Excavation Below Existing Building

[VAD]

It is proposed to excavate below a community building to create basement areas. The building is about 30 years old and most of the foundation elements consist of grade beam on straight shaft bored cast in-place piles. There are also two existing basement locations under the building.One was created with the original construction and the other was done about 6 years after

The sizes of piles are 16 and 24 inch in diameter and the depth of embedment below ground varies from 22 to 27 feet. The shaft adhesion was backfigured to be 612 lbs/ft2. It is proposed (1) to excavate to grade beam and construct basement wall on footings or (2) to stay some distance away from piles say 4 ft and construct basement walls and footings. This would leave a strip of earth around piles.

Option 2 implies that despite the excavation and reduction of overburden stress caused by the excavation, not exposing the piles would allow them to maintain close to their original capacities. This concept suggests that despite the excavation there should be a distance away from the piles which an excavation can be done that does not influence the original capacity of the piles.

The design load of one of the 24 inch piles is 37 kips. The ground water is at 10 feet below ground and the soil is soft to firm silty clay with moisture contents from 35 to 42%. SPT in top 10 ft is 4. Pocket pen below 10 ft varies from 1 to 1.5 tons/ft2, maximum would be 2.

Questions:

1. How would you approach the determination of the capacity of existing piles before and after excavation re total or effective stress evaluation, Assume that end bearing is negligble and shaft adhesion governs. What capacity would you provide before and after.

2. Do you believe that leaving the strip of soil next to the piles, assuming the strip can stand, maintain the original allowable capacity (FOS 2) which is assumed as 37 kips for the pile in question.

3. Would you be comfortable excavating under this building or would you suggest underpinning.

4. If you believe in 2 re leaving strip of soil, how wide should that strip be and reasons.

Previous geotechnical report is unavailable. Recent geo information suggests the shaft resistance of 612lbs/ft2(unfactored/ultimate/without FOS etc) is reasonable. It is assumed that the piles reached their prescribed length.

 

[JAE]

I'm sure there will be some geotechnical replies, but here's my structural thoughts:

1. No my area so no comment here.

2. A strip of earth next to a pile - seems like it really doesn't provide any good capacity in skin friction after excavation - again - I'm not a geotech, but that seems true to me...at least I'd be concerned about it.

3. Keep in mind that drilled piers like this do not have adequate lateral ties to develop a "column" capacity. So beyond the soil capacity of the pier, there will be a serious problem with axial capacity of the remaining concrete section. Most pier ties are roughly #3 ties or hooping at 12" to 18" centers. The 16" diameter may not have anything. So essentially, some investigation will be required, beyond the geotechnical, into the structural capacity of the piling once excavation has taken place.

This can still be excavated, just be sure to consider the effects of the digging. Piers can be banded with steel straps or carbon fiber sheets to deal with the tie issue.

 

[DRC1]

1.After 30 years I would assume most of the drainage will have occured, so you could use an effective stress analysis.(A simple odemeter test would confirm this). A 16 or 24 inch pile is a big pile. even in softer ground it will have a significant bearing value.I think you may be shortchanging yourself not to include the bearing.
2. Leaving a strip next to the footing will nicrease the capacity of the piles over removing it, but since you are lowering the overburden stresswith the excvation, the capacity should be lower than prior to excavation. However since the material was low blow count (4) to begin with, the contribution of the upper portion may not have been as great anyway.
3. If the building is pile supported it should be able to be undermined without damage. assuming you decide the piles have sufficent capacity. I would not worry about the building. I would worry about retaining the soil around the excavation. Anything over 4 ft should be shored. You could do this with steel sheeting. If you dont exceed 12 feet I think you could cantilever, but you would need to check.
$ This is hard to say. You will have to run some numbers and see.

Good Luck - let us know what happens.

 

[JAE]

DRC1 brings up a good point, as you excavate, you could develop some serious lateral earth forces on the perimeter piling and these probably don't have the capacity to resist. take care.

 

[GeoPaveTraffic]

Interesting question(s).

1. I would figure the pile capacity using total strength measurements as they exist today, i.e. at the time of excavation. If I understand the provided information, that would be 1.5 to 2 tsf below 10 feet and less above. I would check the final design using my best estimation of effective stress.

2. The only effect of leaving the strip of soil around the piles is to transfer load from the piles to the newly constructed basement wall. If you leave a 4 ft. strip and construct a 10 foot basement, a significant amount of the pile load will be transferred to the new wall. The exact amount would depend on the relative stiffness of the new wall, the stiffness of the 4 ft. soil strip, and the stiffness of the soil near the piles below the new wall.

3. I would design the new basement walls to act as underpinning for the existing structure and to serve as a total replacement for the existing piles.

4. If you wanted to leave the strip and not transfer load to the new walls, the strip would need to be wide enough to prevent load transfer. Again, depending on stiffness, I would assume a 45 degree distribution as a start.

With respect to the question about the shaft resistance, I would assume an allowable adhesion of about 375 psf assuming that the piles were bored without drilling mud.

 

[VAD]

Interesting replies from all.

Geopavetraffic good response. Your adhesion value of 375 psf is in the ball park. A value of 318 was backfigured as well as 375. Today we will be looking at underpinning the existing piles with helical screw piles and using the same approach to create a retaining wall to retain excavation inorder to construct basement wall under existing grade beams where applicable. Some work may have to be done from inside of the building to initiate the creation of the retaining wall.

DCRI: Regarding end bearing, this would be small and the question lies in relation to movements to mobilize end bearing and how much. This is stll an isssue in the geocommunity. Your lateral load comments are sound.

JAE: Your response on the strip of soil is the view of othres as well. This is debatable in my opinion. I think that theory and practice depart on some of these issues. Just an opinion.

In all fairness there are issues such as piles without grade beams that take roof load etc, space constraints etc. All issues cannot be outlined. One has to review the drawings.

However, I must commend all for their answers. Good thoughts. Shows as we know, that this forum brings good calibre to the fore.

I will let you know on the progress.

 

[dmoler]

I would determine the horizontal spread of the load by zone of influence similar to footings. There was an old book by Grillo with influence charts (one chart is in the NAVFAC 7.01, page 183), which I have used for sand, but dont have the book anymore to see if there is one for clay.

I would assume the loss of stress outside of the excavated area is transferred to the clay surrounding the pile, thereby overstressing the remaining strip by the difference. Then, I would calculate the capacity of the clay/pile from there. An influence chart would be helpful for clay, and I used to have a better reference for that, so I will see if I can find it.