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Frequently
Asked Questions |
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- How high can retaining walls be built without the use of geogrid?
- Retaining walls can be constructed between 2' (.6m) and 6' (1.8m)
high depending on the type of unit, soil conditions, amount of batter
used, and surcharge on top of wall. The best way to determine if your
wall will require the use of geogrid is to consult with A. Jandris &
Sons.
Are concrete footings ever needed or required?- Most retaining walls can be built directly on 4-6" (100-150mm)
of well compacted granular base. However, there may be occasion to consider
the use of a concrete leveling pad. Examples include: applications in
or near water, a taller wall built on soft sub-soils, or a wall that
is very long. By using a concrete footing, the contractor can speed
up the installation process for larger walls.
Can I use sand to help level the units?- Yes. After the road base material has been leveled and compacted, 1/2'
(13mm) to 1" (25mm) of sand may be used to help speed up the leveling
process.
Can adjustments be made on a concrete leveling pad?- Minor inconsistencies on a concrete leveling pad will not usually
create much of a problem. However, if there are noticeable differences
in block height when the units are placed due to low spots in the leveling
pad, a thin layer of sand or mortar may be used to help the leveling
process. High point inconsistencies may require some grinding. Make
all adjustments as gradual as possible. Before you begin laying the
base course, be sure to check that the leveling pad is level front to
back. Make corrections as needed, especially if the back of the footing
is higher than the front. It is important to note that taking the time
to accurately level and finish off the concrete leveling pad will allow
for minimal adjustment time and greatly speed up the installation process.
How many units should be buried?- Typically, 1" (25mm) is buried for every 8" (200mm) of wall
height with a minimum of 6" (150mm) below finish grade. In most
cases, other than where the grade slopes away from the wall at the base,
3 to 4 units are the maximum to be buried on tall walls. Burying more
than that does not create an added benefit. Unit burial is not a function
of building below frost depth in cold climate areas like rigid wall
systems, but is meant to provide a resistance to base exposure form
erosion to grade in front of the wall.
Is there a way to figure how much setback there will be per course before
construction of the wall begins?- Yes. Level three units side by side and install pins in the preferred
wet of pin holes. Set the next course of units on the three you just
leveled and slide the forward toward the wall face so they are in full
contact with the pins. Measure the distance the second course tails
are overhanging the units below. This will give you your true setback
per course.
Should I always begin construction at one end of the wall or is it OK
to start in the middle?- Construction of the wall should begin at your lowest point when ever
possible. If the wall is going to tie into building or structure, measure
the distance from the corner of the unit to the edge of the building
and make sure the distance is in an increment of 18" (457mm) (full
unit width).
What size rock is best suited for filling in and around the units in
the drainage zone?- A clean, angular 3/4" (20mm) rock is best for core fill, if available.
Otherwise, use a clean rock material that is 1/2-1-1/2" (15-40mm)
in diameter. Avoid aggregates that are round in nature. Angular material
will provide the best interlocking strength. Also avoid material that
contains a lot of fine grains because these fine grains can flow with
water through the wall and possibly stain the wall face.
How much rock do I need to use?- Adequately fill all open cores and 12" (305mm) behind the unit
when using the Compac II unit. The additional rick behind the unit provides
for better drainage and eliminates the need for compaction equipment
directly behind the wall. For Standard II units, core fill needs only
to be placed in the open cores to the back of the tail.
How high can units be stacked before placing unit core fill and back
fill?- A. Jandris & Sons recommends adding core fill and backfill after
each consecutive course for the Compac II units. This insures that all
voids are properly filled with rock providing maximum interlocking strength.
I also aids in keeping the wall straight and reduces the amount of wasted
rock.
What type of material should be used to backfill?- Granular materials such as rock and sand are best if available. These
types of material compact fairly easy and won't hold moisture that can
increase the weight of the soil behind the wall. Retaining walls can
be effectively built with silty material and lean clays, but these types
of soils require more compaction and care should be taken not to place
these materials when they are wet. High clay soils that shrink and well
rapidly as well as organic soils should be avoided.
How often do I need to compact the fill soils?- Compacting backfill material in 8" to 12" (200-300mm) lifts
allows you to effectively compact the entire area behind the wall without
putting unnecessary pressure on the units. Thick lifts of soil require
more compaction effort and create a greater force at the back of the
wall, which may cause potential alignment and rotational problems. Consult
with a geotechnical engineer for further compaction criteria based on
specific site soil.
How often should the wall's alignment be checked?- Wall alignment should be checked at least every third course by visually
looking down the wall or using a string line along the pin holes or
tail positions. The wall should also be checked every 15-20' (4.6-6.1m)
to make sure the units are level from front to back. If the bubble on
the level is high to the back, this means the wall is building to negative
batter (leaning forward) and needs corrective measures.
How can Ii fix units that are out of level?- If the units are leaning back toward the embankment, cue to geogrid
thickness or units being thicker in front than in back, this is generally
nor a problem in that the batter is increased. However if the space
is limited on top of the wall, this could be a problem because the wall
is setting back faster that expected. To correct this problem, you may
uniformly insert shims under the tails to bring the units back to level.
The best material for this would be excess geogrid, pieces of asphalt
shingles, or other appropriate non-deteriorating materials. Avoid using
wood or materials that will deteriorate over time. Care should be taken
to make adjustment in small increments. If the units are rotating outward
and higher at the back of the unit, the problem should be addressed
immediately. If the tails are higher than the fronts by more than 3/4"
(20mm), disassembling a portion of the wall should be considered. The
same guidelines and materials for shimming the back of units may be used
for the fronts as well. For minor adjustment, tapping down the back
of the units with a maul for dead blow hammer may also help.
How can units that are out of alignment be fixed?- To determine which units are out of alignment, run a string line across
the pin holes before the pins are placed. Adjust the misaligned units
by sliding back and forth until the pin holes are in alignment with
the string line. Don't be alarmed if you look down the kidney shaped
holes and you see that these are not in full contact with the pins below
due to the adjustment. The gravel fill should adequately fill in around
this area to secure the unit against the pin. If the unit type you are
using has the dual pin option, either position may be used if it helps
the alignment process.
When building gravity walls with terraces, what is the recommended distance
between the terraced walls where the upper walls does not affect the
loading condition of the lower wall?- The distance between terraces (face of wall to face of wall) should
be greater than or equal to two time the lower wall height. Typically
this can also work for lower height soil reinforced walls.With higher
reinforced walls or walls built on a slope, the issue of global stability
must be considered by a qualified engineer in analyzing the terrace
situation.
What are the recommended methods to avoid settlement when building a
wall that has terraces converging into a single wall?- If at all possible, build on bench cut of virgin soil conditions. If
this is difficult to achieve, the next best solution is to build the
base course of the terrace as it approaches the single wall, to a deeper
elevation (either by stepping down to the same level as the single wall
or to virgin soil at a higher elevation). This provides for a deeper
and more stable base condition.
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