ASTM C1372

ASTM C1372 – Standard Specification for Segmental Retaining Wall Units

This specification covers machine made dry cast concrete segmental retaining wall units. Dry cast meaning manufactured by using a low frequency, high amplitude vibration to consolidated stiff or dry concrete.

In general ASTM C1372 requires units be manufactured to a compressive strength of 3000 psi and maximum absorption of 15 pcf for normal weight aggregate (125 pcf or higher). Proof of freeze thaw testing is required when the units will be used in areas prone to freezing and thawing under saturated conditions per ASTM C1262.

Dimensional tolerances are also specified. The overall dimensions for width, height and length shall not differ by more than 1/8”+/-. NOTE: the height tolerance is sometimes altered by DOT’s to be 1/16”.

Finally, ASTM C1372 allows for incidental minor cracks resulting from customary shipping and handling but in general requires all units be sound and free of cracks or other defects that would interfere with the placement of the units or significantly impair the strength and permanence of the construction.

Acceptability is defined as: “chips and cracks shall not be visible from 20 ft under diffused light.” Color and texture must be specified by the purchaser, and the approval based on inspection of no less than 4 representative units.

Water Causes MSE Walls to Fail

Contractors should be aware that water is a major factor in the failure of mse walls. Major storm events where flash flooding is involved can cause damage in multiple ways.

1. Washing away the soil in front of the wall.
2. Washing away the soil behind the wall.
3. Saturation of the backfill soil over time.

Toe restraint of an mse wall is important because it has a limiting effect on displacements of the modular block facing. Washing away the toe and exposing the leveling pad puts the wall at risk. It is important to include enough embedment and use toe protection.

Directing water over the mse walls is not uncommon, but not preferred. Typically a low permeable cap (8” min) is proposed on top of the wall to limit infiltration. An additional measure is to direct the water to drain away from the face or around the ends of the wall using a swale. The use of a swale is especially needed if a large volume of water from a parking lot or hillside is expected to pass over the wall. IN ANY CASE, directing water to one point behind the wall is NEVER a good idea and could result in the backfill soils washing away. The geogrid does not work without soil between it and the facing will fail.

Saturation of the backfill over time can occur from seepage or from water from ponds in front of the wall. Saturated soil puts more pressure on the wall face than anticipated. Drains are recommended to provide a path for water to outlet through the wall face. Also, a free draining reinforced backfill, in the form of chimney drains, blanket drains, or the entire reinforced zone up to the high water level is recommended.

Estimating and Adjusting for Batter

It is important sometimes to estimate and adjust for the total setback of a segmental block wall considering the setback per course. A sidewalk between a building or curb may be located just above the wall. The sidewalk must usually maintain a certain width and the curb usually cannot be moved. Also consider a bridge application, you could have a bridge abutment seat behind the wall that needs to be a certain distance from the back of the upper units. 

In these cases the contractor will need to move the front of the wall at the base outward by the estimated total setback. The engineer should ideally estimate this distance and show it in their shop drawings. The contractor should verify the setback per course by building a test wall and re-calculate the estimated setback using the wall units shipped to the site.

The actual width of the wall on paper is usually wider considering the setback. That is important for a proper wall installation and grading plan layout. In summary, knowing the total setback and/or having a setback to aim for can ensure the wall does not impact other structures on the civil plans.

Reinforced Backfill

When building a reinforced earth wall the reinforced backfill is an important component to the design.

Reinforced backfill is the material that will be placed and compacted between the geogrid layers. The National Concrete Masonry Association in there design specifications recommends a sandy backfill or better.

USC soil types: GP, GW, SW, SP, SM

The gradation is listed below

Sieve Size                            % Passing

4 inch                                    100-75

No. 4                                     100-20

No. 40                                   0-60

No. 200                                 0-35

Plasiticity Index < 20

A strength reduction (higher damage factor) on soil reinforcements is required when reinforced backfill larger than 3/4'” in size is used.

Many times import backfill material is required for the reinforced soil zone. This may represent a significant cost to the project. That is why mse walls are usually better suited as fill walls versus cut walls. A cut wall would require removal and replacement of clayey site soil.

Proposed backfill materials of a silty nature, >35% fines (#200 sieve), would be considered marginal backfill. Clayey soils are not a desirable reinforced backfill material for multiple reasons. They are difficult to work with, they have a tendency to swell and soil reinforcement poorly connects to clay.

Common Geogrid Installation Mistakes

There are some very simple mistakes that contractors and/or homeowners sometimes make when installing geogrid. It is very important to properly install the geogrid because incorrect installation can result in unnecessary movements of the wall after construction. I will outline these common mistakes below.

1. Biggest mistake, installing the geogrid with the strength direction PARALLEL to the wall. This occurs when the geogrid is rolled parallel to the wall face. Most geogrids are uniaxial geogrids and designed to take strength in the roll direction. Geogrid should be rolled out perpendicular to the wall the design length, trimmed, then moved over the width of the roll with a new layer. The strength direction is usually indicated with arrows on the geogrid. This installation method is recommended for a biaxial geogrid to be consistent in the installation procedures. Biaxial geogrid can take strength in both directions.

2. Pull the geogrid taught and stake it down at the ends. Backfill and compact over the geogrid in thin lifts, 8” max.

3. Do not use large compaction equipment within 3’ from the back of the wall face. This will push out the wall.

4. Push the block forward to engage the connection device between units. If this is not done a consistent setback will likely not be established.

I also recommend that contractors review the construction portion of the specifications or installation handbooks. Construction requires lots of checking for setback and levelness of the block or blocks, adjusting as required. A 10' level is recommended.

Segmental Retaining Wall Construction Time

A new contractor, homeowner or estimator may want to know how long it takes to construct a segmental block retaining wall. This answer depends on multiple factors including:

Site conditions

Site access

Wall layout (curves, bends, and obstructions)

Soil types (clayey soils may be more difficult to work with)

Contractor experience

Size of contractor crew

Equipment available

This is the general rule of thumb

100 sq ft per man per day

500 sq ft per crew per day

The base course will take the longest, assume

25 sq ft per man per day

Reasons for a Retaining Wall

A retaining wall is generally used to create level surfaces. This is especially useful as level land is increasingly harder to come by in established communities. All remaining undeveloped parcels usually contain steep slopes or development has come up against hilly or mountainous terrain at the outskirts of towns.

Other more specific reasons include:

Homeowners attempting to create a level space for a patio or backyard that is currently too steep require a retaining wall.

Cities, Counties and State DOT widening road or constructing highway interchanges.

A developer needs a level surface for a parking lot and/or building pad.

Private mining companies creating a truck dump pad for a crusher.

There are generally 2 types of walls, cut and fill walls. There are a multiple kinds of retaining walls including gravity, embedded walls, and tie-back walls. MSE walls are tie back walls and generally easier to construct as fill walls. Cut (cutting into the existing ground) walls require excavation for soil reinforcement. Due to the possible high cost of excavation or property line limitations a different kind of wall may need to be considered in a cut wall situation.