15-5: SEGMENTAL RETAINING WALL DESIGN

Introduction

Segmental retaining walls (SRWs) function as gravity structures by relying on self-weight to resist the destabilizing forces due to retained soil (backfill) and surcharge loads. Stability is provided by a coherent mass with sufficient width to prevent both sliding at the base and overturning of the mass about the toe of the structure under the action of lateral earth forces.

SRWs are durable and long lasting retaining wall systems. The units are dry-stacked (mortarless) and accommodate settlement without distress in the wall. The typical size of SRW units permits the construction of walls in locations with difficult access, as well as allowing the construction of tight curves or other complex architectural layouts.

Segmental retaining walls are used in many applications, including landscaping walls, structural walls for changes in grade, bridge abutments, stream channelization, waterfront structures, tunnel access walls, wing walls, and parking area support. Highway structures are typically designed as critical structures, utilizing higher safety factors. This TEK may be used for most other retaining walls, which are not designed as critical structures.

Types of Segmental Retaining Walls

Conventional (Gravity) Segmental Retaining Walls

Conventional (gravity) SRWs retain soils solely through the self-weight of the units. They can be constructed with either a single depth of units or with multiple depths. The maximum wall height achievable using a single depth unit is directly proportional to its weight, width, and batter for any given soil and site geometry conditions. Wall height can be increased by using tiered construction, or by using multiple depths of units.

Soil-Reinforced Segmental Retaining Walls

Soil-reinforced SRWs are composite systems consisting of SRW units in combination with a mass of retained soil, stabilized by horizontal layers of reinforcement, typically a geosynthetic material. The reinforcement increases the effective width and weight of the gravity mass. Geosynthetic reinforcement materials are high tensile strength polymeric sheet materials. They may be geogrids or geotextiles, though current SRW construction typically uses geogrids.

Figure 1—Soil Reinforced Segmental Retaining Wall

	H = total height of wall
	H’ = exposed height of wall
	Hemb = wall embedment depth
	Hu = height of segmental retaining wall unit
	HR = elevation of a geosynthetic layer above base of wall
	L = total length of geosynthetic reinforcement, including facing connection
	Wu = width of segmental retaining wall unit
	w = wall batter

Figure 1 illustrates a typical soil-reinforced segmental retaining wall, and current design terminology. The geosynthetic reinforcement is placed between the units and extended into the soil to create a composite gravity mass structure. This mechanically stabilized wall system, comprised of the SRW units and a reinforced soil mass, offers the required resistance to external forces associated with taller walls, surcharged structures, or more difficult soil conditions. Soil-reinforced SRWs may also be referred to as mechanically stabilized earth (MSE) walls, the generic term used to describe all forms of fill-type reinforced soil structures.

Design Considerations

Geosynthetic Length and Spacing

For soil-reinforced segmental retaining walls, geosynthetic reinforcement increases the mass of the composite SRW structure, and therefore increases the resistance to destabilizing forces. Length of the geosynthetic is typically controlled by external stability calculations. Increasing the length of the geosynthetic layers increases the SRW’s resistance to overturning, base sliding, and bearing failures. In some instances, the length of the uppermost layer(s) is locally extended in order to provide adequate anchorage (pullout capacity) for the geosynthetic layers. The strength of the geosynthetic and the frictional interaction with the surrounding soil may also affect geosynthetic length.

A sufficient number and strength of geosynthetic layers must be used to satisfy horizontal equilibrium with soil forces behind the wall and to maintain internal stability. In addition, the tension forces in the geosynthetic layers must be less than the design strength of the geosynthetic and within the allowable connection strength between the geosynthetic and the SRW unit. The optimum spacing of these layers is typically determined iteratively, usually with the aid of a computer program. Typically, the vertical spacing decreases with depth below the top of the wall because earth pressures increase linearly with depth. Vertical spacing between geosynthetic layers should be limited to prevent bulging of the wall face between geosynthetic connection points and to prevent exceeding the shear capacity between SRW units.

Drainage System

Drainage is an essential part of a properly designed SRW. Drainage materials are generally well-graded aggregates. A properly designed drainage system relieves hydrostatic pressure in the soil, prevents retained soils from washing through the face of the wall, provides a stiff levelling pad to support a column of stacked facing units, and provides a working surface during construction. Surface water drainage should be designed to minimize erosion of the topsoil in front of the wall toe and to direct surface water away from the structure.

Wall Batter

Segmental retaining walls are generally installed with a small horizontal setback between units, creating a wall batter into the retained soil. The wall batter compensates for any slight lateral movement of the SRW face due to earth pressure, ensuring that the finished wall does not appear to rotate. For conventional (gravity) SRWs, increasing the wall batter increases the wall system stability.

