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Introduction The method of high-frequency vibration driving and extraction of sheet piles at modern construction sites of St. Petersburg is used quite often due to its high productivity, the possibility of using a simple set of technical means and the reuse of the piles themselves, which determines its economic benefits [1-6]. However, its use, especially in weak water-saturated rocks of St. Petersburg, entails the destructuring of the foundation soil and, as a result, the violation of stability of the pit fence itself and additional settlement of the surrounding buildings [7-12]. The upper part of the geological section of St. Petersburg is composed of weak water-saturated soils capable of turning into a quicksand state under external influences of various nature, including vibration, as well as restoring their strength in a state of rest. Dusty-clayey deposits of lacustrine and glacial genesis are covered with such properties [13]. The problems of reaction and behavior of soil under external dynamic load have been studied by many authors, the main of them are D.D. Barkan, S.L. Kramer, E.A. Voznesensky, A.Z. Ter-Martirosyan, A.Y. Mirny, V.V. Sidorov, V.V. Kapustin, V.A. Ershov [1, 6, 14-20] and others. The reaction of soils to the occurrence of dynamic stresses as a result of vibratory driving or extraction of sheet piles depends primarily on the type of soil, its structural connections and physical and mechanical properties [21, 22]. For example, sandy soils become compacted after exposure to dynamic load [14, 23-25], the reaction of water-saturated dusty-clayey soils is expressed in partial or complete loss of stability as a result of the development of such processes as dilatancy and liquefaction - quicksand and thixotropy [26]. Under dynamic action deformation and strength characteristics are significantly reduced. Previous studies on changes in the properties of soils are not enough to identify a clear dependence of their changes. Thus, the problem of assessing the applicability of the technology of high-frequency driving of sheet piles in the difficult geological conditions of the city is relevant. Experimental part The authors carried out experimental studies of changes in the strength characteristics of dusty-clay soils in laboratory conditions after vibration loads exposure. The first series of experiments is devoted to the study of changes in strength parameters depending on the time of vibration exposure, the second - on the time of "soil rest" and the third - on the consistency and type of soil. The first series of laboratory experiments was carried out in order to determine the regularities of changes in the strength parameters of clay soils depending on the time of exposure to a dynamic load commensurate with the immersion time of one sheet pile. The key parameters of the external dynamic stress are frequency, amplitude, and vibration acceleration. The frequency of high-frequency vibration immersion is in the range of up to 50 Hz, the value of vibration acceleration is normalized by Territorial Building Codes50-302-2004, Departmental Building Codes 490-87, Russian State Standard 52892-2007. The time of the dynamic load impact on the soil is not a normalized value and depends on the number and length of the pile to be driven at the construction site. The time of vibration immersion can reach 15-20 minutes in the geological conditions of Saint Petersburg. The experiments were carried out in a single-plane shear device on samples of loam of soft plastic consistency with a moisture content of about 35 % according to the "non-consolidated-undrained" (NU) testing scheme on a single-plane shear device at "normal" loading levels of 50 and 100 kPa. Testing according to NU scheme is called "fast shear". Under testing of poorly permeable water-saturated loamy rocks according to this scheme it is possible to obtain an angle of internal friction φ and adhesion in the unstabilized state. In the course of the experiment, the following time intervals of vibration impact on clay soils were considered: 5, 10 and 15 minutes, which generally corresponds to the time of immersion of a sheet pile in the geological conditions of Saint Petersburg. Thus, a total of 4 tests were carried out, in each of which two twin samples were tested, the first - without vibration action, the second after 5 minutes, 10 minutes and 15 minutes of dynamic action on a vibrating stage at a frequency of about 50 Hz, which simulates the impact of vibration immersion or vibration extraction of sheet piles on the soil of the pile space. The obtained graphs of shear resistance of soft-plastic loam before and after vibration action according to the "NU" scheme are shown in Fig. 1. Parameters of strength properties of loamy soil obtained before and after 5, 10 and 15 minutes of vibration exposure are given in Table 1. The results of the experiment show the reducing trend of internal friction angle at the increased time of dynamic load exposure, thus the value φ showed the reduction from 18 to 16, 13 and 9 degrees; i.e. the decrease ranged from 11 to 50 %. Whilr the amount of adhesion, which depends on the consistency of the soil, did not show any