Granular Backfill at the Bottom of a Footing

[EngrJS1]

Hi,

Based on the geotechnical report, the clayey soil beneath the foundation level shall be replaced with 150mm thick compacted granular backfill. With that, I have a series of questions:
1. Will this backfill affect the soil's bearing capacity?
2.If yes, should I do any adjustments on the bearing capacity obtained from the boreholes? How should I do the adjustments?
3. Is it allowed if I simply design the footing based on the bearing capacity from the borehole logs without accounting for the effect of this backfill?

 

[BigH]

Are you a geotechnical engineer? Bearing "capacity" seldom rules. What is the consistency of the clay? The plasticity?
As to your questions
1. No. to any appreciable amount.
2. Are you basing the bearing on bearing capacity (shear) or allowable pressure (settlement)? In either case, the engineered fill will be a benefit - but I am surprised that it is only 150 mm instead of 300 or more.
3. Based on the borehole logs - what does the geotechnical engineer for the project say? He has established the allowable bearing pressure (or capacity if that rules) through its soils report - why are you trying to "outguess" the geotechnical engineer?

 

[HTURKAK]

A= Negligible effect.. practically NO!!


In general , 150mm thick 150mm thick compacted granular backfill is recommended by Geotechs for the reasons,

- To break the cappillary effect ,

- The excavation surface is disturbed with bucket ..The use of 150mm thick compacted granular backfill is the way for clayey soils . Trimming the disturbed surface beforee 150mm thick compacted granular backfill would be better.

 

[le99]

I think you have a competent subgrade that has the bearing capacity indicated in the soil report. The recommended 150mm gravel backfill is mainly to provide a good platform for compaction (if required), and ease of surface preparation for placing rebars and concrete. This backfill is too thin to influence the subgrade beneath.