Efficiency of trench barriers used to Protect structures from dynamic loads and studyof the Stress — strain State of soils based on strain hardeningand elastic models Текст научной статьи по специальности «Строительство и архитектура»

УДК 624.131.5

V.V. Orekhov, H. Negahdar

MGSU

EFFICIENCY OF TRENCH BARRIERS USED TO PROTECT STRUCTURES FROM DYNAMIC LOADS AND STUDY OF THE STRESS — STRAIN STATE OF SOILS BASED ON STRAIN HARDENING AND ELASTIC MODELS

Wave barriers are intended to mitigate vibration transmission in the soil. They include open and infilled trenches, sheet piles, etc. In this study, a two-dimensional finite difference element analysis was performed using FLAC-2D software as a research into the efficiency of open and infilled barriers exposed to dynamic loading with or without the presence of structures. In this contribution, two constitutive models are considered to study the soil response in the elastic and elastoplastic range with account for yield, failure and potential functions of soil plasticity. The results were assessed with account for reduced soil particle displacements on the ground surface exposed to impulse loading.

The numerical analysis has proven that the results obtained by assigning elastic properties to the soil material fail to comply with the results obtained by analyzing the model having non-linear properties of soils. The presence of a structure produces a significantly larger effect onto the efficiency of barriers by reducing the surface wave energy.

Key words: strain-hardening model, elastic model, wave barrier, attenuation curves.

Wave barriers are used to reduce ground-borne vibrations induced by different sources such as machine foundations, earthquakes, dynamic compaction, high speed trains, etc. Most of these vibrations propagate in the soil in the form of surface waves, and they can run to long distances. Geometry, location and composition of a wave barrier influence the isolation efficiency. In the literature covering ground-borne vibrations, D.D. Barkan (1962) was the first scientist who used screening as protection from vibration waves in case of open trenches. He reported that open trench dimensions were large enough relative to the wavelength of surface motions [1, 2]. In the three past decades, extensive research into the efficiency of surface wave screening in the soils [3—5] was completed by Russian scientists like V.K. Musaev and others.

The finite difference method is, perhaps, the oldest numerical technique employed to solve sets of differential equations, given the availability of initial and/ or boundary values. In this study, the 2D finite difference element model was developed using FLAC software package. The choice of the constitutive model may have a significant influence on the numerical results generated in the course of any analysis of geotechnical problems. Every constitutive model has its strengths and weaknesses.

Models simulating two soil behaviour patterns, including strain hardening (nonlinear behaviour) and elastic (linear behaviour) were analyzed to study the efficiency of open and infilled barriers exposed to dynamic loading with or without

© Orekhov V.V., NegahdarH., 2013

105

a nearby structure. Findings of standard tests were applied to develop general parameters of nonlinear constitutive materials. Properties of the concrete and structures were assumed as linear and in the elastic mode; therefore, only soil materials were assumed as nonlinear. Soil behaviour was simulated in a half-space. Dynamic properties of soil and other materials are specified in Table.

Material Properties: Building-Soil-Trench-Building System

Material Mass density (p), (kg/m3) Shear modulus (G), (E8-Pa) Bulk modulus (K), (E8-Pa) C(kPa) Ф (0)

Initial Residual Initial Residual

Half-space 1.865 0.288 0.625 1 2 20 30

Concrete 2.400 104 138.8

Structure (concrete)* 432 * 18.8* 25.2*

*derived value

In this study, the structure (10 meters wide and 15 meters high) was approximated to an equivalent rectangular shape. The structure was placed on the right side of a barrier at the ground level. The mat foundation of the structure was located 1.0 m below the ground surface. In this paper, the authors assume that the vertical impulse dynamic load of the triangular shape is induced onto the soil surface having a one meter diameter located on the left side of the barrier (exposure duration = 0.1 second). The maximal dynamic load value was P = 1.0 MN. All geometrical parameters of the model are provided in Figure 1.

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Most researches are focused on the effectiveness of barriers in absence of any nearby structures; researchers assign an elastic modulus to soils exposed to dynamic loads of sinusoidal or regular excitation. In this study, we also study the effect of (1) nearby structures and (2) the nonlinear behaviour of soils produced on the effectiveness of barriers exposed to impulse loads. 2D finite difference element models developed by the authors were studied by comparing the results of the analysis of elastic and strain-hardening moduli of the soil, namely, the attenuation amplitude of displacements on the ground surface (Ar) [6].

The structure was assumed to be located at the distance of 25 m from the right side of the trench, and the source of excitation was located at the distance of 8 m from the left side of the trench. Figures 1a, 1b, 1c, and 1d show the decay curve of the ground motion vs. the distance on a semi-logarithmic scale, in the event of exposure to the impulse load. The figures demonstrate that results of the analysis based on the soil model that has an elastic modulus are different from the results of the analysis based on the strain-hardening modulus, and the values of the former are higher than those of the latter. The authors demonstrate that the damping performed by the material has some influence onto the barrier efficiency. The reason for this phenomenon may consist in the fact that the ground motion is suppressed both geometrically and materially; therefore, the soil damping value is relatively high. Another source of discrepancy between linear and nonlinear results is that in the event of dynamic loading substantial strain can induce plastic areas in nonlinear materials, or local heterogeneity of the soil. Thus, the model simulating nonlinear properties can be reliably used to extrapolate the results and to implement an extensive parametric study into the behaviour of open and infilled trench barriers.

