The collapsible soils of Kazakhstan

Brovko I.S., Doctor of Engineering

(M. Auezov South Kazakhstan State University)

Soil conditions of south of Kazakhstan are complex and very diverse. This is primarily due to the prevalence of collapsible loess soils of type I and II according to subsidence, which make up the range from weakly collapsible ones to soils with vividly expressed collapsible properties.

From the point of view of economy the foreign investors find the investment of finances into the construction in the south of Kazakhstan to be attractive. But at the same time, the selected construction sites are often on the most complex in geological aspect areas of urban development.

In this context very typical is the recent example, when the Western construction company "Metro Cash & Carry" was building a multi-functional shopping center in Shymkent. The construction and commissioning of this object must provide the city with food products, to create new jobs and to enrich the market with new imported goods and to promote further qualitative development of trade sphere. In this case the central street of recently built microdistrict "Nursat" has been chosen as the location of the construction site. The largest part of this district is located at 15-20 metres thickness of collapsible loess soils of type II according to the subsidence. The main method of preparing the beds of five-storeyed, more rarely of nine-storeyed buildings of local development is deep seated moistening of the whole subsidence thickness through drainage wells with further compaction of the top 4.5-5 m layer by heavy earth rammers. The degree of dampening in this case was controlled by selection of samples along the depth of the bed, and the reached soil density of the top layer -through the selection of samples by method of the "cutting ring". The greatest difficulty in this method of bed preparation was to create optimum moisture in the soil of backfilling at creating the top layer of compacted soil. In the dry, hot climate it is practically very difficult to create qualitatively a uniformly distributed moisture of the soil in its large massives on the construction site.

The difficulty of achieving optimum moisture in the soil ,compacted by the builders, is compensated by increasing the number of earth rammer strikes on one track i. e. this leads to increased labor intensity and building time. The building time is also influenced by a sufficiently long cycle of moistening, that is why the work only for bed preparation in these soil conditions makes 3-6 months.

Subsidence - is a complex physical and chemical process. Its main manifestation is soil compaction due to the movement and space-saving installation of individual particles and their aggregates, thus reduced the total porosity of soil to the state corresponding to the pressure. In connection with the increasing density of the soil after the drawdown of its strength properties increases. With further increase of pressure compaction process of loess soil in water-saturated condition continues, and with it increases its strength.The above shows that the necessary conditions for the manifestation of subsidence are: a) the presence of the load from its own weight of soil or foundation that can overcome the force when wet ground connection, and b) adequate moisture, which greatly reduces the strength of the soil. Under the combined influence of these two factors is subsidence.

The character of the deformation in time for subsiding soils is determined by the humidity. Due to the fact that the ground subsidence malovlazhnom usually in a state of compressive strain on the external load occurs within a relatively short time. Subsidence, and equally in the water-saturated sediment, occur over a long period of time, as these processes are associated with the filtration of water through the thickness of the soil.

As noted earlier, subsidence occur with increasing soil moisture and the presence of sufficient load, yes, them. In this regard, the character of the ground subsidence in time to a large extent determined by the characteristics of their increasing humidity, the type and source of soaking. to some extent change the load on the ground.

Subsidence may occur at any time during the construction and operation of a building or structure as often soaking or increase soil moisture and appear only after its humidity will increase the initial subsidence humidity within all or part of damp areas. When local emergency soaking subsidence within the deformable zone load the bases usually occurs rapidly in time with the intensity of 1 - 5 cm per day after the termination of a few days soaking comes stabilization. In the event that the initial wetting of soils occur within only a portion of the deformable zone or subsiding column, then each subsequent soaking will cause subsidence long until moistened all deformable zone or sagging thickness. With increasing pressure on ground subsidence increases and its complete stabilization occurs when the maximum design load, or the whole thickness of wetting and collapsible soils.

With continuous development of intensive soaking top subsidence of their own weight over time is determined mainly by promoting wetting down the front and the formation of hydrated zone. In this case, the drawdown begins after wetting front reaches a depth at which to start subsidence of its own weight and ends after soaking the entire thickness of soil subsidence and complete the formation of hydrated zone. Studies show that the rate of subsidence continuous soaking time first increases to a maximum and then decrease. After the cessation of soaking rate of subsidence of soil, as a rule, first increased, causing a reduction in their moisture and create more favorable conditions for self-consolidation of soil, and then stabilize.