Destructuring settlements due to frost heave during the structures’ exploitation are often not taken into account at the designing stage, although they are indirectly related to the bearing capacity of the soils. The objective of this research was analyzing the effect of the number of freezing-thawing cycles on the strength characteristics of soils. A paired experiment with various initial parameters (void ratio, initial moisture content, and the number of freezing-thawing cycles) was carried out. According to the experimental results, the cohesion largely depends on the above parameters which might lead to its decrease by up to three times. The angle of internal friction demonstrated an indefinite behavior during the freeze-thaw cycles, which is confirmed by a literature review. Freezing–thawing cycles significantly decrease the soil bearing capacity: up to 44% after 10 freezing-thawing cycles for soil with $e=0.55$ and $w=16.5\%$. However, in the case of $e=0.75$ and $w=22.6\%,$ it increased by 33%. A program based on the least-squares method was used to calculate the approximation coefficients of the dependence describing the changes in strength characteristics from the abovementioned parameters. Changes in strength characteristics must be taken into account when designing structures, as they can lead to additional settlement or even subsidence of the foundations.

In the conditions of global warming of the climate in the territories of permafrost distribution it takes place the processes of increasing the temperature of perpetually frozen soils and increasing the depth of their thawing. These factors lead to a decrease in the structural safety of the buildings and constructions. Measures to return soils to the frozen state are not always feasible and effective. In such conditions the solution to the problem of decreasing the pile foundation bearing capacity can be the ground strengthening by the cementation using the wristband-pipe injection technology. In the article it is presented the experience of assessing the current technical condition and designing of bases and foundations strengthening of a three-storey residential building in Yamal-Nenets Autonomous District, village Muzhi. The object was given a limited-operational category of the technical condition due to the deficit of the pile foundation bearing capacity. To bring the constructions of the object to the working condition the project of pile foundation strengthening with the cementation by wristband-pipe injection technology was developed. While injecting the soil foundation with the hydraulic fracturing the additional (induced) tension state appears, and it changes the deformation and strength properties of soils, voids ratio, index of liquidity and modulus of deformation. Reinforced joints, soil compaction and stabilization appear due to the injection. While making the hydraulic test and fracturing in the soil body its stress and strain state is changing, foundation piles are stressed with the additional lateral pressure. Besides the soil is strengthening under the pile toe bulb. Analytical calculations are given for increasing the bearing capacity of a pile during cementation of soils due to the increase of horizontal stresses and the work of compacted foundation soil on the lateral surface and under the lower end of the pile.

Deep excavations in dense urban areas often result in additional settlement of buildings that fall within the influence area of new construction. The most sensitive to such settlements, according to the normative literature, are historic buildings. One of the ways to regulate additional building settlement in the influence area of deep excavations is compensatory injection. The hydraulic fracture network created in the foundation creates additional lateral compression of the soil and changes its physical and mechanical properties. The article deals with a real example of an emergency situation at one of the objects in Tyumen, where during excavation of a pit with the designed depth of 7 meters for the construction of a five-storey building with an extended two-level underground space significant excessive settlement of the adjacent historic building and damage to its bearing structures were revealed as a consequence of the additional settlement development. The purpose of the present study is to evaluate the effectiveness of compensatory injection using collar technology to reduce excessive additional settlement of buildings in the zone of influence of deep excavations. A comprehensive analysis of the impact of new construction on the surrounding environment has been carried out by the authors of the article by means of numerical simulation. Analyzing the results of numerical modelling, the authors of the article developed measures to eliminate the emergency situation and stabilise the foundation. Based on the results of geodetic monitoring, it was found that the installation of a geotechnical barrier by injecting soil using the collar technique not only stabilised the foundation but also brought the building into position to meet regulatory requirements.

ИССЛЕДОВАНИЕ РАБОТЫ СПИРАЛЕВИДНОЙ МИКРОСВАИ С ГРУНТОМ