Three locations of the excitation system were chosen to study the influence of the distance to the source of disturbance onto the protective effectiveness of the isolation system: 3.0, 8.0, and 16.0 m from the left side of the trench.

Figures 2a, 2b, 2c and 3 a, 3b, 3 c illustrate the effect of the presence and absence of structures on the soil displacement in the event of exposure to the impulse load. These figures illustrate the maximal horizontal displacement of soil particles on the ground surface with account for the strain-hardening properties of the soil. The structure produces a significantly larger effect on the efficiency of barriers by reducing the surface wave energy and assigning the nonlinear modulus to the soil. As for the models having a structure, the efficiency of trench barriers is higher in case of the presence of a structure and a trench barrier. In this regard, it is noteworthy that if the structure is present, the longer the distance between the load source and the trench barrier, the higher the protective effectiveness of wave barriers due to the suppressed soil particle response.

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Fig. 2: Ground motion following impulse loading in case of presence and absence of a structure, in case of presence and absence of a trench. Trench depth = 10 m; W = 50 cm. a — trench-to-structure distance (L) is assumed as 3 m; b — trench-to-structure distance (L) is assumed as 8 m; c — trench-to-structure distance (L) is assumed as 16 m

Also, the figures also demonstrate a striking difference between the values of maximal horizontal displacements of soil particles of the model that has a structure and the one that has none.

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This study is aimed at provision of several general guidelines for the design of vibration isolation actions using trench barriers. In this study, the protective effectiveness of wave barriers was assessed using the criterion of reduction of the value of displacement of soil particles on the ground surface exposed to the impulse load. The findings can be summarized as follows:

(1) The results demonstrate higher values for soils having elastic properties than those having strain-hardening ones.

(2) Whenever a wave reaches the barrier, the results demonstrate a very steep decay in the nonlinear soil behaviour.

(3) Models having a structure demonstrate their higher efficiency irrespective of the presence of barriers.

(4) It has been identified that the longer the distance between the loading source and the trench, the higher the protective efficiency of a structure in case of its presence.

References

1. Barkan D.D. Dynamics of Bases and Foundations. McGraw-Hill Book Company Inc., 1962, pp. 374—406.

2. Al-Hussaini T.M. and Ahmad S. Numerical and Experimental Studies on Vibration Screening by Open and Infilled Trench Barrier. International Workshop on Wave Propagation, Moving Load and Vibration Reduction. Brookfield, 2000, pp. 241—250.

3. Musaev V.K., Kurantsov V.A. O razrabotke metodiki rascheta sooruzheniy neglubok-ogo zalozheniya pri vnutrennikh vzryvnykh volnovykh vozdeystviy [Development of Methodology of Analysis of Shallow Foundation Structures Exposed to Internal Explosive Wave Impacts]. Vestnik Rossiyskogo universiteta druzhby narodov. Seriya problemy kompleksnoy bezopasnosti. [News Bulletin of Russian Peoples' Friendship University. Comprehensive Safety Problems Series]. 2008, no. 1, pp. 75—76.

4. Musaev VK., Sazonov K.B. Chislennoe modelirovanie bezopasnosti sooruzheniy neglubokogo zalozheniya pri vneshnikh vzryvnykh vozdeystviyakh [Numerical Modeling of Safety of Shallow Foundation Structures Exposed to External Explosive Impacts]. Vestnik Rossiyskogo universiteta druzhby narodov. Seriya problemy kompleksnoy bezopasnosti. [News Bulletin of Russian Peoples' Friendship University. Comprehensive Safety Problems Series]. 2008, no. 3, pp. 6—13.

5. Musaev V.K. Komp'yuternoe modelirovanie bezopasnosti okruzhayushchey sredy s pomoshch'yu polostey pri vzryvnykh vozdeystviyakh v sooruzheniyakh neglubokogo zalozheniya [Computer Simulation of Environmental Safety Using Cavities in Case of Exposure of Shallow Foundation Structures to Explosions] Bezopasnost' i ekologiya tekhno-logicheskikh protsessov i proizvodstv. Materialy Vserossiyskoy nauchno-prakticheskoy konferentsii. Poselok Persianovskiy Rostovskoy oblasti. [Safety and Ecology of Manufacturing and Production Processes. Works of the All-Russian Scientific and Practical Conference. Persianovskiy Settlement, Rostov Region.] Donskoy gosudarstvennyy agrarnyy universitet [Don State University of Agriculture]. 2009, pp. 110—115.

