per unit volume of water--soil mixture and the volume of this mixture placed per unit time [i, 2]. To design a rational system for the standardization of dredging operations, it is necessary to consider factors on which the values of these cofactors depend. In problems involving standardization, it is apparently inexpedient to consider certain factors that characterize the technical level of operation and affect dredge productivity. One should proceed on the following assumptions: the dredging crew is well qualified, the dredge is in good condition and is provided with a standard line of interchangeable replacement parts, a steady supply of electric power is available, and the auxiliary operations are performed at an adequately high level of mechanization. All factors affecting dredge productivity can be divided into two groups: basic and supplementary. Basic Factors. As with any earth excavating and moving machine, the basic factors determining dredge productivity should be distinguished, to wit: the character of the soils, the hourly dredge output under certain specific soil conditions at an optimum feed distance and lift, the distance and llft of the soil feed, and the efficiency of the dredge with time. Supplementary Factors. The following are supplementary factors affecting dredge productivity: the characteristics of the slurry-transport line (the location of pumping stations, the characteristics of the pipeline course as to intersections with water obstructions, highways and railroads, etc.); the soil placement conditions in the fill sections (fills for lowprofile or broad-profile structures, special requirements for placing the soil according to fineness, placing the soil in banks, etc.); the characteristics of the dredge basin (currents, disturbances, horizontal fluctuations, etc.); the height o~ the above-water and underwater sections of the cut; the climatic conditions (temperature, wind, precipitation, fog, etc.); and, obstruction of the excavation pits. Let us examine the basic factors in greater detail. It has been established by many years experience with the operation of dredges, the bottom-deepening fleet, and equipment used for construction hydromechanization that the difficulty associated with their excavation depends on combinations of the following physicomechanical properties: a) soil gradation; b) strength (the density and consistency of a soil under natural conditions); and c) adhesion. Soils can be divided according to gradation into classes, each of which corresponds to specific quantitative ratios of the content of particles of different fineness. This classification of soils according to gradation, which would enable us to refer any soil to be excavated directly to some granulometric class, should be developed on this principle. The granulometric soil characteristics listed in Table 3 of [2] satisfy these requirements in full. Soil strength can be characterized by indicators derived from field studies -- by the resistance to embedment of a probe -- and from laboratory studies for noncohesive soils with a density factor K d under natural conditions and for cohesive soils with a consistency factor K c under the same conditions. K d is determined from the relationship

where 6n, ~Z, and 6d are the bulk masses of the soil skeletons for the natural, ultimately loose, andultimately dense states; K c is determined from the relationship

Kr (Wn-A)/~'. Translated from Gidrotekhnicheskoe Stroitel'stvo, No. I0, pp. 39-41, October, 1977.

0018-8220/77/0010-1031507.50

9 1978 Plenum Publishing Corporation

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B. M. SHKUNDIN

~0

J

,/5

JJ",2!

N 2s E 2~ ,0

I lO00

I q'~~ 2000

,

r45

I 4t700

qO00

5000

6000

Delivery of soil pump, m 3 / h

Fig. i. Working position of 20R-II soil pump with slurry delivery at different elevations, i) Performance curve of slurry line; 2) performance curve of soil.

where W n is the natural moisture content of the soil in %, A is the moisture content of the soil at the lower limit of plasticity (the plastic limit of the soil) in %, $ is the plasticity index of the soll ( $ = F - - A ) , and F is the moisture content of the soil at the upper limit of plasticity (the liquid limit) in %. The adhesion force of the soil (g/cm2), which is determined by experimental means on an apparatus employed in Okhotin's system, is taken as the adherence indicator of cohesive soils. The classification of soils according to the difficulty of their excavation by dredges should be established so that the soil being excavated could be referred directly to any group according to difficulty of excavation as a function of a combination of its granulometric class, strength, and adherence. The table should not contain any indication of the water consumption required to excavate a unit volume of soil from any group of difficulty, since this quantity will vary over extremely wide ranges as a function of the type of dredge employed. One possible form for tables classifying soils according to the difficulty of dredging them is presented below. Soll group based on difficulty of excavation

Soil designation

Granulometric class

Density coeff~ K d of noncohesive soils

Consistency coeff. K c of cohesive soils

Soil resistance to probe embedment

Adherence

The output of a dredge depends on the character of the soils, and the water output of its soll pump, as well as on the structural efficiency of its soil-lntake equipment, working hoists, pile travel, and a number of other subassemblies. Since a large number of different types of dredges distinguished by different technical levels and capacities of the basic subassemblies themselves are found in service in hydraulic construction, it is apparently necessary to determine the output individually for each type of dredge. We will treat it thusly in developing norms for sea-going, bottom-deepening dredges. In production norms for marine bottom-deepening operations, all working dredges are divided into 18 different types and individual productivity norms are developed for each. In our country, dredges have worked ~ 160 years on bottom-deepening operations, and only 40 years on construction projects; it is therefore completely logical to utilize the experience that they have gained. In dividing dredges built in our country for construction purposes into individual types, there must apparently be some agreement as to the minimum number of monotypic dredges for which all-union norms should be established. Certain dredges are produced in small series (from one to five); in this case, local departmental norms should be restricted.

