Choice of structural and technological solutions for engineering preparation of bases for foundations

. The object of the research is a comprehensive assessment of the choice of structural and technological solutions for the engineering preparation of soil foundations using the methods of dynamic compaction with heavy rammers. It is proposed in the studies to choose the optimal solution according to the criteria of both efficiency and preference with sequential or selective implementation of certain target groups: multi-purpose choice from a set of goals; optimization on a set of conditions and in dynamics; multi-vector optimization, etc. Location of the options according to preference, the main directions for optimizing the options for constructive and technological solutions and the best option for the engineering preparation of the base for the foundations were identified on the basis of the proposed method for the experimental site.


Introduсtion
Limited territories with favourable geotechnical conditions for construction necessitate the development of complex, in engineering and geological terms, construction sites.
A certain problem when choosing the optimal design solution in terms of structural and technological parameters is created by the need for test compaction of soils at the construction site and multiple uncertainties: initial state of soils; influence of technological parameters on the change in soil properties during the compaction process; peculiarity of taking into account soil compaction in time, etc. [9]. It should also be noted that traditional methods for solving single-criterion problems that implement optimizing factors when restrictions are imposed on all other, other alternative constructive and technological solutions only in terms of economic indicators are not always legitimate, since the cost estimate is most often nonlinear in terms of the optimality of the decision taken [10]. _________________________________________ *Corresponding author: klebanyuk.dmitri@gmail.com It should also be noted that traditional methods for solving single-criterion problems that implement optimizing factors when restrictions are imposed on all other, other alternative constructive and technological solutions only in terms of economic indicators are not always legitimate, since the cost estimate is most often nonlinear in terms of the optimality of the decision taken [10].
The authors [4] propose to use multivariate models which allow to take into account to a large extent the direct and feedback links of the main factors, their interrelation, as well as the uncertainty of many subjective and objective conditions and parameters of the compaction of soil massifs.

Identification of interdependencies and interrelationships of conditions and factors
Production experience and most studies [11][12][13][14] allow us to note that: -with an increase in the deformation modulus of soils in the natural state, a decrease in the diameter of the rammers is required, but not less than 1 m, since in this case the soil loosens with the formation of soil lifting zones; -when compacting soil strata of high thickness, simultaneously with an increase in the diameter of the rammer, it is necessary to increase its mass and discharge height; -the greatest efficiency of compaction is achieved with optimal moisture content and content of clay particles; -the more homogeneous the soil the more efficient the compaction with the same energy consumption; -the increase in the rammer weight often does not lead to a high degree of compaction and uniformity of the compacted base; -with an increase in the impact energy, the density of the soil increases most significantly at the initial stage of compaction; -the quality of the compaction is largely determined by the layout and the distance between the compaction points; -the smaller the diameter of the rammer (at constant impact energy) the smaller the size (radius and depth) of the compaction zone; -the greater the deformation modulus of natural soils the smaller the tamping diameter should be; -uniformity of the soil structure in the compaction zone can be achieved by increasing the initial density of dry soil; -to achieve the required density it is necessary to ram additional volumes of soil into the base at relatively low density of natural soil; -for soft soils the greater their power, the greater should be the mass of the rammer or the height of its dropping and diameter; -the greatest depth of the compaction zone while minimizing energy costs can be achieved through the use of rammers of large diameters (2 m or more) at the initial stage of compaction with additional compaction with rammers of a smaller diameter upon reaching failure; -the greater the thickness of the compacted layer, the distance between the points of compaction of the soil should decrease and at certain values of the thickness of the compacted layer compaction must be performed without the presence of bridges between the indentations, i.e. continuous throughout the site; -in the presence of a solid underlying layer within the compacted strata, an increase in the radius of the compacted zone is characteristic, which makes it possible to increase the distance between the points of soil compaction; -with an increase in the depth of the imprint of the rammer and its diameter, the distance between the sealing points increases proportionally; -the formation of the soil compaction zone is greatly influenced by the shape of the rammer sole. The use of a rammer with a stepped bottom allows to significantly increase the depth of the compaction zone in comparison with a rammer with a flat bottom; -the maximum depth of the compaction zone with a sufficiently uniform degree of compaction can be achieved using a two-stage technological process and rammers with a concave spheroid and a convex spherical bottom.

Research method
Hence, selection of the optimal solutions should be carried out on a group of criteria of  Using the method of expert evaluations to determine the significance of performance indicators and taking the Bernoulli criterion as the most reliable criterion for the optimality of a constructive and technological solution where value ij X of the j indicator for the i option which implements the principle of fair absolute concession and allows one to switch from vector criteria to scalar ones, reducing the multicriteria problem to a single-criterion one.
For significant indicators of the effectiveness of constructive and technological solutions, the following were taken: height of discharge of heavy rammers (H, m); energy consumption where C is the average index of agreement, and d is the average index of disagreement.
As the significance of performance indicators are taken according to [ The initial matrix was formed for an experimental construction site in the geological structure of which technogenic formations are involved, represented by dumps of soil from sands of various sizes, mixed with sandy loam with inclusions (up to 3%) of gravel. The dumping time is more than 10 years, which determines their sufficiently compacted state. The thickness of the deposits is from 0.5 to 0.7 m. The bedrock deposits are represented by fine and medium sands of low strength and medium strength. The thickness of the sandy strata is from 2.0 to 5.0 m. Loams occur from a depth of 7.0-8.0 m. Groundwater -free-flow with a steady level in the range of 4.6-5.1 m.Water-bearing soils -fine and medium sands with the following averaged physical and mechanical characteristics: Based on the production capabilities of the general contractor, the following indicators of the effectiveness of constructive and technological solutions were adopted (Table 1).  No less important is the question of ordering options according to their preference. We have used the most acceptable method of ordering which is suitable for a set of cardinal (numerical) and ordinal indicators of performance of the compared options.
As an estimate of the ordering of the variants   and then the agreed (the best) ordering will be mo, for which the value βq is the largest. It should be borne in mind that the most consistent ordering is identical to the series of preferable options. Significant cardinal and ordinal indicators are shown in Table 2.  III  III  II  II  0,06   Relative energy costs  X7  II  III  II  II  0,07  Operational reliability  X8  III  III  II  III  0,05  Degree of dynamic impact  on existing adjacent  objects   X9  III  III  III  II  0,03   Internal factors beyond the  control of the designers  Х10  II  III  III  II  0,02   Internal factors beyond the  control of the contractor  Х11  III  I  II  III  0,02   External factors of the  features of the compaction  devices   Х12  I  III  III  III  0,01   External factors of  qualification of work  performers   Х13  II  II  III  III  0,02   Factors of randomness and  uncertainty  Х14  I  II  I  II  0,05 Note. Ordinal indicators I, II and III for levels and conditions characterize respectively "high-medium-low", and for needs -"small-medium-large" The preference of the options was analyzed on the basis of entropy [16,17]