Excavation and backfill

What it does

Create and close an excavation while supporting its sides and the new basement wall.

Coordinate excavation support, wall construction, drainage, bracing and backfill.

When it fits

Learning the dependencies in basement construction.

The trade-offs

Groundwater, adjacent loads and soil condition can make a generic excavation arrangement unsuitable.

Context. An educational model. Geometry, material assumptions and the reference building are described in the illustrated study.

A basement is built in a hole, and the hole is the first structure on the site. It has to stand open for weeks while people work at its bottom. It has to be exactly the right size, square and level, because everything the house becomes is set out from what is cast in it. Then it has to be filled back in without harming the wall it took so long to build. The order of the work is not habit: each step makes the next one safe or possible.

Lines in the air

The surveyor’s stakes mark the corners, but they stand exactly where the excavator is about to dig. So before anything is dug the crew sets batter boards: pairs of stakes with a level board across them, driven well outside the hole on the line of each wall. All of them are set to one height taken from the site’s benchmark, here 4 ft above the top of the finished wall. A string stretched from board to board marks the outside face of a wall in the air, and where two strings cross a plumb bob drops the corner to the ground. The strings come down while the machine digs and go back up on the same nails afterwards, so the corner can be found again at the bottom of the hole.

Two checks prove the layout. A rectangle is square when its diagonals are equal; for this 36 × 24 ft house both measure 43 ft 3 in (13.19 m). Within a corner the crew uses 3-4-5 at a larger scale: marks 6 ft along one string and 8 ft along the other must be exactly 10 ft apart. Good layout closes to about ¼ in (6).

A hole that cannot fall in

A cubic yard of soil weighs about 3,000 lb (1,360 kg). A wall of it that slides into a trench pins whoever is standing there before they can move, which is why cave-ins are among the deadliest accidents in construction. In the United States the rules are OSHA’s: any excavation 5 ft (1.5 m) or deeper needs a protective system unless it is entirely in stable rock, and beyond 20 ft (6.1 m) that system must be designed by an engineer.

OSHA type	The soil	Steepest slope	Benching
A	Cohesive, unconfined strength 1.5 tons/ft² or more: hard clay	¾:1 (53°)	Allowed
B	Cohesive, 0.5 to 1.5 tons/ft², angular gravel, silt, anything A that is fissured or dug before	1:1 (45°)	Allowed
C	Granular soil, or any soil that is wet, seeping or submerged	1½:1 (34°)	Not allowed

The stiff silty clay under the reference house is Type B. Dug 10 ft (3.0 m) down at its uphill corner, the banks run about 9 ft (2.7 m) back from the over-dig at 1:1. Change the soil in the figure and the banks lie back or stand up, and the volume follows: about 470 cu yd for the whole basement in hard clay, 526 in ours, 651 in sand, before the soil bulks up by a quarter in the trucks.

Over-dig: 2 ft (610) of working room beyond the walls on every side, for forms, coating and the drain.
Spoil: Piled at least 2 ft back from the edge, as are machines: their weight on the edge is what starts a bank sliding.
Getting out: In trenches 4 ft deep or more, a ladder or ramp within 25 ft (7.6 m) of anyone working.
Inspection: A competent person looks at the banks every day before work, and again after every rain.

How do the sides of the dig stay up?

Decided by soil type · depth · room on the lot · groundwater · cost

Slope the sidesThis house
The banks are laid back at the angle the soil can hold: ¾:1 in hard clay, 1:1 in medium soils, 1½:1 in sand or anything wet.
Choose it when there is room on the lot: a 10 ft deep dig in Type B soil runs about 9 ft past the over-dig on every side, 14 ft in Type C; the soil is dry and stands on its own; the dig is under 20 ft deep; deeper than that, an engineer must design the protection.
The catch the most soil to dig and to put back: going from 1:1 to 1½:1 adds half again to the banks; spoil piles, machines and trucks have to stay at least 2 ft back from a wider edge; rain softens the banks, so they are checked after every storm and before every shift.
The banks, by soil type No. 11 →
Bench the sides
The banks are cut as steps no more than 4 ft high, set back so their edges stay on the safe slope.
Choose it when the soil is cohesive, OSHA Type A or B, and stands vertically for a few feet; the dig is deep enough that a straight slope would run a long way back; workers and small machines need somewhere to stand partway down.
The catch not allowed at all in Type C soil: granular or wet ground runs out of the risers; each riser is a short vertical cut that can still slough when it gets wet; it saves only part of the digging a slope takes, and takes more care to cut.
Benches in clay No. 11 →
Shore the sides
The sides are cut straight down and held: steel soldier piles with timber lagging, interlocking sheet piles, or a trench box.
Choose it when the lot is tight: a neighbour's foundation, a street or a lot line lies within the width a slope would need; the soil cannot stand on its own: Type C sand, or ground below the water table; the dig is deep enough that the banks would take up the whole yard.
The catch an engineer designs it, and a rig drives or drills the piles before the dig begins; it costs several times as much as laying banks back on an open lot; the lagging is usually left in the ground, and pulling the piles can loosen the backfill.
Soldier piles and lagging No. 11 →

Here An 80 ft lot with 22 ft side yards has room to lay stiff silty clay, Type B, back at 1:1: about 9 ft of bank beyond the over-dig at the deepest corner. It is the cheapest safe hole, and the spoil stays on site for backfill.

