drainage detail at sheet pile wall

[structdb]

We are working on a permanent sheet pile wall with precast concrete facing. Does anyone have a good detail for collecting the subsurface drainage behind the wall. I am thinking about drilling 2" weep holes every 8' thru the sheet pile and collecting the water in a 6" or 8" drain in front of the sheet piling. I have 2 concerns: drilling the hole in the sheet pile web (diagonal) and cutting a trench 5' in front of the sheet pile. Anybody have any experience with this?

 

[PEinc]

I would design the wall for the condition with 5 feet of extra excavation - First, so that you can install the drain; Second, so that the drain can be repaired in the future. I would extend the facing to the bottom of drain trench excavation so that the facing never gets undermined and hangs eccentrically on the sheeting. Also, 5 feet sounds awfully deep for the drain pipe. Can't you just use weep holes through the precast facing just above finished grade? However, it is common for some state DOT's to require the wall be designed for an additional 5 feet of height to accomodate future utility excavation. Can you just use weep holes through the precast facing just above finished grade? Is there that much water that it needs to be collected?

How high is the finished wall? How much ground water is there? Is there any special reason you are using steel sheet piling? Many walls are built with driven or drilled-in soldier beams, lagging, and pre-cast or cast-in-place concrete facings. Frequently, a wall needs to be tiedback if the soldier beams or steel sheeting can not cantilever. This would probably be the case if you add the extra 5 feet of excavation.

I would not worry too much about cutting weep holes through the sheet piling if they are near the neutral axis of the sheet piling. I would be more concerned with making a proper connection at the sheet pile wall so that water can properly get into the drain pipe without clogging or loss of soil into the stone backfill behind the precast panels. You may want to use slotted pvc pipe installed through the cut weep holes or perforated PVC pipe wrapped with a geotextile. These pipes could then be extended to the outlet drain pipe.

 

[structdb]

Thanks for your suggestions. Actually, the plan is to put a 6" plastic pipe running parallel to the sheet piling and located 5.5' horizontally from the front face of the sheet pile. The invert of the proposed pipe will be 2' below grade.

The wall was designed by others and our firm is providing construction inspection and technical assistance. The sheet pile wall is in place - generally 15' high. The bottom (exposed) 18" of sheet piling is constantly wet with water pooling in front. Of course this is the rainy season and it will probably dry up in the summer.

The weep holes will be located 6" below the front grade elevation and empty into a pocket filled with 1/2" stone. We were going to put heavy filter fabric in front of the hole and rely on the rock fill to hold it in place. We will have perforated laterals at 9' centers running from the rock pocket to the 6" pipe running parallel to the wall.

The slope of an imaginary line from the ground elevation at the base of the sheet pile to the invert of the 6" drain will be about 20 degrees from the horiz.

 

[PEinc]

Generally, precast wall facings are entirely backfilled with stone so that compaction is not needed. Also a perforated drain pipe is placed in the stone backfill, running along the wall, between the precast and the sheeting. Then, the water is carried out the end of the wall to another drain pipe.

Sorry I misunderstood you about the 5 feet. But, like I said, some highway departments require the wall to be designed for an extra 5 feet just in case a future utility needs to be installed.