Live Cribwalls

1. CATEGORY

1.0 – River Training (longitudinal structure)

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Log or timber cribwall. Note: vegetation, primarily transplants and rooted plants, can also be inserted into the frontal openings of several proprietary concrete cribwall systems, e.g., Criblock and Evergreen Walls (Jaecklin, 1983). These structures are known as vegetated cribwalls.

4. DESCRIPTION

A cribwall is a type of gravity retaining structure consisting of a hollow, box-like inter-locking arrangement of structural beams, e.g. logs. The interior of the cribwall is filled with rock or soil. In conventional cribwalls, the structural members are fabricated from concrete, wood logs, and dimensioned timbers (usually treated wood). In live cribwalls, the structural members are usually untreated log or timber members. The structure is filled with a suitable backfill material and live branch cuttings are inserted through openings between logs at the front of the structure and imbedded in the cribfill. These cuttings eventually root inside the fill. Once the live cuttings start to root, the growing roots gradually permeate and reinforce the fill within the structure; they should eventually root in the native soil behind the structure as well.

5. PURPOSE

Live cribwalls protect the toe and help retain steep streambanks. Live cuttings placed within the crib structure reinforce the cribfill. The tips of vegetation that protrude outside the crib face provide cover and shade for aquatic organisms and promote sediment accretion or build up at the toe.

6. PLANNING

Useful for Erosion Processes:

	Toe erosion with upper bank failure
	Scour of middle and upper banks by currents
	Local scour
	Erosion of local lenses or layers of noncohesive sediment
	Erosion by overbank runoff
	General bed degradation
	Headcutting
	Piping
	Erosion by navigation waves
	Erosion by wind waves
	Erosion by ice and debris gouging
	General bank instability or susceptibility to mass slope failure

Hydrologic / Geomorphic Setting

	Resistive
	Redirective
	Continuous
	Discontinuous
	Outer Bend
	Inner Bend
	Incision
	Lateral Migration
	Aggradation

Conditions Where Practice Applies:

Log cribwalls may be useful in areas where mass instability is the predominant mechanism of failure and where a near vertical structure is required to protect an eroding streambank. A live cribwall system is helpful at the base of slopes where a low toe-wall can be used to:

Protect a general reach of stream where floodplain encroachment has occurred

Complexity:

Moderate to High. Live cribwalls are relatively complex structures to design and to construct in comparison to other soil bioengineering techniques. They must be built to withstand both hydraulic loading (from stream scour) and lateral earth forces (from bank oversteepening).

Design Guidelines / Typical Drawings:

Crib structures must have adequate external and internal stability. External stability requires that a crib structure as a whole have sufficient bearing capacity and resist lateral forces that cause sliding and overturning. Internal stability requires that individual structural members be capable of resisting shear, moment and compression forces placed on them by both internal and external loads. Standard designs and specifications for conventional concrete and treated timber crib structures can be found in Gray and Leiser (1982) and Schwarzoff (1975). In live cribwalls, the structural members are usually untreated log or timber members. Limited design specifications for some types of log cribs can be found in Gray and Leiser (1982). Additional information about log cribwalls is provided by Washington State (2003). A schematic or conceptual design for a cribwall streambank protection can be seen in Figure 1. For the most part, live cribwalls have been designed and built in practice by a trial and error process without the benefit of rigorous analysis and promulgation of design specifications.

Figure 1. Schematic diagram/conceptual design of live cribwall (from USDA, 1996)

Live Cribwalls Typical Drawing
.dwg	.dgn

7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

A vertical structure at the toe minimizes or may avoid the need to flatten the bank and encroach on the land behind the crest. A live cribwall has a more natural appearance and is less visually intrusive than a structural treatment alone, e.g., conventional cribwall, steel bulkhead, or gabion wall. Well established vegetation in a live cribwall screens the structural elements from view and gives the bank the appearance of a naturally vegetated steep bank. A well vegetated (or revegetated) bank can improve aquatic and riparian habitat in addition to providing important functional benefits (Coppin and Richards, 1990). The vegetation protruding beyond the cribwall face provides cover, overhang, and shade for fish and other aquatic life near the water's edge.

