The volumes, and then the carrying
The arithmetic is simple and the answer is not. A trench 90 ft long and 12 in wide through a 4 in slab removes 90 × 1 × 0.333 = 30.0 cubic feet of concrete. Going 12 in below the slab adds 90 × 1 × 1 = 90.0 cubic feet of soil. That is 120 cubic feet of material in place. Broken up it bulks — 25 percent is a reasonable working figure for mixed rubble and dug clay — giving 150 cubic feet loose, which is 1,122 gallons. In five gallon pails that is 225 trips.
| Line | Default 90 ft trench | Arithmetic |
|---|---|---|
| Saw cut | 90.0 ft | 90 ft × 1 cut |
| Slab removed | 30.0 cu ft | 90 × 1.00 × 0.333 |
| Excavation below slab | 90.0 cu ft | 90 × 1.00 × 1.00 |
| Spoil in place | 120.0 cu ft | 30 + 90 |
| Bulked 25% | 150.0 cu ft | 1,122 gallons |
| Five gallon pails out | 225 | 1,122 ÷ 5 |
| Pipe volume displaced | 9.94 cu ft | 90 ft of 4.5 in OD |
| Stone | 80.06 cu ft | 90.0 − 9.94 |
| Stone plus 10%, in tons | 4.57 tons | 3.26 cu yd × 1.4 |
| Patch concrete | 33.0 cu ft | 30.0 × 1.10 |
| 60 lb bags at 0.45 cu ft | 74 | 33.0 ÷ 0.45 |
Read the last two rows together with the first. Two hundred and twenty five pails of rubble go up the stairs, four and a half tons of stone come down, and 74 bags of concrete follow them — around 4,440 pounds of dry mix carried by hand into a basement and mixed there. The material handling is the job. The saw cut takes an afternoon.
Deriving the stone rather than guessing it
The stone quantity is the trench volume minus what the pipe occupies, calculated rather than estimated, so the two always agree. Ninety feet of 4.5 in outside diameter pipe has a cross section of π × 0.1875² = 0.11045 square feet and displaces 9.94 cubic feet — 11 percent of the trench. Ignoring it would put you 11 percent over on the stone, which on a 4.5 ton order is nearly half a ton of material you then have to move twice.
The patch is the slab section exactly, because the stone fills to the underside of the slab and the concrete fills the channel through it. That is why the patch volume and the slab-removed volume are the same number before waste is added. If your detail differs — a thicker patch, a lip, a reinforced edge — adjust the slab thickness field rather than adding a fudge somewhere else.
What the page will not decide for you
Whether an interior drain is the right answer is not an arithmetic question. It moves water that has already got inside the wall to a pump, which is the correct response to some problems and an expensive way to ignore others. Run the water entry organizer first: if the water arrives during rain, from one spot, and there is a downspout discharging at that wall, grading and gutters are a fraction of the cost and sometimes the whole fix. An exterior drain handles the same water outside the wall.
Downstream of the trench there has to be somewhere for the water to go, which means a pit and a pump — the sump pump calculator works out the inflow, the drawdown and the cycle count from a rise you time yourself. And nothing here says anything about excavating near a footing, which is a structural question for an engineer rather than a volume calculation.
Questions people ask
How much concrete do I have to remove for an interior drain tile?
A 12 inch channel through a 4 inch slab removes a third of a cubic foot for every foot of run. Over 90 feet that is 30 cubic feet, which is 1.11 cubic yards or roughly 4,350 pounds at 145 pounds per cubic foot. The excavation below the slab is usually much larger — a foot of depth over the same channel adds 90 cubic feet — so the soil, not the concrete, is most of what goes up the stairs.
How many buckets of spoil come out of a basement drain trench?
On the default 90 foot run with a 12 inch channel, a 4 inch slab and 12 inches of depth, the spoil in place is 120 cubic feet. Bulked 25 percent it is 150 cubic feet, or 1,122 gallons, which is 225 five gallon pails. If a wheelbarrow can get to the stairs it is about 25 barrow loads instead, but the pails are what most basements actually allow.
How much gravel does an interior perimeter drain need?
Trench volume less the pipe. Ninety feet of one foot square trench is 90 cubic feet, and 90 feet of 4.5 inch outside diameter pipe displaces 9.94 of it, leaving 80.06 cubic feet or 2.97 cubic yards. Adding 10 percent and taking 1.4 tons per cubic yard gives 4.57 tons. Ask your supplier for the tons per yard figure on the actual stone, because washed drainage stone varies between about 1.3 and 1.5.
How many bags of concrete patch a drain trench?
The patch volume is the slab section — 30 cubic feet on the default run — because the stone comes up to the underside of the slab and the concrete fills the channel through it. Adding 10 percent gives 33 cubic feet, and at 0.45 cubic feet a bag that is 74 sixty pound bags, about 4,440 pounds of dry mix. Read the yield off the bag you are buying rather than using the default; 80 pound bags are commonly around 0.60 cubic feet and would take 55 instead.
Is cutting the slab next to the foundation wall safe?
That is a structural question and this page does not answer it. Excavating alongside a foundation wall or below the level of the footing can undermine what the house is standing on, and how close and how deep a trench may go depends on the wall, the footing and the soil. A structural engineer or a foundation contractor who has looked at your basement decides that. Separately, cutting concrete produces respirable silica dust in a space with nowhere for it to go, which is a hazard for everyone in the house and not just the person holding the saw.