BEST PRACTICES – GEOGRID REINFORCEMENT

The National Concrete Masonry Association (NCMA) has put together a collection of practical, field proven solutions. This excerpt is for best practices when using geogrid reinforcement.

Reinforcement Empirical Limits:

Minimum grid length =
- 60% H static,
- 60% bottom/middle and
- 90% upper layers for seismic
Absolute minimum grid length = 4.00 feet
Minimum anchorage length (beyond failure plane) = 1 foot
Maximum vertical spacing =
- 16 inches if % passing #200 is >35%
- 24 inches if % passing #200 is <35%
Maximum vertical spacing = twice (2x) the block depth
Minimum soil cover between layers = 3 inches
Maximum height to first layer of geogrid = 18 inches

For more information on best practices for segmental retaining walls, download the NCMA guide here.

Construction, Designs, Earth Retention
August 18, 2022

PDEAS, SOIL NAILS & HELICAL ANCHORS…OH MY!

Earth anchors are often used in earth retention designs when geogrid reinforcement is not an option. But these amazing earth retention solutions come in a variety of shapes and sizes and it can be overwhelming or intimidating to determine which option is best for your project. Let us help you down the yellow brick road to success by breaking down the differences between some of the most common earth anchor options available. Learn more about Percussion Driven Earth Anchors (PDEAs), Soil Nails and Helical Anchors—how they work, their benefits and challenges here:

Percussion Driven Earth Anchors (PDEAs)

What are they: Metal rod with a mechanical anchoring system at one end to tie back or stabilize the ground for vertical construction in either temporary or permanent structures. (Examples: Duckbill, Manta Ray, Platypus.)

How it Works: A PDEA is impact driven by hammering a driving to force the anchor end and rod/cable into the ground. Once the determined anchoring depth is reached, the rod/cable is pulled back on from the ground surface to engage and rotate the anchor head, commonly referred to as “anchor locked”. The load required to pull and lock the anchor is monitored with a device called a load locker which is monitored for capacity and is stopped when the anchor has reached the required design capacity. A bearing plate is then secured to the end of the anchor and distributes the surface load to suffice pullout requirements.

When to use: Erosion Control to anchor different types of geosynthetic & metallic mats and meshes, Retaining Walls, Tree & Pole Anchoring, Landfill Capping, Hurricane Proofing, Slope stabilization, temporary structures and other construction techniques.

Benefits: Often times, the least expensive of all systems and several of them on the market. Fast/easy to install and corrosion resistant.

Challenges: Least reliable of all systems as soils in the anchoring zone may be uncertain or undefined and anticipated design loads may not be attained. Often not allowed on DOT projects for permanent structures.

Soil Nails

What are they: There are two different types of soil nails:

Hollow steel rod with tube inside to that allows grout injection for full length of “nail”.
Solid bar or strand used in open hole installation which is the grout injected to create the nail bonded to the adjacent soil stratum.

How it Works: A hole is drilled or rod pushed into the earth at engineered depths and locations. The soil nail is inserted into the hole. Grout is pumped into the tube and the rod is slowly retracted as grout fills the hole behind it leaving a “bulb” at the base of the rod and along its sides to act as a nail, securing soil around it. A plate is then fastened to the end of the nail.

When to use: Erosion Control to anchor different types of geosynthetic & metallic mats and meshes, Tunnels, & Retaining Walls, Slope stabilization, temporary structures and other construction techniques.

Benefits: Very reliable system in engineering terms. Accepted by agencies such as DOT’s, FHWA, and AASHTO. Can be done in more confined sites where there are property line or construction easement limitations. Process is fast and there is no height limitation.

Challenges: Cost can be comparable to Helical Anchors. Can’t be used in water applications or where ground water is a concern. Need specialized contactor to install. Plates and nails can be corrosive, but non-corrosive options are available as well.

Helical Anchors

What are they: Also known as screw anchors or screw piers/piles. Metal tube anchoring systems that is “screwed” into the ground.

How it Works: Helical anchors are twisted into the ground using hydraulic rotary equipment that can place the anchor very deep below the ground surface. The anchor is installed in sections that allows variation in flight sizes with increasing width nearer to the surface to achieve the engineered capacity requirements. Since the anchors are hollow, they can be filled with grout to increases their frictional capacity.

When to use: If surficial foundation conditions cannot support the load of the intended structure, especially when alternate construction techniques are not feasible for cost or schedule. Used to repair an existing structure’s foundation that has sustained damage without need for large excavations.

Benefits: Most reliable of all systems in engineering terms. Several feet can be drilled in a very short amount of time. Increases bearing of poor or loose with minimal environmental impact. Can be used to resist vertical uplift forces such as those created by permafrost.

Challenges: Commonly most expensive of all systems. Cannot be used in dense/rocky soil types. Need specialized contactor to install.

Designs, Earth Retention
May 23, 2022

DESIGNING FOR WATER APPLICATIONS

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.

Best Practices, Designs, Earth Retention
February 28, 2022

NCMA & AASHTO WALL EMBEDMENT REQUIREMENTS

Embedment Rules:

Embed to a depth that provides 4-feet horizontal bench to daylight (i.e. For a 4H:1V slope, embed the toe of wall 1-foot minimum)
AASHTO requires 24” minimum for slopes steeper than a 4H:1V
AASHTO requires 24” minimum when the wall is in a water application with no permanent erosion prevention measures such as rip-rap or a hardened liner above grade.
Embed toe of wall below curbing or other site components to be installed after wall construction.

Best Practices, Designs, Earth Retention
February 28, 2022

BEST PRACTICES – MSE WALL BACKFILL RECOMMENDATIONS

For Walls Up To 10 Feet:

Recommended Backfill Types = GW, GP, SW, SP, SM, SC
Maximum Atterberg Limits (LL and PI) = PI < 20, LL <40
Minimum Compaction = 95% of Standard Proctor Density
Moisture Content = +/- 2% of optimum moisture
Recommended Material Gradation:

SIEVE SIZE	PERCENT PASSING
1 in.	100
No. 4	20-100
No. 40	0-60
No. 200	0-35

For Walls 10 Feet to 20 Feet:

Recommended Backfill Types = GW, GP, SW, SP, SM, SC
Maximum Atterberg Limits (LL and PI) = PI < 6, LL <40
Minimum Compaction = 98% of Standard Proctor Density
Moisture Content = +/- 2% of optimum moisture
Recommended Material Gradation = Same as walls shorter than 10 – feet

For Walls Over 20 Feet:

Recommended Backfill Types = GW, GP, SW, SP
Maximum Atterberg Limits (LL and PI) = PI < 6
Minimum Compaction = 98% of Standard Proctor Density
Moisture Content = +/- 2% of optimum moisture
Recommended Material Gradation:

SIEVE SIZE	PERCENT PASSING
1 in.	100
No. 4	20-100
No. 40	0-60
No. 200	0-15

Designs, Earth Retention
November 18, 2021

REINFORCED SOIL SLOPE (RSS) DESIGNS

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!

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