8. HYDRAULIC LOADING

Uncertain effect on bank roughness, particularly as vegetation becomes established in frontal openings of wall and/or flow velocity changes. Presence of well established vegetation likely increases roughness at low velocities and decreases at high (as vegetation bends over and flattens against front of wall). Roughness can be enhanced by incorporating roughness elements such as rootwads into the cribwall’s construction; however, this may also cause some addition local scour due to exaggerated turbulence around them (Washington State, 2003).

9. COMBINATION OPPORTUNITIES

Can be used in combination with mid and top of slope treatments (see Live Staking, Live Fascines, Brushlayering, Erosion Control Blankets, Turf Reinforcement Mats, etc) and techniques specifically suited to protect/defend the toe of streambanks (see Live Siltation, Vegetated/Modified Riprap, Large Woody Debris Structures, etc.)

10. ADVANTAGES

Does not require slope flattening and regrading. Protects toe while increasing bank roughness (with establishment of dense vegetation in wall and at low velocities). Well vegetated cribwall promotes siltation and provides overhanging shade and cover.

11. LIMITATIONS

Relatively complex and costly compared to other soil bioengineering treatments. Design requires both geotechnical and structural analysis. A cribwall must have a stable foundation and must be capable of resisting lateral loads and overturning forces. The structure may be subject to undermining by scour; which will require ancillary rock toe protection at the toe. The vertical nature of cribwalls make them somewhat comparable to bridge abutments, and studies have shown (Washington State, 2003) that vertical bridge abutments incur twice the scour that sloped abutments do. A cribwall requires time to design and construction is difficult and impractical during high-flow events.

12. MATERIALS AND EQUIPMENT

The inert construction materials consist of logs or timbers ranging from 10 cm to 15 cm (4 in to 6 in) in diameter (or greater depending on the height of the structure and expected loading). Log cribwalls should be constructed using logs that retain their strength for an acceptable period of time and are resistant to rot. Cedar and spruce logs have a relatively high resistance to rot, while soft wood, such as alder and pine, should be avoided. Galvanized fasteners at the corners, e.g., spikes and lag bolts, must be of sufficient size and diameter to withstand internal stresses resulting from the cribfill and external forces acting on the structure. The cribfill can consist of native soil, provided it is relatively cohesionless or coarse grained (sand, silty sand, sandy silt, or non plastic silt). Live construction material consists of the live cut branches placed inside the crib; they are normally 12.5 mm to 50 mm (0.5 in to 2 in) in diameter and long enough to extend beyond the crib into the backfill and native soil behind the structure. Site preparation and construction of cribwalls can be accomplished with hand labor, but a small excavator is helpful for both. Logs can be cut and notched using a chain saw; a power drill is needed to make the holes into which pins (or large spikes or reinforcing bars) are driven to hold the log structure together.

13. CONSTRUCTION / INSTALLATION

Live cribwalls use dimensioned timbers or logs; preferably durable material such as redwood or cedar. They are not intended to resist large, lateral earth stresses, and should be constructed no higher than 1.8 m (6 ft). The following guidelines and procedures should be followed when constructing a live cribwall system. Detailed installation/construction guidelines can be found in Gray and Sotir (1996) and USDA (1996). Additional information is provided by Washington State (2003).

Starting at the lowest point of the slope, excavate loose material 0.6 to 0.9 meters (2 to 3 ft) below the ground surface until a stable foundation is reached. The footing base should be inclined into the slope so that the structure will have a batter that provides additional stability to the structure.

Place the first course of logs or timbers at the front and back of the excavated foundation, approximately 1.2 m to 1.5 m (4 to 5 feet) apart and parallel to the slope contour. Place the next course of logs or timbers at right angles (perpendicular to the slope) on top of the previous course to overhang the front and back of the previous course slightly.

Each course of the live cribwall is placed in the same manner and fastened to the preceding course with spikes, lag bolts or reinforcing bars. This erection procedure results in an interlocking, box-like structure as shown in Figure 2.

