huangyhg

piled ground beam formwork
we normally specify on the drawings that the sides and bottom of the ground beams have a type a finish to bs8110,
thus some sort of formwork is required. typically permanent forms such are cordeks groundform system or similar would be used.
a client, after talking to a contractor, has come back with the question do we need to use formwork or can we just cast directly against the ground, which is stiff clay (no heave precautions necessary).
personally i dont like this as the concrete may escape into the surrounding ground (although unlikely as this is clay). also bits of clay may fall into the concrete contaminiating it. there may also be issues with cover to the rebar. what are you thoughts on this. do you allow ground beams to be cast directly against the ground and do you see any other potential problems in doing this.
thanks
mostly sand where i am but always formed sides with bottom cast against soil.
i wouldn't recommend it due to the contamination risk.
as above, no problems for the bottom form being deleted, but with extra concrete cover for extra tolerance.
we do it all the time in clays and silty clays. "neat earth forms" is the term used. trenched footings are what we call them.
here in the states, when we do this, it then requires 3" clear cover on any reinforcing bars due to irregularity of the earth.
if neat earth forms can't be achieved, then of course supplemental formwork is required.

we have done trenched footings; usually the footing/wall is combined and reinforcing is minimal. often dowels are placed and the top portion (foot or so) is formed. i am not familiar with some of your terminology, but when you say 'ground beams', i think of grade beams. these are usually heavily reinforced and this method (trench and pour) is more difficult, ie. how to support and maintain reinforcing location, and cover. in this latter case i agree with comments that bottom can be cast against earth and sides formed.
we cast against the bottom and sides all the time where i come from. as long as the soil stands up, is not moisture sensitive as to volume change, and the appropriate cover is used, there is no problem. weather permitting, of course. taking the reinforcement out to clean the excavation after a downpour is not fun.
we generally put 50mm blinding concrete in the base. if casting the sides against bare earth bs8110 calls for a minimum of 75mm cover to reinforcement.
if you are checking the reinforcement on site, you would need to keep an eye on the cover.

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