ptdgeo
Geotechnical
- Dec 13, 2007
- 20
I realize this topic has appeared and been discussed here many times, but for myself I would like to bring it up one last time to assure myself I’m thinking correctly when explaining this topic to fellow structural engineers that call regarding this question.
Most recently I had a site that was in a valley setting where the soils at the site were alluvial deposits composed of a sequence of fat clay, lean clay and sand over shale. The fat clay had formed a desiccated crust over the underlying lean clay, that was normally consolidated to just slightly overconsolidated, and the sand below was medium dense to dense.
The structures for the project consisted of a single story mechanical building and reinforced concrete sedimentation basin with 20’ tall walls and a concrete roof. The floor slab for the mechanical building needed to be a minimum of 5 feet above grade for flood protection.
Long story short, total settlements needed to be less than 1 inch and an increase in the vertical effective stress greater than 300 psf calculated more than an inch of settlement. The client did not want to use deep foundation or any type of ground modification technique because the “local” contractor wasn’t equipped to handle them.
We ended up giving recommendations for “contact” pressures for a mat foundation established at depths ranging from 5 to 10 feet below the existing ground surface with anticipated total settlements of 1 inch or less. I used the term “contact pressure” because the structural engineer and I could not agree on the meaning of “ net allowable bearing pressure”.
The structural’s argument was, that if bottom of the mat was at 10 feet below existing grade, then he could use the net allowable pressure I was giving and add the weight of the soil removed. His explanation, “you give a net allowable pressure of 1500 psf for a mat at 10 feet deep, so to size the mat I can use at least 2500 psf, by taking advantage of the soil removed.” My reply was, “No, not if you want to keep settlements to less than 1 inch.” So I was informed that I was not giving him a “net allowable pressure” because a net allowable pressure is that pressure in excess of the overburden. I agreed, with half of his statement, and explained myself by saying:
“A net allowable bearing pressure is that pressure in excess of the overburden and it does account for the depth of overburden but what that means is that this is the total change in effective vertical stress that the soil below the foundation can experience to keep settlements within serviceable limits. The 1500 psf is the 10 feet of soil (gamma total 120 pcf) plus the 300 psf increase in effective stress at that depth. So this pressure is what needs to be used to size foundations and should be the total of the structures dead load and live load divided by the given pressure.”
After a little more banter, back and forth, regarding the meaning and definition of bearing capacity versus settlement and that the strength of the soil was not the issue because from a bearing capacity standpoint the soil probably could support 2500 psf without a bearing failure but it was the soil’s compressibility characteristic that was governing, so we agreed to use the term “contact” pressure.
My question is, does that make sense? Even if I were to have called the term a net allowable pressure of 1500 psf, technically wouldn’t that have been correct? By the structural's interpretation the net allowable bearing pressrue should have been 300 psf., I didn't agree.
Sorry about the long message.
Most recently I had a site that was in a valley setting where the soils at the site were alluvial deposits composed of a sequence of fat clay, lean clay and sand over shale. The fat clay had formed a desiccated crust over the underlying lean clay, that was normally consolidated to just slightly overconsolidated, and the sand below was medium dense to dense.
The structures for the project consisted of a single story mechanical building and reinforced concrete sedimentation basin with 20’ tall walls and a concrete roof. The floor slab for the mechanical building needed to be a minimum of 5 feet above grade for flood protection.
Long story short, total settlements needed to be less than 1 inch and an increase in the vertical effective stress greater than 300 psf calculated more than an inch of settlement. The client did not want to use deep foundation or any type of ground modification technique because the “local” contractor wasn’t equipped to handle them.
We ended up giving recommendations for “contact” pressures for a mat foundation established at depths ranging from 5 to 10 feet below the existing ground surface with anticipated total settlements of 1 inch or less. I used the term “contact pressure” because the structural engineer and I could not agree on the meaning of “ net allowable bearing pressure”.
The structural’s argument was, that if bottom of the mat was at 10 feet below existing grade, then he could use the net allowable pressure I was giving and add the weight of the soil removed. His explanation, “you give a net allowable pressure of 1500 psf for a mat at 10 feet deep, so to size the mat I can use at least 2500 psf, by taking advantage of the soil removed.” My reply was, “No, not if you want to keep settlements to less than 1 inch.” So I was informed that I was not giving him a “net allowable pressure” because a net allowable pressure is that pressure in excess of the overburden. I agreed, with half of his statement, and explained myself by saying:
“A net allowable bearing pressure is that pressure in excess of the overburden and it does account for the depth of overburden but what that means is that this is the total change in effective vertical stress that the soil below the foundation can experience to keep settlements within serviceable limits. The 1500 psf is the 10 feet of soil (gamma total 120 pcf) plus the 300 psf increase in effective stress at that depth. So this pressure is what needs to be used to size foundations and should be the total of the structures dead load and live load divided by the given pressure.”
After a little more banter, back and forth, regarding the meaning and definition of bearing capacity versus settlement and that the strength of the soil was not the issue because from a bearing capacity standpoint the soil probably could support 2500 psf without a bearing failure but it was the soil’s compressibility characteristic that was governing, so we agreed to use the term “contact” pressure.
My question is, does that make sense? Even if I were to have called the term a net allowable pressure of 1500 psf, technically wouldn’t that have been correct? By the structural's interpretation the net allowable bearing pressrue should have been 300 psf., I didn't agree.
Sorry about the long message.
