6.1 Analysis of calculation results of actual site soil profile
This study employed the actual soil profiles established in Section 3.1 as the calculation model. The seismic wave A, scaled to a magnitude of 0.1 g via the El Centro wave, served as the input ground motion. It progressed to a comparative analysis between the maximum, minimum, and average values of the dynamic shear modulus ratio and damping ratio of soft soil in the Tianjin area from Table 1 and the measured values of silty clay in Table 3. Furthermore, a comprehensive comparative analysis was undertaken, encompassing aspects such as the shape of the response spectra and characteristic parameters of the design response spectrum. Ultimately, this article furnished recommended values for the dynamic shear modulus ratio and damping ratio for soft soil based on the analysis.
Table 6 Deviation between characteristic parameters obtained from different dynamic parameters and those obtained from measured values(%)
|
Average
|
Maximum
|
Minimum
|
Characteristic period
|
0.99
|
0.99
|
16.83
|
Platform values
|
6.98
|
7.30
|
14.61
|
Analysis of Fig. 2 and Table 6 reveals the following observations:
(1) The deviation in the characteristic period remained consistent at 0.99% when considering the maximum and average values of various dynamic parameters. However, the deviation markedly escalated to 16.83% when factoring in the minimum value. Consequently, the minimum value was disregarded for determining the characteristic period.
(2) Upon examination of platform values, it is evident that the deviation progressively increased from the smallest to the largest values when accounting for the average, maximum, and minimum values.
In summary, it is noteworthy that the response spectrum characteristic parameters derived from the minimum values significantly diverged from the actual measured values, whereas those derived from the average values exhibited the least deviation. Therefore, the average values were selected as the recommended values for the dynamic shear modulus ratio and damping ratio for soft soil in the Tianjin area in this study, as evidenced by Table 7.
Table 7
List of Recommended Values in this article
Parameter | Shear Strain(×10− 4) |
0.05 | 0.1 | 0.5 | 1 | 5 | 10 | 50 | 100 |
G/Gmax | 0.9957 | 0.9915 | 0.9591 | 0.9223 | 0.7146 | 0.5643 | 0.2204 | 0.1309 |
λ(%) | 3.47 | 4.30 | 7.02 | 8.63 | 13.18 | 15.19 | 18.65 | 19.48 |
6.2 Analysis of results for uniform single soil layer profiles based on varying thicknesses of soft surface soil
In this study, we utilized ideal soil layer profiles consisting of 10 different thicknesses of soft surface soil layers, ranging from 2 m to 20 m, as established in section 3.2, to serve as calculation models. The seismic response analysis of soil layers was conducted using the one-dimensional equivalent linearization software SOILQUAKE. For this analysis, seismic waves A and B, adjusted to intensities of 0.1 g and 0.2 g, respectively, following modulation by the El Centro waves, were employed as input ground motions to shed light on the influence of soft surface soil’s thickness on ground response spectra. Tables 4 and 5 list the parameters of soil density, shear wave velocity, dynamic shear modulus ratio, and damping ratio. The recommended values for the dynamic shear modulus ratio and damping ratio of soft soil were determined based on this study.
Analyzing Figs. 3 and 4, the following observations can be made:
(1) The thickness of the soft surface soil wielded a dramatic impact on the characteristic period. Under the same input ground motion, the characteristic period initially diminished with escalating layer thickness. Nevertheless, upon reaching an 8 m layer thickness, the characteristic period commenced to ascend with further increments in layer thickness. In conditions of identical layer thickness, the numerical value of the characteristic period rose with the heightened intensities of input ground motion.
(2) Regarding platform values, when subjected to the same input ground motion, the platform values initially increased temporarily with increasing layer thickness. However, once the layer thickness hit 6 m, a gradual descent trend emerged. Under the same layer thickness conditions, the platform values exhibited a more complex behavior. For larger layer thicknesses (16 m, 18 m, 20 m), the platform values of seismic wave B (0.2 g) were lower than those of seismic wave A (0.1 g). Conversely, for smaller layer thicknesses (2 m, 4 m, 6 m, 8 m, 10 m, 12 m, 14 m), the platform values of seismic wave B (0.2 g) and seismic wave A (0.1 g) alternated in an increasing trend.