Increasing Foundation Capacity

[eric1037]

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!

 

[BigH]

First, you might try to analyse the existing foundation to see if it will take the additional loading. If you are on sand, then the original bearing pressures were probably governed by settlement considerations. The existing pressures will have densified the sandy soils so that new loadings will experience less settlement than at the time of initial consideration (e.g., increasing modulus value due to the existing footing pressures). It may work - also, the actual pressures put on may not be "design" pressures - I have seen cases where the actual loadings are considerably less than the original geotechnical (or even as given on the design drawings) pressures permitted.

 

[JAE]

Many helical piles are installed with a bobcat system. You might check with local Chance or Atlas installers to see what kind of equipment they use.

 

[eric1037]

Thanks for the input. The building is relatively old. Some parts of it are 100 years old. I think this portion of the building is about 40 to 50 years old.

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.

 

[GeoPaveTraffic]

Helical anchors can be installed with hand equipment. Not as fast as using a skid steer, but you can do it anywhere.

 

[cbosy]

I would core the slab and advance a bucket auger and/or a portable geoprobe and a portable static cone penetrometer to obtain soil samples and density and allowable bearing pressure. We do this all the time in these Chicago warehouse to condo rehabs with great success. Normally the sand is dense enough where nothing is required.

 

[ashjun]

Hi,
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

 

[ashjun]

Sorry missed this on the case history; in addition to the high grouting pressure being the cause; it was noted that the moisture content of the soils had increased after grouting (water table was "miles" below the foundation).

 

[PEinc]

Ashjun could be correct. Underpinning may be a very good option. The soils sound OK for underpinning and the water table is 10 feet below the footing. Is this a wall footing, an interior column footing, an exterior column footing, or a pilaster type column with a widened footing? What size is the existing footing and what percent load increase are you expecting? If the building is 40 to 50 years old, there should be a concrete footing as opposed to a rubble stone or other masonry footing. Are you sure that the existing footing is not on piles? Not knowing more about the building makes it difficult to know the best method. Is the building occupied? Is it an office or a warehouse? Can you remove the floor and excavate without too much disruption of business? How deep are the top and bottom of footing below finished grade? All of these questions, and probably more, need to be answered in order to find the best solution to the problem.

 

[eric1037]

The foundations (there are 2 of them) are interior column footings in the basement of a hospital. The existing column footing in 4 feet by 4 feet in plan. The bottom of footing is 2'-6" below the top of slab. The footing is 1 foot thick. I am not sure about the load increase at this point.

The hospital is currently occupied. Due to non-structural concerns (mostly utilities), we can't disturb the area too much.

 

[Geocontractor]

This may be old news, but have you considered jet grouting below the footing to increase the baering capacity of the soils? The soil sounds like it may be ideal for this type of work. I have done it in the kitchen of an operating hospital and it has worked well.

 

[J1D]

eric1037
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.