Figure 2. Log cribwall under construction showing second course of logs being placed perpendicular to slope (after Gray and Sotir, 1996)

Figure 3. Log cribwall showing placement of live cuttings in the interior cells between logs (after Schiechtl and Stern, 1997)

When the cribwall structure reaches slightly above the existing ground surface (at the base), place live branch cuttings on the cribfill perpendicular to the slope as shown in Figure 3; then cover the cuttings with more cribfill and compact.

The live branch cuttings or branches should be placed at each course to the top of the cribwall structure with the growing tips extending from the back toward the front face. Follow each layer of branches with a layer of compacted soil to ensure soil contact with the branch cuttings.

Placement of a cribwall at the toe of a slope may require some excavation at the toe to prevent encroachment of the structure into a stream channel. Unfortunately, excavation at the toe can also cause stability problems in very unstable slopes. This situation may require dry weather excavation, or alternatively, excavation and construction in “slots” or increments. If the slope above the wall is flattened or graded back, the scaled material can often be used as backfill behind a toe-wall. It may also be possible to use the scaled material within the structure itself (e.g., as cribfill).

14. COST

Live cribwalls are relatively more complex and expensive than other soil bioengineering techniques. Cost comparisons are provided in the table below.

UNIT COSTS FOR SOIL BIOENGINEERING MEASURES

Method Installed	Installed Unit Cost¹
	Metric		US
	1994 Dollars	2003 Dollars	1994 Dollars	2003 Dollars
Live Staking	$1.50 - $3.50 per stake	$1.86 - $4.34 per stake	$1.50 - $3.50 per stake	$1.86 - $4.34 per stake
Joint Planting	$2.00 - $9.00 per stake	$2.48 - $11.17 per stake	$2.00 - 9.00 per stake	$2.48 - $11.17 per stake
Live Fascine	$16.40 - $29.50 per lineal m	$20.35 - $36.60 per lineal m	$5.00 - 9.00 per lineal ft	$6.20 - $11.17 per lineal ft
Live cribwall	$107.60 - $269 per m² of front face	$133.50 - $333.75 per m² of front face	$10.00 - 25.00 per sq. ft of front face	$12.41 - $31.02 per ft² of front face
Brushlayer - Cut	$26.24 - $42.64 per lineal m	$32.56 - $52.90 per lineal m	$8.00 - 13.00 per lineal ft	$9.93 - $16.13 per lineal ft
Brushlayer - Fill	$39.36 - $82.00 per lineal m	$48.83 - $101.74 per lineal m	$12.00 - 25.00 per lineal ft	$14.89 - $31.02 per lineal ft
Vegetated Geogrid	$39.36 - $98.40 per lineal m	$48.83- $122.08 per lineal m	$12.00 - 30.00 per lineal ft	$14.89 - $37.22 per lineal ft
Live Slope Grating	$269 - $538 per m² of front face	$333.75 - $667.49 per m² of front face	$25.00 - 50.00 per sq. ft of front face	$31.02 - $62.03 per ft² of front face

¹ Installation includes: 1) harvesting, 2) transportation, 3) storage, and 4) placement

The unit costs of log cribwalls constructed in Washington State during the period 1995-2000 ranged from $825-$1,000 per m ($250-300 per ft) (Washington State, 2003). These costs included materials and construction only; not design and post construction costs. Approximate unit costs (in $1999) for live cribwalls constructed in Maryland ranged from $118 to $300 per square m ($11 to $28 per square ft) of front face (Maryland, 2000). These cost data are approximate and site specific; they should be used primarily for order-of-magnitude estimates and comparison purposes only.

15. MAINTENANCE / MONITORING

Inspect for evidence of excessive erosion from stream scour and undermining at the toe of the cribwall. If erosion is excessive, the toe may need to be armored with rock. The condition of the logs should be inspected periodically for premature rotting. Eventual rotting is to be expected and should not diminish the efficacy of the live cribwall system, because as vegetation becomes well established, its root system provides the stability that would otherwise be lost as log crib members decay over time.