Forms, steel and concrete

The footing is formed first: boards on edge, staked to line from the strings, two steel bars on chairs inside them, and a bevelled 2×4 hung down the middle that leaves a keyway for the wall to lock into. The inspector sees the open forms and the undisturbed soil at their bottom before any concrete arrives. Then the wall forms go up on the footing. Wet concrete presses on them like a liquid half again as heavy as water, as much as 1,200 lb on each square foot at the foot of an 8 ft wall poured quickly. So the two faces are tied to each other through the wall every 2 ft with snap ties, stiffened by walers and braced plumb from inside.

The wall is poured in one continuous pass round the house, about 23 cu yd (18 m³) of concrete for the walls and 7 for the footings and column pads: three or four mixer trucks. It is vibrated so no voids are left against the forms, and the anchor bolts are pushed into its wet top. A day or two later the forms come off and the tie ends are snapped back inside the wall. The concrete keeps gaining strength for weeks: about two thirds of its 28-day strength after a week, provided it is kept moist and above freezing.

What are the basement walls made of?

Decided by crews and suppliers nearby · height of soil against the wall · water · insulation · season · cost

Poured concreteThis house
8 in of concrete cast in rented forms on the footing, bars at the corners and openings, keyed into the footing.
Choose it when a ready-mix plant and a forming crew are within reach: the walls are formed in a day or two and poured in an hour; the wall holds back a lot of soil, or water: one solid piece has no mortar joints to leak or crack along; the basement will be finished: the walls come out straight, plumb and flat.
The catch forms, pump and concrete trucks must arrive together, and freezing weather needs heaters and blankets; it needs about a week before it is strong enough to backfill against; insulation has to be added afterward, inside or out.
Forming and pouring the wall No. 11 →
Concrete block
8 × 8 × 16 in hollow units laid in mortar on the footing, the cells with bars grouted solid, parged and dampproofed outside.
Choose it when no ready-mix within reach, or a small crew working a course at a time; an addition or an awkward plan where forms would be wasted; masons are easier to find than forming crews.
The catch hundreds of mortar joints: more ways in for water, and a stair-stepped crack where soil pushes too hard; slower: about 1,000 blocks for this house, laid by hand; weak in bending unless reinforced and grouted, which adds labour.
The block wall No. 11 →
Insulated concrete forms
Hollow foam blocks stacked dry, braced and filled with concrete; the foam stays on both faces as insulation.
Choose it when the basement will be lived in and must be warm: about R-22 comes with the wall; a small crew without forming gear builds the walls itself; a cold climate, where heat lost through a basement wall adds up.
The catch the foam outside must be covered above grade, and watched for termites in warm regions; a form bursting during the pour is messy: pour rate and bracing matter; usually costs more than a poured wall with insulation added.
Foam forms, filled No. 11 →
Permanent wood foundation
Pressure-treated 2× studs and plywood standing on a gravel footing, the whole basement drained through gravel to a sump.
Choose it when concrete is scarce or dear, or the season is too cold to pour; a framing crew can build the basement too, in any weather, fast; a finished basement: the stud bays take insulation and drywall like any upper wall.
The catch everything depends on the right preservative treatment and on stainless or hot-dip galvanized fasteners; drainage is everything: gravel under the whole floor and a working sump; less familiar to inspectors, lenders and buyers in many places.
The wood foundation No. 11 →

Here Ready-mix and forming crews are everywhere in the region, 7 ft of clay presses on the back wall, and one solid piece of concrete resists both water and bending best for the money.

Keeping water off the wall

Water reaches a basement wall three ways: as moisture wicking through damp soil, as rain soaking down through the backfill, and as groundwater rising round it in a wet season. Dampproofing, a bituminous coat from the footing to finished grade, deals only with the first. Against water that can press on the wall, the code asks for waterproofing: a continuous membrane, lapped and sealed, that bridges the cracks a concrete wall will eventually have.

