Increasing Foundation Capacity
Increasing Foundation Capacity
(OP)
I have a project that involves adding additional load to an existing foundation. The soil below the foundation is a medium dense, fine to medium sand with little silt. Groundwater is about 10 feet below the bottom of footing elevation.
The existing foundation is located in an area that is not accessible by large machinery, although it may be possible to get a skid steer (bobcat) size vehicle in there.
I am trying to evaluate possible ways of increasing the capacity of the foundation assuming it is suitable from a structural standpoint.
At this time I am considering the following:
1) minipiles
2) helical piers
3) compaction grouting
4) chemical grouting
I have a feeling that we may not be able to get the equipment to do minipiles or helical piers into the area. As for compaction grouting vs. chemical grouting, I have the following concerns:
1) Chemical grouting may be more appropriate for a temporary condition and not long-term support.
2) Compaction grouting may result in heave of the existing foundation.
3) In both cases you don't really get to see where the grout goes.
Am I completely missing an alternative? Are there any other concerns with the above alternatives?
I will patiently await your responses.
Thanks in advance for the input!
The existing foundation is located in an area that is not accessible by large machinery, although it may be possible to get a skid steer (bobcat) size vehicle in there.
I am trying to evaluate possible ways of increasing the capacity of the foundation assuming it is suitable from a structural standpoint.
At this time I am considering the following:
1) minipiles
2) helical piers
3) compaction grouting
4) chemical grouting
I have a feeling that we may not be able to get the equipment to do minipiles or helical piers into the area. As for compaction grouting vs. chemical grouting, I have the following concerns:
1) Chemical grouting may be more appropriate for a temporary condition and not long-term support.
2) Compaction grouting may result in heave of the existing foundation.
3) In both cases you don't really get to see where the grout goes.
Am I completely missing an alternative? Are there any other concerns with the above alternatives?
I will patiently await your responses.
Thanks in advance for the input!





RE: Increasing Foundation Capacity
RE: Increasing Foundation Capacity
RE: Increasing Foundation Capacity
I had the same idea about the previous settlement and the columns loads densifying the soil. Unfortunately, due to access restrictions, it is relatively hard to get good in situ soil data. I may be able to core through the floor adjacent to the existing foundation and do hand augers and portable DCP probes there. Not very accurate, but better than nothing.
RE: Increasing Foundation Capacity
RE: Increasing Foundation Capacity
RE: Increasing Foundation Capacity
Hang on, let me chip in too! Firstly, your question on grouting. Grouting should cautiously be used. You need proper monitoring both during and after grouting. Preferably, you may have to do trial tests to evaluate the correct grouting pressure. This is very very important. I have a case, where the SPT value decreased after the grouting; although it was not clear why; but it was estimated that the grouting pressure was probably high that it moved the dense sands and silts to less dense (remember, the grouting was conducted to increase the bearing capacity under the existing building so that additional floors could be added).
Coming back to your original question; have you considered trench underpinning? It is cheap, effective. The existing foundation can be lowered and widened by excavating below the foundation in stages (alternative trenches to be excavated), concreted. Something like mass concreting the trenches. Follow this step for the next section and so on.
Regarding the subject raised by BigH on the increase in densification and henceforth the bearing capacity due to the loads of the existing building. I would say yes. This would be expected in sands. But a big BUT. There is no standard theory which can be used to predict this behaviour. At least I have not heard of any. All theories are based on assumption that any footing is placed on undisturbed soil and miles away from anything.
Rgrd
RE: Increasing Foundation Capacity
RE: Increasing Foundation Capacity
RE: Increasing Foundation Capacity
The hospital is currently occupied. Due to non-structural concerns (mostly utilities), we can't disturb the area too much.
RE: Increasing Foundation Capacity
RE: Increasing Foundation Capacity
With site test and load calculation, it might be okay without doing anything. Foundations should have some capacity preservation. If we consider settlement is the main issue of allowable soil pressure and the foundation is well standing there for 50 years without much movement, then you can somehow anticipate what will happen with the addition load.
A thought about capacity increasing: how about increasing the bearing area with new concrete around the footing and dowels into the old concrete, at the same time increase the thickness of footing to 1’-6” or more (since there is a space between slab and footing). I hope this doesn’t sound weird.