6. Orekhov V.V., Negakhdar Kh. Nekotorye aspekty izucheniya primeneniya tran-sheynykh bar'erov dlya umen'sheniya energii poverkhnostnykh voln v grunte [Particular Aspects of Research into Application of Trench Barriers Aimed at Reduction of the Surface Wave Energy in the Soil]. Vestnik MGSU [Proceedings of Moscow State University of Civil Engineering]. 2013, no. 1, pp. 98—104.

Поступила в редакцию в феврале 2013 г.

About the authors: Orekhov Vyacheslav Valentinovich — Doctor of Technical Sciences, Professor, Department of Soil Mechanics, Beddings and Foundations, Moscow State University of Civil Engineering (MGSU), 26 Yaroslavskoe shosse, Moscow, 129337, Russian Federation; [email protected];

Negahdar Hassan — postgraduate student, Department of Soil Mechanics, Beddings and Foundations, Moscow State University of Civil Engineering (MGSU), 26 Yaroslavs-koe shosse, Moscow, 129337, Russian Federation; [email protected].

For citation: Orekhov VV, Negahdar H. Efficiency of Trench Barriers Used to Protect Structures from Dynamic Loads and Study of the Stress — Strain State of Soils Based on Strain Hardening and Elastic Models. Vestnik MGSU [Proceeding of Moscow State University of Civil Engineering]. 2013, no. 3, pp. 105—113.

В.В. Орехов, Х. Негахдар

ЭФФЕКТИВНОСТЬ БАРЬЕРОВ ПРИ ЗАЩИТЕ СООРУЖЕНИЙ ОТ ДИНАМИЧЕСКИХ ВОЗДЕЙСТВИЙ И ИЗУЧЕНИЕ НАПРЯЖЕННО-ДЕФОРМИРОВАННОГО СОСТОЯНИЯ ГРУНТОВ НА ОСНОВАНИИ МОДЕЛЕЙ МЕХАНИЧЕСКОГО УПРОЧНЕНИЯ И УПРУГОГО ДЕФОРМИРОВАНИЯ

Защитные барьеры предназначены для подавления волн, распространяющихся в грунте. Защитные барьеры представляют собой открытые и засыпные траншеи, шпунтовые сваи и пр. В данном исследовании проведен двумерный анализ элементов методом конечных разностей с помощью программного обеспечения FLAC-2D. Программное обеспечение использовалось для определения эффективности открытых и засыпных траншей при воздействии динамических нагрузок, как при наличии, так и в отсутствие сооружений. Рассмотрены две конструктивные модели упругой и упругопластичной реакции грунта. При оценке результатов учитывался параметр смещений поверхности грунта под воздействием динамической нагрузки.

Ключевые слова: модель механического упрочнения, модель упругого деформирования, волновой барьер, кривая затухания.

Библиографический список

1. Barkan D.D. Dynamics of Bases and Foundations. MacGraw-Hill Book Company Inc., 1962, pp. 374—406.

2. Al-Hussaini T.M. and Ahmad S. Numerical and Experimental Studies on Vibration Screening by Open and Infilled Trench Barrier. International Workshop on Wave Propagation, Moving Load and Vibration Reduction. Brookfield, 2000, pp. 241—250.

3. Мусаев В.К., Куранцов В.А. О разработке методики расчета сооружений неглубокого заложения при внутренних взрывных волновых воздействий // Вестник Российского университета дружбы народов. Серия проблемы комплексной безопасности. 2008. Вып. 1. С. 75—76.

4. Мусаев В.К., Сазонов К.Б. Численное моделирование безопасности сооружений неглубокого заложения при внешних взрывных воздействиях // Вестник Российского университета дружбы народов. Серия проблемы комплексной безопасности. 2008. № 3. С. 6—13.

5. Мусаев В.К. Компьютерное моделирование безопасности окружающей среды с помощью полостей при взрывных воздействиях в сооружениях неглубокого заложения // Безопасность и экология технологических процессов и производств : Материалы Всеросс. науч.-практ. конф. Поселок Персиановский Ростовской области : Донской государственный аграрный университет, 2009. С. 110—115.

6. Орехов В.В., Негахдар Х. Некоторые аспекты изучения применения траншейных барьеров для уменьшения энергии поверхностных волн в грунте // Вестник МГСУ. 2013. № 3. С. 98—104.

Об авторах: Орехов Вячеслав Валентинович — доктор технических наук, профессор кафедры механики грунтов оснований и фундаментов, ФГБОУ ВПО «Московский государственный строительный университет» (ФГБОУ ВПО «МГСУ»), 129337, Россия, г. Москва, Ярославское шоссе, д. 26; [email protected];

Негахдар Хассан — аспирант кафедры механики грунтов оснований и фундаментов, ФГБОУ ВПО «Московский государственный строительный университет» (ФГБОУ ВПО «МГСУ»), 129337, Россия, г Москва, Ярославское шоссе, д. 26; hassan_ [email protected].

Для цитирования: Orekhov V.V., Negahdar H. Efficiency of Trench Barriers Used to Protect Structures from Dynamic Loads and Study of the Stress — Strain State of Soils Based on Strain Hardening and Elastic Models // Вестник МГСУ. 2013. № 3. С. 105—113.