I will try to keep this as short as I can.
A few weeks ago I asked some questions and received valuable feedback regarding the consolidation of soft clay due to lowered water tables, and the possibility this may contribute to the numerous cases of residential foundation distress in a certain area of the Northeast. Since then I have met with the Town Building Commissioner to determine whether the Town had requested or obtained any data or information regarding water table fluctuations (none), consolidation test data (none), sensitivity (none), etc. What they do have from the design professionals here is limited to Atterberg limits, moisture content and expansive index test results, and no detailed analysis or concrete design computations.
With regard to the possibility that lowering the water table may have resulted in the observed foundation problems, I now believe that while this may be a contributing factor, I would expect to see a more widespread and uniform distribution of the problem, and not only in residential foundations. Also, I believe that lowering the water table does not necessarily result in a proportional increase in effective sress at depth, especially if an underlying confined water bearing zone is present. We just don't know at this time.
While in the Commissioners office I viewed a few photographs of foundation failures, many times consisting of diagonal cracking and a broken cantilever shape that appears to confirm what Robert168 rightfully pointed out, it is differential settlement that we seem to be experiencing here.
In March, 2003, the Town began requiring builders to place vertical reinforcing steel in accordance with the guidelines included in the State building code, plus 2 horizontal rows of 2-#4 bar in the wall and one horizontal row of 2-#4 bar in the footing. Since the requirement for horizontal steel is more stringent than the State Code, the Town required permission from the State Code Commission to enforce it. Earlier this week, the Town and the Building Association (who is fighting the default use of this more stringent requirement) argued their case in front of the Commission, and the Town lost.
Now, I have gotten to the point where I want to see more than one borehole at a building site to check for variability in the thickness and elevation of the soft clay, including "undisturbed" sampling for laboratory testing of the their compressive properties. I will also ask that a small pourous tube piezometer be placed in the completed borehole for continuing water table measurements. I have also spoken with four different architects and engineers who are preparing these home designs (some costing over $700,000 to build). I am told the design footing pressures (using the Town required minimum 24-inch wide footer) for one of these homes varies between 1700 to 1000 psf along the length of the footer. There is no attempt by the engineers I have spoken with to balance these pressures to help address the possibilty of differential settlement. In fact, these folks are specifying reinforcing steel only because they have to and without computing shear, tensile or bending stress etc. One engineer I spoke with does not have a copy of the ACI Code. I am somewhat taken back by the lack of engineering being practiced here.
My goal now, is to particiapte in developing, or finding an existing, methodology that begins with the collection of the proper amount and type of subsurface information, and incorporates this with site specific residential design data that will establish the proper (plain or reinforced) concrete foundation design. I am imagining a bracketing of the probable soil reactions given the initial design footing pressures, to develop shear and bending diagrams useful in fundamental reinforced concrete design.
I beleive this is not a simple task, but also cannot imagine this has not been done before. I am surprised the people I have sploken with have not done this type of thing, and am having some difficulty getting other local engineers on board with this idea. Am I on the wrong track? Any ideas?
A few weeks ago I asked some questions and received valuable feedback regarding the consolidation of soft clay due to lowered water tables, and the possibility this may contribute to the numerous cases of residential foundation distress in a certain area of the Northeast. Since then I have met with the Town Building Commissioner to determine whether the Town had requested or obtained any data or information regarding water table fluctuations (none), consolidation test data (none), sensitivity (none), etc. What they do have from the design professionals here is limited to Atterberg limits, moisture content and expansive index test results, and no detailed analysis or concrete design computations.
With regard to the possibility that lowering the water table may have resulted in the observed foundation problems, I now believe that while this may be a contributing factor, I would expect to see a more widespread and uniform distribution of the problem, and not only in residential foundations. Also, I believe that lowering the water table does not necessarily result in a proportional increase in effective sress at depth, especially if an underlying confined water bearing zone is present. We just don't know at this time.
While in the Commissioners office I viewed a few photographs of foundation failures, many times consisting of diagonal cracking and a broken cantilever shape that appears to confirm what Robert168 rightfully pointed out, it is differential settlement that we seem to be experiencing here.
In March, 2003, the Town began requiring builders to place vertical reinforcing steel in accordance with the guidelines included in the State building code, plus 2 horizontal rows of 2-#4 bar in the wall and one horizontal row of 2-#4 bar in the footing. Since the requirement for horizontal steel is more stringent than the State Code, the Town required permission from the State Code Commission to enforce it. Earlier this week, the Town and the Building Association (who is fighting the default use of this more stringent requirement) argued their case in front of the Commission, and the Town lost.
Now, I have gotten to the point where I want to see more than one borehole at a building site to check for variability in the thickness and elevation of the soft clay, including "undisturbed" sampling for laboratory testing of the their compressive properties. I will also ask that a small pourous tube piezometer be placed in the completed borehole for continuing water table measurements. I have also spoken with four different architects and engineers who are preparing these home designs (some costing over $700,000 to build). I am told the design footing pressures (using the Town required minimum 24-inch wide footer) for one of these homes varies between 1700 to 1000 psf along the length of the footer. There is no attempt by the engineers I have spoken with to balance these pressures to help address the possibilty of differential settlement. In fact, these folks are specifying reinforcing steel only because they have to and without computing shear, tensile or bending stress etc. One engineer I spoke with does not have a copy of the ACI Code. I am somewhat taken back by the lack of engineering being practiced here.
My goal now, is to particiapte in developing, or finding an existing, methodology that begins with the collection of the proper amount and type of subsurface information, and incorporates this with site specific residential design data that will establish the proper (plain or reinforced) concrete foundation design. I am imagining a bracketing of the probable soil reactions given the initial design footing pressures, to develop shear and bending diagrams useful in fundamental reinforced concrete design.
I beleive this is not a simple task, but also cannot imagine this has not been done before. I am surprised the people I have sploken with have not done this type of thing, and am having some difficulty getting other local engineers on board with this idea. Am I on the wrong track? Any ideas?
