Horizontally loaded piles in saline soils

. The influence of the aggressiveness of the environment and the variability of the saline soil properties on the operation of horizontally loaded piles is studied. The design scheme is taken in the form of a multi-span beam with a variable cross-section formed due to concrete corrosion (Winkler model) and stiffness coefficients. The calculated value of the variable modulus of deformation and the stiffness coefficients of the pile are determined based on the conditions of the construction area and the properties of saline soils.


Introduction
The model for calculating a reinforced concrete pile for horizontal loading has sufficient "flexibility", which makes it possible to take into account the variable characteristics of soils as a result of prolonged soaking and leaching of salts.In addition, the material of piles working in close contact with an aggressive environment undergoes a number of qualitative and quantitative changes [1-7, 8, 13-14].Corrosion processes in concrete and their uneven distribution along the length of piles reduces the strength and deformation properties of concrete, as a result of which the nature of the structure's operation under horizontal loads changes [9][10][11][12]14].
The solution of this type of problem was previously considered in relation to the calculation of pile columns [1].In this paper, the problem is solved taking into account the variable stiffness characteristics of soil and piles, which allows us to take into account the features of pile operation in saline soils, possible soaking of foundations during the operation of buildings and structures, as well as the corrosive effect of aggressive salts on the foundation material [2,9,10,11].

Fig. 1. Calculation scheme
The calculation scheme is presented in the form of a multi-span beam (Winkler model) [4,15,16], supported on the ground at infinite points with a variable stiffness coefficient witho(xk), the change of which obeys the power law: Value of the coefficient of linear soil stiffness of K-th dummy supports where, С0C0(кxk) is the soil stiffness coefficient at deptho zo ; ini(xk) -the size of the cross-section of the pile at the i-th support unit, taking into account the predicted depth of concrete corrosion; lsv -the distance between the bonds; n isa dimensionless parameter that takes into account the physical nonlinearity of thepileground"system (according to G. Klein) The stiffness coefficients Co(xk) , taking into account the degree of salinization of the soil around the pile and the decrease in the load-bearing capacity of the soil due to short-term and long-term moistening, can be taken: -direct testing of the pile with the use of strain gauges and mesdoz and withvay-probes in the soaked state of the ground; -based on the results of stamp or compression filtration tests of soil samples from different depths.
Of these, the first method is the most reliable, where numerical values of stiffness coefficients can be established directly from pile testing materials.The first method for determining the stiffness coefficient of elastic supports is lengthy and time-consuming.Therefore, we will consider the possibility of describing the stiffness coefficient кof the k-th conditional support located at a depth of x, in terms of the modulus of total soil deformation E(kxk).In this case, the calculation of a horizontally loaded pile according to M. Gorbunov-Posadov [17][18][19] and B. Laine [15] agrees with the analytical solutions of the Winkler model.Based on this, the stiffness coefficient of elastic supports [15] is: Here: ko -experimental parameter, for clay soils, ko = 0.65.µo(xk) -Poisson's ratio for the-kth conditional support; EI(xk)= EoZ nmodulus of soil deformation of the-kth conditional support, kPa; nis the coefficient with which the plot of the distribution0 of E0 over depth becomes constant (n = 0), convex parabola (n<1<1), triangle (n=1) concave parabola (n>I) (Fig. 2).
E0 -modulus of soil deformation at a depth of 1.0 m.F = hebi(x) -effective surface area of contact of piles with the ground,m2; he is the effective length of the contact surface, m.The effective length of the ground contact surface with the pile can be determined by the expression: he = zo -aпред.(4) Here: zoo-height to the zero displacement point, m. aprev = 1.5 b(x)is the maximum thickness of zero ground resistance at the surface [15].bi(x)-width of the pile cross-section, taking into account the corrosion damage of concrete, m: bi(x)= b0 -2 δк (1-ai / l) (5) where, andi is the height of the i-th section under consideration, m. l -length of the pile, m. δkк -expected depth of concrete corrosion, m. z-depth of the considered layer, m.The coefficientof stiffness of elastic supports With0(xk) for medium and heavy construction conditions(Table.1) in case of possible long-term soaking of the soil, it is accepted according to the characteristics of the desalinated state of the soil, and in other casesas for a short-term soaked base(Fig.2a.b).
Note: The degree of compressibility variability of β substrates for medium and heavy construction conditions can be determined by the expression β =S+Ssl+Ssf/S (Sis the average building draft on saline soil of natural humidity, Sslis the soil draft under load during short-term soaking, Ssf is the suffusion draft during their desalination defined by [5].Taking into account theabove, the balance equation for calculating a horizontally loaded pile in matrix form has the form [1]: Here: C -matrix of stiffness coefficients for the pile-ground calculation.
The matrix of pile stiffness coefficients is defined as the sum of the matrices of pile stiffness and saline ground stiffness of the where Csv is the matrix of pile stiffness coefficients, has the form: The soil stiffness matrix Withgr corresponding to i-bonds will be: From where, The load matrix is written as follows:а is expressed in the following form: The following transposed matrix is created for the displacement and rotation angle that fall on each conditional support:  (15) where, Vk , kdisplacement and angle of rotation of the K-th pile node; P -horizontal load, kN.
To determineеделенthe values of the rotary and linear reactions that occur at each K-th bond, taking into account the change in the pile cross-section due to corrosion, the method of displacementsis applied[...]: (16) Coefficient that takes into account the shift in the pile cross-section: Where, Icb(xk) is the bending stiffness of piles, determined taking into account the predicted depth of concrete corrosion; K -coefficient of the shape of the pile cross-section, equal to 1. Formula ( 17) for piles with a constant cross-section is taken as: ( The reaction values at the K-th bond level are determined by : ¯   =  gr  ; Expected corrosion depth of concrete L( ) with moderate to severe construction conditions (Table.1) for the estimated period of operation of buildings or structures [2,12]: where: coefficient that determines the resistance of the aggressive environment of the material (concrete) in the underground part of the structure, determined by the laboratory method [3].
[ ]-standard period of building maintenance, one year.
-coefficient that takes into account the influence of lateral soil pressure in the near-pile massif on the corrosion damage of concrete (Table 2).
-correction factor that takes into account the water-cement ratio of concrete (Table 3).. = 0.8 -confidence factor that takes into account the heterogeneity of concrete, the variability of the concentration of aggressive media and the spread of experimental data.It should be noted that in real conditions, corrosion processes in concrete occur unevenly.Themost intensedamage to concrete occurs in the zone of alternating moistening and drying.In an area where the structure constantly interacts with an aggressive environment, the process proceeds slowly.
We introduce (taking into account ground conditions, the position of the UGV, etc.) the functions of variability of the depth of corrosion damage along the length of the pile shaft.Taking into account that the upper horizons of the soil layer are most intensively corroded, we assume a linear law of change in the depth of corrosion along the length of the pile (other dependencies established for real construction conditions are also allowed).Then, changes in the cross-section along the length of the pile due to corrosion can be calculated according to the dependence: where, aai -height of the ith section under consideration, m, l -pile length, m.

