huangyhg

groundwater issue
i have a client who needs to place concrete caissons 42" below grade (frost) for a new deck but he has groundwater in the bottom of the holes. what is the best thing for him to do? how would i determine an allowable soil bearing pressure?
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assuming the deck is lightly loaded, call a local geotech and he'll hopefully help you through a rough bearing capacity based on soil description. the water in the bottom can be taken care of by overexcavating the "mush" and replacing with gravel (usually no. 57 or no. 89 stone).
water in the excavation is not necessarily bad if the soil is not clay, silty clay or silt. concrete can be placed under water using tremie.
by all means, discuss with a local geotech.
in my area (alberta), we wouldn't consider anything as shallow as 42" because the frost can go much deeper than that, but i don't know whether or not that is pertinent to your site.
if you overexcavate and replace the "mush" with gravel, be absolutely sure you are below the depth of frost penetration or you will get heaving when the reservoir freezes.
perhaps drilled concrete friction piles or steel screw piles are easier alternatives.

ba

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