serious changes. Fig. 1. Results on in-single plane shear of loam of a soft-plastic consistency. Blue color - the soil was tested without the impact of dynamic load; orange color - the soil was tested after 5 minutes of exposure to dynamic load; gray color - the soil was tested after 10 minutes of exposure to dynamic load; red color - the soil was tested after 15 minutes of exposure to dynamic load Рис. 1. Результаты на одноплоскостной срез суглинка мягкопластичной консистенции. Синий цвет - грунт испытан без воздействия динамической нагрузки; оранжевый цвет - грунт испытан после 5 мин воздействия динамической нагрузки; серый цвет - грунт испытан после 10 мин воздействия динамической нагрузки; красный цвет - грунт испытан после 15 мин воздействия динамической нагрузки Table 1 Parameters of strength properties of loamy soil obtained before and after 5, 10 and 15 minutes of vibration exposure Таблица 1 Параметры прочностных свойств суглинистого грунта, полученные до и после 5, 10 и 15 минут воздействия вибрации Name Angle of internal friction j, degrees Reduction of internal friction angle, % Adhesion, kPa Vibration-free 18 0 2-7 Vibration exposure time - 5 minutes 16 11 2-7 Vibration exposure time - 10 minutes 13 27 2-7 Vibration exposure time - 15 minutes 9 50 2-7 The second series of experiments was also carried out according to the "NU" scheme in a single-plane shear device in order to determine the change in the strength parameters of the soil as a result of thixotropic reduction. In the course of the experiment 3 samples of undisturbed loam composition of soft-plastic consistency were tested in 2, 4 and 10 days of "rest" after 10 minutes of their dynamic exposure by vibrating table. The amplitude-frequency effect of the vibrating table on soil samples did not exceed 50 Hz, which is comparable to the effect of vibration immersion or vibration extraction of sheet piles on the soil of the pile space. According to different authors, the duration of thixotropic restoration and strengthening of structural bonds varies significantly for clay soils of different chemical and mineral composition, dispersion and humidity. So, if the soil is a purely thixotropic system, it will fully restore its strength to the initial level, regardless of the dynamic load parameters. In contrast to a quasi-thixotropic system, the final strength of such systems after dynamic action either slightly does not reach or slightly exceeds the initial value [26, 27]. For young loams and clays, the recovery process ends in the first day, for more compacted clays - in the first ten days. Also, in addition to the composition and condition of the soil, the restoration process is influenced by the impact parameters, which cause partial or complete destruction of structural bonds. The restoration of property parameters is of decisive importance in the problem of vibration extraction of sheet piles [28]. Vibration extraction with the purpose of sheet pile reuse can significantly reduce the cost of work, which in its turn determines the economic attractiveness of this method. Thus, as a result of thixotropic restoration of the foundation soil the parameters of strength properties obtain their initial values and this fact signifies the safety of further work on pile extraction. In other words, the immersiom and extraction of piles without any temporary intervals will lead to a catastrophic decrease in the angle of internal friction and adhesion, the transition of the soil to the state of heavy liquid. The obtained graphs of shear resistance of soft-plastic loam after 2, 4 and 10 days of "rest" according to the "UN" scheme are presented in Fig. 2. Fig. 2. Results of in-plane shear test of soft plastic loam. Orange color - after 2 days of "rest", blue color - after 4 days of "rest", gray color - after 10 days of "rest" Рис. 2. Результаты испытания суглинка мягкопластичного на одноплоскостной срез. Оранжевым цветом - после 2 сут «отдыха», синим цветом - после 4 сут «отдыха», серым цветом - после 10 сут «отдыха» The parameters of the strength properties of loamy soil of soft plastic consistency obtained after 2, 4 and 10 days of "rest" of samples of 10-minute dynamic impact by a vibrating table depending on the time of "rest" of the samples are presented in Table 2. Table 2 Parameters of strength properties of loamy soil after 2, 4 and 10 days of "rest" after dynamic impact Таблица 2 Параметры прочностных свойств суглинистого грунта после 2, 4 и 10 сут «отдыха» после динамического воздействия Angle of internal fiction φ, degrees Adhesion, kPa Time of "rest" - 2 days 11 2-5 Time of "rest" - 4 days 18 2-5 Time of "rest" - 10 days 25 2-5 The results showed that during the so-called "rest" the angle of internal friction restores the initial value after 4 days, and after 10 days of "rest" the value exceeds the initial one. The third series of tests was carried out by the in-plane shear device, also according to the "NU" scheme, in order to determine the dependence of changes in the parameters of strength properties after dynamic action in different types of soils. Samples of loam of various consistencies and dusty sandy loam were tested. Three samples were tested. The first is a sandy loam of undisturbed composition