The slurry composition is a 3/4 sack sand slurry. I'm looking for the engineering criteria that would require a change in the trench bedding zone of a buried 60" water line.
For instance, when is it permittable to use a sand bedding, a slurry bedding, a concrete saddle bedding, etc.
slurry is primarily used for reasons of constructibility. It allows the contractor to bed the pipe quickly before the trench walls cave, without the need for compaction. It provides a very high quality bedding / backfill and is very quick. Very little labor is required to place it. The savings in time and labor and potentially traffic control may offset the additional cost to furnish and place the slurry.
Slurry also provides other advantages for the owner over sand such as:
1) it is relatively impermeable which prevents water from flowing through the pipe bedding along the trench.
2) Assume another trench is dug (without shoring) near an existing waterline (or gas or sewer) shaded with sand and assume the trench wall starts to cave - the sand can flow into the new trench and cause movement and potentially a break in the existing waterline.
3) it protects the pipe during future excavation. The backhoe operator will know when he finds the waterline trench as he will see and feel the hardened slurry before he actually strikes the waterline with the bucket.
So the soil classiifcation encountered in the trench would not change the trench bedding design? For example, If I went from an OSHA "B" soil to an OSHA "C" soil the trench bedding detail would not have to be modified?
I do not consider myself an expert in this subject, however, I believe I have also seen at least some flowable-fill type backfills employed when there are a lot of spaghetti-like utilities and cables etc. in a congested area (and it might be difficult or risky to compact with mechanical equipment).
my thought is that slurry backfill will provide some type of structural strength in the backfill. "Slurry" is a general term. The mix design will really determine what the intended use is... but typically you will slurry backfill a 60" pipe when it is under a slab or pavement section and then use a sand or AB backfill when in other situations.
The soil classification encountered in the trench would change the trench bedding design. It is necessary to provide a stable formation before pipe laying. Such conditions commonly occur in peat, silty ground soft clays, running sand, or in fill material.
Although sometimes trench formations are filled with concrete, this is unlikely to assure long term stability in all cases, and a form of flexible bedding construction is the preferred method of dealing with the situation.
The trench formation should be over excavated by 24" to 30", depending on the bearing strength of the soil. Gravel reject material is then compacted in layers to form a firm trench bottom. A 2" thickness of lean-mix concrete is then placed as blinding. The pipe is then laid on granular bedding material.
Slurry is usually used as a blinding layer. Clays and silts are highly susceptible to softening when in contract with water. Clay formations are protected with blinding concrete or with foundation concrete as soon as possible after completion of the excavation. If not protected, the clay will swell and then must be removed.
Slurry is also sometimes used to protect and stabilize excavated sloped areas from storm water.
