Chapter 18.
Walls calling
A main function of walls is to hold the roof up.
Walls separate heat from cold and wet from dry. They also separate
good from bad, at least until such time as the bad get out on parole. Walls
make doors necessary and windows appropriate, and are quite useful to
hold the roof up. Walls should be in every Hall of Fame, which indeed
they are, because without walls there would be no halls.
Walls that hold back soil are called retaining walls until such time
as they quit retaining and fall over, which is the topic of this chapter.
Gravity walls. Anybody attempting a retaining wall should be made
aware of the gravity of the situation and the need for some heavy lifting.
Most retaining walls are called gravity walls because gravity, which acts
to hold them down, also acts to hold them up. Nobody said this was going
to be easy.
The successful gravity retaining wall plays the game like a football
lineman, heavy enough and wide enough that it is no pushover when the
ball is snapped. The wall that is too light or too thin either bulges, slides,
or tips over. These matters constitute a design problem, which is a happy
circumstance for an engineer. Because not everybody gets that pumped up
over equations, we will skip the design part and get right to the principles.
That does not mean that everybody then will know how to build a wall;
better to say that they then will know better how not to build one.
Foundations. Every wall, like every house, requires a firm
foundation so it does not sink into the ground. In addition, the foundation
for a retaining wall must be deep enough and strong enough to keep the
wall from kicking out at the toe. Short walls can be tied down to timbers
or a poured concrete pad that extends down into firm soil that holds the
toe of the wall in place. For high walls, the foundations are as carefully
designed--actually more carefully designed--than they are for a house, and
in some cases are put on piles.
Landscape walls. The garden-variety retaining wall often is laid up
with treated wood timbers, old railroad ties, or even hollow concrete
blocks. These elements seem heavy when you lift them, but are
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lightweights compared with the soil that they are supposed to retain. For
example, wood that floats on water must weigh less than 62.4 pounds per
cubic foot, which is the density of water, whereas a typical wet soil
weighs in at about twice that much. A standard concrete block would
weigh over 90 1b if it were solid, but only weighs about 55.
A simple test that will predict if the landscape wall will be stable is:
Can the soil stand that high by itself, without any wall? The wall then
becomes a decoration and defense against erosion, a kind of steep version
of a layer of sod. If the soil can stand up unaided, the chances are good
that it also can stand without a prop, so long as the prop does not prevent
water from draining out and making the soil heavier.
With most soils, the maximum safe height for a plain timber wall is
about 3 feet. Plain rock walls may go 4 feet because rock is heavier. In
either case the wall should be
sloped back, or battered
Welcome to our cooking
show.
Many commercial Soil
landscaping walls are
available and come with
directions on the box. Some
walls use simulated stone
blocks cast from concrete,
with protrusions at the back
that prevent slippage and automatically build in a tilt or batter. Other
types create soil-filled boxes, discussed farther along in this chapter.
Built with a tilt; the flatter the batter. Anybody who ever has
pushed a stalled car knows you have to lean into it. The same applies to
retaining walls; they should have some lean or batter in order to work.
There is a simple reason for this: A bit of a lean puts the center of weight
of the wall farther back from the toe, giving better leverage against
tipping. The flatter the batter or wall angle, the longer the lever arm
against tilting, and the
safer the wall is from tilting. Timber or tie walls
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Bod
Batter increases the lever
Active pressure and
orm, xwall
movement. A secret help for
retaining walls is that soil and wall
work together to hold each other Toe buried to prevent kick—out up. Soil
has internal friction from grains rubbing and pushing against each other
like bubbles in a tub. Intemal friction is what allows sand to pile up
instead of running out flat like water. In fact, the maximum angle that a
sand pile can make with the horizontal is called the angle of internal
friction.
A soil must move slightly in order to engage the clutch on its
internal friction, which means that a wall built to retain it must be allowed
to move a bit, too. Retaining walls therefore are built with a tilt in order to
tip straight. If they are built straight, they will tip and make an overhang
that is psychologically unsatisfying.
