Retaining wall design engineer,landscaping ideas to cover utility boxes,retaining wall wood beams - Downloads 2016

05.09.2014
Proper installation of small and large retaining walls is critical to assure that the wall does not fail. Retaining walls are frequently thought of as a stack of blocks or wood timbers with soil dumped behind those blocks or timbers.
The purpose of this introduction to the Maytrx Retaining Wall Book is to give information that applies to retaining walls. Large retaining wall projects are normally called Mechanically Stabilized Earth [MSE] retaining walls while small retaining walls are normally called Gravity retaining walls. Mechanically Stabilized Earth retaining walls built today use some of the same knowledge that was used in the Great Wall of China built as much as 1600 years ago. To build a competent retaining wall of either type, gravity or MSE that will look good and serve its purpose of retaining soil for decades requires an understanding of many aspects of where the wall is to be built and how the wall will be constructed. The two types of walls will be discussed separately to eliminate confusion that may come from mixing a discussion of both types of walls in one all encompassing discussion.
Gravity type walls are normally for landscape applications and are usually built without geogrid or other stabilization material in the soil behind the face stones. The installer of a gravity type retaining wall needs to start by considering the location where the project is to be built. When the leveling pad is finished the first layer of retaining wall stones can be set making certain that the stones set flat and level. For the top layer of the retaining wall stones it is advisable to use an adhesive manufactured specifically for retaining walls to glue the top stones to the layer below the top layer. Design the swale such that it will flow surface water away from the wall rather than allowing the water to soak into the soil behind the retaining wall and saturating the compacted soil and rock behind the retaining wall.
A drain tile system can be designed if a swale cannot be built to carry surface water away. Following the proper installation methods will allow the gravity retaining wall you build to last for decades. Mechanically stabilized earth retaining walls are normally larger projects but earth stabilization with geogrid materials may be necessary in walls as short a 3 feet or less. The select soil, geogrid, crushed rock and face stones are a complete structure that is the retaining wall. The know-how that is needed to design a mechanically stabilized earth retaining wall and to design the area where the mechanically stabilized earth retaining wall will be built is of utmost importance. Information concerning the area, in front of and behind the wall structure, must be obtained to determine if the soil that is currently in place where the wall is to be constructed is of sufficient strength to support the mechanically stabilized earth retaining wall.
If the soil at the location will not adequately support the mechanically stabilized earth retaining wall then the retaining wall engineer will design a soil base for the mechanically stabilized earth retaining wall that may include soil replacement or soil treatment to strengthen the base soil or some type of caissons for structural support or other methods to strengthen the base soil. If a ground water zone is found, the engineer will design a system that will capture the water and carry it away from behind the mechanically stabilized earth retaining wall. With all external data known the retaining wall engineer can design the mechanically stabilized earth retaining wall structure.
Front bench – For walls that have sloping soil in front of the wall, a bench that is a minimum of 4 feet wide should be constructed in front of the wall. Foundation soil – The foundation soil must carry the huge load of the complete mechanically stabilized earth retaining wall structure as previously discussed. Finished grade – The finish grade and any structures that are or will be built above the retaining wall are necessary to the design of the mechanically stabilized earth retaining wall.
Retained backfill – The retained backfill is the non-reinforced soil that is behind the mechanically stabilized earth retaining wall structure. Original ground surface – The original ground surface may require a dig-out to build the mechanically stabilized earth retaining wall structure. Limits of excavation – The limit of the excavation defines the amount of soil that will need to be removed, and possibly hauled away from the site to properly construct the mechanically stabilized earth retaining wall structure.
The preceding points that discuss mechanically stabilized earth retaining wall structures do not cover all aspects that may be critical to any retaining wall structure but do give a idea of the value of the design engineer, the geotechnical engineer or engineering geologist, the contractor or installer and the oversight engineer all working to provide the owner with a properly built retaining wall that will have a design life of 75 to 100 years. For the owner of a mechanically stabilized earth retaining wall structure the following rules-of-thumb can be used to ask questions to check the design of your retaining wall structure. All SRWs will be constructed utilizing only products, such as adhesive or pins that are supplied or approved by the company that designed and licenses the units for production. The information will be given for segmental block walls although much of this information can be applied to walls built with other face materials such as wood timbers.
