Category talk:605 Underdrainage

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Our office has found what appears to be a conflict between the Standard Plans and the current online spec book.

The standards show that the type 1/5 base placed on top of the underdrain. The spec states that the type 1/5 is to be placed and compacted first. Since the EPG, says to review the Standard and spec prior to installation. The two of these need to agree or be clearified.

http://www.modot.mo.gov/business/standards_and_specs/documents/60510.pdf

605.10.2.2 Trenching, placement, and backfill of underdrains shall be performed only after Type 1 or Type 5 base is placed and compacted. Backfill material shall be compacted by three passes of a vibrating pad or drum-type compactor approved by the engineer.

Keith L. Smith: A Construction and Materials Liaison Engineer responds:

The general intent of the spec is for white base compaction to occur prior to installation of pipe aggregate edge drains. The drawings for the Type 5 base examples in Sheet 2 of 605.10H do not indicate a visible break in the base material, but it is implied that the base has been trenched and recompacted. In the case of the permeable treated base/Type 1 base combination the trench actually extends through the Type 1 base in order to interface with the permeable treated base, so from a constructability aspect, it wouldn't be very practical to place the trench before the Type 1 base.
However, in the Type 5 base situation the trench is completely below the base, making it appear more practical to trench and install first and lay base second, so why don't we do it? Because we don't want to crush the pipe. Running heavy equipment back and forth directly across the trench greatly increases this risk. And we do perform (or are supposed to) a QA video inspection of 10 % of the longitudinal pipe drain before acceptance.
So perhaps our reasons could be explained a little better, but the spec does require what we should be doing.