Geologic and Geotechnical Investigation for Seismic Design of Foundation

Dear reader we have already discussed about seismic site classification and seismic design criteria according to international building code. We know seismic design categories are category A,B,C,D,E and F. At first we will learn about investigations required for foundations that support structures assigned to seismic design categories C to F.

The structures that are assigned to the seismic design category C, D, E and F according to section 1613, IBC 2009, geotechnical investigation should include determination of seismic and geologic hazards as follows:

1. Stability of slope of foundation is located in relevant site condition.

2. Liquefaction potential; dear reader we have discussed many posts about liquefaction hazard, analysis of hazard and some mitigation against this natural phenomenon.

3. Differential settlement

4. Displacement in surface due to lateral spreading or faulting.

Relation between faulting and occurrence of earthquake, fault geometry and paleoseismology have already discussed in previous posts.

These requirements are for seismic design category C to F; but when the structure is assigned to the category D to F, that is D, E or F, some additional geotechnical investigations are required; that is for category C only above four investigations are enough. But for category D,E and F more information is required which include

-Lateral earth pressure

-Soil strength reduction

-Recommended mitigation measure

We need to include following additional information to design a foundation supporting structures assigned to these categories:

1. Lateral pressure on retaining wall or foundation walls as a consequence of earthquake motion. There have many record of failure of earth retaining work during excavation and foundation construction work.

2. Liquefaction potential and strength loss of corresponding soils for

–PGA

–Magnitude

–Source characteristics

This parameter must be consistent with design seismic ground motions.

In calculation of peak ground acceleration (PGA), soil amplification factors are derived based on site specific studies. (Reference ASCE7, chap-21). 

Alternatively PGA can be taken as S_DS /2.5, 

where S_DS=Maximum considered seismic spectral response accelerations (for short period, ref: 1613.5.4 IBC-2009).

3. With assessment of liquefaction potential, consequences of liquefaction and subsequent strength loss should be included. Which includes:

–Differential settlement

–Lateral movement

–Lateral loading on foundation

–Reduction in bearing capacity of foundation soil

–Increased lateral pressure on earth retaining wall

–Floating potential of buried foundation element or structures.

4. At last, how to mitigate the consequences of above phenomenon? This discussion of measure to mitigate should include at least –Ground stabilization –Selection of proper foundation depth and types as well –Selection of proper structural system; the objective is to accommodate expected forces and subsequent displacement –Requirement of combination above measures and appropriate way to account in designing foundation and structures.