As the Tire Store turns.
Excavating the storm water detention system in the front parking lot. 30" cut from existing top of asphalt, not a big excavation. A friend and trucking owner had a place for the dirt to go close by so it was a win/win.
The system is designed to have Mirafi non-woven fabric on the bottom and up the sides with a 6" layer of sand on the bottom, 1.5' of #4 crushed limestone with an 1" of #57 crushed limestone on top. The fabric doesn't wrap burrito style, the pervious concrete pours directly on top.
In between the #24 stone layer there is a 4" HDPE perf. pipe that dumps into a 6" SDR 35 line that runs into an existing storm inlet. The idea behind the design is the parking lot water goes through the pervious concrete and fills up the sand/gravel layer, slowly leaching out through the 4" HDPE, then to a 6" SDR 35 and finally dumping into a storm inlet.
Personally I don't like the idea as it will saturate the surround soil and possibly compromise the sub-base soils. The 4" HDPE is designed to be 21" off the bottom of the excavation which means 21" of water has to soak into the surrounding soil.
We don't paid to design it, we get paid to put it in the ground..
Also on this job was a short run of 15" HDPE and a grate inlet. Here's the head scratching part - the existing inlet we tied into was a standard single wing inlet draining this part of the parking lot. The engineered removed that inlet top making a blind junction box and installed the new inlet 10' away.
Yesterday we had a frog strangling afternoon shower that flooded the job. The guys did a good job mudding up the connection as it was still holding water today.