If a retaining wall simply cannot be allowed to budge, as where it
ties into a building or bridge abutment, the wall must be made strong
enough to compensate for idleness of the soil and its corresponding lack
of strength and self-esteem. That is because with zero wall movement, the
soil does not achieve the active state, and exerts higher pressures that
engineers call earth pressure at rest.
Tiebacks and deadmen. One way to get better stability in a timber
wall is to turn an occasional board 90 degrees so it juts back into the soil.
The backward projecting timber is a tieback, and must be tied into the
wall. The other part is held in place by friction from the soil. In
landscaping walls, tiebacks should be set slightly below the mid-height
and spaced about every 4 to 6 feet along the wall. They should be at least
as long as half of the wall height, plus some extra length for better
anchorage.
A timber wall that is Better thon botter thus tied back still should be
built no higher than about 5Tiebock or 6 ft
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without some Steel spikes attention to design, and not Friction ever where
a wall failure could be perilous.
Tiebacks also are used in high walls designed by engineers for each
specific site and purpose. The tiebacks may be steel cables or rods that are
fastened to a soil anchor called a deadman, a further indication of how
risky it can be around a construction site. "Deadman" refers to a buried
plate, pipe, log, car axle, or something else tumed crosswise in the soil to
make an anchor. More common nowadays is to use an augerlike plate that
is screwed into the soil, or to set ends of the rods or cables in soil using a
grout.
Uneven pressures on Distribution of soil pressure walls. Just as the
pressure from water is not even on the
Woll Clossicol
sides of a bucket but is highest at the
tilt to theory
bottom,theory pressure from soil is
uneven measurements against a Soil
retaining wall, and is not arching highest
at the bottom. Most design ond
equations and computer energize
programs still assume that it is, soil which
shows the futility of
measurements once a belief has become firmly established. Measurements
made by Terzaghi in the 1920's showed that soil pressure is highest near
the wall mid-height, and observations indicate that walls bulge out at the
middle before they kick out at the bottom. The most recent theory to
explain this is based on something called soil arching and seems unduly
complicated.
A timber retaining wall should be held together with more than
good intentions. Timbers can be drilled and held together with vertical
steel rods or spikes. Hollow concrete building block, a poor choice but still
a favorite among do-it-yourselfers who have blocks left over from a
project, can be tied together with steel rods set vertically through the holes
and grouted in with cement-sand mortar. Poured concrete walls routinely
incorporate steel reinforcing rods. Stone walls are laid up with soil or
cement mortar in the horizontal joints, with vertical joints being left open
for better drainage. Another approach is to number each block so when
they fall down they can more readily be put back up again.
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Widening the base. Tying a wall together does not keep it from
flopping over, but only helps it to go over in one piece. Defenses against
overtuming include batter and tiebacks, already mentioned, and
broadening the base of the wall, A simple way to do the last is to lay
timbers in a zigzag pattem. Even more effective is to make boxes that are
filled with soil. ms can quadruple the cost of the wall, but has the
advantage of permanence. Some commercial methods integrate special
concrete blocks into boxes that extend behind the wall and hold soil. Your
local landscaper should be able to tell you more about it.
Height limitations. Wall height is critical, because the higher the
wall, the higher the soil pressure. ms may be why bar stools are made tall,
so people will not embarrass themselves when they stand up and
experience a sudden increase in pressure. One of the most effective ways
to make a wall flop is to make it higher. Another method is to pile more
soil behind it, or slope the soil up from the top of the wall.
A few years ago in a picturesque town in northwestern Illinois, the height
of an existing stone wall was increased from 20 to 30 feet without due process,
meaning that no engineers were involved, only attorneys. When the wall crashed
to the ground, stone blocks weighing 6 tons were thrown down with the
acceleration of a drag racer and the impact of a locomotive, smashing two
automobiles down to curb height with a low curb. The results were sudden, tragic,
and irreversible for unsuspecting occupants of one of the automobiles. Another
person escaped certain death by a matter of a few seconds.
