Wet silt 3

[eric1037]

I have a little construction issue. We have a project where we did the geotechnical evaluation for a single-story, slab-on-grade structure. We encountered medium dense silty fine sand to a depth of about 3-4 feet below the ground surface. The sand was underlain by loose sandy silt to a depth of about 9 feet. The silt was underlain by a relatively clean, medium dense sand. Groundwater was encountered at a depth of about 5 feet below the ground surface.

The foundations were designed to bear at a depth of about 4 feet below the ground surface.

During foundation excavation, it appears that the silt is becoming very disturbed. To make matters worse, it appears that the groundwater is about 1 foot higher than it was in the soil borings. Therefore, it is at, or just below, the foundation bearing level.

For a case like this, we normally recommend undercutting slightly, then pounding 1-inch to 3-inch size crushed aggregate until it firms up.

Does anyone have any other ideas? We have considered dewatering, but the silt is relatively difficult to dewater due to its low permeability. I have also considered placing geogrid at the bottom of the crushed aggregate to help establish a good working surface.

Thanks in advance.

 

[SlideRuleEra]

I'm sure you designed for a proper (low) bearing pressure because of the silt. If so, this is more of a construction issue, the aggregate in the bottom of the excavation should provide a suitable working surface. The low permeability can work in your favor - a suitable diaphragm (mud hog) pump should be able to easily keep most of the water out of the excavation until the concrete is placed.

After all, once you start digging deeper - looking for good soil, you may find yourself on a "Journey To The Center Of The Earth"

http://www.SlideRuleEra.net

 

[eric1037]

My thoughts exactly. We recommended a low bearing pressure. We just need to have a good, stable working surface in order to form the foundations and not muck it up by having workers walking on it.

In my view, one of the most important jobs of the geotechnical engineer is to make sure the contractor can actually do what you recommend.

I have already told them that they should excavate more than about 6 to 8 inches below the groundwater.

My experience with the silt is that after the foundations are constructed, the confinement cause the silt to become much more stable.

We have even placed pvc pipe in the foundations to be able to test the soil below the footing after it has been constructed. The dynamic cone penetrometer blow counts go way up after the foundation is placed.

Thanks again for your comments.

 

[BigH]

Not sure how big your structure is, but in the past, we have dug down to near the 'silt' layer, then carry out careful excavation by hand in sections and place a mud mat or blinding concrete for a working platform. A thin mat of crushed stone after cleaning before the mud mat can help in preventing disturbance. You can then walk on it without disturbing the silt. Another method is to dig perimeter and internal ditching deep enough to take the water to just below the foundation bottom level. But on a big site this would be problematic - as you know.

 

[oldestguy]

Question for Eric 1037:

Why did you ask for 4 ft. depth for foundations if you anticipated a construction problem by going that deep?

Was it due to expected frost heave protection? Local code requirement for "frost walls"?

Have you tried "floating slab" insulating around the slab to keep, the "cold" out from under the slab? I've called for burying a 4 ft. wide pink sheet below grade all around the structure, plus some on the outside edge of concrete, protected from sunlight. These have worked out great. No need to excavate deeply.

Could not the slab be designed as a "raft" or thickened edge slab.

Just asking about the alternatives considered in your report preparation.


Otherwise, my experience is that the "disturbance" usually is temporary (excess pore pressure).

The next way has worked on some jobs recognizing the improvement achieved by delaying:

Waiting a day and quickly placing clean coarse sand quite thick, via off-site rig (no skid steers) and then waiting a day to add more and compact has worked out OK. The sand is placed in the water, no pumping. Skip the "thin layers" for fill spec. at the bottom area. Coarse sand (saturated) stays stable when compacted in a thick lift, but not so for fine sand fill.

A common "disturbance maker" is a front end loader doing the work. Sticking to back-hoe excavator with flat plate covering the teeth has helped prevent major problems.

Your clear rock also has been done a lot and it works OK also, but leaves questions (not mine) about voids left open to later fill and cause settlement. My spec. says tamp until mud shows up in the voids. Is it followed if I'm not there? Who knows.

BigH's mud sill working platforms also are common here, but usually where a big structure is to go above (like a 35' deep lift station). I have not used this for common small structure foundations at light bearing pressure.

 

[eric1037]

oldestguy:

I am in Michigan. The building code requires that the foundation bear at least 42 inches below the ground surface for frost heave considerations. We have attempted to use insulation and other methods to protect the foundations from frost, but the building departments reject the design.

