Vibratory Stone Column Installation Problem in Soft Silt

[SUSA12]

I am involved with a small project in which vibratory stone columns were selected to improve the soils for support of shallow foundations. The soils consist of a silt (ML) in the upper 20 feet. (Avg N=5, M% - 22-28, DD = 95 pcf, no atterbergs were run). Below that the soils continue to be soft with increasing clay content. Water table at 12 feet (approx).

As the stone columns are installed the silty soils surrounding the stone columns become very soft. In several areas where multiple stone columns have been installed in close proximity water has been observed to bubble to the surface. Water has not come out of the stone column itself, but water could be seen in some of the stone columns a few feet from grade during installation. When standing on the ground next to the stone columns I can pump the ground about 6 inches. The contractor attempted to load test one of the VSC (3x3 ft on a 2ft dia VSC) after a couple days and had about 1.5 inches at 30 kips (not good). They let it sit another couple days and attempted to load it again and got about 0.5 inches at 20 kips. They also tested another stone column that had been in the ground for 3 days and got 3/4 inch of deflection at 20 kips.

The contractor has installed stone columns spread out across the site and has pulled off to give the pore water pressures time to dissipate and plan to do more testing next week. They tried to get a CPT cone out to measure pore water pressures, but the equipment was unavailable.

Has anyone seen these conditions? If so, how was it addressed.

 

[rchanke]

SUSA12 - I've seen this before and your problem is simple. Vibratory stone columns should NOT have used on this site. Silts (ML) and clays (CL) can be reinforced with stone columns but not improved or densified, especially if this is a full displacement stone column without any pre-augering. Vibro-replacement will activate and remold these soil types and quite often leave them in a weaker state. Conversely, silty sands and sands (free draining granular soils) can be improved with the vibration and displacements associated with stone column construction. It sounds like the contractor bit off more than he can chew and will learn from his mistake. Waiting for pore pressures to dissipate may not be practical. Regarding bearing capacity, a remedial measure could be that each stone column will need to be drilled and grouted (cement slurry) to increase it's stiffness modulus. This depends on what the soil conditions are at the tip elevation of the columns. Only a few columns should be grouted with varying grout placement parameters and then load tested to verify performance.

 

[BigHarvey]

SUSA12 -2 comments. The first one is that you are constructing "floating" stone colums. Stone columns should be conlayerstructed down to a "competent layer" and not stopped in a "soft" layer.
The second one is that you are experiencing a soil liquefaction. Hence no lateral support and no possibility of compacting the stone and eventually a failing load test. You should work in several phases with grid becoming smaller and smaller and monitor pore water pressure in order to make sure you don't install stone columns in a liquefied soil.

 

[SUSA12]

I went out to the site this morning and they had not installed any stone columns in the last 3 days. The ground surface was very stiff (especially right around the stone columns) and the surfical soils were not pumping.. They did a plate load test up to about 30 kips with a little more than a quarter inch of movement and thought they would get better results if it sat longer. They are planning to start installing stone columns again today. Quite a difference from last week.

I know they are running the stone columns up till they get a certain pressure on each lift.