The General parameters section contains parameters regarding the calculation of Cyclic Stress Ratio (CSR). CSR is calculated according to the following equation, which was first introduced by Seed and Idriss, in 1971:
CSR = 0.65 ? amax ? rd ? σv / σ'v
where:
| • | 0.65 : reduction factor converting the (single / one time) peak cyclic stress to the "equivalent uniform shear stress". |
| • | amax : the peak ground acceleration in units of g. |
| • | rd : stress reduction coefficient accounting for the flexibility of the soil profile. |
| • | σv : total vertical overburden stress. |
| • | σ'v : effective vertical overburden stress. |
Earthquake & Ground Water Table (G.W.T.) parameters
where:
| • | P.G.A.: Enter the peak ground acceleration for which you wish to perform the analysis. |
| • | Moment magnitude: Enter the earthquake moment magnitude (Mw is dimensionless). |
| • | G.W.T. during test: Enter the depth of the water table during the in-situ test as a positive number if below the ground surface level, and negative number if above the ground surface level. |
Magnitude scaling factor MSF
The CSR value calculated from the above relationship for the MW indicated, must be converted to a CSR for a duration typical of an "average" event of Mw = 7.5. This is done by using a factor named Magnitude Scaling Factor.
A variety of different approaches and functions for MSF value can be found in the literature. LiqIT allows you to use one of the three most widely used relationships. However, some methods require the use of a specific equation. In this case you will not be allowed to make any selection. You can visualize these functions by clicking on the chart icon.
Stress reduction factor rd
The stress reduction factor is used to take into consideration that the actual shear stresses induced at any depth are less than the theoretical "rigid body" shear stresses.
LiqIT allows you to use one of the three widely recommended relationships. However, some methods require the use of a specific equation. In this case you will not be allowed to make any selection. You can visualize these functions by clicking on the chart icon.
Effective overburden stress correction factor Kσ
The adjusted CSR for an earthquake moment magnitude of 7.5 should be further adjusted for the effect of non-linear increase of liquefaction resistance with increased effective initial overburden stress.
The numeric value of this correction factor is determined by the most widely recommended relationships. If you choose to make use of the Youd & Idriss relationship, the software offers a range of methods for determining the relative density (Dr) of the soil, which is needed for the calculation of Kσ. These options appear only if the in-situ test data is set to SPT. For the CPT test data the relative density of the soil is determined by the relationship provided by Jamiolkowski et al., (1985).
Other parameters
As liquefaction potential assessment methods are only applicable on saturated, non cohesive soils you may wish not to test data that lay above ground water table. Furthermore, it is well accepted in Geotechnical Engineering practise that liquefaction cannot occur on depths greater than 20 meters below free surface (all evidence until today prove the correctness of this assumption). You can instruct the software accordingly by checking or unchecking the appropriate checkboxes under the General parameters for liquefaction calculation title.
| • | Test data above G.W.T.: By default the software will not perform any test for data points above the ground water table. If you wish to test these points e.g. you believe that the soil above the G.W.T. is considered to be fully saturated then you can check this option. |
| • | Limit analysis depth at: Using this option you may prevent LiqIT from testing data below a certain depth value. |
Note The fully adjusted and normalized CSR can be further adjusted by multiplication with a user defined factor of safety. The larger the value of the factor of safety, the more conservative the results of the selected procedure will be. A value of 1.00 is highly recommended by the author of the software. |
Calculation for a reinforced system
Stone columns offer both rigidity and significantly increased resistance to liquefaction. They prevent liquefaction due to their capability to reduce the excess water pore pressure in the soil almost instantly.
LiqIT provides a method for computing the factor of safety against soil liquefaction for a reinforced system with stone columns (vibro replacement) utilizing the calculation method from Priebe. This method calculates a reduction factor for the CSR which seems to be adequate with regard to the favorable performance of vibro replacement in seismic events. More information can be found here.