Bedding of Structures
Bedding of Structures
the rock masses that directly absorb the load of structures. Strains develop in the bedding because of the load from the structures. All types of rock can be bedding: solid rock (bedrock bedding) and loose rock (soil bedding). Bedding formed by rock under its natural, native conditions is called natural bedding; if the rock is compacted or reinforced, the bedding is called artificially reinforced bedding. The load absorbed by bedding is transmitted to it from the structures by the foundation structure. The proper choice of the type of bedding and foundation not only ensures durability and normal service conditions for a structure but also is of great economic importance. In modern construction, expenses for preparation of the bedding and foundation account for 15–20 percent of the total cost and 12–15 percent of the labor cost. Erection of the underground part of a structure requires 20–35 percent of the time required to build the entire structure.
In the USSR, advances in the design and construction of bedding have been achieved through substitution of the limit-state design for calculations based on permissible pressures (which do not completely take into account the interaction between a structure and its bedding), and also through standardization of the structural elements of foundations and the use of efficient work methods. The limit-state method for calculating beddings, which is an achievement of the Soviet school of soil mechanics and foundation construction, is based on the objective characteristics of soils, their bedding conditions, and the features of the planned structure. The use of this method provides better service qualities of the structures, full use of the supporting capacity of the soils in the bedding, and more efficient expenditure of materials.
For construction on soil bedding, two kinds of limiting conditions are considered—the supporting capacity of the bedding (restricting the load to limits that prevent failure of the bedding) and the strain in the bedding (restricting the strain in the structures above the foundation, when strains in the bedding occur, to limits that ensure the preservation of strength and normal service conditions of the structures).
Exhaustion of the supporting capacity of the bedding (loss of stability) is accompanied by the formation of slip surfaces in the soil for which the relationship between the normal stresses (σ) and the tangential stresses (τ) from the structural load and the dead load of the soil itself is expressed by Coulomb’s formula, τ = σ tan ɸ + c, where ɸ and c are soil parameters (the angle of internal friction and the cohesion) that characterize its shear strength under given load conditions. Experiments have proved the validity of Coulomb’s formula for most soils under pressures up to about 700 kilonewtons per sq m (KN/m2), or 7 kilograms-force per sq cm (kgf/cm2). For highly compressed soils, with a modulus of resilience E ≦ 5 MN/m2, or 50 kgf/cm2, the function τ = f/(σ) is a curve; in such cases, methods of nonlinear soil mechanics are used to solve problems of the stability of bedding.
The combined strains of the bedding and structure and their limit values may take the following forms: absolute settling of the foundation, average settling of the structure, relative unevenness of settling of adjacent foundations, tilting of a foundation or structure as a whole, relative sag of part of a structure, relative twisting of the structure, and horizontal shifts of a foundation or structure. Uneven strains in the bedding (flexure, twisting, and so on) can cause structural damage, whereas uniform settling and tilting of a structure affect only its service qualities. Construction standards and rules establish the limit values for each type of bedding strains for various structures.
The settling of beddings under specific foundations is determined, by appropriate design methods, as the settling of the centers of gravity of their bases. For beam foundations or foundations in the form of solid slabs, the problem of calculating structures on elastic (compressible) bedding is solved by assuming S(x, y) = W(x, y), where S(x, y) is the settling of the soil’s surface under the foundation at a point with coordinates x and y, which is in contact with the base of the foundation, and W(x, y) is the vertical displacement of the points on the base with the same coordinates. The solution of the problem is based on a system of two equations that describe the flexure of the structure and the settling of the bedding upon loading of its foundation. Simultaneous solution of the equations for flexure of the foundation beam or slab and the settling of the bedding is done by approximate methods. Electronic computers are widely used for this purpose. By using the iteration method (method of successive approximation) it is also possible to find solutions in the case of complex laws—including nonlinear laws—for the variation of the properties of the bedding soil (with respect to both depth and extent). Special problems in the calculation and design of bedding arise when the bedding is composed of permafrost soils, soils of high deformability (”weak” soils, such as silts, muddy soils, and peats), or soils that subside and expand when wet. The transmission of the load to the bedding from structures with pile foundations is also of a particular nature; this is taken into account in calculating the stability of the foundations. However, the standards for the limiting strains of the bases for this type of foundation are the same.
Solid rock is used as the bedding mainly in the construction of transportation structures (such as bridge piers) and hydraulic-engineering structures (bedding for dams). In this case allowances are made for the natural irregularity of the bedrock (the complicated orientation of stratified rock and the differing mechanical properties of the layers) and the presence of fissures and, in certain cases, of cavities. In building hydraulic-engineering structures it becomes necessary to prevent water from seeping into the bedding. This requires compacting and reinforcement of soil bedding or cementing of fissured rock.
REFERENCESFlorin, V. A. Osnovy mekhaniki gruntov, vols. 1–2. Leningrad-Moscow, 1959–61.
Terzaghi, K. Teoriia mekhaniki gruntov. (Translated from German.) Moscow, 1961.
Osnovaniia i fundamenty. Moscow, 1970.
Tsytovich, N. A. Mekhanika gruntov: Kratkii kurs, 2nd ed. Moscow, 1973.
N. A. TSYTOVICH and R. S. SHELIAPIN