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Главная Переработка нефти и газа CHAPTER IX , , Bases and foundations on permafrost soils whose frozen state is retained or intensified The researsch in frozen soil mechanics, dealt with in the preceding chapter necessitated to substantiate methods of construction on permafrost soils, finds* extensive use in designing bases and foundation. Firstly, on the basis of concrete data on the mechanical properties of the frozen soils and the geocryological conditions of the territory being built up, as welll as the temperature conditions of the buildings and structures to be built the principle of consruction is chosen. This may be either «construction with the use of permafrost soils in the frozen state» or «construction with the use of such soils-in the thawing and thawed states». The present chapter deals with the calculation and design of foundations according to the first of these principles. Proposed by the author as far back as 1928 and fully verified in practice, the most expedient method of conserving the frozen state of the base soils in the great majority of cases (the only exceptions being small buildings in subartic regions) is the provision of cellars that can be ventilated either in the winter or all year round. A method (developed by the author) is given for determining the area of the ventilation openings of the cellars required to keep the permafrost soils in the base in the frozen state. A more exact solution of the same problem, proposed by Dr. G. V. Porkhaev is also given. Measures are discussed for intensifying the frozen state of the base soils. Up-to-date methods are set forth in detail for calculating the bearing capacity and strength of foundations for structures built on permafrost soils. These methods are based on relationships previously proposed by the author and only modified somewhat at the present time. The last section of the chapter is devoted to a procedure for checking foundations for heaving (also based on a method proposed by the author). This procedure takes into account the most up-to-date data on the magnitude of the heaving forces and the shear strength of frozen soils along the freezing surface. The chapter ends with practical examples of the design and building of structures according; to the principle of conservation of the frozen state of the soil bases. CHAPTER X Bases and foundations on thawing soils Structures can be built on thowing soils only under the condition that the design settlements of the foundations upon the thawing of the permafrost soils do-not exceed the limiting permissible values. The pressure exerted by the structure on the thawing soils should also be within the design value. The latter may betaken equal to the initial critical pressure on the soil at the design parameters of shear strength corresponding to the unconsolidated and undrained state of the thawing soils, i. e. at the natural density and moisture content that the soil has in thawing. Moreover, the value of the adopted design strength should be a certain fraction (approximately 0,5 to 0,7) of the limiting load on the soil which is also determined (according to the formula of Prandtl, Berezantsev or others) for the unconsolidated and undrained state. The design of foundations built on thawing soils consists in determining the overall dimensions of the foundations and finally selecting their cross sections according to the limiting settlements of the thawing bases. These settlements are determined for the design depth of thawing of the permafrost soils under the structure calculated by formulas, given in Chapter VIII which are presented in the tabulated and alignment chart form in this chapter. Also used to calculate the design depth are the known parameters of the equation for the settlement of thawing soils, these parameters being establiched on the basis of the data of geocryological engineering research. The variation in the settlement with time for coarse-skeleton and other water permeable soils depends only on the thawing depth. For clayey soils it is also necessary to take into account the degree of consolidation of such soils in thawing and their further compaction after being completely thawed. This is described by a relationship in the given section. To illustrate the above-mentioned, a numerical example of the determination of foundation settlements on thawing soils is discussed in detail. CHAPTER XI Preconstruction thawing and features of earth work in permafrost soils Conditions are established, at the beginning of this chapter, under which preconstruction thawing of permafrost soils can be efficiently employed. In assigning a thawing depth, it is chosen so that the average settlement and the difference in settlements of the thawing base are less than the limiting permissible values. Also discussed are methods of carrying out preconstruction thawing and methods for compaction and stabilization of the thawing bases. These methods are applicable when the soils have an insufficiently high bearing capacity or impermissible deformability. The end of this chapter is devoted mainly to a generalization of industrial -experience and special research on the methods of carrying out earth work in building various kinds of structures on permafrost soils. Especial attention is given to the earth work performed in building dams of local materials under conditions of permafrost soils. CONTENTS PART ONE RESEARCH AND THEORY FOREWORD.......................... 3 INTRODUCTION (Difficulties in the construction of structures or» permafrost soils; principal concepts and definitions; establishment and development of frozen soil mechanics in the USSR; basic principles)......... 5 Chapter I. Nature of frozen soils, their distinctive features and physical properties.......................22 § 1. Frozen soils as multicomponent and multiphase systems of interconnected particles ................22 § 2. Principal components of frozen soils...........23- § 3. Special features of the freezing of the pore water in soils 28 § 4. Characteristic regions in the phase transitions of water into ice, and the unfrozen water content in frozen soils .... 35- § 5. Dynamic equilibrium of unfrozen water and ice in frozen soils.............................3 § 6. Physical properties of frozen soils............ § 7. Types of ice in frozen and permafrost soils and their significance.........................51 Chapter 11. Migration of moisture in the freezing of soils, and the forces of frost heaving.....................58 § 1. State of the art..................... 58 § 2. Moisture distribution in frozen soils........... 61 § 3. Motive forces of migration................ 69- § 4. The migration principle................. 7 § 5. Prediction of moisture migration and frost heaving in freezing soils........................ 74 § 6. Forces of the frost heaving of soils in their action on the foundations of structures........• • ..... § 7. Methods of frost heaving prevention and control..... 99 Chapter 111. Rheological processes in frozen soils...........lOT § 1. The internal bonds in frozen soils............107 § 2. Initiation and development of rheological processes in frozen soils...........................Ill § 3. Rheological equations for the stressed-strained state of frozen soils.........................117" § 4. Stress relaxation in frozen soils, and their long-term strength . ......................124 § 5. Determining the creep parameters of frozen soils.....131 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 [ 144 ] 145 146 147 148 |
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