As we all know, saturated fine-grained soils are very susceptible to frost. I'm not sure if I would be comfortable with insulation for this case any way.

To accomodate the voids, we usually recommend that the coarse-grained material be pushed down into the subgrade so it is completely infilled. We then recommend, a smaller aggregate be used on top. This way we don't violate filter requirements and it gives the contractor something smaller to pound in his form pins. Ocassionally, we will recommend a filter fabric around the coarse stone.

I actually went to the site yesterday to take a look. Fortunately, it looks like the area of silt does not cover the entire building. One footing line (about 200 ft. long) bears in this material. We recommended installing fabric-wrapped perforated pipe a couple of feet below the bottom of footing and having several sump pits to attempt to dewater some of the silt.

Thanks for your replies. Keep them coming.

 

[sjhgeotech]

If you are dewatering the silt below the footings, aren't you concerned about settlement of the footings after they are installed? I would recommend dewatering before installation of the footings.

Was there any consideration for using drilled piers (caissons) and using grade beams?

 

[eric1037]

sjhgeotech:

We are dewatering before foundation construction. Foundations haven't been constructed yet.

We didn't consider drilled piers for a few reasons:

1) Soils were relatively dense.
2) There are no drilled pier contractors in our area.
3) There are similar issues with construction of drilled piers. You have to use casing and, most likely, slurry to drill.

All of these make drilled piers a more expensive alternative even considering the subgrade preparation.

 

[PEinc]

Have you considered a helical pier foundation? You would not have to dig below water or do an undercut. You say there are medium dense sands at 9' deep. Your building doesn't sound heavy or big.

 

[eric1037]

Actually, around here, auger-cast piles are more economical than helical piers. There are at least 3 auger-cast contractors nearby.

The only time I recommend helicals around here are for underpinning projects. Even then, several of the mini-pile contractors can be competitive.

 

[PEinc]

Where are you located? I'm surprised that auger-cast piles would be cheaper than helical piers on a smaller job (if your job is indeed smaller in size). Helicals have much lower mobilization costs.

 

[MRM]

We've run across the same problem you describe, Eric. You've got a silt or silty sand with a relatively shallow groundwater table, you excavate several feet which removes several hundred psf of overburden, the silt dilates (due to a slight amount of heave), pulls a little extra groundwater into the voids, and becomes unstable. I've often noticed what I'll call the "tomato soup skin" feel on the surface of these soils.

I've approached the matter using the undercut routine followed by geotextile and crushed stone with success under certain conditions. This provides a stable subgrade for construction in the short term, and after the foundation is constructed and backfilled, the silt will once again regain strength (with a small amount of associated settlement). However, you can really come out looking like the hero by checking the soil out closely:

Check the plasticity, dilantacy, and gradation of the soil. This is done in the field by hand. Determine how silty it is, and whether it is likely to regain strength quickly after the load is reapplied. Next, I like to probe, hand auger, and DCP to help determine the extents of the disturbance and dilation. The level of disturbance is very much time dependent and could also vary with depth depending on the factors above.

Depending on the structure type and everything else, if the soil has an appreciable amount of clay, and the depth of disturbance is great, i.e. several feet below subgrade level, removal of these materials may be required followed by placement of an aggregate of your choosing. In certain situations, the undrained strength of the soil could become a concern. If the soil has almost no clay, it behaves like a silt, and/or the depth of influence does not appear to be very deep, it could be possible to excavate slightly place a specific geotextile followed by a small amount of stone. All the time, with the idea that the silt will regain strength in short order after the foundation is constructed.

It all depends on the soil, and a good amount of judgement is necessary.

 

[eric1037]

PEinc:

I am located in West Michigan. With the relatively high cost of steel and the proximity of at least three, hungry auger-cast contractors, I have ended up looking like a chump recommending helicals on several locations. This is even when there are helical contractors close to the site.

MRM:

What I am dealing with is indeed silt and/or silty fine sand with almost no clay content. I have suggested doing a test section of footing with PVC through it to test if the confinement increases the stability of the silt. Our client and the contractor have been reluctant because they don't want to have to rip out a section of foundations if it doesn't work.

I am hoping that dewatering and aggregate will stabilize things enough to appease everyone.

It is tough convincing people when you tap the soil with the excavator bucket and it looks like you are beating on a water bed.