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

Selection of logs that rot too soon and do not maintain their structural integrity for an acceptable period of time.

Undermining by stream scour at the toe of the wall.

Inadequate transition at upstream and downstream ends of cribwall. If smooth transition is not created, turbulent eddies will result, and erosion of adjacent banks may occur.

17. CASE STUDIES AND EXAMPLES

A live cribwall was used in conjunction with other soil bioengineering treatments to stabilize and protect a streambank on a small stream in Paulding County, GA, known as Raccoon Creek. The stream has a mean daily flow of 15 m³/sec (45 cfs). Raccoon Creek drains a watershed of about 78 sq. km (30 sq. mi.) that was undergoing transition from rural residential or agricultural to suburban land use. The live cribwall was used to protect a portion of the streambank adjacent to a transmission tower that was located very close to the bank edge (see Figure 4).

Figure 4. Live cribwall nearing completion. Note interlocking, box-like construction and live branches protruding between logs.
Racoon Creek, Georgia (from Gray and Sotir, 1996)

The bank at this location was originally protected by metal sheet piling and rock riprap. This bank protection system was constantly being repaired and rock replaced during a twelve-year period, prior to replacement with soil bioengineering bank protection methods. The channel top width at this location was approximately 15.25 m (50 ft), bottom width was about 9 m (30 ft), and bank height varied from 3 to 4.5 m (10 to 15 ft). Before installing the live cribwall system, the riprap was removed and the sheet piling cut off to allow good drainage behind the structure. Additional systems installed upstream and downstream of the live cribwall at this site included a live boom, brush mattressing, live fascines, and joint planted (live staked) riprap. This bank protection system has performed satisfactorily since its installation in the early 1980s. No further bank erosion has occurred at the site, and other than periodic brush removal in the immediate vicinity of the tower, no maintenance has been required since installation.

Please visit the Photo Gallery for more pictures.

18. RESEARCH OPPORTUNITIES

Determine the minimum period required for log longevity so that loss of strength of structural members is compensated by mass stability provided by monolithic earthen mass consisting of root permeated and reinforced cribfill and natural soil. Determine in greater detail how establishment of vegetation in cribwall face affects roughness over time and under varying flow regimes.

19. REFERENCES

Coppin, N. J., & Richards, I. (1990). Use of Vegetation in Civil Engineering. Butterworths: Sevenoaks, Kent (England).

Gray, D. H. & Leiser, A. (1982). Biotechnical Slope Protection and Erosion Control. Van Nostrand Reinhold, New York, N. Y.

Gray, D. H. & Sotir, R. (1996). Biotechnical and Soil Bioengineering Slope Stabilization. John Wiley and Sons, New York, N. Y.

Jaecklin, F.P. (1983). Retaining beautifully. Architect and Builder, May 1983.

Maryland Department of the Environment, Water Management Administration (Follweiler, J eds.) (2000). Maryland’s Waterway Construction Guidelines, Section 3 Channel Stabilization and Rehabilitation Techniques, Baltimore, MD. (pdf)

Schiechtl, H.M & Stern, R. (1998). Water Bioengineering Techniques for Riverbank and Shore Stabilization. Blackwell Science, London, England

Schwarzoff, J.C. (1975). Retaining wall practice and selection for low-volume forest roads. Transportation Research Board Special Report # 160, pp. 128-140.

USDA Soil Conservation Service. (1996). Chapter 16: Streambank and Shoreline Protection. Part 650, 210-EFH, Engineering Field Handbook, 88 pp. (pdf)

Washington Dept of Fish & Wildlife (2003). Integrated Streambank Protection Guidelines, published in co-operation with Washington Dept. of Transportation and Washington Dept. of Ecology, June 2003. (Chapter 6 pdf) (Appendix L pdf) (Appendix H pdf) http://www.wa.gov/wdfw/hab/ahg/ispgdoc.htm (April 2003)

	Instream
	Toe
	Midbank
	Top of Bank