AboutRobert Cummings, P.E. Expertise I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.
Experience 30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.
Organizations Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.
Publications Mining Engineering, AEG Bulletin.
Education/Credentials BS and MS Geological Engineering
Question We excavated our hole for our home to be built on a 8" high footing and a 24" stem wall. The bottom of the excavation is bedrock (limestone mainly).
Before footings were poured, we noticed some water in the hole after rains, but were not worried. The footings were just poured with some 12" steps due to the bedrock being uneven in the hole. The contractor poured even though there was some water still in the hole. He said all was OK as long as I kept pumping the water out of the hole.
As we were pumping with 2 pumps, we could see water seeping in the bedrock bottom in a few places. Ultimately, we gave up pumping as we were just wasting gasoline. The water was coming in too fast.
Now, after a few days of rain, the hole is 3/4 full of water. It is 1/2 way up the footing on the high side and the other side footing is under water.
I shot some elevations and about 100 - 125 feet away, the rock drops about 2 feet below the lowest level of my hole.
My issue is the route of any drain I would install goes right across where my driveway will go...
How would I best run a drain in this situation. Should I hoe-ram a channel and fill with washed rock? How can I avoid compression from the driveway? A regular perforated pipe probably would not last, I assume.
Any ideas?
Answer Why would the pipe have to be perforated under the driveway? It sounds like you need just to just drain the footing area. Presumably you would lay your perf pipe or french drain all around the interior of the stem wall at the top of the footing, and discharge it through the stem wall to a discharge pipe that would grade downward to the outlet you have chosen.
How big an outlet drain pipe do you think you'll need? Once the house is in place and surface drainage is no longer contributing to the water issue, you'll only have the fracture flow due to the limestone. Unless it's a real big area or the issue is a whole lot more severe than it sounds, a 3 inch outlet pipe should be more than sufficient. By the way, even slotted 3 inch PVC pipe should stand up under a paved driveway provided you bed it all around in crushed stone (1/2 inch minus to avoid point loading) at least 18 inches deep and you do a decent job of compacting the soil above it.
If you are very worried about the collapse of the driveway, use heavy ABS or DIP (ductile iron pipe) under the driveway. It won't be a french drain, but if for some reason you have to include the driveway subgrade in your drainage plans, line the pipe trench in a geotextile separation fabric to exclude fine soils, bed the ABS or DIP in clean crushed limestone (#57 stone or equivalent) vibrated tightly into place, wrap the separation fabric over the top of the stone, and compact the soil to grade, per the geotech specifications (usually 95% of the standard Proctor). At the outlet end of this french drain, provide a second outlet pipe, say 1.5 inch Schedule 80 PVC, and route it to an outfall along with the ABS or DIP.
Make sure the outfall is a location that will not eventually clog with debris or soil.
