Stiffness modulus
The stiffness modulus E s (in Switzerland compression modulus M E ) is a material parameter for calculating the settlement of a soil in soil mechanics and of subsoil deformations. The stiffness modulus has about the same meaning for soils as the modulus of elasticity (E-modulus) for solid materials, but depends much more on the load range than the E-modulus.
The stiffness modulus is u. a. used in the constrained modulus method. It is used for illustration of the bottom in a floor model and the design of foundations .
detection
The stiffness modulus is determined from compression tests with the oedometer with handicapped lateral stretching. To determine the stiffness modulus, the pressure-settlement line of a soil (also called pressure-settlement diagram or load-settlement curve) must be known. The stiffness modulus is the gradient of the tangent to the pressure-settlement line and therefore the ratio of the pressure difference to the settlement difference between two load levels. The stiffness modulus, like pressure, therefore has the dimension of force per area. The pressure-settlement line is obtained in the oedometer test by showing the measured deformations as a function of the associated effective stresses .
The stiffness modulus depends on the stress range; therefore the pressure-settlement line is not linear. Therefore, an oedometer test must be carried out in several load levels. The pressure-settlement curve is assumed to be linear between the load levels. Since the compressive stresses increase with depth due to the soil's own weight , the stiffness modulus also increases approximately linearly with depth. To simplify matters, however, a mean stiffness modulus is often used.
When a floor is loaded for the first time, the stiffness modulus is still low, because the floor is initially partially irreversibly compressed during its consolidation , whereby the storage density increases. If the soil is relieved again and then loaded again, the stiffness modulus is significantly greater.
Differentiation from the bedding module
The stiffness modulus must not be confused with the bedding modulus , a system parameter that is defined as the ratio between the normal stress in the bed and the settlement. This module has the dimension force per area and per length. The bedding module is a parameter of the bedding module method with which the compliance of the subsoil is approximately taken into account in the static calculation and dimensioning of foundations and foundation slabs.