Type "C" Soil Classification for Excavations 2

[jmh77]

I am overseeing a 6' deep trench for the purpose of laying a 6" fire water line. Ingress will be required for the pipe layers to perform their connections. This project is in the Pacific Northwest where our groundwater table is approximately 5' below ground surface.

The soil strata consists of a thin layer ~1' of non-cohesive fill followed by native clayey material which has not be characterized in a lab.

Visual and manual observation of the native material gives our engineers strong confidence that the material is atleast deserving of a Type 'B' classification. In preparation for ingress, we have benched the excavation according to OSHA requirements for Type 'B' soil and the formation is stiff under foot and seemingly very reliable.

Despite the type 'B' qualities of the soil, the bottom of the excavation is just below the water table and we are seeing slow but steady seepage of water into the trench bottom.

According to OSHA, the soil must be declassified to Type C if "water is freely seeping through the sides of the excavation".

My question is, how should I interpret the meaning of "freely seeping". Is the presence of slowly flowing groundwater at the bottom of our trench immediate grounds for declassification from Type B to Type C conditions?

The soil is clearly very stiff outside of the seepage zone and meeting Class C excavation requirements is burdensome (for various site specific reasons),however, we want to adhere to the intent of OSHA's requirements and be conservative above all else.

 

[coloeng]

You should interpret "freely seeping" to mean if water is seeping through the walls of the trench in any amount if it is slow and steady. That type of trench is the most dangerous because you can loose strength in the bottom layer and have a layer chunk of the sidewall break off and trap or kill one of the workers. Get some shoring or a trench box and use it. It may add time and cost to the project, but nothing like you would we see if you injure or kill one of the workers.

 

[chicopee]

Better be safe than be cited with withfull intent.

 

[dicksewerrat]

The nice sand/gravel on the top will hide the sidewall crack as it forms. Once you see it, the wall will have killed your laborer. A 6 foot high by 3 foot wide trench box will be an easy and imho a good bit of insurance. It will also tell your workers you are very concerned for their safety. You won't die in the trench, they will.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[jmh77]

Thanks for the replies. We all want to make a conservative decision on the matter and worker safety is the highest priority.

Ironically, in my experience, it's often the craftsmen and laborers, not the management, who push back against more more conservative determinations with regard to trench safety and soil classification.

 

[dicksewerrat]

That is why you have to preach, preach safety to them. Trench work is right up there on accidents per 1000 workers. You can also call OSHA for a consultation without getting a citation. I know you can in Minnesota. Of course all your ducks should be in a row for anything you are not calling about.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[cvg]

if you have your OSHA certified, Competent Person on site, like you are supposed to have, then you really don't need to call OSHA out for the consultation...

 

[jgailla]

If you have groundwater at the trench bottom, you have problems.
All of my specs require a dry bottom, which seems reasonable.
Your groundwater at construction level is asking for a lawsuit.

 

[jmh77]

jgailla can you expand on your point? Here in the PNW there isn't a hole to dig that isn't going to be subject to groundwater infiltration, this time of year our water table is 1'-4' below ground surface depending on tides.

We are actively dewatering the trench during construction and personnel ingress.

cvg, yes our on-site Safety Reps are Competent Persons, they have inspected the excavation and echoed the points that you all have made here on the forum.

 

[jgailla]

jmh77,
I'm in North Florida, where the groundwater is typically at about 1'-4' also.
The sandy soil will usually allow for dewatering.
I have done work in South Florida where the porous limestone soil will not allow for dewatering. In this case the workers wear wetsuits and lay the pipe in the trench under a few feet of water. The pipe is covered with 57 stone up to the water surface and then covered with soil. It seems to work for them.
My point was that dewatering should be done unless it is infeasible.
If you can dewater, you should. Seepage from the bottom of the trench indicates to me that dewatering has not been properly accomplished and should be continued until the groundwater is at least a foot below the trench bottom. This is both safer and will allow for the soil to be compacted properly.
My point about the lawsuit, although overstated, is that it is easy to get hurt in a situation where groundwater is not properly controlled. coloeng put it better than me.

 

[chicopee]

Good point by CVG. Excavations qualified under the OSHA standard will require a competent person.
If the layer under the type B layer is weaker than yes it is type C

 

[pelelo]

Regarding Soil type C,

I am working on a shoring project that involves Silty sand (SM), silty gravel (GM), clayey sand (SC) and clayey gravel (GC). The fines content are somewhere between 25% and 40%. At the same time, the fines are classified as silt (ML) and clay (CL).

It is not very clear to me if these soils are Type C, as according to OSHA manual Type C classification are for granular soils with less than 15% silt and clay content and no plasticity.

On the other hand, type B soil clasification are for Granular COHESIONLESS soils with more than 15% of clay and silt content and no plasticity. Since they soils have cohesion and do have plasticity I am not sure if this classifies as type B either.

Please correct me if I am wrong.

Thanks

 

[cvg]

pelelo
not sure where you are coming up with 15% fines. My OSHA manual does not give a percentage. If you consider it to be "cohesive soil", you must have compressive strength estimates to further classify it as Type B. Otherwise, I feel you must classify it as Type C to be on the safe side.

 

[pelelo]

One more question about Soil type C,

In case water level is located at the middle depth of my excavation, for sure it is Type C. Soil properties belong to a Type B soil. In case I de-water, water is not supposed to drain freely through the walls of the excavation since water level is lowered. Does this de-watering procedure changes my soil to type B, as water is not present anymore, Or will it still remain as Type C?,

What do you think?

 

[cvg]

even if you dewater, you still need to meet the other requirements for Type B.

[ol i]
[li]cohesive soil with unconfined compressive strength greater than 0.5 tsf[/li]
[li]granular cohesionless soils including: angular gravel (similar to crushed rock), silt, silt loam, sandy loam and in some cases, silty clay loam and sandy clay loam[/li]
[/ol]

 

[Hoaokapohaku]

jmh...Just where are you in the PNW? I'm curious because I grew up in the area but have never worked there.

 

[dirtsqueezer]

Yeah, same here. If your clay material at the bottom is hardpan then you might be edging toward type B, however if that margin is lubricated, trench shoring is a must. Also, how are you going to backfill in water? I used railroad ballast in a project in Centralia until I got above the water table and compaction was great on top of that with CSTC. Let us know.