Construction and Geotechnics

In recent years, due to the increasing of urban construction extent, there has been a tendency to exhaust of territorial reserves, suitable for construction in large cities. Increasingly, it is necessary to use territories with unfavorable soil conditions for the construction of civil and industrial buildings. In such cases, various pile foundations are used, which have significant technical and economic advantages compared to foundations on a natural bases. Among the progressive designs of pile foundations, a special place is occupied by piles in punched holes with broadening (SPSu), which ensure the improvement of the physical and mechanical properties of the foundation during the production of works, as well as the reduction of material, energy and labor costs during their installation. Piles of this type are designed using the developed regulatory framework ISO 36554501-018-2009 "Design and installation of pile foundations and reinforced foundations from packed piles in punched holes". At the same time, the task of improving the quality of work on the design, arrangement, as well as the assessment and prediction of the bearing capacity of piles becomes particularly relevant. This task is solved with a high degree of accuracy by conducting large-scale tests of piles with a static load tests. These tests are carried out in real conditions of the construction site and allow to observe the changes pile`s settlement under load. The article is devoted to solving the problem of increasing the efficiency of assessing the bearing capacity of piles according to static load tests. Using the example of the test results of single SPSu, the authors of the article assess the effectiveness of the existing domestic and foreign methods for determining the bearing capacity of piles. As part of the solution of this issue, the results of the bearing capacity determining according to SP 24.13330.2021 were compared with the data obtained during the use of Davisson, Chin, De Beer, Hansen, Decourt and Eurocode 7 methods. The advantages and disadvantages of this approaches used are noted. Based on the accumulated practical experience of testing the SPSu in various ground conditions, the authors of the article proposed an alternative method of interpreting the "settlement - load" graph.

Presently, nonlinear mechanical soil models are widely used in the geotechnical practice. They are implemented in geotechnical finite-element software PLAXIS, MIDAS GTS NX, Z-Soil, Optum, etc. Notwithstanding the fact that International regulatory documents mainly recommend nonlinear models for calculating foundations, they do not specify methods for defining their parameters because nonlinear models are actually commercial products of various software manufactures and cannot be normalized in any country. For this very reason, specialists of Gersevanov Research Institute of Bases and Underground Structures (department of JSC "Research center of Construction") developed Standard of Organization STO 36554501-067-2021. The Standard is based on the world experience in assessing nonlinear model parameters defined with using advanced laboratory equipment. The article describes the most significant provisions of this Standard. Thus, depending on the problem to be solved, four groups of models are introduced: I - a model includes volumetric isotropic hardening (Soft Soil); II - a model includes volumetric isotropic hardening and rheological properties of the soil (Soft Soil Creep); III - a model includes double isotropic hardening (Hardening Soil) and IV - a model includes double isotropic hardening and stiffness at small strain (Hardening soil with small strain). Requirements to the necessary and sufficient types and scope of tests, which define model parameters, are reported. Taking into account that only laboratory tests define model parameters, Standard provides requirements for specimens' quality evaluation performed basing on measurements of the volumetric strain under effective natural stress. Specimens of poor and very poor quality cannot be considered. It is shown that power-law coefficient m , which indicates the relationship between soil stiffness and stress-strain-state in the models of III and IV groups can be received from triaxial compression and oedometer tests and shall be prescribed depending on prevailing deformation (shear or compression). Stiffness parameters at small strains can be defined wit in-situ tests (seismoacoustic profiling) or with laboratory tests (resonant column test, seismoacoustic profiling of specimens in triaxial cell equipped with bender element and strain gauges on the local base). The developed Standard specifies requirements for laboratory definition of stiffness at small strains.

