Variable foundation depth

[grace h]

Hi,

We're digging foundations for a 9mx6m extension. We were aiming for 1m trench foundations, but the ground has been very variable, and in areas we have had to dig deeper to get to solid ground, whereas in others we are hitting solid rock at <1m.

We can cope with the additional concrete costs of just filling it all rather than a stepped foundation but I wondered whether we still need to use defined steps between levels in the base of the trench or whether a slope is acceptable?

We're waiting on building control this week on whether the depth is acceptable where we've hit rock but would be helpful to know if we should be forming some defined steps in the meantime.

Thanks!

Grace

[ProDave]

I have only ever seen trenches with flat bottoms and steps.

[Jeremy Harris]

In general, steps are preferred, as they don't translate vertical loads into horizontal slippage.  With a slope at the base of the trench foundation, when a vertical load is imposed the foundation will want to move sideways down the ramp formed at the base.

[bassanclan]

Needs to be flat at the bottom.

The building inspector won't sign it off otherwise

[grace h]

Thanks all.