Results
Taking into account the above, the calculation of horizontally loaded piles on saline soils is performed.The stiffness characteristics of saline soils in their natural, soaked, and leached state were taken as initial data.The most unsuitable condition for the operation of piles was the characteristics of desalinated soil.Some tasks were solved taking into account the possible depth of corrosion damage to concrete over the estimated life of buildings.Checking the strength of the pile cross-section for horizontal forces is performed according to the instructions of the current regulatory documents.
Based on the above, variable soil properties (Figure 4) and corrosion effects are taken into account when calculating reinforced concrete piles for horizontal loads in areas with heavy and medium construction site conditions.
The calculation results show the values of displacements vi(xk) and the angle of rotation i(xk) of the piles for each conditional support.According to the nature of pile deformation, the load-bearing capacity of piles is estimated by deformations.Based on the displacement plot or the angle of rotation of the piles, we determine all the necessary forces that occur in the pile shaft.The distribution of ground resistance along the length of piles will be: Where, v0 is the movement of the pile head.YY0 is the distance from the surface to the point where Qi = 0.
The maximum transverse shear forces Qmax can be taken from the dependence: Where, k' is a dimensionless parameter determined depending on the ground stiffness coefficient of the base, the depth of driving piles, the height of their aboveground part and cross-section, and the concrete grade.For single piles up to 3 m long, the coefficient k' varies from 1.6 to 1.85.On average, k' can be taken equal to 1,825.
When taking into account vertical loading, the described algorithm does not change, only the beam coefficientschange [2].Taking into account [2], the stiffness matrix of driven piles immersed in saline soil, taking into account the pinching effect of the floor and the influence of the grillage, is taken: (25) Cpol-matrix of responses to rebuffing the floor.Cp is the grillage stiffness matrix.