Unit Size and Shear Capacity

In conventional (gravity) SRWs, where the stability of the system depends primarily on the mass and shear capacity of the SRW units, increasing the SRW unit width or weight provides greater stability, larger frictional resistance, and larger resisting moments. In soil-reinforced SRWs, heavier and wider units may permit a greater vertical spacing between layers of geosynthetic.

All SRW units provide a means of transferring lateral forces from one course to the next. Shear capacity provides lateral stability for this mortarless wall system. This is accomplished by shear keys, leading lips, trailing lips, clips, pins, or compacted columns of aggregate in open cores.

Wall Embedment

Wall embedment is the depth of the wall face that is below grade. The primary benefit of wall embedment is to ensure the SRW is not undermined by erosion of the soil in front of the wall. Increasing the depth of embedment also provides greater stability when site conditions include weak bearing capacity of underlying soils, steep slopes near the toe of the wall, potential scour at the toe (particularly in waterfront or submerged applications), seasonal soil volume changes, or seismic loads.

Surcharge Loadings

Often, vertical surcharge loadings are imposed behind the top of the wall in addition to the load due to the retained earth. These surcharge loadings generate an increased lateral pressure on the SRW structure. The surcharge loading can be caused by a sloped backfill behind the wall, a uniform surcharge due to buildings, parking lots, etc., or by line or point loads due to heavy isolated footings or continuous footings close to the wall.

Design Tables

The following tables summarize design and analysis results for a series of typical generic single depth and soil-reinforced segmental retaining walls. These tables were generated using conservative, generic properties of SRW units and geosynthetic materials. They are not a substitute for project-specific design, since differences between properties assumed in the tables and project-specific parameters can result in large differences in final design dimensions or factors of safety. Although wall heights up to 7.5 feet for conventional (gravity) walls and 20 feet for soil-reinforced walls are presented, properly designed walls can exceed these heights.

For a detailed discussion of design and analysis parameters, the Design Manual for Segmental Retaining Walls (ref. 1) should be consulted.

Table 1—Soil Reinforced Segmental Retaining Walls, 3:1 Sloped Backfill
Select an exposed height of wall for a given soil, then read down the column to determine geosynthetic length and spacing.
	Soil angle of internal friction, f = 28°						Soil angle of internal friction, f = 34°
Exposed height, ft:	4	6	8	10	12	14	4	6	8	10	12	14	16
Elevation, HR:	Length of geosynthetic, L, ft:						Length of geosynthetic, L, ft:
0'-8"		5	6	7.5	9	10.5					7.5	9	10.5
1'-4"	4				9	10.5	3	4	5	6.5		9	10.5
2'-0"			6	7.5						6.5	7.5
2'-8"	4.5	5			9	10.5	4	4	5			9	10.5
3'-4"			6	7.5		10.5					7.5		10.5
4'-0"					9					6.5		9
4'-8"		6.5	6	7.5		10.5		6	6		7.5		10.5
5'-4"					9							9
6'-0"						10.5				7	7.5		10.5
6'-8"			8	8	9				7.5			9
7'-4"						10.5							10.5
8'-0"											8.5	9
8'-8"				10	10	10.5				9			10.5
10'-0"												10.5
10'-8"					11.5	11.5					11		11
11'-4"
12'-8"						13.5						12.5	12.5
15'-4"													15

Design Parameters for Tables 1 and 2:

• Width of SRW unit, W_u< , 12 in.
• Height of SRW unit, H_u , 8 in.
• Wall batter, w , 3º. Base inclination , 0º.
• Geosynthetic long term design strength, 700 lb/ft
• Minimum connection strength between SRW units and geosynthetic reinforcement, 200 lb/ft
• Angle of friction between SRW units and geosynthetic, 40º
• Minimum shear capacity between SRW units, 400 lb/ft
• Angle of friction between SRW units, 30º
• Soil and block unit weight, 120 pcf
• Surcharge is initiated two feet from back of wall face.
• Typical values of f for various soil types are contained in Reference 1.

Design Parameters:

• Level backfill, no surcharge.
• Typical values of f for various soil types are contained in Reference 1.
• Required wall embedment at the toe, H_emb , 6 in.
• Soil and block unit weight, 120 pcf
• Wall batter is for unit setback per course. Base inclination, 0º.
• Internal friction coefficient for base SRW unit sliding on bearing soils, 0.6
• Minimum shear capacity between SRW units, 400 lb/ft
• Angle of friction between SRW units, 30º
• The analysis includes the effects of sliding, overturning, shear between units, and global stability of the SRW system.

References

1. Design Manual for Segmental Retaining Walls (Modular Concrete Block Retaining Wall Systems), 2nd edition. National Concrete Masonry Association, 1996.

Provided by: CCI Industries Ltd.