Weeping tile drainage near train rail mat foundation.

[CanuckPE]

I'm working on a project for a mat foundation under train rails for some rail equipment. The area we've excavated is traditionally wet, and we specifically designed a drainage system that would mitigate the typically wet surface. I've attached an image with a depiction of our proposed weeping tile location(drainage path is purple, top image) and the proposed weeping tile location from the contractor (drainage path is purple, bottom image). For reference in the images, the black rectangle is a rail loadout facility, the black parts with the hatch is the rail, the blue rectangle is our newly placed mat foundation with 30" of clean granular fill below, and red arrows depict the natural slope (which is about 1% from top of the image to the bottom). The water traditionally runs off of an adjacent road and towards the rails per the red arrows. The contractor "forgot" to place the weeping tile, and now has ground prepared, and reinforcing steel placed for the mat. I caught this on the field review.

I did soil mechanics training in a master's degree, but never really focused heavily on hydrology. I'm hoping there are some experts out there to provide some guidance.

The fact of the matter is that surface water from precipitation will likely hit the gravel, dive under the mat through the permeable clean gravel layer, and make its way into the weeping tile as proposed by the contractor. I would have preferred our original arrangement to the contractor's proposed alternative such that ground water was conducted under the mat via the weeping tile rather than through the gravel under the foundation. It's a more direct path.

Questions:
1) Should I capitulate and allow the contractor to place the weeping tile as proposed? Are there any technical drawbacks to this approach? The bearing pressure on the ground is minimal (in the 1000 to 1500 psf range), so the additional ground water shouldn't have a substantial drawback on strength/stiffness of the granular fill.
2) Would the granular fill conduct the water just as well/quickly as the weeping tile?

 

[SlideRuleEra]

The area we've excavated is traditionally wet...
...contractor's proposed alternative such that ground water was conducted under the mat...

In the sketch, please show relative elevation of the water table.

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[dhengr]

CanuckPE:
Is this some sort of an unloading site which you are trying to keep dry? The granular fill should work, if done right. But, it should be wrapped in a geotextile material so it doesn’t plug up with fines over time. The drain tile should be at the bottom elev. of the fill, and be sloped to drain to daylight, or it should be arranged to be pumped out at a sump of some sort. I don’t really understand what you are trying to show in your sketches.

 

[CanuckPE]

Thanks for your replies.

SRE, I've attached a cross section of the arrangement with the slab. Grey = existing soil, Red = granular fill, Green = a thin sand layer, and the black dots are the potential drain tile locations. Granular fill is 28" and concrete is 15". The water table is currently at the bottom of the excavation. There's a geotextile layer between the top surface of the excavation and the bottom of the granular fill.

We're in a semi-arid climate, but considering the weather recently the WTL is likely near the typical elevation. The wet area concern was brought up as this is a relatively flat area with a road and building draining towards the rail. In the past, the rail tended to be wet (so I'm told) and we're slightly lower (3") with concrete now compared to the previous arrangement. It's an existing facility. There was no drainage tile in the previous arrangement.

dhengr, Yes this is a rail unloading area. Where would the textile material typically wrap, or rather along which boundaries? Just the bottom, "wrapped" around the top as well? The road and building discharge toward the rail. I agree, the tile would ideally go to the bottom of the fill, but we're seeking to daylight the discharge rather than sump, and the elevation chosen will allow us to discharge to daylight within about 300'. The sketch was meant to display two plan views. One with our proposed arrangement with the tile on the side that would typically receive the precipitation from the road and building, and another with the contractor-proposed arrangement in which the surface water would need to pass under the mat to reach the weeping tile.

 

[SlideRuleEra]

...flat area with a road and building draining towards the rail.
...we're slightly lower (3") with concrete now compared to the previous arrangement.
It's an existing facility. There was no drainage tile in the previous arrangement.

I would not use any drainage tile. Successful shallow foundations can be on either "dry" soil or "wet" soil. Trouble starts when soil alternates between "dry" and "wet", especially if the (occasional) surface water is intentionally routed under the foundation. That causes the change between two conditions to be even more extreme. IMHO, as much trouble and expense it might be, adjust the grade of the new facility for natural surface water drainage. Other than that, leave both the surface water and water table alone.

http://www.SlideRuleEra.net