Raft/Mat question from a non-geotech engineer

[LearnerN]

Hello,

I'm not a geotechnical engineer, but am a civil engineer working in mechanical engineering. But I'm planning to take the geotech civil PE exam, and I only had two geotech classes in college, so I'm having to relearn some things.

One of the sample problems is about a mat/raft foundation for an 80'x40' building, and the raft is 8' below ground. Could someone find an image online and paste a link here so I can visualize something like this? Would the mat be 8' deep and then have the columns coming up from it to support the building??

Thank you.

 

[Ron]

nwink....one purpose of a mat foundation is to decrease the concentration of load from the building columns and allow lower magnitude loads to be transmitted to deeper soil layers for reduction of settlement.

For your example, the mat is placed at a depth of 8 feet. The mat is not 8 feet thick, but is as thick as necessary by structural analysis (usually in the 2' to 4' range) to cause the column and wall loads from the building to act on the soil as a unit, not as individual loads.

Placing the mat at 8 feet below grade allows a greater bearing capacity of the soil (overburden increases the bearing capacity) and allows the structural engineer to use a potentially larger coefficient of subgrade reaction ("k") for his design.

Removing 8 feet of soil and placing the building back on it at a lower depth might even give a net reduction in load on the soil, but will at least reduce the load from what it would be if placed at ground level.

 

[hokie66]

Another possibility to Ron's, and one that is common, is for the mat to be at or near a basement level. It may be a carparking area under an office or residential building.

 

[LearnerN]

Ron,

Thank you for that detailed explanation. One thing I'm uncertain about what you said, though, is: "placing the mat at 8 feet below grade allows a greater bearing capacity of the soil (overburden increases the bearing capacity)." Do you mean the mat would be constructed at 8 feet below ground, and then fill would be placed over the mat up to the grade elevation (around the columns, of course)? I'm just trying to understand how your example would have any overburden if there wasn't any soil put back over the mat...like if there was parking under the office, like hokie66 suggested. Thank you!