Model of Soil Deposits - part 1 | part 2 | part 3
Dynamic properties
Effects like settlement, liquefaction and slips are caused by ground particular characteristics, such as the material type or the saturation degree [Seed, 1974].
However, in the case of Managua, the importance of these parameters decreases due to the proximity of the seismic activity [Shah et al., 1975], so the soil failure is not, generally, an important source of problems [Saint-Amand, 1973].
But the ground characteristics affect in a less evident way by exerting influence in the intensity of the ground movement and, therefore, in the structural damage, even if the ground under the structures remain perfectly stable during the earthquake [Seed, 1974]. The soil deposit operates as a filter for the incident wave, so that it incorporates to the signal the characteristic of resonance of the soil deposit itself [Sarria & Bernal, 19??]. Resonance between the structures and the ground is major cause of damages.
To determine the possible response of the ground during an earthquake, the dynamic properties of the different types of soils should be determined first.
The main parameters for the analysis of the dynamic response are shear modulus and damping, which are interrelated with the density, the velocity of the shear wave, the Poisson's ratio, etc.
When the soil response analysis is carried out, independently of the method used, the non-linear characteristics of the stress-strain relationships of the soils are necessary to be taken in account [Seed, 1974] [Seed et al., 1970].
The incorporation of the non linear behaviour of the soils is achieved using shear module and damping values compatible with the strains developed in the strata, in such a way that the characteristics of the materials vary with the intensity of the ground movements and the corresponding strains [Seed et al., 1970].
Several procedures exist, for both laboratory and field, to estimate the value of the shear module of the soils.
The field tests seek to assess the velocity of seismic waves. These consist on generating seismic waves, usually with explosives or by the impact of heavy loads on the ground. Some of those tests have been applied in Managua: the downhole method [Faccioli et al., 1973] [SOP, 1973], seismic refraction and, more recently the SASW [Ekholm & Norberg, 1998].
The shear module is then calculated from the equation:
G=ρVs2 ρ=γ/g
where G is the shear modulus, ρ is the density of mass and γ is the volumetric weight.
The laboratory methods, such as the cyclic triaxial test, the resonance column or the torsion pendulum, generally determine G from the analysis of a stress-strain unitary curve [Dowrick, 1995].
There are also indirect methods to estimate the shear module, by means of empiric relationships between G and the number of blows for each 30 cm in the standard penetration test (ASTM D-1586).
The relationship developed by Imai, Fumoto and their collaborators (Japan, 1975) was chosen to calculate the shear wave velocity, and then to calculate the shear module of the soils:
Vs=89,8N0,341
This equation has been used previously (Astacio & González, 1996; Escobar & Corea, 1998) even though the values obtained are presumed to be high compared with the real conditions of the ground in Managua (H. Taleno, pers. communic.).
The damping of the soil is its capacity to dissipate energy in the recurrent processes of load and unload that the
earthquake induces into the soils.
The internal damping, as well as the shear module, occurs mainly for hysteresis of the soil. This means that such parameters don't suffer significant changes if the loads are applied at different frequencies; they only depend on the strains.
The data published on damping relationships are scarce and deduced from tests in a short number of samples, or theoretical estimates, and no determinations have ever been made in situ. Therefore, the damping relationships may only be used as reference [Dowrick, 1995].
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