Mat Foundation Bearing Capacity and Settlements Question

[whitejuv]

I have a question regarding common practice of mat foundation (B = 20m, L = 40m) design. I have four boring logs (one on each corner of the building). These boreholes each have different elevations (BH1 elevation is 160m, BH2 elevation is 163m, BH3 elevation is 160.5m, and BH4 elevation is 162.8m). So, if the mat foundation depth on the BH1 side is 5m, the depth on the side of BH2 will be 8m (and so on…).

I normally do the calculations for each borehole (use the strata thickness, density, cohesion, friction angle, and elastic/oedometric modulus), and get different bearing capacities and settlements for each borehole location.

Since the elevations of boreholes are different, bearing capacities and settlements are going to be quite different because of foundation depth (especially settlements – around 5 cm difference from BH to BH, which doesn't make sense since the soil strata are fairly equal in thickness).

My question is, am I treating the problem right? Should I have leveled the surface before doing the calculations (take the lowest borehole elevation as a base and subtract the difference from other boreholes)? This way, the foundation depth would have be the same on each borehole location.

what do you think?

 

[dik]

I would have started from a level surface.

Dik

 

[oldestguy]

I assume some sort of computer program is being used. Does it take into account the full depth of significant stress; down to where the added pressure from the mat is less than 10 percent of the earth gravity pressure? In any case, it will even out differential settlements so use the average conditions for one calculation.

 

[whitejuv]

Yes I am using a computer program for each calculation. Each borehole has a depth of 30, so I am calculating the settlements up to that depth (where i know the stratography). THe layers are mostly silty clay (CH and CL), however, the upper borehole has a thicker layer of sand (classified as SC) compared to the others. It still should not result in differential settlement since the Oedometric Modulus and Elastic Modulus are 11 MPa and 16.5 MPa for clay and sand respectively.

So, I guess I should definitely level the surface to get an accurate calculation. Am I right?

 

[Okiryu]

Talk to your civil designer and check the FFE of the building. This is important. You may be needing fill and your calculations are not capturing the weight of the fill. I always check the FFE first before done any calculations.

 

[EireChch]

What loading are you using for your calculations?

What is the level of the base of the mat?

If you are founding 5-8m deep you are removing a lot of load from the soil. Assuming 20kn/m3 * 5m depth - thats a reduction of load on the soil at foundation level of 100kPa. The net increase in load should be considered.

 

[MotorCity]

You need to know the bearing elevation of your mat. Typically, your geotechnical engineer will give you the allowable bearing pressure and expected settlement based on info you provide (loads, foundation type and size, bearing elevation, etc.) The allowable bearing pressure is based on the borehole with the least desirable soil properties at your elevation with an appropriate safety factor applied

(Based on the soil reports I receive, I think they typically use a safety factor of like 24 or something......just kidding)

 

[whitejuv]

Thank you for your answers! I meant I want to calculate the bearing capacities and settlements myself. In my case, we are talking about a mat at 8m depth.

I don't have a problem with bearing capacity calculation (i calculate it for all four boreholes and consider the most unfavorable condition. The question I have is calculating settlements. If i don't level the surface, each borehole calculation results in different settlements (which don't make very much sense to me).

I don't have much experience in the geotechnical world, and wanted to see what more experienced engineers do in this case.

@EireChch, I am not removing 5-8m from the soil profile. I only removed the difference in elevation between each borehole so I start with a level surface, and then perform calculations for a foundation depth of 8m.

Am i going the right way?

Thank you!

 

[EireChch]

Settlement is caused by an increase in effective stress. If you remove X amount of soil or overburden and then replace it with a foundation and load that is less than previous stress then settlements should be very minimal.

I don't think you will have a settlement issue considering you are founding at 8m depth. Will the load applied to the raft create a bearing pressure if more than the previous over burden.

If so then you could do a settlement calc for each borehole but you would consider the net increase at each borehole (i.e the raft bearing pressure - the overburden at that location ). That would give you a range of settlements. But being engineers you could just design the structure to accomodate the worst

 

[BigH]

My take - yes, your site varies in elevation but your slab on construction will not. What is the slab elevation compared to the ground surface. A metre here or there would not/should not be that critical to a mat foundation. I am surprised a bit that you do a "settlement" and a "bearing capacity" computation for each borehole! What if the bearing capacity is different on each one - are you going to assign an allowable bearing capacity at each borehole location? - No, you will not. Your allowable bearing pressure will be based on settlement; very very doubtful that a bearing capacity (or shear) will govern - very doubtful.

You have not indicated the type or nature of the soil that you have. Again, it quite doubtful that you will find any meaningful difference of settlement under the slab for settlement computations - sure, the removed thickness will be a bit more in a few locations - but overall, unless you are founding on poor soils - and it is doubtful you are or you might be using a piled raft - this will be what? a few millimetres??

You do realize that your boreholes do not go deep enough for a mat foundation of that size - unless of course, you have encountered bedrock or glacial till.

Just some thoughts to consider.