Features of calculation and design of a bulwark at the condition of non-liquefaction of soil under seismic load

Abstract

Introduction. Currently, new port complexes are being built and existing ones are being reconstructed in seismically dangerous construction areas of Russia, such as Petropavlovsk-Kamchatskiy and Primorsky Krai, Sakhalin Region, including the Kuril Islands, the Republic of Crimea, Astrakhan Region, the Republic of Dagestan, Donetsk Region and others. Calculation and design of bulwarks according to Russian regulatory documents under static load is carried out at an angle of internal friction of sandy soil of 28-30 degrees, and when calculating for seismic load - is not regulated. This corresponds to loose soil composition, which is not allowed when constructing structures in seismically dangerous construction areas. In seismically hazardous construction areas, the calculation of bulwark-type sheet pile walls without taking into account the possibility of liquefaction of backfill soils can lead to a significant increase in the horizontal load on the sheet pile wall, leading to the breakage of anchor rods and the destruction of the bulwark structure under seismic load. Objective. The article considers the fundamental aspects of calculating a bulwark to prevent liquefaction of the backfill soil. A specific example shows the main dimensions of the bulwark structure and the calculation results. A general assessment of the calculation results is given when constructing a bulwark in seismically hazardous construction areas. Materials and methods. To identify the conditions for dangerous liquefaction of water-saturated foundation soils under dynamic action, the relationship proposed by prof. Seed H.B. (USA) is used, which he calls the cyclic loading coefficient c, linking the shear dynamic loads dyn. from the external load and the static stresses st in the foundation soil skeleton. Taking into account the loads acting on the retaining wall, the expression for checking the stability of sand backfill to the soil structure liquefaction process is transformed into the dependence of the ordinate eас of the active lateral pressure of the soil and water per unit volume of soil at a given soil depth and vertical stresses ( gr) in the backfill soil skeleton (taking into account the value of the useful load q on the soil surface). The calculation of the bulwark includes the calculation of seismic loads at a given earthquake intensity, determination of the bending moment in the sheet pile wall, the tensile force in the anchor tie and the sheet pile driving depth. The range of sheet piles is selected according to the maximum calculated bending moment M_max^(s calc), and its diameter is selected according to the tensile force in the anchor tie R_a^calc. Results. The test of the backfill soil stability at the reservoir bottom level to the soil liquefaction process showed that the cyclic loading coefficient was 0.753, which significantly exceeded the permissible value. To meet the condition of non-liquefaction of the backfill soil, the sandy soil should be compacted in the underwater laying zone to an internal friction angle of 26 to 34 degrees. In this case, the cyclic loading coefficient value decreased to 0.61. In the calculations, the internal friction angle of the backfill soil in the above-water zone is taken to be 36 degrees (instead of 28 degrees before compaction). The calculations showed that the free height of the bulwark (from the cordon mark to the reservoir bottom) should not exceed 12 m at a seismicity of 9 points. which is limited by the existing range of sheet pile grades according to the calculated value of the moment of resistance. When modeling the bulkhead structure, the approximate sheet pile driving depth tsh should be taken within the range of (0.8 – 0.9) H. Conclusions. When calculating the bulwark for seismic load, it is necessary to check the stability of the sandy soil of the backfill to the soil liquefaction process, which can lead to the destruction of the bulwark structure. The calculations have established that the free height of the bulwark should not exceed 12 m at a seismicity of 9 points. Taking into account that when calculating the bulwark for static load, the free height of the bulwark does not exceed 15 m, it can be assumed that the free height of the bulwark should not exceed 13 and 14 m at seismicity of 8 and 7 points, respectively. To prevent the liquefaction process of the bulwark backfill, dynamic methods of soil consolidation should be used. In the new edition of SNiP on aseismic construction, it is recommended to provide a section on the calculation of seismic loads when calculating retaining walls, including bulwarks, which includes checking the possibility of liquefying sandy backfill soils.

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