SIMULATION OF SUBGRADE EMBANKMENT ON WEAK BASE Purpose

Dep. «Tunnels, Bases and Foundations», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (050) 708 50 69, e-mail petrenko1937@mail.ru, ORCID 0000-0002-5902-6155 Dep. «Tunnels, Bases and Foundations», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 79, e-mail i-svjatko@yandex.ua, ORCID 0000-0002-7099-2637


Introduction
The new technologies that allow working on strengthening soil quality and reliability are appeared in the bundle of knowledge and experience of native scientists.One of the leading-edge technologies is jet grouting, which allows implementing various constructions enhancement the soil base.The paper presents the main parameters of the numerical analysis of the foundations of the subgrade base using amplification-based jet technology.
When choosing a method of strengthening soil it should be based on its technological and purpose of application.The soil cementation is a penetration and filling of cavities in the soil by means of solution for injection under some pressure.In this regard it is necessary to thoroughly consider the following factors: the lack of damage which inflicted by high pressure building, located near the zone of cementation, if they are available; possibility of squeezing cement at considerable speeds of groundwater or the presence of cavities in the soil and the degree of strength solution for injection.
The jet grouting method includes works on improving soil quality by energy liquid jet for destruction, mixing and replacement of the original natural soil cement slurry, followed by the formation of strong soil cement.It is the formation of cylindrical piles by means of soil's destruction of cement's jets by pressure 200 … 400 kg/cm² (20 … 40 MPa), emerging from monitor's nozzle with diameter Ø 2 … 3 mm, attached to the drilling equipment to further mixing and filling of soil with cement.

Purpose
The aim of this work is to study the impact of a weak base on general deformation of subgrade.Another goal is to identify and optimize parameters subgrade through research by numerical simulation.

Methodology
The theoretical research of the stress-strain state of the roadbed's base and embankment by simulation software LIRA was conducted to reach this goal.The finite element method is among the most contemporary and efficient methods for calculating structures of various purposes, which allows combining two tasks of geomechanics problems: deformation and limit state.The program complex LIRA is a program of numerical simulation processes which occur in soils during the influence of different pressures and also allows exploring the process of deformation and zones development of limiting condition of the soil.During the simulation may be received a complete picture of the stress-strain state of the investigated area, and the importance of breaking load, precipitation, etc.
The follows can be determined by using the software system: − subsidence of soil and pile base with a given load and engineering-geological conditions; − limit compressed thickness in accordance with existing regulations; − the difference subsidence and rolls of existing buildings including those which projected; − coefficients of the bed soil base according to soil models of Winkler and Pasternak [5,7].
Winkler model is the simplest model that has gained widespread use.This model is based on the fact that the draft stamp proportional loading, deformation and elastic enough after unloading disappear.The only mechanical parameter is the coefficient of bed.
Elastic subsidence of surface under the stamp D under pressure p according Winkler model is defined as where C -coefficient of subgrade reaction.The Pasternak's model is used for research deformation of subgrade crest outside of the load transmitted from the rolling stock.
Elastic subsidence of surface around the stamp D under pressure p according Pasternak model is defined as where C 1 -ratio compression, kgs/cm 3 ; C 2 -offset coefficient, kgs/cm; r -distance from the center of stamp to point out of stamp definition subsidence, cm.

Review of recent research sources and publications
The problem addressed in many publications.Numerical modeling, particularly in the software LIRA is described in a lot of works [4,6].Payment schemes of most commonly used pile bases' models are given in investigation [2].The problem solutions of subgrade strengthening are devoted at the work [3,9,7,10].

Findings
Calculation of the roadbed on the weak basis based on the solution of the plane problem of elasticity, but the body of the embankment stability calculations and their slopes are performed by the classical theory of limit equilibrium.In these methods is not taken into account that the occurrence of plastic zones occurs long before the limiting condition in the body of the embankment and soils weak base.In these areas accumulate residual strain that caused vibro dynamic impact load and excessive moisture soils.In weak bases can be slow viscous and plastic flow of soil, aimed at slopes of subgrade, which can cause dangerous shear and expansion joints and cracks.
Thus, for weak soil the most appropriate is nonlinear bases model considering of existing areas as elastic and limit equilibrium that is mixed problem of elasticity and plasticity.
To obtain reliable results, it is necessary to follow those recommendations: to determine deformation and strength characteristics of soils that make up the body of embankments and foundations with sufficient accuracy; define the parameters of the rolling load distribution; establish dimensions of cross-section given its boundaries loose soil with a weak base.Payment schemes that are used in full to more accurately show the stressstrain state of the roadbed than the schemes adapted to the axis of symmetry.
Under the influence of load in a weak base appears significant horizontal tension because border calculation scheme (prohibition of horizontal displacement) should be placed as farthest from the place application of the load.Also there are prohibited horizontal movement of the left and right boundaries and vertical movement of the lower nodes of boundary.Splitting cross-section is performed with increasing density of finite element mesh in areas where the expected increase of stresses and displacements values.
The values of loads from rolling stock in the calculation scheme adopted according to the experimentally obtained values.The values of vertical deformation in the event of plastic zones in the soil depend on the nature of the loading.In the absence of plastic deformation load of rolling stock may take one step.When using the finite element method, the determining soil characteristics and embankment foundations upon this depends largely on the stress-strain state of the real subgrade plays an important role.
In the design scheme of layers of soil are taken according to the results of geological surveys.
Estimated parameters used in the numerical simulation should be taken from Table 1.So to calculate the embankment on the weak basis is performed on calculation scheme with the size of the real profile, creating finite-element mesh, determination of forces and loads.Also there is performed choose of the design characteristics of the system «embankment-weak base» and simulated process system load rolling stock.
Determination of the total subsidence occurs by layered adding using scheme linear deformation half-space (Boussinesq's task).
The achievements limits cramped thickness H c is controlled of zp zg k σ = ⋅σ by using the coefficient depth of cramped thickness k, which is defined.
After making the necessary calculations perform building fields of a subsidence, borders cramped thickness, bed's coefficients of Pasternak and Winkler.The diagrams construction of vertical stress performs at any point of load application.
Also, using the software system may perform peer review subsidence, rolls railroad tracks in natural and consolidated basis.

Originality and practical value
There was defined the algorithm of forming design scheme for the calculation of the embankment on the weak basis by finite element method.The method of the selection strength characteristics of soils and calculated parameters for using in numerical modeling is offered in the article.The modeling process of load system by rolling stock was grounded.
Analysis of the stress-strain state of the system «weak base-roadbed» allows you to see the basic patterns of soil and take the necessary steps for strengthening of soil-cement elements.

Conclusions
Based on the above, in the condition of the growth rate of the rolling stock and traffic on Ukrainian railways, the investigations allow using software LIRA for the analysis of roadbed operation of rolling stock in the railway, which is reduced on a weak base.
With increasing the load on the weak basis by building of the second track, adding embankment or increasing axial load when changing the rolling process of settling and consolidation can be continue again.Therefore, one of the viable and future options design and reconstruction of embankments on weak bases are strengthening the base by using jet grouting.Covering soil cement piles allows to provide a short time strengthening roadbed, the ability to quickly enter areas of track in operation and small size of subsidence.