The real defence is to give water an easier way out. A perforated pipe at least 4 in (100) across lies beside the footing, holes down, on stone that runs a foot beyond the footing and 6 in above it, wrapped in filter fabric so the clay cannot wash in. Water that reaches the stone falls into the pipe and runs to an outlet: open ground downhill, a storm sewer, or, as here, a sump pit under the basement floor whose pump lifts it out. Above all of it, the ground slopes away from the house, so that most rain never gets near the drain at all.

How is water kept out of the basement?

Decided by how high groundwater rises · how fast the soil drains · how the basement will be used · the grading around the house

Dampproofing and a footing drainThis house
A sprayed or rolled bituminous coat on the wall below grade, with a perforated drain in washed stone beside the footing.
Choose it when groundwater stays well below the basement floor all year; the ground is graded to shed surface water away, so little reaches the wall; the basement is for storage and machines, or will be finished with care inside.
The catch it stops moisture wicking into the concrete, not water pressing on it, and it does not bridge cracks; it fails if the drain clogs or the ground settles toward the house.
Dampproofing No. 11 →
Membrane and drainage board
A rubberized sheet or thick fluid membrane bonded to the wall, and a dimpled board over it that lets water fall freely to the drain.
Choose it when groundwater can rise to the floor in a wet season; the basement will be finished living space, where a damp wall ruins the finishes; clay backfill holds water against the wall for days after rain.
The catch three to five times the cost of dampproofing; it needs a clean, dry wall and careful laps and edges to work; a stone in the backfill can puncture it: backfill against it with care.
Membrane and board No. 11 →
Interior drain and sump
A perforated pipe along the inside of the footing, under the slab, collecting water that comes through or under the wall and leading it to a pumped sump.
Choose it when an existing basement leaks and digging outside is impractical; a high water table, as a second line behind an exterior system; soil gas or radon also needs a sealed sump and a vent.
The catch water still reaches the wall and the underside of the floor before it is taken away; it depends on a pump, and on power: a backup pump or battery is wise; added later, the edge of the slab has to be broken out to lay it.
The sump No. 11 →

Here The water table is 14 ft down, below the 10 ft dig even in a wet spring, and the ground is graded to fall away with a swale uphill. Dampproofing and a footing drain led to a sump pit are enough.

Why the soil waits

Backfilled, a basement wall is a beam standing on end. Soil presses on it harder the deeper it goes; codes design for 60 psf per foot of depth against silty clay, 30 against clean sand and gravel. Against the 7.2 ft (2.2 m) of clay on the back wall that is 1,541 lb on every foot of wall. With the first floor framed and the slab poured, the wall spans between them: the deck holds its top with about 470 lb per foot, the slab its foot with about 1,070, and the bending in the wall peaks at 1.6 kip·ft per foot.

Push the soil back before the deck is on and the wall must stand alone, cantilevered from its base. The same soil now bends it 3.7 kip·ft per foot, more than twice as hard, and the plate compactor working the lift beside it adds as much again. A week after the pour, an 8 in plain concrete wall cracks at about 3.5. So it cracks along the floor line and leans in, and the crack is very hard to repair. Switch the figure to backfill before the deck to see it. That is why the code lets no backfill go in until the wall is anchored to the floor above or braced, and why the crew frames the deck first.

Backfill in lifts

Soil dumped in from a machine lies loose and full of voids. Left like that it settles for years, and the ground that was graded to fall away from the house ends up falling toward it. So backfill goes in lifts of 8 to 12 in (205 to 305), each run over with a plate compactor before the next. Against the wall the material is clean and free-draining, with no stones that could puncture the coating, no frozen clods and no construction debris, and heavy rollers stay several feet back. The top foot is capped with the clay, which sheds water better than sand, and shaped to rough grade: 6 in down over the first 10 ft, then into a swale that turns the water coming down the lot round the house. It is set 8 in low, for the topsoil that comes back last. How the whole lot is shaped is the subject of cut and fill.

How the figure decides

Ground	The reference lot: grade falling 8 % toward the street, topsoil 8 in, stiff silty clay to 12 ft, sand and gravel below, water table 14 ft down
Depth	To the underside of the footing, 98 in (2,490) below finished grade; the interior to the underside of the slab’s stone
Banks	From the over-dig line at OSHA’s steepest slope for the soil; benches 4 ft high with their edges on that slope; shoring vertical behind 3 in lagging
Volumes	The same rules mirrored round the whole 36 × 24 ft basement on the real ground, integrated on a fine grid; swell 25 %, trucks of 12 cu yd
Push	Equivalent fluid pressure from finished grade to mid-slab; a plate compactor taken as 120 psf more over that height
Wall	8 in plain concrete, 3,000 psi at 28 days and 65 % of that at a week; cracks at 7.5√f′c in bending
Leaning	Drawn larger than life, nothing at the corner where the walls brace each other, most at the cut