of plastic consistency. The second is the undisturbed loam of soft plastic consistency. The third is a sample of the disturbed composition of fluid-plastic loam. The test samples had a density of 1.78-1.83 g/cm3 and a humidity of 25 to 37 %. The tests were performed on 2 samples of plastic sandy loam at a load of 50/150 kPa and 100/150 kPa, on 2 samples of soft plastic loam and 2 samples of fluid plastic loam at a load of 50/100 kPa [29]. One sample was tested without prior vibration and the other was subjected to vibration for 10 min at a frequency greater than 50 Hz. In total, 2 samples of dusty plastic sandy loam, 1 sample of fluid plastic loam, and 2 samples of soft-plastic loam were tested [29]. Tests of sandy loam and fluid loam were carried out earlier and described in [29]. The strength parameters obtained before and after exposure to vibration are presented in Table 3. The results of soft-plastic loam testing on the in-plane shear according to the "UN" scheme are presented in Fig. 3. Fig. 3. The results of soft-plastic loam testing on the in-plane shear. Blue color - the soil was tested without the impact of dynamic load; gray and orange color - the soil was tested after 10 minutes of exposure to dynamic load Рис. 3. Результаты испытания суглинка мягкопластичного на одноплоскостной срез. Синий цвет - грунт испытан без воздействия динамической нагрузки; серый и оранжевый цвет - грунт испытан после 10 мин воздействия динамической нагрузки Table 3 Parameters of strength properties of dusty clayey soil before and after high-frequency impact Таблица 3 Параметры прочностных свойств пылевато-глинистого грунта до и после высокочастотного воздействия Type of soil Number of sample, time of vibration impact 10 minutes Angle of internal friction j before vibration impact, degrees Angle of internal friction j after vibration impact, degrees Reduction of internal friction angle, % Adhesion, kPa Plastic sandy loam 1 30 23 24 0 2 42 29 31 0 Cumuloplastic loam consistency 1 5 2 60 0 Loam of soft coplastic consistency 1 18 13 27 5-7 2 18 9 50 5-7 The parameters of strength properties of dusty-clayey soil obtained before and after exposure to vibration according to the "NU" scheme are presented in Table 3. Laboratory tests according to the "NU" scheme showed a tendency to reduce the angle of internal friction after exposure to vibration from 24 to 60 %, depending on the type and consistency of the rock, while the amount of adhesion, depending on the consistency, did not change. The data obtained by the authors coincide with the results of studies carried out by other researchers. For example, tests on loess-like loam under vibration effects at MGSU showed a change in strength by 8-17 % [30]. Interesting data were obtained at 2 sites in St. Petersburg, when soils "before" and "after" vibration immersion were subjected to a static sounding test [29]. The results of static sounding showed a decrease in the resistance to probe immersion by 30-70 % at a depth of 1.5-6 m from the day surface during the vibro-immersion of sheet piles and a decrease of 15-85 % during the vibro-extraction of sheet piles. The results of field and laboratory studies showed good convergence. Conclusion 1. Studies on the determination of strength parameters by the method of in-plane shear according to the "NU" scheme after dynamic action showed a decrease in the angle of internal friction in sandy loam by 30 %, in soft-plastic loam by 30-50 %, in fluid-plastic loam by 60 %. As it turned out, the amount of adhesion does not react to vibration. 2. Laboratory studies aimed at determining the strength parameters of soft plastic loam according to the time of exposure to vibration load from 5 to 15 minutes showed a decrease in the angle of internal friction from 11 to 50 %. The magnitude of the adhesion did not change significantly and was in the range of up to 7 kPa. 3. Studies on determination of the strength parameters of soft-plastic loam after 2, 4 and 10 days of "rest" showed an increase in the angle of internal friction to the initial value after 4 days, and after a 10-day "rest" the value exceeds the initial value, which indicates the repackaging of particles and the restoration of structural bonds. Thus, weak clay rocks after complete or partial destruction of thixotropic-coagulation structural bonds by dynamic load are able to increase strength to or even higher than the initial value. And this fact characterizes these soils as a quasi-thixotropic system. The process of restoring the structural ties of weak water-saturated dusty-clayey soils of St. Petersburg plays an important role in the process of extracting sheet piles for the purpose of their reuse. For example, the repeated application of a dynamic load during high-frequency vibration extraction can lead to a catastrophic decrease in parameters and the transition of the soil to the state of a heavy liquid as well as the loss of stability of buildings falling into the zone of influence.

About the authors

M. B. Zavodchikova

Saint Petersburg State University of Architecture and Civil Engineering

A. D. Cheremkhina

Saint Petersburg State University of Architecture and Civil Engineering

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