Технические науки/2. Механика

D.tech.scien. Artamonova E.N.

Saratov State Technical University n.a. Gagarin Y.A., Russia

Analysis of soil bases

In this paper the conditions for the modeling of facilities' ground bases are summarized. In some models the V.Z Vlasov – N.N Leont'ev theory of calculation for ground bases is used. It based on the fundamental equations of mechanics and the solution of differential equations for the expected draft at the point.

In this study, subgrade strength variability and flexible foundation and pavement designs are evaluated for reliability. Reliability is an important factor design to consider the variability associated with the design inputs. Parameters such as mean, maximum likelihood, median, coefficient of variation, and density distribution function of subgrade strength are determined [1]. The approach is based on an extensive literature review of current damage concepts included in current mechanistic-based design procedures, soil permanent deformation laboratory data. Design outputs are compared in terms of reliability and thickness using these design procedures. It is shown that the provides higher reliability values compared to the probabilistic procedure. All the existing subgrades fail distress reliability such as rutting and top down cracking reliabilities. Currently uses a single design P value to deal with variability associated with subgrade strength design.

Is used to generate full scale subgrades response and performance data for development and verification of subgrades design criteria. The physical properties of subgrades structures significantly influence both the response of the subgrades to applied loads and the long-term performance.It is, therefore, of the utmost importance that full scale test subgrades be constructed with uniformity in material properties, layer thicknesses, and other considerations for which non-uniformity might result in nonrepresentative and nontypical behavior and failures [1]. Current mechanistic-based design methods for the design of subgrades use vertical strain criteria to consider foundation rutting.

A considerable number of measurements of the physical properties test pavements were made at all stages of construction and after construction was completed. The measurements were made for three purposes: construction quality control, construction acceptance, and material characterization. The material characterization tests were performed to provide information for theoretical modeling and were not related to construction and contractual requirements.Tests were conducted on the subgrade materials, base subbase, and surface layers. For a basis of model building we take the model of elastic foundation, Vlasov - Leont'ev [1] (fig.1).

figure 1

Here [2]:

u(x,y,z)=0; v(x,y,z)=0 ;

₍₁₎

; ;

D₃=D₁μ₁+2D_k; D_k=Gh³/12.

Tests performed during construction consisted of measuring insitu moisture content and density. Tests were performed to characterize the variation of subgrade strength with depth and over a tight horizontal grid. Width of the subgrade surface was divided into equally sized quadrants and a location within each quadrant determined by randomly selected x and y coordinates. The choice of the appropriate type of foundation is governed by some important factors such as: the nature of the structure; the loads exerted by the structure; the subsoil characteristics; the allotted cost of foundations. Therefore to decide about the type of foundation, subsoil exploration must be carried out. Then the soil characteristics within the affected zone below the building should be carefully evaluated. The allowable bearing capacity of the affected soil strata should then be estimated. Theory of elasticity analysis indicates that the stress distribution beneath footings, symmetrically loaded, is not uniform. The actual stress distribution depends on the type of material beneath the footing and the rigidity of the footing. For footings on loose cohesion-less material, the soil grains tend to displace laterally at the edges from under the load, whereas in the center the soil is relatively confined.It is shown in this study that single design strain value for a roadway section does not yield an effective design regarding target reliability [2].

References:

1. Petrov V.V. Dimensional model of nonlinear deformable heterogeneous base // Interuniversity scientific collection.- Saratov: SSTU, 2007.- P.6-12.

2. Artamonova E.N. On the design of slabs on the basis of a non-uniform // Moskau: INGN, 2012.- P.4.

In this paper we propose a mathematical model of destruction (the relations connecting parameters of efficiency at the time of fracture characteristics material), based on the relationship of both these approaches to allow for the dependence of the limiting critical conditions at which the destruction, the time of stress, temperature environmental exposure, exposure, etc. This is especially typical for polymers [1]. An examination of these experimental data one can draw conclusions that should be taken into account when constructing the mathematical correlations for the conditions of fracture:

Mechanical properties and the process of destruction of polymer materials substantially depend on time and operating conditions.

Destruction is a two-stage process. At the first stage the degradation of the properties of the material, the accumulation of damage, microcracks occur. The stage ends at a time when the merger of microdamage formed macroscopic crack. This moment is short-lived and by their physical nature is a loss of stability of equilibrium microdefects.

Because of the irreversibility of the process of destruction is determined not only the current values of parameters characterizing it, but the entire prior history change of these parameters.

Because of the private nature of the experimental data on the effect of medium on behavior of plastic the composition of the general mathematical for all materials the phenomenological description of fracture based on mechanical ideas due to the difficulties and serious shortcomings. Therefore it is necessary and the molecular interpretation of macroscopic changes in the material. Thus, the phenomenological theory of time dependence as would provide a common framework, which must fit the theory of material behavior, and that put a detailed mechanical theory of change of macroscopic and microscopic properties of the polymer. This need arises in the interpretation of the parameters of the phenomenological equation, allowing you to identify not only the common features, as well as the difference between the materials.

Because of significant time effects for polymers the process of their destruction more difficult than traditional materials, the phenomenon of viscous and brittle fracture occur simultaneously. Fracture criterion in this case must take into account the achievement σ, ε of the instantaneous and destructive values σ р, ε р, at the time tразр., and their dependence on the development of degradation of material properties ω (t).

Figure 1.

Analysis of experimental data (Fig.1) suggests characteristics of the temperature dependence of relaxation processes and fracture for viscoelastic polymers with the same value of energy activation for each material.Both aspects of the strength of polymers depend on the local structural changes that primarily can be linked with the process of accumulation of damage, education grid hairline cracks. Combining different approaches to describing these processes, i.e. formulation of a general mathematical theory of deformation and fracture of polymers depends on the study of the relationship of deformation, destruction and action of strain, temperature, aggressive factors in the whole time interval of operation of the element.

According the survey of the literary sources for the analyzing of long-term durability of materials and elements made of them two alternative approaches are basically exist: mechanical (benchmarking) and kinetic.

According the first approach we model the generalized condition for material destroying:

Ф (θ1, θ2, θ3 ) = Ф р.

Here Ф - the functional is some combination of the components of the stress or strain. The functional Ф depends on the accepted theory strength or given empirically and then the functional contains parameters determined experimentally.

Ф = ∫dV [1/2 ρu?iu?i– λ/2 ∂ui/∂xi ∂uj/∂xj - µ/2( ∂ui/∂xj ∂ui/∂xj + ∂ui/∂xj ∂uj/∂xj)].

1.The strain tensor can be represented as a sum of tensors of elastic deformation of inelastic deformation:

ε ij = ε ij¹ + ε ij².

2. For description the strain state and fracture in the framework of a generalized model of inelasticity is necessary to consider the history of deformation of the sample depends on the loading path and on time. For different loading paths for the processes of varying duration results will be different.We give a physical explanation of the above stated hypothesis.

References:

1. Suvorova J.V., Ohlson N.G., Alexeeva S.I. An approach to the description of time-dependent materials //Materials and Design, Vol.24. Issue 4, June 2003.- P.293-297.