Expert: Arshad Khan - 4/20/2004

Question
Dear ARSHAD,

I am Adel and I am working on a project in which we have a reinforced concrete box structure which is going to be constructed below the ground surface. The bottom and roof level of the structure are 13m and 4m below the ground surface respectively...in a few words, it is a reinforced concrete box with the bottom slab and wall thickness of 60cm and roof slab thickness is 40 cm.there are also columns in between...the construction process is to excavate the ground,build the structure and then fill and compact the soil around it.To reduce the weight over the structure , light backfill will be used on the roof.the dimensions of the structure in the plan are 14m x 41m

My question is, according to the fact that the structure is well below the ground, is the earth quake force apllied to it? My judgement is that the structure will move with ground and there will not be relative movement so there will not be shear force applied to the structure..

What is your idea about it? could u please give me your opinion?

I really appreciate your concern about my problem

Thank you very much in advance

sincerely yours,

Adel Hesari

Answer
Hi Adel,

You are correct in principle, but it also depends on what type of soil is backfilled and what the groundwater table is in relation to the structure. What I am getting at is this: if the earthquake causes liquefaction of the soil, then you may have to design against the resulting horizontal pressures. Also, if there is any vertical acceleration (as is usual), then the roof slab (and floor slab if it is acting as a true culvert-cum-foundation) will have additional vertical loads.

If you are confident that the back-filled soil is compact and unlikely to liquefy, then all you need to worry about is the additional vertical loading on the roof and slab (usually approximated as about 30% of horizontal response, but it could be a lot higher- depends on your local hazard profile.

However, you must be aware that for passive resistance to mobilize, there has to be some movement in the walls, so it would be wise to cater for the stresses induced by such strains in the walls.


I hope this helps!

Kind regards,

Arshad