dirtandrock
Geotechnical
I’ve got a general foundation question. I’m trying to figure out how to best list the maxiumum allowable bearing capacity for a single type of soil, but for multiple types and sizes of foundations.
In general I can say that the foundations will be on a very dense and consolidated till (phi = 35, insitu density = 23 kN/m3 [145 tsf]) which overlies bedrock.
The depth to bedrock from the foundation base will vary between 3-8 m (10 – 30 ft) generally.
The items under consideration with some guessed design sizes are:
1. At-grade conveyor on 0.5 x 0.5 m (1.5’ x 1.5’) spread footings 3 m (10’) off-center
2. Elevated conveyor on 1.0 x 1.0 m (3’ x 3’) spread footings 6 m (20’) off-center
3. A 1.5 m (5’) thick mat foundation which is 10 x 10 m (30’ x 30’) and supports a vibrating crusher load.
4. A 1.5 m wide (10 m length) strip footing (5’ x 30’ length) which supports a pre-engineered building
Additionally there are other miscellaneous buildings mat foundations and spread or strip footings which will likely need to be covered conservatively by the geotech recommendations report.
Now if you take Terzaghi’s bearing capacity equation it and ignore cohesion and embedment it relies heavily on the bearing width, since the other factor (N-y) is defined by the foundation soil internal friction angle. So for the at-grade conveyor we find a maximum ultimate bearing capacity of 100 kPa, which is inadequate and for the design bearing load BUT for the same soil and the mat foundation we find a q-ult of 2000 kPa.
Questions:
1. How should I convey a maximum allowable bearing capacity and minimum width without having a tiny bearing capacity? (eg. if I define in my report as the dense till having a allowable bearing capacity of 100 kPa for a minimum width of 0.5 m, this will likely be useless for most designs of a strip footing that supports a large pre-eng building).
2. Is there a better formula for small footings, such as a small conveyor footing/sleeper described above? Meyerhof requires spt n-values and design tolerable settlement which I don’t have for the future structures.
3. In cases were the bedrock depth is shallow and within the zone of influence the Terzaghi assumption of an infinite depth medium doesn’t apply and the bearing capacity should be approaching that of the bedrock. Is there a good reference on how to approximate the weighted bearing capacity?
Final note:
I no doubt expect some to say that I shouldn’t be providing bearing capacities for unknown structures, but unfortunately this is hardly ever the case in reality. The structural engineer needs a bearing capacity (among other parameters) to provide a final design.
In general I can say that the foundations will be on a very dense and consolidated till (phi = 35, insitu density = 23 kN/m3 [145 tsf]) which overlies bedrock.
The depth to bedrock from the foundation base will vary between 3-8 m (10 – 30 ft) generally.
The items under consideration with some guessed design sizes are:
1. At-grade conveyor on 0.5 x 0.5 m (1.5’ x 1.5’) spread footings 3 m (10’) off-center
2. Elevated conveyor on 1.0 x 1.0 m (3’ x 3’) spread footings 6 m (20’) off-center
3. A 1.5 m (5’) thick mat foundation which is 10 x 10 m (30’ x 30’) and supports a vibrating crusher load.
4. A 1.5 m wide (10 m length) strip footing (5’ x 30’ length) which supports a pre-engineered building
Additionally there are other miscellaneous buildings mat foundations and spread or strip footings which will likely need to be covered conservatively by the geotech recommendations report.
Now if you take Terzaghi’s bearing capacity equation it and ignore cohesion and embedment it relies heavily on the bearing width, since the other factor (N-y) is defined by the foundation soil internal friction angle. So for the at-grade conveyor we find a maximum ultimate bearing capacity of 100 kPa, which is inadequate and for the design bearing load BUT for the same soil and the mat foundation we find a q-ult of 2000 kPa.
Questions:
1. How should I convey a maximum allowable bearing capacity and minimum width without having a tiny bearing capacity? (eg. if I define in my report as the dense till having a allowable bearing capacity of 100 kPa for a minimum width of 0.5 m, this will likely be useless for most designs of a strip footing that supports a large pre-eng building).
2. Is there a better formula for small footings, such as a small conveyor footing/sleeper described above? Meyerhof requires spt n-values and design tolerable settlement which I don’t have for the future structures.
3. In cases were the bedrock depth is shallow and within the zone of influence the Terzaghi assumption of an infinite depth medium doesn’t apply and the bearing capacity should be approaching that of the bedrock. Is there a good reference on how to approximate the weighted bearing capacity?
Final note:
I no doubt expect some to say that I shouldn’t be providing bearing capacities for unknown structures, but unfortunately this is hardly ever the case in reality. The structural engineer needs a bearing capacity (among other parameters) to provide a final design.
