Layer-wise Summation Research Articles

Stress-Strain State Assessment of Building Subfoundations Having Flexible Constructive Scheme in Irregular Soil Stratification

Introduction. One of the main problems in the operation of buildings and structures is possibility of their damage due to non-uniform deformation of subfoundation soil caused by the various natural and technogenic factors. To establish the possible character and degree of damage of the above-surface structures, it is necessary to calculate the value of subfoun- dation relative differential settlement at the level beneath the foundation bottom. The subfoundation settlement is calculated within the compressible soil thickness using the specific gravity values and soil deformation modulus. The increment of settlement is induced by the increase of vertical stresses in the massif and increase of soil deformability upon growth of moisture. The purpose of the study is to establish the dependence of the magnitude of the relative difference in settlement on the deformation properties of soils with uneven occurrence of layers in order to predict the nature of damage to above-ground structures.Materials and Methods. An example of subfoundation deformation calculations at the bottom level of the free-standing foundations of a building having flexible constructive scheme has been studied. A model of stratification of two heterogeneous geotechnical elements with a 1:4 boundary slope and differing physical and mechanical properties has been used. It has been assumed that one of the geotechnical elements has natural properties enabling the building foundations to rest on it without prior site preparation. Another geotechnical element has significantly worse properties. The calculations on determining the settlement have been made in accordance with the layer-wise summation methodology.Results. The regularity in alteration of the predicted settlement value depending on the ratio of firm and soft soils thickness in the irregular stratification has been established. It has been proven that the difference between the specific gravity values of the heterogeneous beds within 10 % does not affect the depth of compressible soil thickness if the calculation of settlement is made using the layer-wise summation methodology.Discussion and conclusions. The calculation results analysis of the subfoundation settlement of the equidistant column foundations was carried out, the engineering solution was proposed to ensure reliability per limit state groups.

Design Index and Model for Controlling the Permanent Deformation of the Granular Base Course

This study proposes a design index and model for controlling the permanent deformation of the granular base course for asphalt pavements by adopting two types of graded macadam. Three levels of confining pressure stresses and different stress ratios were set as the test loading conditions. The permanent deformation shakedown characteristics of the graded macadam were examined by conducting triaxial repeated load testing based on shakedown theory. Statistical regression and curve fitting were applied to the test results to overcome the limitation on the number of repeated loads. In accordance with the fitting results, the testing curves were extended to analyze the limit stresses for the shakedown under different shakedown statuses of the permanent deformation of the aggregates. A new design model was established for controlling the permanent deformation of the granular base course in zone A (plastic shakedown status). The permanent deformation of granular base courses of typical asphalt pavement structures was calculated by adopting the layer-wise strain summation and employing the evaluation model currently adopted by the United States Mechanical-Empirical Pavement Design Guide for the permanent deformation of granular base courses. Consequently, the critical standard values of the permanent deformation and the permanent strain of granular base courses were determined. The stress ratio is recommended as a design index for controlling the permanent deformation of the granular base course. The control model of the regression formula between the stress ratio and the number of axial loading was originally created based on laboratory tests.

Analysis and Improvement of Fitting Models for Predicting Subsidence Under High-Speed Railway Lines

A number of methods for predicting land subsidence and monitoring deformation under high-speed railway tracks exist, and are divided into three categories: layer-wise summation, numerical calculations based on consolidation theory, and curve fitting. One of these, curve fitting, including the hyperbola, expanded hyperbola, three-point fitting and Asaoka methods, is widely used because it is computationally simple and applicable in many situations. In this paper, we analyze the performance of the four classical curve fitting methods using field data and propose a novel approach to estimate land subsidence. The new method integrates three-point fitting, which is computationally simple whilst stringent in terms of correlation restrictions, with the Asaoka method to significantly improve performance in practical applications. Our experimental results indicate the average relative error of the modified method is reduced by 35.3 % than that of three-point fitting, and the mean correlation coefficient remains within acceptable bounds and even was enhanced by 1.48 %, so that this modified method can substantially improve prediction precision.

Nonlinear Analysis of Foundation Settlement by Modified Secant Modulus Method

A new technique namely modified secant modulus method（MSMM）considering time factor for nonlinear settlement computation of foundation is proposed. The principle of MSMM is using correction technique of reverse solution to solve modified secant modulus (MSM) of subsoil under different additional stress level with load-settlement test data, and the additional stress is solved with Boussinesq elasticity theory, then layer wise summation method is adopt to calculate the total settlement of foundation. The loading test results of footings performed by Briaud and Gibbens (1993) on sand foundation are analyzed with MSMM. Normalized settlement curves over width versus pressure for all footings can be obtained in normalization processing, and the calculation curve agrees well with the normalization curves. The loads of Q25 and Q150 corresponding to the settlements of 25mm and 150mm respectively of different size footings are calculated and the errors of them are within 10% compared with testing results, and the creep exponents of 3m south and north footing for 30 minutes load test were 0.036 and 0.030, then the calculated total settlement in 2014 (20 years later) will be 39.6 and 36.7mm respectively.