SRW Design: Expert Level

Aaron Fisher | March 29, 2021

Are you an architect, engineer, or contractor looking for more technical information as you design and install a segmented retaining wall (SRW)?

Guidance Manual

NCMA Guides

One of the best places to look for more information on SRW construction is the National Concrete Masonry Association (NCMA)

Installation Guide
Best Practices Guide (2016) Some parts of the Best Practices Guide have been pulled out and highlighted below to address common questions.
NCMA SRW Design Manual (2009)
Design Details (CAD files)
Inspection Guide

Software Tools

Interested in designing an SRW, but don’t want to do the reading. There are several good software packages out there

SRWall – NCMA’s design software package
GeoSlope
Ressa
Slide

Design Standards of SRW Block (ASTM C1372)

Dimensional tolerances: ±1/8 in. (except for architectural finished surfaces)
Minimum Compressive Strength = 3,000 psi
Maximum Absorption = 15 – 18 pcf

Freeze-Thaw Resistance of SRW Block (ASTM C1262)

Exposure Zone	SRW Properties	Freeze/Thaw Testing (C1262)
Zone 1 and Non-Roadway Applications	ASTM C1372	None
Zone 2 – no/negligible de-icing salt exposure1	ASTM C1372	Proven field performance or test in water: •≤1% wt. loss in 5 of 5 samples after 100 cycles; or •≤1.5% wt. loss in 4 of 5 samples after 150 cycles.
Zone 2 – de-icing salt exposure	ASTM C1372, plus • Targeted compressive strength: 4000 psi • Targeted absorption: 7 pcf	Test in 3% saline solution: •≤1% wt. loss in 5 of 5 samples after 20 cycles; or •≤1.5% wt. loss in 4 of 5 samples after 30 cycles.

Note: Only Zone 1 & 2 are shown because Ernest Maier does not service any regions that fall in Zone 3. Please see the Best Practices Guide for more information.

Minimum Design Standards

Appropriate design of an SRW should include a minimum safety factor. These are the minimum recommended safety factors.

Geosynthetic Reinforcement

Design Guidance

Use these design tables to understand how much reinforcement to place in your SRW

Wall Height ft (m)	Reinforced Zone Material			Design and Layout Criteria
Wall Height ft (m)		Gradation	Plasticity	Reinf. Spacing, Max.	L/H, min.	Gravel Fill Thickness in. (mm)
H ≤10 (H ≤ 3)	Recommended	See Table Below	Moderate	24 in.	0.6	24 in. (610 mm) from face 12 in. (305 mm) behind unit
H ≤10 (H ≤ 3)	Alternate	See Table Below #200 waived	Moderate	16 in.	0.7	30 in. (762 mm) from face 18 in. (457 mm) behind unit
10 < H ≤ 20 (3 < H ≤ 6)	Recommended	See Table Below	Low	24 in.	0.6	Top 10 ft (3 m) same as above, remainder 36 in. (914 mm) from the face, 24 in. (610) behind unit
H > 20 (H > 6)	Recommended	See Table Below	Low	24 in.	0.6	Top 10 ft (3 m) and lower 10 ft (3 m) to 20 ft (6 m) same as above, remainder 48 in. (1219 mm) from the face, 36 in. (914 mm) behind unit

Sieve Size	Percent Passing (Short/Medium Walls)	Percent Passing (Tall Walls)
1 in. (25 mm)	100	100
No. 4	100 – 0	100 – 20
No. 40	0 – 60	0 – 60
No. 200	0-35	0-15 with PI<6

Material Choice

The proper use of geosynthetics, can compensate for many other imperfections, mistakes, or oversights in the design of an SRW. Be sure to select materials that provide the recommended material strength and are permeable to water.

An interesting application that showcases the power of geosynthetic reinforcement is: Geosynthetic Reinforced Soil- Integrated Bridge Systems (GRS-IBS). These are basically just SRW blocks with geotextiles every layer that are used for smaller bridges <140’ spans. These solutions are accepted by the FHWA and DOTs around the country.

Geosynthetic Reinforced Soil Integrated Bridge System

Drain Design

There are several drainage design options

Blanket Drain- runs underneath the reinforced soil zone- used to address the presence of migratory subsurface water
Toe Drain- located behind/below the SRW blocks- removes water from behind the blocks
Heel Drain- located at the base of the interface of the reinforced and retained soil zones- drainage structure to further reinforce the
Chimney Drain- located at the interface of the reinforced and retained soil zones- used to address high water tables or the presence of migratory subsurface water

Toe and Heel Drain Detail

Chimney and Blanket Drain Details

Ernest Maier, Gomoljak Parker Block, and Skyline Brick have professional sales representatives who in many cases have served as contractors and installers ourselves. Feel free to reach out with your questions.