Backfilling
basics
Backfilling the wrong way often costs
more than doing it right
BY R. T. BUCK BARTLEY
ackfilling—an essential step in basement construction—is all too often the beginning of
basement destruction. Done improperly,
backfilling causes problems that may start before the builder leaves the site and may remain long
after he’s gone. But the problems can be avoided by taking a few simple precautions.
There are four key elements to proper backfilling:
• Protecting the foundation wall from damage during
backfilling
• Using the right backfill materials
• Compacting the backfill
• Final finishing the subgrade to ensure that water
drains away from the foundation
Done correctly, these four steps help to ensure that a
well-built basement functions the way it’s supposed to,
providing the homeowner with dry, usable living space.
B
Protecting the wall during backfilling
The surest way to protect walls from damage during
backfilling is to backfill only after the basement floor
slab and the first-floor deck are in place. For the deck,
sill plates should be bolted down and joists nailed to the
sill plate. This provides the restraint needed to make the
wall strong enough to resist earth pressures caused by
backfilling.
Home builders seldom want to wait until the deck is
in place, howe ve r. Early backfilling speeds the framing
process by making the foundation more accessible for
carpenters so they don’t have to ramp in.
An alternative is bracing the walls before the deck is
built. We do this for all straight walls longer than 24 feet
that don’t have offsets or pilasters. Corners, offsets, and
pilasters add structural stiffness to the foundation wall.
But we figure as a rule of thumb that the stiffening effect
is negligible more than 12 feet away from the corner.
Hence straight spans longer than 24 feet need bracing.
We use ve rtical bracing—steel braces fabri c a t e d
f rom 3-inch-diameter schedule 40 pipe—which we
quick-bolt to the wall and slab or pier pad as shown in
Figure 1a. We also modify some shoring jacks from a
form manufacturer to do the same thing. Another op-
Figure 1. Vertical or horizontal bracing can be used to
protect walls from damage during backfilling. Vertical
braces can be made from steel pipe (drawing a) or lumber
(drawing b). Horizontal bracing (drawing c) works well for
straight walls up to 48 feet long.
tion is bracing with lumber using
scaffold boards or 3x12s as shown
in Figure 1b.
If the slab and pier pads can’t be
used for bracing, horizontal bracing
as shown in Figure 1c is usually adequate. Attach 3x 12s to all four
walls, parallel to and about 4 feet
above the footing. Then toenail
3x12s diagonally across the corners
as shown. Add 2x4 cleats and kickers
to prevent slippage and flex. This
system works well for walls up to 48
feet long. Beyond that length additional intermediate vertical bracing
is needed, attached to pier pads,
footings, or wood
stakes firmly driven into the
ground.
Although most
building codes
require
that
foundation walls
be adequately
braced
before
backfilling, most
home builders
don’t use braces
for poured concrete walls. They
believe that the
concrete is strong
enough to resist
p re s s u re caused
by
backfilling.
And they may get
by without damaging the wall.
But too often a
wall gets pushed
in (Figure 2).
Sometimes this
happens during
the winter when
concrete doesn’t
gain
strength
quickly because
of low temperat u re s. Sometimes
it’s because the excavating contractor’s equipment
gets too close to
the wall. And
sometimes rain
saturates the area
around the foundation soon after
it’s backfilled, inc
reasing soil presFigure 2. Backfilling pushed in this unbraced wall and
s
u
res enough to
cracked it. The diagonal crack was repaired by the home
cause a wall failure.
builder but the wall still leaked when the owner moved in.
Fixing the problem required excavation, pushing out the
Whatever the
wall, and installing new waterproofing at a cost much
cause, the home
greater than doing the job right the first time.
builder is left with
a costly problem. Fixing it will require pushing out the bow in the
wall and repairing the crack with
epoxy injection. As insurance
against a wall failure, having two
carpenters work a couple of hours
bracing the wall is a good investment.
Choosing the right backfill material
Some people recommend backfilling with a free-draining granular
material. I’m not so sure that’s always a good idea. When you dig a
hole in the ground for a foundation,
water moves toward the hole just as
it does in a well. A granular fill
makes it easier for water to move toward the foundation wall. And if the
drain tile system gets plugged, that
could cause problems.
I think the object should be to restore equilibrium, as nearly as possible, to the soil moisture system.
That usually means putting back in
the same soil that came out. But not
if it’s a poor soil. If heavy, moistureladen, or expansive clays are removed during excavation, granular
backfill with an exterior drainage
system would be the best choice.
Also avoid backfilling with large
clumps of clay or with soil full of
roots, tree branches, or other organic materials (Figure 3). These materials won’t compact well and will
hold a lot of water even if the
ground slope is steep enough to carry surface water away from the
house.
The amount of overdig for a foundation is about 3 feet. Splash blocks
for downspouts don’t usually carry
water more than a foot or two away
from the foundation (Figure 4). So
most of the water that comes off the
roof may still end up next to the
foundation if the backfill is porous.
Compacting the backfill
As backfill soil settles, drainage
patterns change and water may flow
t ow a rd the house or pond next to
the foundation. Compaction is
needed to reduce the amount of soil
settlement that occurs with time.
Fa i l u re to compact the soil can
Figure 3. Backfilling with soil full of tree roots leaves a
porous fill that can cause basement leakage. The fill can’t
be properly compacted and will settle with time.
Figure 4. Splash blocks often don’t carry water from
downspouts beyond the backfilled area. Then water from
the roof saturates the backfill.
trough caused by
settlement and
s t a rted leaking
t h rough shri n kage cracks in the
foundation wall.
Co r recting the
faulty dra i n a g e
stopped
the
leaking .
Be careful when
compacting backfill. Some compactors are powerful enough to
damage the wall.
One way to avoid
this problem is to
place the backfill
in 6-inch-thick
lifts and have the
laborers tamp it
with foot pressure
or hand rammers.
Vibrating plate
compactors also
can be used for
granular backfill
without damaging
the wall. It takes
more time to compact thin layers,
but the method
provides insurance against wall
damage
and
against call-backs
related to drainage
problems.
Final finishing
to the correct
grade
Di ve rting surface water away
from the foundation is one of the
most
effective
Figure 5. Settlement can occur rapidly in poorly compacted
ways
to
pre ve n t
backfills. Water collecting in the low spots may cause
basement
leakbasement leakage even before the house is completed.
age. The minimum slope for
cause basement leakage even befinished grade should be 1/2 inch
f o re the house is completed. Fi gper foot for at least 10 feet away
u re 5 shows the results of poor
from the foundation. This is espeg rading and no backfill comcially important if there are landpaction. In this case, during rainy
scaping beds near the house. Waterwe a t h e r, water collected in the
ing plants in low-lying areas close to
the foundation saturates the soil
and makes basement leakage more
likely.
If the house is set high enough on
the lot, getting the correct grade for
drainage isn’t difficult. Too often,
howe ve r, houses are built too low.
Then instead of the code-re q u i re d
8-inch minimum between the sill
plate and exterior grade level, the
house might be built with the finished grade only 4 to 6 inches below
the plate. To avert this problem,
avoid making the excavation too
deep and set footing elevations high
enough.
Avoiding basement leakage is a
lot like preventing a roof from leaking. You need to keep the water
moving and prevent it from collecting in one spot. Correct backfilling
methods help to accomplish this
goal.
Buck Bartley is a general contractor
in Silver Spring, MD. He’s a former
p resident of the Poured Concre t e
Wall Contractors’ Association, now
called the Concrete Foundation Association.
Publication # C910315
Copyright © 1991, The Aberdeen
Group. All rights reserved