STANDARDIZING

THE OPERATION OF FLOATING DREDGES

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Considering the continuing work on the modernization of dredges, the rated individual productivity of dredges cannot be considered constant. It is apparently necessary to allow the ministry and department heads to correct the individual productivity of certain dredges on the basis of data derived from engineering tests performed on these dredges after modernization. In cases where new, more modern dredges, which are not included in the norms, are employed, their use should be prohibited. In this case, standardization should be done in accordance with individual local norms. The individual output of the separate groups of dredges must be determined for the optimum placement of the soil pump. We must remember that the distance and lift for delivery of the water-soil mixture have a direct effect on the amount of displaced soil. Actually, the lift of the soil pump is determined by the point of intersection of the Q--H performance curve and the performance curve of the soil pipe. It is demonstrated in Fig. 1 that any one soil pump employed to place the embankment of a dam will have a different output depending on the elevation to which the water--soil mixture is delivered. In placing fill at an elevation of 12.5 m, therefore, the delivery of the soil pump is 3900 m3/h, or in terms of soil, the productivity is 390 m3/h for a consistency of i:i0. On placing fill at an elevation of 32 m, the effective position of the pump is displaced markedly toward the left, and the delivery diminishes to 2000 m3/h, while in terms of soil, the productivity drops correspondingly to 200 m3/h. It is obvious that a change in the distance over which the soil is delivered will also cause the same displacement of the effective position, and, consequently, a change in dredge productivity. The example that we have just cited demonstrates convincingly that in standardizing dredges it is necessary to determine the mean-suspended position of the effective point of the soil pump on its Q--H performance curve. Assigning a dredge efficiency in terms of time, it is necessary to consider that the amount of technologically required downtime may vary within extremely wide ranges, depending on the specific conditions under which the dredge is working. For example, the frequency of slurry-line anchor replacement and, correspondingly connections to shore lines may vary markedly, depending on the area of the cut section. In our opinion, the assignment of dredge efficiency for the working period based on the averaging of statistical data is completely inadmissible. The normalized efficiencies of dredge working time must be oriented toward an adequately high organization-technical level of their use. All supplementary factors (with the exception of the first) affecting standard dredge productivity must be ccnsidered with corresponding factors as is done in the current Construction Norms and Specifications. The effect of pumping stations on dredge productivity should be accounted for by seeking a new effective point for the entire hydraulic-transport system. In all cases where pumping stations are present, the current Construction Norms and Specifications prescribe the use of factors that reduce the standard productivity. This is incorrect, since it is easy to show that the cutting in of a pumping station frequently results in a significant increase in dredge productivity, shifting the effective point to the right. Values of the factors that account for the effect of all residual factors should be differentiated as a function of the technical characteristics of the dredge for which the standard productivity is being determined. One can, e.g., assign such factors as a disturbance in the working basin, the geometric characteristics of the excavation pit, the climatic conditions, etc. It is perfectly obvious that they will all have a different effect on the standard productivity of different dredges. The development of new productivity norms for dredges is a major and complex, but necessary task, since the current Construction Norms and Specifications neither stimulate a rise in the organizational-technical level of dredge operation, nor the implementation of technical advances. This task should commence not with a review of the values of the individual indicators contained in the tables provided in the Construction Norms and Specifications, but with the treatment of scientifically based systems of standardizing dredges as earth excavating and moving machines with consideration given to all basic and supplementary factors affecting their productivity [4-6].

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B.M.

SHKUNDIN

LITERATURE CITED i. 2. 3. 4. 5. 6.

B.M. Shkundin, Excavating Pumps and Dredges [in Russian], Gos~nergoizdat, Moscow (1961). Production Norms for Marine Bottom-Deepening Operations [in Russian], Transport, Moscow (1964). B.M. Shkundin, Dredges [in Russian], ~nergiya, Moscow (1968). B.M. Shkundin, Dredges [in Russian], Energiya, Moscow (1973). B.M. Shkundin, Machinery for the Hydromechanization of Earthwork [in Russian], Stroiizdat, Moscow (1974). V.V. Erofeev, "Classification of soils according to difficulty of excavation by floating dredges," Gidrotekh. Stroit., No. 5 (1976).