MЕCHANICAL PROPERTIES OF REINFORCED SOIL CUSHIONS ON SOAKED SUBSIDING SOIL IN THE TERRITORY OF MONGOLIA

MЕCHANICAL PROPERTIES OF REINFORCED SOIL CUSHIONS ON SOAKED SUBSIDING SOIL IN THE TERRITORY OF MONGOLIA

Nyamdorj Setev

Doctor (Ph.D) Frofessor Mongolian State University of Scence and Technology,

Mongolia, Ulaanbaatar

Dashjamts Dalai

Doctor (Sc.D) Frofessor Mongolian State University of Scence and Technology,

Mongolia, Ulaanbaatar

ABSTRACT

Introduction: Non-genic loess deposits of the Quaternary age, widespread in Mongolia, are often used as the foundation of buildings and engineering structures, most of which are classified as loess-like subsidence soils, mainly of type I by subsidence. To improve their construction properties by the method of applying rational innovative solutions of reinforced soil cushions of shallow and relatively lightly loaded foundations, taking into account the regional peculiarities of the base soils, it is an important task of construction in the appropriate soil conditions in Mongolia.

Materials and methods: This article discusses the results of stamping (A = 2500 cm²) field tests to determine the deformation moduli of highly compacted soil cushions made of crushed stone-sand mixtures and local sandy loam soils with horizontal geosynthetic reinforcements from flat geo-shots and geotextile, simulated in 4 different combinations on a preliminary soaked subsidence bases. To compare the results obtained, 2 soil cushions were also prepared from a crushed stone-sand mixture and local sandy loam soils without reinforcement and stamp tests were carried out.

Results: The data of measurements of stamping tests were obtained and, on their basis, the numerical values of the modulus of deformation of reinforced and unreinforced soil cushions from different soil materials were determined, taking into account the moisture and density of the latter. Comparative analyzes of the numerical values of the stamping modulus of deformation depending on the variable factors of modeling soil cushions that have been carried out.

Conclusions: According to the test results, it was established that the numerical values of the modulus of deformation of soil cushions, depending on the moisture regime of the base soils, the type of soil materials of the cushion and geosynthetic materials for reinforcement, can be in wide ranges towards improvement.

Keywords: loess-type soils, technogenic soil wetting, bearing capacity, subsidence, geosynthetic materials for reinforcement, periodic consolidation, stamping test.

INTRODUCTION

In recent years, experimental and theoretical studies have been actively conducted in the world to improve traditional soil cushion solutions in the direction of using geosynthetic materials for horizontal and vertical reinforcement. Neogenic losse sediments of the Quaternary age, common in Mongolia, are often used as the foundation of buildings and engineering structures, most of which are classified as loess-like subsidence soils, mainly of type I in terms of subsidence. To improve their construction properties by the method of applying a rational innovative solution of a reinforced soil cushion for shallow, lightly loaded foundations, taking into account the regional features of the foundation soils, it is an important task for the construction of Mongolia [1;2;3]. Under normal soil conditions, the estimated cost of foundation and foundation works is 13-15% of the total estimated cost of the building, and for buildings built in difficult soil conditions, including on the subsiding soil of Mongolia, it reaches 30%.

Based on this provision, the issues of reducing the cost of material, technical and labor resources are an important task. To solve these problems, different methods of using pile and other foundations are used, as well as preparing an artificial foundation [4; 5]. Of these, the soil cushion method without reinforcement and with geosynthetic reinforcement in a horizontal and vertical position and from different soil materials. The use of a highly compacted soil pad can be a reliable simple method, from an economic point of view, the most acceptable method that does not require expensive imported machines and equipment for special construction work on the base and foundation [6; 7; 8].

MATERIALS AND METHODS

For a long time, mankind has used reinforced soil structures for the construction of various buildings and structures, the earliest of which are the upland buildings of the city of Dur-Kurigatsu and the Great Wall of China and others. Before our Era, the Romans used the method of soil reinforcement for the construction of the soil dam of the city of Rome [7;9]. Since the beginning of the 20th century, research work has begun in the United States on the use of the method of strengthening soil structures with reinforcement from various materials. In the 1960s, the French engineer Henri Vidal first proposed the use of metal tapes as soil reinforcement, and he is also the founder of the idea of ​​using polymeric materials to reinforce various soil elements and structures [10;11].

