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Retaining Perfect Proportions

The use of gravity earth retention systems and segmental retaining walls (SRWs) to alter the landscape, control erosion, and increase the area of usable land, dates back to the introduction of primitive agriculture some 10,000 years ago.

The basic concept of using gravity and friction resistance based on materials and shape was employed in many historic civil engineering and construction projects. The Egyptian pyramids, for example, constructed from huge blocks of stone ranging in size from five to fifty tons, are actually carefully shaped segmental gravity walls.  The Great Wall of China, (which is actually a raised roadway), was built by constructing two gravity retaining walls several meters apart.  The space in between the walls was later filled with soil and stone to create a roadway.

In more recent times engineers typically used wooden timber walls, cast-in-place concrete and precast concrete panels to construct earth retention systems, provide erosion control, or create steep or vertical surfaces. These materials and construction techniques generally contributed to an imposing industrial appearance and, in the case of timber walls, had a life span of only 10 to 15 years before replacement was required. With the development of more durable and quickly constructed SRWs, traditional building materials and systems like masonry, timber and reinforced concrete began to lose their appeal.

Rapid urbanization and the need to soften vertical landscapes while providing effective erosion control has accelerated the acceptance of SRW's, which typically require less space and provide an attractive facade.  SRW's also provide design flexibility, improved performance and reduced cost. These systems have also been successfully used as an erosion resistant embankment and slope retention for lakes and rivers. In addition, SRWs have enabled widening and improvement of storm water channels and erosion control measures within existing rights-of-way.

There are three main stability modes for SRWs that include internal, external, and overall stability. Internal stability considers the mechanics working within the structure to resist all forces such as pullout, tensile strength and localized stress between grid layers. External stability considers local modes of failure such as overturning, base sliding, bearing capacity, and global stability. Overall stability considers the effects of the environment surrounding the wall, such as slope stability and settlement. Factors affecting the external stability are the wall height, the pressure and forces on the retained soil, and the weight of the wall.

EARTH RETENTION

SRWs have features that make them practical for earth retention projects. The destabilizing pressure of the retained soil mass is nullified by the combined weight of the individual solid concrete blocks when a gravity SRW system is in use. (The SRW approach is generally referred to in the industry as limit equilibrium analysis).

Life Cycle Cost Analysis

Life cycle cost analysis involves a comparative economic assessment of design, material and construction alternatives to determine the best value for funds invested.  An analysis of this type reviews the initial installation cost and, in addition, considers other factors such as maintenance, rehabilitation, inflation, interest and user costs. In many cases studies of this nature demonstrate that the best value for earth retention systems is not determined by initial cost.

Life Cycle Cost Analysis Expected Life Span Treated Timber 15 to 20 yrs. SRW 50 years minimum
 Initial Installed Cost $165.00/sq. yd  $270.00/sq. yd.
Replacement Cost (Over 50 years)* $528.00/sq. yd.  $0.00/sq. yd.
Life Cycle Cost (Over 50 years)** $693.00/sq. yd. $270.00/sq. yd.

*  Typically, treated timber wall will last an average 17.5 years.  Therefore, it will require replacement 3.2 times.

** Initial Installation & Replacement.

These estimates do not include annual maintenance costs associated with timber walls and do not consider inflation.  The actual life span of segmental retaining walls is likely much longer.

RETAINING PERFECT PROPORTIONS

SRWs have demonstrated significant benefits for designers, engineers, builders and property owners.  The features and benefits of SRWs are:

• Reduced costs for material, labor, equipment and installation.

• Design versatility and flexibility.

• Colorful, esthetically pleasing designs that soften the landscape and effectively eliminate the stark vertical appearance.

• Increased speed of construction due to the absence of curing time and tie-backs.

• Can be constructed quickly under virtually any weather conditions.

• Elimination of tie-backs and geo-grids that enables construction in narrow construction envelopes. In most cases existing site soils can be utilized, reducing the need for balancing cut and fill sites and eliminating the need for imported backfill soil.


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June 18, 2019, 8:47 am PDT

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