Post-subsidence deformations of loess soils

The investigations established that subsidence deformations are not short-time processes and at times develop over several months and even years. Termed as post-subsidence deformations, these are connected With consolidation, creep, and desalination of soils. The characteristics of development of subsidence and post-subsidence deformations are determined by a set of factors: rate of moisture migration in the soil, physicomechanical chemical properties of the soil, structure of the structural bonds of the soil and rates at which these bonds are weakened with wetting of the soil, and effective pressure. It is assumed that the process of development of post-subsidence deformations of loess soils containing small quantities of water-soluble salts (less than 5%) is theological with a decreasing rate. Post-subsidence deformations of soils with more than 5% salt content are, to a significant extent, connected with desalination and suffosion compression. In graphs of the dependence "subsidence-time" t = f(tsl), we can distinguish three sections with approximately a constant rate, i.e., the starting, intermediate, and final stages of subsidence. The increases and rates of subsidence deformation in all three stages vary over a wide range due to the variety of soil conditions and other factors which have an effect on the subsidence. A large part of the total subsidence takes place in a relatively small (in duration) initial stage. Subsidence rates do not exceed 3-5 mm/day under conditions of slow development, reach 110-130 mm/day with rapid development of subsidences at the base of stamps and in the compression apparatus, and 40-50 mm/day at the base of foundations and below the floors of wetted foundation pits. The subsidence rate does not exceed i0 mm/day at the intermediate stage. At the final stage, it decreases to 0.1-1.5 mm/day in the compression apparatis amd to i-3 mm/day at the base of foundations, stamps, and below the foundation pits floor. Characteristic graphs of the dependence "subsidence-time" for comparatively short-time (up to 80-120 days - curves i-3) and longer geodetic observations (up to 240-460 dav- curves 4-7) on subsidence of the soil surface below the foundation pits floor are given in Fig. i. Curves i-6 are based on test data and, 1'-6', calculated by the extrapolation formula. Deformation due to post-subsidence compaction can be distinguished conditionallv in curves 4-7 at the third stage of subsidence with its rate of less than I mm/day not earlier than half a year after start of subsidence. Curves 4 and 7 are characteristic for cases of slow subsidence and, curves 5 and 6, for comparatively rapid development at the starting and intermediate stages. In this, increments in deformations at the third stage (these deformations can be considered as post-subsidence) are from 30 to 140 mm and are from 5 (curve 6) to 30% (curve 4) in reference to the deformations attained at the preceding stages.