While the earth between the parallel walls that make up the Great Wall was stabilized with water reeds laid horizontally between layers of soil, current Mechanically Stabilized Earth retaining walls are characterized by placing layers of metal straps or strong synthetic grid material in a zone of earth behind the face stones. The greater the set back slope [batter] of the gravity retaining wall, to a limit, the greater the ability of the stones to retain the soil placed behind the stones.
The following information does not fulfill all knowledge necessary but will serve as an alert to owners, contractors and installers of what is important in retaining wall design. These walls normally do not exceed 3 feet in height and if tired landscape walls are built the concepts of gravity walls may not apply.
It is important that the installer study the area to determine if the soil upon which the retaining wall will rest is original virgin soil or can be compacted to carry the load of the new retaining wall. The drain tile may collect surface water behind the wall and carry the water away through tubes that are installed under the footing of the wall.
The main body of this retaining wall, that provides most to the strength to hold the load of the retained material in place, is the geogrid reinforced select soil zone. Segmental retaining wall engineers and geotechnical engineers work together in the design of the mechanically stabilized earth retaining wall and the area where the wall will be built.
Typical geogrid depths behind the face stones may be from 4 to 12 feet although the design engineer will determine the geogrid depth. If the base soil is not adequate to support the mechanically stabilized earth retaining wall the structure can settle downward more than an acceptable amount or there may be global failure of the complete structure as shown here. If this water is not removed it can saturate the reinforced zone and cause failure of the wall by changing the internal strength of the reinforced soil zone of the mechanically stabilized earth retaining wall and by adding a large horizontal load behind the retaining wall. In this brief presentation it is impossible to cover all aspects of the mechanically stabilized earth retaining wall design yet many of the important design factors can be presented followed by rules of thumb. Stones are to be manufactured to specific standards, usually designated as a 3000 pound crush test and less than 5 % water absorption after 24 hours. This will prevent the front soil from sliding away and reducing the designed embedment of the wall.


Infiltrate water is water that comes from internal to the soil behind the wall with a minimum of surface water such as rain water or sprinkler system water. Upward sloping soil and structures such as roads and buildings above the wall are additional loads to the retaining wall and must be incorporated into the wall’s design. These rules-of-thumb are not all encompassing but can initiate conversation to address all steps that are covered in a mechanically stabilized earth retaining wall structure’s design. The unit will not impair the strength or performance of the SRW [segmental retaining wall].
Stresses that are imposed upon units that are artificially leveled with shims may result in a SRW that does not meet the design engineer’s criteria. The formulas used by the engineers to design Reinforced Soil Slopes are a different set of formulas. These 16 pages do not encompass all aspects but are intended to give an initial and basic understanding of proper retaining wall construction. If the installer does not understand the critical aspects of constructing a retaining wall, large commercial walls or small residential landscape walls, it is probable that one of the following types of retaining wall failure will occur.
Segmental block walls are normally built with individual stones ranging in size from about ¼ square foot for the face area of each stone to 1 square foot face area or more for each stone.
The principal reason is because, unlike unsaturated and compacted soil that imposes a small horizontal force on the face stones of a retaining wall, water that invades the retained soil zone or the select soil zone will put a horizontal force on the face stones that is equal to the water’s vertical force or weight.
An understanding of the value of a competent design engineer for Mechanically Stabilized Earth retaining walls should be gained from this limited information. Tired landscape walls may require design as would a single wall that is of the total height of the multiple tired retaining walls. There must be no Karst topography under the retaining wall or the open cavities of the Karst topography must be stabilized. Additional work is needed at the surface behind the wall to direct surface water such as rain water away from behind the retaining wall.
A generalized view of a cross-sectional drawing of a mechanically stabilized earth retaining wall structure is given here. The blocks that are used to face the retaining wall are a minor structural element of the complete retaining wall.
It is obvious from the drawing given above, that the mass or weight of the mechanically stabilized earth retaining wall is large.
An example of a drainage design called a Chimney Drain is shown here but other designs may be given by the engineer.
Most stones are designed with a layer by layer set back that will develop a batter in the face of the wall.
