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Maximum trenching depth?

Maximum trenching depth?

Maximum trenching depth?

Hi All

We are in the conceptual design phase of a deep (6 - 9 meter) 1200 mm diameter sewer tunnel and I am doing the options costing: deep trenching/open cut versus tunneling/trenchless.

My question is "What is generally the maximum depth for open cut/trenching"?


RE: Maximum trenching depth?

Most standard excavating equipment can reach down 10'-14', so if you go deeper than that you need to widen and bench the trench, which drives up the cost. Plus you have OSHA requirements for trench sideslopes, assuming you have the need to put workers in the trench, which also generally requires additional benching below a certain depth.

So I would say the maximum standard trench depth is about 12'.

RE: Maximum trenching depth?

theoretically, no limit with shoring or laying back the slopes. at some point your equipment will not be large enough to excavate from the trench bank and will need to enter the trench. For example, you might use a Cat 390F L which can go a deep as 38 feet vertical (theroretical). but practically speaking, for trench safety and with an unshored face, could dig only about half of that unless it goes into the trench

RE: Maximum trenching depth?

For a sheeted trench, the deeper the trench, the more wales and cross braces you need. The more cross braces you use, the harder it is to install the lower wales and braces and the harder it is to dig the trench with a backhoe. Deeper trenches could be more easily excavated by a crane with a clam bucket. I rarely see trenches deeper than about 30 feet - open cut or sheeted.


RE: Maximum trenching depth?

we mobilized a backhoe to a project for a slurry wall that had a maximum depth of 50 ft. We were installing a bentonite slurry wall to surround a landfill.

No idea how you'd install a utility in such a trench though. . .


ípapß gordo ainÆt no madre flaca!

RE: Maximum trenching depth?

It appears the intent of the OPs question was for cost-comparisons of open trench vs. boring/trenchless construction. While it's definitely possible to develop trenches upwards of 30'-50' deep the cost to do so will almost always exceed the cost to go trenchless.

RE: Maximum trenching depth?

without a lot more information, no way to estimate feasibility or cost. (length, soil conditions, traffic control, other utilities, depth of water table, schedule, etc.)

you could do some fairly deep trenching for less than the the cost to mobilize and operate a microtunneling rig to install a 1200 gravity sewerline.

RE: Maximum trenching depth?

I would look at microtunnel or bore and jack. But 30 feet deep is not a big project either. Stacked trench boxes 15-20 feet long, subcut the entire path about 5 feet deep, 0 feet wide. Then slope the sides down 5 more feet. This will give you about 15 feet in the center. Start using the 10 foot high boxes with end plate on machine end.

Richard A. Cornelius, P.E.

RE: Maximum trenching depth?

civilman72....don't forget that this is an international forum......OSHA only applies in the US

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