Soil pressure, like water pressure, increases with the height of the
wall. Because force equals pressure times area, height gets its licks in
twice. Thus, doubling the wall height increases the soil load against the
wall by a factor that is: double x double = 4.
But that's not all: Increasing the wall height also increases the
height of the center of pressure, that is, the length of the lever arm causing
overturning shown by y in the sketch. That means that height gets its way
three times, each time as a
multiplier. Thus doubling the
height increases the tendency the better
wall weight
for the wall to tip over by a
pressure
factor of double x double x hod better
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Hinge point Sofety factor Wx/py
=
double 8! Actually, things are not quite that bad because the wall itself is
heavier.
In order for a gravity wall to be safe, as it is made higher it must be
made proportionately thicker at the base. Making it twice as high means it
should be twice as thick, increasing its weight and cost by a factor of 4.
Then the resisting lever arm, shown by x in the sketch, also will increase
by a factor of 2. This in combination with quadrupling the weight should
increases stability by the required factor of 8.
Putting more weight on soil behind the wall, whether from a
building or a parking lot, will decrease wall stability. This effect can be
minimized by using a horizontal setback distance that should at least equal
the height of the wall. The other option is to make the wall stronger.
A predictable misfortune that sometimes occurs during basement
construction is when a loaded ready-mixed concrete truck gets too close to an
unsupported basement wall, so the wall collapses. Plhere is an added urgency if
the wall is freshly poured concrete that must be removed before the concrete sets.
A fruck should come no closer than the height of the wall.
Cutting some corners. Because concrete is expensive, one way to
save big money is substitute soil
wall
weight
keep from
over-
Hinge point
weight for concrete weight by the use
of some clever design tactics. A
concrete flange can be extended out
underneath the soil, making what is
called a "cantilever wall." Bin walls
are simply rectangular or cylindrical
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bins filled with soil to make them
heavy.
Reinforced Earth is a patented
method that originated in France and now is widely used throughout the
world: The wall is composed of vertical concrete panels held in place by
horizontal metal strips running back into the soil so the strips act as
tiebacks, tying the soil to the wall. Numerous friction strips are used,
arranged throughout the height of the wall, and are encased in sand to give
good friction.
This same bootstrap approach has been extended to include the use of
molded plastic strips instead of steel, or even old tire sidewalls wired together.
'Ihese all are fastened to the wall and project back into the soil in layers.
Generally, the wall thickness defined by the length of the reinforcement should be
at least 80 percent of the wall height, so a 20-foot high wall will be 16 feet thick.
This still is a spacesaver compared with no wall at all.
Gabion walls. "Gabion" is French for cage or basket. A gabion wall
is made up of wire baskets
that are fastened together and
filled with stones. Gabion
walls drain well, but require
hand labor to assemble and
fill.
Drainage. Saturation of soil behind a retaining wall decreases the
soil weight by buoyancy, but adds water pressure undiminished by intemal
friction. The result is to almost double the total force on a typical wall.
That is why every retaining wall must provide some kind of drainage, or
the wet year is when the wall will fall over.
Timber tie walls drain freely, which is an advantage of this type
wall. Walls laid with mortar should have vertical joints left open to give
drainage. Solid concrete walls must incorporate either drains or "weep
holes" at the bottom, and a free-draining soil or a synthetic fabric drainage
mat called a geotextile behind the wall.
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One way to subvert How to droin o wolldrainage
is put drain tile in shallow,
because a tile can only drain (Whot
the soil that is above it.
Depth is a critical factor for
the same reasons that wall
height is a critical factor, and
putting the drain half-way up
the wall is only
it riaht!
about half as effective as putting the drain at the bottom. Putting the drain
at the top is like putting a water spigot at the top of a bucket; regardless of
whether the spigot is open or closed, no water comes out and the bucket
stays full.
