Vegetation cover on slopes is well known to limit and control soil erosion. The benefits result primarily from the ability of foliage and plant residues to intercept precipitation and absorb/dampen the energy of raindrops and localized runoff, to increase surface roughness and slow runoff velocities, and to enhance soil porosity and permeability at the ground surface to help deter runoff. Additionally, root systems bind soil particles together and provide increased soil permeability throughout the root zone.
Although increased permeability in the root zone helps diminish surface runoff and erosion, it can adversely influence slope stability by enhancing infiltration and saturation potential in the root zone, particularly in shallow soils that overlie bedrock or low-permeability subsoil horizons. Increased infiltration makes it far easier for unsaturated soils to become saturated and lose an important component of their shear strength—matric suction, or pore-water tension.
This stabilizing stress, a component of soil cohesion, is due to the water tension that develops in soil pores
as water menisci “stretch” across the gaps at grain boundaries in an unsaturated soil. When soil pores fill with water, this tension is lost, significantly reducing the cohesive strength of the soil. This likely is the primary reason that even heavily vegetated slopes can experience shallow debris slides and mud flows during intense and/or long-duration precipitation events.
Stability Strong Points
In most situations, the advantages of vegetation for enhancing slope stability outweigh the adverse impacts on stability, especially in three key areas:
- Foliage intercepts precipitation, which leads to absorptive and evaporation losses that reduce the water available for soil infiltration.
- Roots extract water from the soil and use it for plant maintenance and growth.
- Roots reinforce the soil, increasing shear strength. When deep enough, they may anchor into firm strata (fractured bedrock or subsoil horizons) to “pin” and help stabilize the overlying soil mantle
Numerical models based on evapotranspiration, water-balance and/or seepage computations can be used to address the first two areas and predict water conditions in the soil mantle—particularly how a perched water level (saturated zone) can develop above a low-permeability strata underlying the primary root zone. When the vegetation cover is damaged or destroyed due to fires or extensive timber harvesting, representative models should indicate an immediate response in the form of increased infiltration and greater groundwater response.
Practical engineering models of root strength typically are empirically based and often result in a simplified estimate of root cohesion, a stress that can be applied directly in limit-equilibrium stability analyses for shallow slope failures. Historically, root cohesion has been estimated by four parameters:
- Tensile strength measurements of individual plant roots;
- Direct-shear tests conducted on natural or artificially molded soil-root masses;
- Field pullout tests of large root masses or whole trees; and
- Back-analysis of existing slope failures that can be carefully mapped and described.
In the case of individual root tensile strengths, the overall root strength per unit area of soil (i.e., root cohesion) is estimated by summing the average tensile strengths of roots in given size categories times the number of such roots, divided by the cross-sectional area of soil used in the root-sample count. But because all roots do not mobilize their maximum tensile resistance at the same time during a soil shearing event, investigators have long held that these calculated root strength values should be reduced by 25% to 50% when applied as root cohesions in slope stability models. Other proposed mathematical conversions include tangential and normal components of the overall root tensile strength.
Direct-shear tests and field pullout tests have provided estimates of root cohesion that generally concur with those from root tensile-strength methods. Six important engineering observations made over the past 30 years regarding root strength are summarized below:
- The shear strength of soil-root masses increases with higher numbers of roots present.
- Depending on soil and slope conditions and on plant species, estimated root cohesion values typically range from 0.5 to 2.4 kPa (10 to 50 psf) for deep-rooted grasses and small woody shrubs, 1.4 to 7.2 kPa (30 to 150 psf) for large woody shrubs and sapling trees, 3.4 to 12 kPa (70 to 250 psf) for small trees and 7.2 to 28.7 kPa (150 to 600 psf) for larger trees.
- Vegetation “monocultures” that consist primarily of one species or several like species with similar root morphologies induce a potential failure surface at the base of the root zone, leading to shallow slope failures when heavy precipitation events cause infiltration that partially saturates the root zone.
- The combined morphology of soil layers and root masses directly influences shallow slope stability.
- Roots provide a cohesion component to the shear strength, but not an increase in the friction angle of the soil. Thus, root cohesion is independent of the normal stress applied to a shear surface and can be a major factor in stabilizing shallow soil mantles on slopes.
The typical shear strength model is given by t = cr + cs + σ’'tan φ, where t = shear strength, cr = root cohesion cs = soil cohesion, σ’ = effective normal stress and φ = soil friction angle. As such, root cohesion provides a much greater advantage for slope stability when the potential sliding mass is thin, as shown by safety factors computed using the infinite slope model (see Figure 1).
- Loss of vegetation not only increases the potential for increased water infiltration to surface soils, but it also causes significant reductions in root cohesion as the plants die and decay. Studies have shown that tree-root tensile strength reductions of 30% to 85% occur within 10 years after tree cutting (with most of the reduction in the first four years), with some pine species having a root strength “half-life” of about 14 months. The strength reduction for small woody shrubs with smaller diameter roots is even greater.
The Right Approach
In the current state of practice, engineers and land-use managers have practical tools to evaluate and model slope stability and asses the impact of vegetation loss due to fires, timber harvesting or land development. Diminished vegetation can impact both shallow and deep-seated slope instabilities directly by allowing increased water infiltration into the near-surface groundwater system. It also has a significant destabilizing effect on potential shallow failures (debris and mud flows) due to root-strength reductions within 2 meters of the ground surface.
Probabilistic modeling of slope stability using variable inputs, along with confirmations obtained through back-analysis of prior slope failures,is recommended; this approach provides practical, realistic output for effective land management to prevent and mitigate slope instability impacts.