I need expert advice on drain installation in clay soil.
I am installing channel drains and a low retaining wall (300mm H) in front of my house, but the soil turned out to be very heavy clay.
What I have done so far:
1. dug about 4-500mm deep X 600mm wide trench which filled with water. See pic 1
2. put 2-300mm MOT Type 1 compacted with rammer, then poured 200mm C-20 slab with rebars. Laid 2 rows of concrete blocks on the flat side.
There is a gas pipe that crosses the middle where I left a 300mm opening. So the concrete slabs are in two parts 4m long, and V-shaped section 3m&2m. See pic 2 & pic 3
3. The triangle section has about 200mm deep MOT (rammer compacted) and temporarily some leftover concrete about 30-50mm thick to avoid the MOT being waterlogged while work was ongoing. See pic 2 & 4
Problem:
After I installed the drain channels with mortar, water started to fill the triangle section.
the water mainly came up from the gap between the two foundation slabs.
The back of the retaining wall is also filled with more water. Before installing the channels, the triangle was fairly dry. See pic 4 & pic 5
My original plan next was:
To fill the triangle section with another course of MOT to raise the level and pour concrete slab (about 100mm) to finish off.
So the triangle will have:
• MOT (200mm deep) and temporary concrete layer (30-50mm) – already installed
• some more MOT (hand compacted), and final C20 slab pavement – plan to do
Questions:
• Is it ok to simply fill the space with MOT and concrete? Or would it trap water there and damage the foundation of the wall or worse the foundation of the house?
Or if I install a drainage system as suggested below, would it create an uneven moisture level in the front and the back of the house and causes damage to the foundation?
• Is it better to keep the temporary layer of concrete in the triangle intact or break it up like hardcore so the water can flow?
• Since the water is mainly coming through the gap between the slab, is there any point installing a perforated drain pipe behind the wall which is higher than the gap?
Solutions I can think of are:
a. Remove part of the drain channel I installed and put French drain next to the gas pipe and triangle section and connect it back with the rest of the drain channel.
I would like to avoid this, if possible. See pic 6
b. As the water mainly comes up from the gap for the gas pipe section and the triangle. I can drill some holes in the bottom of the drain channels and turn the section into like French drain. See pic 6
c. Or fill the triangle as I originally intended and ignore the water.
I also added pic 7 (a Cross-section view) for reference.
Could you please advise what is the best option forward?
I have been working on it for six months and really want to finish it soon.
Thank you
pic 1
pic 2
pic 3
pic 4
pic 5
pic 6
pic 7