Soil type and drainage. The best soils for good drainage also have
high intemal friction, such as coarse sand or crushed stone. The worst soil
for drainage also has the lowest intemal friction, clay. The clay problem is
compounded if the clay is expansive, because if and when it expands, the
wall will bulge or tip. The safest walls therefore are backfilled with sand
or crushed stone, and clay is used only if absolutely necessary, or where
nobody cares much or notices if the wall bulges and tips through the
decades.
Basement walls.walls Why need
Basement walls are retaining basement
reinforcementwalls that get some steel lateral
support from floors at the top and bottom,
to oppose soil pressure that is applied near
the middle. Poured concrete walls
therefore incorporate
steel to prevent bowing inward.
block walls may have steel rods Soil
Il reinforcing
Concrete
running
down through the holes and pressure grouted in
place with mortar--or they may not.
There is a strong analogy BUI ge between a
concrete block wall and a juggler
performing a box trick, where a stack of boxes is held by pressure applied
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at both ends of the stack. In a block wall, end pressure comes from the
weight of the house. If the house is gone or partly lifted by a strong wind
or tomado, the wall may crack or buckle.
Sometimes a basement wall is buttressed on the inside with
"pilasters" or vertical columns against the wall. These are effective only if
they are tied into the wall and have steel reinforcement to take tension, as
they tend to buckle inward.
A few worthwhile suggestions. Low walls, about 4 feet or less in
height, can be built without benefit of an engineering design provided that
when they fall over they won't hurt anybody. It still is necessary to
provide a
good foundation, good soil, and good Getting some height
drainage. Plans are available in while minimizing
risk_
do-it-yourself books and university extension
bulletins.
Heights higher than 3 or 4 feet can be
achieved by terracing--that is, 4 feet up, then a
4 foot-wide level place; then another 4 feet up,
and so forth--but we still must be aware that
more than a couple of steps can increase the
possibility for a deep adjustment called a landslide.
Wall heights up to about 6 feet often can be achieved with
commercial systems where the seller provides the design and may build
and guarantee the wall. Any heights over 6 feet, or any wall where failure
could be perilous, probably will require design services of a professional
civil engineer. In complicated situations, two kinds of civil engineer may
work on a wall problem, a geotechnical engineer to determine the soil
factors, and a structural engineer to design the wall.
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SlJNffvfARY OF n€PORTANT
Il Height has a tremendous influence on stability of retaining
walls, and doubling the height can quadruple the cost.
Il Drainage is the second most important factor because water
pressure that is undiminished by intemal friction directly adds to the
lateral force pushing on the wall. Because height also affects water
pressure, drains must be at the bottom of the wall.
Il The completed wall must be allowed to slightly tilt or bend with
the pressure in order to mobilize the full soil strength and decrease
pressure on the wall.
Another important consideration is surface load behind the wall,
which can include ground sloping upward behind the wall. This increases
pressure on the wall.
Il Simple timber or rock walls intended for landscaping should be
no more than 3 or 4 feet in height so when they fail over they won't hurt
anybody. A safe height may be indicated by the height that a soil will
stand without any wall holding it back. Larger heights can be achieved by
altemating short walls with terraces at least as wide as the wall is high, or
using tiebacks.
Il Commercial wall systems involving interlocking stones or
blocks are available with valuable design suggestions for do-it-
yourselfers.
QUESTIONS
1. Does a seawall hold back soil or does it hold back the sea?
--Heathcliff Ans. Both, from each other.
2. We want to plant flowers behind our rock wall, and will have to
keep them watered. Will that hurt the wall?
--Flower Power
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Ans. Put a layer of plastic undemeath the flower bed or else plant cactus.
3. Our next-door neighbor has a 5-foot wall made out of concrete
block. The wall is bulging and leaking water. It is right next to our
driveway.
What do you recommend? --Wesley Meek
Ans. Make your neighbor park his car in the driveway until after he gets
the wall fixed.
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