Soil Excavated that was supposed to be driven onto by Piles

[X-Wing]

The title already said it all: This is a Bridge Project. Single Span. 30m. On RC Piles foundations.

The soil, about 5meters, that was not supposed to be excavated, and that depth, where the piles should be driven, was excavated. I don't know the reason yet.
Given that, they tried to onto the remaining layer, then they were able to drive about 1.5m, and they said it cannot be driven anymore, maybe because it is also refusal.
i've seen the geotech report, at that layer, it is already Coring, with RQD about 10 to 20%.

Any remedies? Piles are supposed to be driven 6m minimum (correct me If im wrong). i'm considering to have a spread foundation. I'll be there tomorrow to inspect.

i Don't know whether the other abutment was already excavated.

Any suggestions? Thanks!



Very Truly Yours,

- andru18

 

[oldestguy]

Explain your position (inspector?) and the specifications, if any regarding minimum pile embedment in rock. Under such circumstances some agencies require a 3 meters deep drilled hole filled with concrete and then drive the pile into that.

 

[Ron]

Is this only an abutment or is it in the water? If not in the water, continue the excavation to solid bearing material and use shallow foundation. If in water, use a different pile system that will penetrate to give adequate lateral resistance.

 

[hokie66]

Your problem may now be lateral stability of the vertical supports. A new design is required.

 

[ztengguy]

can you use some CLSM around the piles? Will that help? What does the geotech/pile engineer say?

 

[dcarr82775]

Driller piers instead of driven.

 

[X-Wing]

The problems are the abutments, not on water.
Im not the inspector of the project, we are just here to help solve the problem.

We instructed the contractor to dig a square hole up to the layer where the tip of the rc piles stopped and we are getting sample to verify the geotechnical report.

Followup question: What is the maximum permissible N-Values that RC piles can penetrate onto?

Very Truly Yours,

- andru18

 

[oldestguy]

You usually use piles when the material at the usual footing grade is erodible and/or of low bearing capacity. Since you are not in water and on a rock of sorts, the use of piles for support violates that principle. The usual footing grade can be a variable thing, depending on the site. Driving any pile, other than H piles with reinforced tips into rock, seems stupid to try. Since you are not the owner's representative, inspector, and apparently not the engineer of record, why not leave it up to them?

 

[X-Wing]

Technically sir we are not the project inspector but their office asked for our assistance into that matter, their office is practically "under" our office, (they are the District Office and ours is Regional Office, in which they are under our office).

Based on the geotechnical report, at level 7m (where practically the tip of the piles are design to have bearing) that level have sandstones, with N values about 30 to 50. Beyond that, about 9 below, recovery says that the sample are cored.

Maybe the designer opted to use RC piles because the foundation will be massive in case of using the bearing layer of 7m an it will encroach the river. At the upper layers there are N values of 7 & 9.

Thanks for the reply sir!





Very Truly Yours,

- andru18

 

[ztengguy]

Even as the regional office, why is it the inspectors job to fix a problem?

 

[FixedEarth]

This is best solved by the geotechnical & structural engineers of record. To a pile, that is piercing through soils, the capacity reduction is the same whether the the pile is pre-augered by 3 m from the top or refusal is met at 3m above design depth at the bottom.

I will give you two cases where we had constraints in pile/pier embedment and how we solved it, it might help you.

In the first case, we encountered cobbles at the bottom soils prior to reaching design depth for a driven pipe pile. So in the field, you could see the pipe piles were sticking up above the top of design elevation. We calculated the reduction in axial & lateral pile capacities and determined we needed the upper 5 ft to be 24" drilled pier. So we welded shear studs to the pipe pile at one foot interval in the upper 5 ft and then poured 4ksi concrete around the pile and cut of the sticking above pile portions.

In the second case, we needed 12 ft design depth for a 2.5 ft diameter drilled pier. We could only auger down to 8 ft depth. So we instructed the contractor to make 8 ft cube excavation which we filled with 300 psi (low strength) concrete. Then couple of days later we augered a 4 ft diameter pier in the center of the cube down to the 8 ft depth. This provided us the required axial and lateral capacities.

In both cases, I knew of the geotechnical properties and of the foundation loadings. You need both pieces of information to come up with your fix or just call the designers of record. Your case may further be complicated by surface surcharge loadings like an embankment. These surcharges, if present, induce a distributed lateral stress loading on to the RC piles in addition to your regular foundation reactions. The designers of record would know this information.

http://www.soilstructure.com/

 

[racookpe1978]

Comparable to the above: Seems like you need to "build" a pier on top of the excavated soil. Give up on the piles, they are "in mid-air" right now. You need a designed abutment on top of dirt. Now, that designed abutment may need some piles or anchors down into the new dirt surface to prevent sideways movement, but also retaining walls, fill, and a cover "plate" to carry the load down through the abutment to the exposed soil.

 

[BigH]

Why did the contractor over-excavate? That is the basic question. Did the inspector/client/engineer of record tell him to do so? If not, the contractor should be forced, in my view, to develop at his cost a new design to make his work compliant with the specifications and construction drawings. Of course, the EoR would be involved to ensure that a remedial design would be suitable - and, again, all of this should be at the contractor's cost.

It seems to me that the contractor figured it would cost him less money to dig out the 5 to 7 m of soil and place a spread footing - compare cost to bringing in a pile driver, precast piles, etc - for how many piles? The design seems to ask for very short piles - given only 5 to 7 m to rock and with a pile cap down about 2 m, the piles would only be 4 to 5 m long to the top of the rock. You didn't say how "deep" into rock the piles were to penetrate - and I do not think that RC piles driven to rock like you have would be suitable anyway - always problems with cracking of the piles due to over-driving. So it seems that penetration into rock was not envisaged.

I once was involved in supervision of a project that "required" precast pretensioned concrete piles driven to rock - and the rock was only about 2 m below bottom of pile cap - or, at one pier, the rock was up inside the pile cap. Needless to say, driven piles would have been ridiculous. Ended up going with 2 m diameter hand dug caissons founded about a metre into rock with a slight bell. Three of these more than made up for 10 300x300 precast concrete piles.