Belled Drilled Piers in Sand 2

[CE475]

Is it possible to construct a belled drilled shaft in sand. I am having problems reaching my allowable bearing load and a bell seems to be the only solution. However since i'm in sand I was curious if it was possible. A spread footing seems to be the only other solution, but a quick calc shows an extremely large foundation which will be costly and the client has a preference with drilled shafts.

 

[JAE]

I had a drilled/belled pier design all ready to go on a project northwest of Houston some years ago. The piers were 24" to 42" in diameter with 60" to 90" bells.

The geotech had indicated that the sandy clay soils were OK for bells but once the contractor started drilling, the belled out piers caved in. We quickly had to convert to a straight shaft system (yes - 90" diameter shafts) and go from there.

So be careful with bells in sand. Its very difficult to know whether a sandy soil will hold up long enough to allow cage insertion and concrete placement. You may have to use a tremie as well to avoid the falling concrete from hammering the sidewalls of the shaft and initiating collapse of the bell.

 

[Zambo]

I'm surprised you are finding a spreadfooting design is giving you a higher load bearing capacity than the bored piles. For your project did the geotechnical report give a recommendation, and was it for bored piles? What about trying the design with longer piles, 60-100m 1.5m diam piles are not unsual.

As for the tremie I doubt any contractor would attempt to place concrete in bored piles without a tremie.

 

[JAE]

Zambo - most of the time you are right - but I've seen it done many times.

 

[BigH]

Why not use expanded base (Franki-type) piles?

 

[BigHarvey]

You will never bell a pile in sand. however if you have a solution with spread footings, you can certainly find a soil reinforcement solutions either with stone columns or rigid inclusions depending on many criteria (type of load, intensity of load, thickness of compressible layer, etc.)

 

[Zambo]

I have always worked with bored piles constructed using bentonite or polymer as excavation support. This being the case, placing the concrete without a tremie can only end up with a disaster.

CE475, I assume this is a stupid question but are you calculating using shaft friction along with end bearing capacity for the bored piles.

 

[CE475]

I am using end bearing & skin friction, the foundation is for a transmission A-Frame structure, and a combined spread footing will be very large and deep in the ground due to long anchor bolts provided by the fabricator. Franki-type piles could be a solution but they would have to be 5' in diameter and extend a min. of 30' into the ground.

 

[GeoPaveTraffic]

How about using augered cast-in-place piles or driven piles? There are a lot of options, but to concurr with the others, you can't bell a drilled shaft in sand.

 

[Zambo]

30' is only just over 9m, hardly worth getting a piling rig there for that. How deep have you got a soils report for? GeoPaveTraffic suggests driven piles, this might be a solution they could typically be square, say 450x450mm or prestressed spun tubes, say 800mm diameter. Lengths of 30-50m are normal and easily installed.

Of course everything depends on the soils, but on my latest project (near a river and silt/clay soils) we have 800mm diameter driven precast spun piles which are designed for a working load of 350 tonne with a length of 50m.

 

[BigH]

I'm surprised at the diameter of the Franki-pile you quoted. In Atlantic City, most of the high casinos are on Franki-piles with loads up to 300 tons (if I am remembering correctly) and they weren't 5 ft in diameter.

 

[BigHarvey]

Given the size of your piles, it might be more economical to use barrettes as foundations ( for the same cross section you will have a lot more friction area and if you have horizontal efforts, you can set them in the direction with the greatest inertia )

 

[eric1037]

In my experience in the power industry, drilled shafts are the most common method of support of towers. It seems like the controlling factor was alway uplift and overturning, not downward loading.

Since these structures are often constructed in out-of-the-way places, drilled shafts become more desirable than forming large spread footings. I have seen drilled piers up to 8 to 10 feet in diameter for these kinds of structures and nobody blinked an eye.

I have seen helical piers used frequently in the power industry. For A-Frame type structures, I have designed combined foundations supported by them in the past.

As a side note, AB Chance first developed their helical piers for anchoring guy wires for transmission structures. AB Chance produces a wide variety of power-related products and helical piers are only a small part of that.