The leveling pad will allow the minor settlement that will occur after the wall is constructed. Rain and any other surface type of water must be directed away from the retaining wall at the surface to prevent saturation of the soil behind the wall and near certain retaining wall failure. The retained backfill must be compacted in adequately thin lifts to obtain the design engineers specified density of the retained soil throughout the total height of the wall structure. As shown and discussed previously the wall design engineer will design a system that will capture the water behind the stabilized zone and carry this water away from the mechanically stabilized earth retaining wall. Characteristically, small residential retaining walls are built with the smaller stones while large commercial walls are built with the larger stones. The face stones are a minor structural component of the stability of a Mechanically Stabilized Earth retaining wall. It is obvious that gravity retaining walls that are built with 0 degrees of batter, straight up, will fail with little horizontal force from the soil zone behind the face stones. Also, too much water in the select soil zone will reduce the frictional characteristic of the select soil and then the select soil cannot provide the designed strength of the engineer’s design.
When the full bottom layer of stones are set and level, a perforated drain tube should be placed behind the stones with at least one end of this tube open to an area lower than the retaining wall’s leveling pad. Whether the soil is flat or slopes upward, build a swale in the soil behind the wall as shown here. This large mass must be supported by the soil below the mechanically stabilized earth retaining wall.
The use of modular blocks for the face of the retaining wall allows the minor settlements without a noticible change to the appearance of the face of the wall.
Around the tube and for at least 12 inches behind the retaining wall stones, install graded gravel of 1 inch or 1½ inch size.
Failure to install the rock fill layer by layer will cause the tie between the face stones, the rock, the soil and the geogrid behind the face stones to be significantly lower than the test data used in the design of the retaining wall. The geogrid must be laid on a flat surface of face stone-clean gravel-select soil fill to prevent damage to the grid that will reduce its strength below the manufacturer’s stated strength that is used by the design engineer. A swale built behind the wall near the face stones that is properly terraced will flow surface water away from the wall rather than allowing the water to soak into the soil behind the retaining wall and saturate the compacted soil behind the retaining wall. The engineer responsible for construction oversight has the responsibility to assure that this zone is properly constructed. There are situations where the design engineer will use RSS formulas with tired modular block walls. The installer can use paint or other marking material to mark the ground where the retaining wall will be built.
In this case it appears that water flowed into the area directly behind the wall saturating the soil behind the wall. It is the owner that is responsible for assuring that competent personnel are working on the project and that personnel are employed to cover all aspects of the wall’s design and construction. The leveling pad may have step-ups or step-downs along the length of the wall face depending on the grade of the soil.
This single size gravel will allow any water that does infiltrate into the gravel zone to percolate down to the drain tube and away form the retaining wall. Control of water in any of the soils and rock behind the face stones is perhaps the single most critical factor of both gravity and mechanically stabilized earth retaining walls. Infiltrate water is water that comes from internal to the soil behind the wall and small amounts of surface water such as rain water or sprinkler system water. It is critical that not only the design work be competent but that the construction of the mechanically stabilized earth retaining wall structure is completed as designed.


All of these designs rely on friction to hold the geogrid in place after the mechanically stabilized earth retaining wall is finalized.
As the slope of the soil changes from one end of the wall to the other end the step-ups or step-downs must allow for the design embedment of the mechanically stabilized earth retaining wall.
To build the wall to the design strength it is imperative that the clean backfill gravel be placed as every level of stones are set. The oversight engineer is to assure that all openings between stones and stone cores are full of clean gravel and that the gravel and soil fill behind the face stones is dead level to avoid wrinkles and cutting by the back of the stones. In large commercial retaining walls it is critical that the engineer have experience and cover, as a minimum, the items that are discussed in this paper.
Generally speaking, the separation between using retaining wall design formulas and slope design formulas is that if the overall slope is less than 30 degrees off straight up then retaining wall formulas are used to design the structure and if the overall slope is greater than 30 degrees off straight up-slope formulas are used. Typically, when the batter of a wall or series of tired wall exceeds 30 degrees off straight up the structure referred to as Reinforced Soil Slope [RSS] and is designed with different formulas than the formulas used to design retaining walls.
