Water applications have unique concerns that must be addressed during design of earth retention structures.  These may include increased depth of foundation, requirement of free draining backfill materials, rapid draw down and hydrostatic pressure, custom construction techniques, drainage control both surficial and internal, and less common situations such as ice forces, wave action, channel flow rates, or scour potential. 

Design Requirements

It is necessary to understand the type of water application one is dealing with when designing an earth retention structure.  As a rule of thumb for most water applications, the following minimum design requirements/considerations will apply:

• Minimum one foot embedment to ensure base is protected over time
• Possibly wrapping base and backfill with filter fabric to prevent mitigation of fines into well-draining backfill and gravel base
• Possibly using multiple drainpipes depending on multiple HWLs
• Using well-draining granular soil as backfill to reduce risk of hydrostatic pressure behind wall
• Increasing depth of drainage column to allow water to flow more freely through system
• Surficial water control by directing water away from wall as much as possible

Application Types

The following is a list of the types of water applications encountered and some additional concerns/design modifications the engineer may consider in each situation:

• Retention Ponds – These are meant to permanently hold stormwater runoff on a project site.
• Detention Ponds – These are meant to temporarily hold stormwater runoff on a project site.
  • Rapid drawdown
• Bioretention Ponds/Rain gardens/Permeable pavers – Similar to a retention pond but used to slow and treat on-site stormwater runoff though various media by physical, chemical, and biological means.
  • Greatly increased depth of embedment to obtain pond capacity and enable filtering through various media
• Shoreline Applications for Lakes – A physical barrier to prevent erosion along shoreline.
  • Dewatering to construct wall
  • Wave action
  • Ice forces
  • 2’ embedment for scour and riprap
• Shoreline for Rivers/Channel Lining – A physical barrier to prevent erosion along embankment.
  • Flowrate
  • Damage from large debris/ice
  • 2’ embedment for scour and riprap
  • Rapid drawdown
• Wetlands/High ground water – Water is visibly present or becomes present during excavation in proposed wall location.
  • Dewatering to construct wall
  • Loose/soft soil with low bearing capacity of base soils requiring sub cut
  • Chimney drain/drainage blanket
  • Wetland encroachment buffers

While these may be some useful guidelines and items to consider, every site is unique and will have its own unique set of variables that need to be factored into the design.

Embedment Rules:

Reinforced Soil Slopes (RSS) are very similar to conventional Mechanically Stabilized Earth (MSE) retaining walls however they provide a more economical approach to addressing grade differences.

Depending on the soil used, most permanent slopes greater than a 2H:1V will require some sort of mechanical stabilization for both deep seated and surficial failures. FHWA defines reinforced soil slopes as reinforced soil structures with a face angle flatter than 70 degrees. Much like MSE walls, RSS structures consist of compacted fill that is reinforced with geosynthetic reinforcement placed in horizontal layers relative to the face of the slope. Depending on the steepness of the slope in question, the designer selects a facing to limit erosion and promote vegetation if desired.

Oftentimes Reinforced Soil Slopes are used in a wide variety of applications by transportation agencies as part of their roadway designs. During new construction they may be used to reduce fill requirements or meet right of way (ROW) limitations without the significant price tag of a retaining wall. They can be used to stabilize existing unreinforced slopes or to repair landslides often reusing the same soil.

Next time you are driving along the highway, look at those steep slopes as you pass, because there is a lot of planning and engineering that’s gone into them that you didn’t even know about!

To protect employees working in excavations, OSHA (Occupational Safety and Health Administration) regulates the slopes at which sites can be safely excavated.  These slopes are measured as the horizontal distance to the vertical rise. The maximum allowable slope is the steepest incline that can is acceptable for the site to prevent cave-ins, while the actual slope is the real slope at which the site is excavated and should not be steeper than the maximum.

In addition to soil and rock types that are commonly used to determine the maximum allowable slope, there are a few additional items to consider when determining excavation limits and wall design.

Existing Structures

Existing structures may affect not only the wall design, but the excavation needed to safely construct the wall.

The Terra

The terrain is important to consider when determining how water moves on the site.  How the site is draining can have an impact on the wall design and the type and location of excavation.

Property Lines

The location of property lines is an important factor to take into account as they may limit the accessibility of the site for both digging and construction of the wall. 

While each site is unique and the excavation limits will vary given the layout and the geotechnical properties of the site, having an understanding of these additional factors and taking them into consideration early in the design phase, can provide the safest and least disruptive way to excavate and ultimately construct a solid wall.

Contact us for more information on excavation limits and how they affect your next project.

Did you know we handle welded wire forms? Like the Swiss army knife of MSE retaining walls, welded wire forms may be utilized in a variety of earth retention applications. Some of those applications are:

• Temporary MSE Walls in lieu of steel shoring
• Permanent MSE Walls in soft soils
• Plant-able reinforced slopes
• Load relief for foundation walls and tip up panel buildings
• Vegetated shoreline repair
• Limited site access and staging area

Want to learn if welded wire forms can be used on your next project? Contact us today for design assistance.  Looking for Welded Wire Forms (WWF)? ERS-MidWest stocks both black steel and hot dip galvanized welded wire forms for use in permanent or temporary conditions.

We are excited to announce that we are now offering residential and commercial geotechnical investigation to provide the next level of assistance from beginning to end of your projects! We are currently offering:

• Hand Augers
• Dynamic Cone Penetrometers (DCPs)
• Test Pits
• Soil Classification
• Lab Testing (Direct shear, Moisture Density, Sieve, etc.)
• Geotechnical Reports

Begin and end your next earth retention project with a single design firm – Contact GeoWall Designs today to schedule!

Founded in 2018 by Kyle Huerd, PE, GeoWall Designs is a growing veteran owned engineering firm specializing in the design of earth retention structures and soil engineering.

Value Engineering for Out-Of-The-Box Solutions

With our relationships and experience within the earth retention industry, our design team has been able to thrive doing what we do best, allowing us to become a leader in value engineering and providing “out-of-the-box” solutions given the most complex constraints. We pride ourselves on our ability to blend structural and geotechnical engineering, allowing us to limit design scope gap and provide the most complete engineering package to our customers.

We Approach Every Project Like It Was Ours

The approach that we take with every project is to treat it as if we were to construct it ourselves. We strive for the most clear and concise design optimized to the needs of each individual project.

Engineering Services to Fit Any Project

Our design services cover preliminary and final engineering for residential, commercial, and DOT projects including design build contracts. In addition to design services, we also provide product support for geogrids and various block systems provided by ERS-MidWest. Contact us to learn more about how we can assist you on your next project!