Grading over peat

[dtrisler_imported]

I have a project that will be grading over peat. I'm wondering about special earthwork techniques to make sure we don't bog down too much equipment.

The site is covered with 2.5 - 3 feet of firm to stiff silt over 10+ feet of peat. The plan is to excavate up to 5-6 feet below grade to create stormwater detention ponds. The ponds cover about 2 acres. The peat will be overexcavated about 12-18 inches and then capped with the native silt. We plan on trackrolling the silt over a geofabric. Also, some dikes will be built up on the downstream side, so these areas will be excavated to the geofabric level and then built up with select fill.

 

[SirAl]

Where is your location?

 

[VAD]

Hello dtrisler:

A couple of things you need to know:

1. What material is below the peat and what is its condition re consistency. It can either be so called "loon shit" or soft,medium or stiff clay etc.

2. Where is your water table

3. What equipment is planned for use

4. Can this work be done when freeze-up starts. I assume you are in such a climatic region

Generally light tracked equipment travelling at slow speeds is generally recommended but these days heavy equipment is the norm. Heavy wheel load scrapers tend to cause a problem especially if the material underlying the peat is weak eg saturated fine sand and silt (so called loon shit).

This may be a job for excavating with a backhoe and loading to trucks -not heavy duty rock trucks. However, the assesment generally comes from an understanding of the overall foundation conditions.

It may be necessary to install perimeter drains if the site is water laden. Sometimes this has to be lived with as there is no positive drainage.

Laying of geotextile may have to be done manually to avoid equipment bogging down. Silt cover over geotextile may have to be done by end dumping and spreading with a cat D5 crawler tractor etc

A lot depends on the understanding of conditions and past experience with construction in peat areas. If done by contract then contractor want to get in and out as fast as possible and can cause grief if specs are not clear.

Others may have some additional input or differing concepts.

 

[dtrisler_imported]

The project is in the Portland, Oregon area. We don't get freezing conditions. Groundwater is about 8 to 10 feet below grade. The peat is underlain by soft to medium stiff clayey silt. Construction is a few months out, we are in the midst of preparing plans and specs. I'm trying to make sure we think of as much as possible, so that we don't get contractors coming back and complaining about site conditions.

We are specifiying low-bearing pressure, tracked equipment only, though weren't necessarily going to specify models. I did assume the fabric would be manually laid out, though we should probably specify that. I guess we should also specify that they place soil and track out over it, as opposed to tracking directly atop the fabris.

The perimeter drains to temporary pumps may be a good idea, depending upon the finished elevation and groundwater levels.

I think the ideal would be excavators working from native grade, with no equipment tracking the peat. However, I think it will be difficult to stage and too slow.

Thanks for the advice, keep it coming if you've got more.

 

[KRSServices]

VAD is on the right track.

The key is minimal disturbance on the peat and silt. The problem with end dumping is the fact that the cloth and clay/silt layer (spec'd) may not be able to support the weight of a fully loaded end dump. The lift may have to be thicker to enable support of the truck. Once dumped, the material can be ealily spread by an excavator with widepads. The problem will be compaction, if it is required, or if the silt/clays get wet without sufficient drainage provisions.

KRS Services

http://www.krs-services.com

 

[dicksewerrat]

We did a few of these in Mpls. We used a Komatsu 650 with 7 yd. bucket. The machine started in the middle of the pond area and worked back out of the pond. this machine can be put on wide tracks. Two scoops per truck. It is large enough to dig all the way down. Get the longest boom and dipper arm that the machine can use and still dig with out trying to pull itself into the hole. Dig out a trench from 45 degrees on either side of the machine as you back out.
Move machine and repeat till the hole is the size you want.