The article notes that changes in the loads acting on the drainage pipe (pipeline) in accordance with the relative ability of the drainage pipe and the ground filling to deform are essential. To determine the magnitude of the loads on the drainage pipeline laid in the trench, the formula proposed by A. Marston most accurately corresponds to the results of experiments and is convenient for engineering practice. The perforation of the walls of the drainage pipe affects their strength not only when the pipes are crushed, but also for the conditions of transportation and installation of pipes. The contact interaction of the cross-section of the drainage pipe with the soil base is investigated, in this case, the interaction of cylindrical (having a semicircular contact) or reloid (having a contact of the circle sector) shells lying on an elastic soil base is considered. A differential equation is considered to describe the deformation of an elastic shell, which includes the stiffness parameter, linear differential operators, components of displacement of the neutral axis of the drainage pipe, external load and dimensionless coordinates. For the elastic base models of E.Winkler and V.Z.Vlasov, a general solution of the differential equation describing the deformation of the drainage pipe is proposed. The solution of this equation and the external load is presented in the form of double or single trigonometric series, depending on the task (two-dimensional or one-dimensional). A horizontally lying cylindrical (or reloid) shell (drainage pipe) supported on a ground base is considered. The contact pressure is determined by summing the Fourier coefficients. This solution additionally takes into account the distribution capacity of the ground base of the drainage pipe.

The paper considers the main problems of design and calculation of deep foundations of high-rise buildings in the form of piles-barrettes. The results and methodology of field tests of barrette piles for the foundation of a 56-storey residential building in Moscow are presented. The characteristics of engineering-geological conditions of the future construction site and design solutions for pilot tests are presented. Given a large cross-section of the barrette, which involves a significant amount of pile-type soil mass with the formation of a complex stress-strain state, to obtain reliable results and verification of the calculated model is recommended to perform numerical tests in full compliance with the field experiment, taking into account the anchored piles. The paper describes the method and the results of the numerical calculation of the load-carrying capacity of a barrette taking into consideration the modeling of an anchoring system in the Midas GTS NX program complex by the finite-element method and shows the basic possibility of using the program complex and the described method for practical purposes with an admissible accuracy. The paper contains diagrams of vertical displacements of a barrette head as a function of the applied load in full-scale tests and a general evaluation of the carrying capacity of a barrette pile obtained by numerical solutions. It is found that by taking into account the anchoring system (beams and piles) in the numerical simulation, the calculation results obtained are the most accurate to the field tests in describing the behavior of the pile under load in comparison with the calculation of a single barrette pile. In spite of the fact that the barrette bottom is embedded into the rocky soils, the barrette piles are referred to the hanging piles according to the interaction conditions with the soil due to the significant load transfer along the lateral surface.

The article presents experimental studies of the mechanical properties of the downstream wedge of a low-pressure dam under conditions of a rapid increase in the level of flood waters, since ensuring the stability of the slope of bulk hydraulic structures during increasing natural and man-made disasters, taking into account changes in climatic and seismic conditions, is a paramount task for specialists in design and operating organizations of the water management complex. Currently, despite the increased interest in landslide factors, there is still insufficient information on how the size of drainage devices located in the downstream wedge of a low-pressure dam can affect the stability of its slope in the event of a rapid drawdown of the flood water level. In order to analyze the influence of the size of the drainage devices of a low-pressure dam on the stability of its slope with a possible drawdown of the level, numerical studies were carried out in a flat setting. The performed mathematical modeling made it possible to assess the stability of the dam slope due to the rapid drawdown of the flood water level and to determine the effect of the drainage size of the low-pressure dam. An analysis of the simulation results showed that in the scenario of a rapid drawdown of the water level in the river after passing the flood peak, the stability of the slope of the low-pressure dam facing the river decreases. The pore pressure in the upstream of the embankment decreases with the increase in the size of the drainage of the downstream wedge, while it increases in the downstream. With an increase in the size of the drainage, the values of the slope stability coefficient increase. The factors identified in the process of mathematical modeling that affect the stability of the dam depending on the length of the drainage in the downstream wedge area of the low-pressure dam can be used for predictive purposes to assess their reliability.