We are in the process of reviewing a large proposed multi-family residential development in our jurisdiction. This project is on a site where the bedrock is over 100 ft below grade and has a great potential for soil strength loss and liquefaction. In addition, the soils engineer is stating that the total settlement will be over 7in.

Since drilling piles/caissons is not feasible due to the depth to bedrock, the developers are proposing to still build on the site and not really address the total induced seismic settlement due to liquefaction (although the buildings could be rendered unsafe).

We are trying to address and gather information about:

1. Whether or not certain governing agencies have a “stated” threshold condition (i.e. inches of settlement) for the maximum amount of seismically induced settlement that may be allowed.

2. What is considered an acceptable differential and total settlement (i.e. 1 in. 30 ft differential settlement, 4 in. total settlement, etc.)?

I would appreciate if you could provide us with some guidelines either as a policy or standard care. The site is located near the beach in California.

Answer:

There is no limit for seismically induced soil settlement in the CBC or IBC. It may be constructive to consider practices for hospital buildings in California. OSHPD does not have an amendment to Chapter 18A of the CBC which sets any limits. For new hospital buildings, OSHPD generally limits total seismic settlement to approximately 1 in. and differential ½ in. in 30 ft. To mitigate liquefaction or excessive seismic settlement at a site, soil strengthening methods may be used for new hospital buildings and may consist of dynamic deep compaction on 100% of the building area or use of stone columns that are closely spaced (such as 4 ft on center each way). Such soil strengthening systems are not in the code and require approval under CBC/IBC Section 104.11.

Existing non-conforming hospital buildings (SPC-2) are permitted much larger total seismic settlements, on the order of 4 in. However such non-conforming hospital buildings would be removed from service by the year 2030.

A valuable reference for the effects of differential soil settlement on buildings is “Ground-Movement-Related Building Damage” by S. J. Boone, Journal of Geotechnical Engineering, ASCE, November 1996 and Discussion in the May 1998 Journal issue.