In the works of Russian scientists Abelev M.Yu., Dalmatov B.I., Konovalov P.A., Krutov V.I., Morareskula N.N., Mangushev R.A., Ponomarev A.B., Usmanov R.A. ., Tsytovich N.A. and scientists from other countries Brand H., Perrier H., Alexiew A., Girard H., Giroud J.P., Paul A., Koerner R.M., Schlosser F., Schwerdt S., Sobolewsk J., Uscimura T., Vidal H. and others reflect the results of experimental and theoretical studies on the method of calculations and modeling, the introduction of the design of reinforced and non-reinforced soil pads and other soil elements.

At present, in the conditions of Mongolia, research work on the introduction of reinforced soil structures has not been carried out due to the lack of a corresponding provision in BNbD 50.01-16 and other regulatory documents. In international practice, ground pads with geosynthetic reinforcement are widely used for the following purposes [12;13;14]:

- to exclude subsidence of the base during technogenic soaking and thawing of frozen soil;

- increase in the bearing capacity and stability of the base;

- reducing the depth of laying a lightly loaded foundation up to 2 times;

- reduction of the mutual influence of closely located foundations and redistribution of stress in the soil located below the base of the foundation;

- decrease in the value of the compressible thickness of the water-saturated underlying soil of the base;

- reducing the degree of heaving of the soaked clay soil;

- the possibility of using energy-saving insulated shallow foundations as a modern green technology.

Engineering-geological conditions of the experimental site. The experimental site is located on the territory of the Darkhan branch of the Mongolian State University of Science and Technology, where:

EGE-1. bulk soil with a thickness of 0.3 m;

EGE-2. subsidence silty sandy loam ),  type I in terms of subsidence, with a thickness of 0.3-6.8 m;

EGE-3. gravelly loam (a-p), with a thickness below 6.8m.

When drilling a well to a depth of 15.0 m, no groundwater was found. The general view of the experimental site is shown in Figure 5, The depth of seasonal freezing is -3.40 m, the physical and mechanical properties of the soils are given in Table 1.

Table. 1.

Physical and mechanical properties of soils

Photos of equipment and devices for testing to determine the modulus of deformation of reinforced and unreinforced soil pads from different soil materials are shown in Figure 1÷4.

Figure 1. Flat geogird

Figure 2.Geotextile

Figure 3. Soaking the pit soil

Figure 4. Stamp tests

Methodology for conducting a field stamp test. In the program of test work, the depth of the pillow sole was modeled at -1.50 m, the elevation of the foundation sole was -0.7 m, based on the idea of ​​designing shallow foundations with insulation in vertical and horizontal positions, taking into account the regional features of the climatic conditions of Mongolia, from the “Thermoplex” panel ".

The purpose of the size and design of soil pads and reinforcement from geosynthetic materials and without reinforcement are shown in Figures 4 and 5, taking into account the works of A.B. Ponomarev [15], R.A. Usmanov [16], D.G. Zolotozubov [17]. The test program for experimental soil pads was compiled on the basis of the methods of R.A. Mangushev [7], Rawal A., Shah T.H., Anand S.C. [10], Recommendation [11], V.I. Klevko [18], D.A. Tatyannikova [19], S.Schwerdt [20], Jones C.J., Lamont-Black J., Glendinning S. [21].

After excavation of 6 experimental pits, they were filled with pipeline clean water for 7 days to simulate a wetted weak subsidence base, the total amount of water for soaking each pit was about 10 m3. In each pit, where there will be reinforced pillows, on the border of the soaked soil of the base and the pillow, 1 layer of geofabric was laid out to avoid the separation phenomenon [22;23;24]. Design options for reinforced and non-reinforced soil pads are shown in Table-2.

In the process of preparing soil pads, they were compacted in layers at  with a thickness of 20 cm (Fig. 5), after compaction of each layer, the density ρ and W of the compacted soils were determined in the laboratory. SBR ratio, frontal resistance , adhesion force c, angle of internal friction φ according to the dynamic device for determining the compactibility of soil materials of the cushion “PORTABLE BEARING CAPACITY TESTER. MODEL MIS-244-062. MARUI CO., LTD. OSAKA (JAPAN)” (Fig. 6).  Calculation schemes of soil pads in Figures 7 and 8, photos of stamp tests in Figure 4.