Design and construction of a swale at the surface behind the face stones of either type of retaining wall is critical if the surface soil does not slope away from the face stone toward the rear of the wall area.
The depth and width of the dig out must allow for the leveling pad’s thickness, usually about 1 inch thick for each 1 foot of wall height above the leveling pad. Rain and any other surface types of water must be directed away from the retaining wall at the surface to prevent saturation of the soil behind the wall.
It is likely that this application required a design engineer that would have designed the wall with geogrid to hold the surcharge of the parking lot and that a drainage system to assure that water did not collect in the soil behind the wall. Construction oversight by the design engineer’s company or a third party with knowledge of proper installation of mechanically stabilized earth retaining walls is critical.
That the geogrid is pulled taunt and staked at the rear to assure that the geogrid will start stabilizing the total wall mass immediately after construction of the wall. In small landscape retaining walls, specifications are not normally written yet the installer, homeowner or landscaper, must know and follow similar procedures to build a structure that will perform and look good for decades. The swale must be designed to capture rain or other surface water and carry it away from the retaining wall.
Sometimes these underground water flows are not located until the dig-out for construction of the mechanically stabilized earth retaining wall.
The drawing below shows the major elements of a mechanically stabilized earth retaining wall. Completely filling core stones and placing the gravel behind the stones as each layer is placed will provide for the structural interlock of the geogrid and rock that is necessary to meet the design strength of the mechanically stabilized earth retaining wall. Non-taunt geogrid will not stabilize the total wall mass until the geogrid is pulled taunt by movement of the wall mass ultimately making the geogrid taunt.
Around the tube and for approximately 12 inches behind the retaining wall stones, install graded gravel of 1 inch or 1½ inch size. This small retaining wall’s failure shows that knowledge of retaining walls and proper construction is critical to all retaining wall applications. Using this drawing some critical aspects of the construction of a mechanically stabilized earth retaining wall will be given.
For example, many people believe that a physical lip on a retaining wall block is of value in preventing retaining wall failure. The top of the leveling pad should be below the landscape grade in front of the wall by about 1 inch for each 1 foot of wall height above the leveling pad allowing for a toe of the retaining wall that is below the front grade. This single size gravel will allow any water that does infiltrate into the gravel zone to percolate down to the drain tube and away from the retaining wall.
Inexperienced may lead an installer to think that gluing all stones together or the use of lip type stones will prevent a failure of a retaining wall like the one shown above. General information will follow that will allow the owner to ask questions of the engineer, contractor and person providing daily installation oversight of the construction. For the first three feet behind the face stones the wall installer is to use hand compaction equipment, heaver equipment can be used behind this zone. Fill material should be placed on the geogrid material ahead of the tires or tracks of the wall building equipment.
The overriding technological factors are proper construction of the retaining wall, including the fill zone behind the face stones.
The hydrostatic pressure would have pushed this wall over regardless of gluing and use of lip type stones. In special applications such as water front retaining walls the type of geogrid is potentially critical. Lack of specification enforcement during construction occurs too frequently when the owner expects to save cost by reducing or eliminating wall construction oversight. Also, fill any grove in front of the bottom layer of stones and compact to hold the toe of the wall in place. Only proper design will prevent a retaining wall failure of the type shown in these photos. Proceed by setting additional layers of retaining wall stones, placing 12 inches of single size gravel behind the face stones and the in the cores if core type stones are used.
Modular blocks in a water front environment may cause long term high alkalinity near the wall face. Polyolefins appear to degrade only in high acidic environments and may be preferred for waterfront wall structures built with modular blocks.
Local government inspection is not present continuously and cannot assure that all layers of the retaining wall are built as specified by the design engineer. Placing and compacting the fill soil layer by layer as the wall is built is critical to prevent future settlement that can cause wall failure behind the wall face stones. The proper construction of a retaining wall as described in the engineer’s plans may seem to include unnecessary steps to some contractors and wall installers. During construction of the wall some steps may be eliminated or modified and the result is that the design strength of the retaining wall is reduced which may lead to a failure of the retaining wall. Compacting the fill soil behind the retaining wall also reduces the possibility that this soil will become saturated with water that will cause the retaining wall to fail.



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