Discussion
Using the above method, a numerical analysis of the operation of piles with various geometric and physical characteristics, including the stiffness coefficients of elastic supports corresponding to the natural and desalinated state of the soil, is carried out.The effect of static and stepwise increasing loads is considered.The calculation showed that the stressstrain state of the pile foundation depends on the horizontal load, the strength properties of saline soils, as well as the bending and shear stiffness of pile structures.Changing the deformation properties of soils due to soaking of the bases changes the overall picture of the deformed state of piles.
Reducing the deformation properties of saline soils significantly affects the horizontal movement of piles.The effect of changes in the strength and deformation of the pile material is less significant in this case.In most cases, in low-moisture saline soils, the maximum loadbearing capacity of the pile occurs more often on the material of the trunk than on the ground.In this case, the flexibility of short driven piles is more pronounced, and plastic deformations occur more intensively.
We have studied different forms of the stiffness coefficient distribution adopted depending on the applied forces.The results of the analysis allowed us to recommend the values of stiffness coefficients for saline soilsdepending on the geometric characteristics of piles and the deformation properties of the soil Changes in the deformed state of the pile and the distribution of forces significantly depend on the nature and magnitudeof the applied forces.As the load increases, the degree of influence of changesin the stiffness coefficient on the calculation results increases.In general, the limit of their movement of piles is up to 10 mm.An error in calculating the stiffness coefficient of up to 30% does not significantly affect the calculation results.Usually, the calculated load-bearing capacity of piles is set at these displacement limits.
In addition, corrosion processes in concrete under the influence of aggressive media significantlyaffect the operation of horizontally loaded piles.At the same time, the effect of the depth of corrosion damage up to 10 mm on the operation of piles is insignificant.Further increase significantly affects the load-bearing capacity of piles on the material of the trunk and the ground.However, such corrosion processes can occur in highly aggressive environments (heavy construction conditions); in other conditions, concrete corrosion proceeds much more slowly.Based on etoro, in difficult construction conditions, the loadbearing capacity of the pile is set taking into account the possible depth of concrete corrosion duringthe estimated period of operation of buildings.In the case of piles with anti-corrosion protective coatings, the effect of corrosion damage to concrete is insignificant.

Conclusions
Taking into account the shift of the pile material during bending, changesin the strength and deformation characteristics of soils due to their desalination allows us to most realistically assess the natureof deforming short driven piles.
Changes in the deformationpropertiesof soils dueto grinding and desalination qualitatively and quantitatively change the nature of the deformed state of the pile foundation.
The movement of the upper end of the pile increases,распреbending moments, rebound and cutting forces are redistributed.
forces along the length of the pile shaft.
КоCorrosion processes in concrete under the influenceof aggressive media significantly affect the operation ofhorizontallyннloaded piles.With an increase in the depth of corrosion, the loss of load-bearing capacity of piles occurs on the material of the trunk more than on the ground.Corrosion damage to concrete in the range of up to 10 mm does not significantly affect the operation of piles.The developed method for calculating short piles, taking into account the predicted depth of corrosion, is used for soil conditions of medium and heavy salinity.In the case of piles made of sulphate-resistantconcrete or in the presence of anticorrosive impregnation, the influence of the aggressiveenvironment in relation to concrete is not taken into account.
The proposed calculation method fairly closely reflects the actual operation of short driven piles in saline soils.The convergence of the calculation results with experimental data in the case of soaking of the bases and taking into account the shift of the pile shaft material is up to ±16%.

Fig. 2 .
Fig. 2. (a) Modulus of soil deformation over depth in the natural (1), wet (2) and desalinated (3)state of saline soil, (c) the nature of the distribution of the stiffness coefficient over depth depending on the steps of the applied forces (for example, Central Ferghana).

Table 1 .
Assessment of construction conditions

Table 2 .
Correction factor g, which takes into account the influence of lateral soil pressure in the near-pile massif on the corrosion damage of concrete

Table 3 .
Correction factor M, taking into account the water-cement ratio of concrete