Cut-Slopes Design Guidelines

The Geotechnical Engineer shall design the cut slopes in accordance with the West Virginia Department of Transportation, Division of Highways Design Directive 400 series. For large cut slopes, one or two benches may be eliminated based on the modeling discussed below. Nonetheless, the 400 series Design Directives shall not be considered simply as guidance, and shall be performed first before considering eliminating a bench. Based on the modeling criteria below, the maximum distance between benches may be increased to a maximum of 80 feet with an increase in the bench width(s) immediately below that portion of the slope. If the modeling is not performed as discussed below, then intermediate benching is required for cut slope heights greater than 50 feet.

For all slopes greater than 25 feet, a bench shall be required when a resistive rock layer (Type 1 and 2 sandstone and limestone) at least 5 feet thick is underlain by a soft rock (Type 3 and 4 shale, claystone, or coal) 3 feet or thicker. When frequent alternating hard and soft layers are encountered in the borings, then a slope flattened to 3/4:1 (horizontal to vertical) may be substituted for this bench.

The design requirements with regard to maximum backslope ratio and minimum bench widths in the Design Directives shall be followed; however, when the dip of the rock bedding or jointing is greater than 20° and dips downward toward the traveled way, and the strike angle is less than 45° to the alignment, then kinematic rock slope analysis shall be used to flatten the backslope ratios and widen the benches to provide a factor of safety of 1.5 or greater.

When the overburden at the top of the cut is over 10 feet thick, a minimum 10-foot wide bench shall be constructed at the soil-rock contact. The backslope within the soil portion of the cut is preferred to be 2H:1V; however, when the soil thickness is over 10 feet a minimum factor of safety of 1.3 shall be demonstrated for the steepened soil slope.

Criteria for extending backslope height or eliminating a bench follows: the combination of slope ratio, height between benches, bench width, and catchment area shall result in 90% of rocks retained before entering onto the pavement as determined by the Colorado Rockfall Simulation Program (CRSP). In the absence of the rock fall data nearby, a 2.5-foot diameter spherical shape shall be modeled with a minimum of 500 rocks simulated. A total of five (5) simulations shall be performed for the proposed slope configuration and the one with the most rocks passing onto the pavement shall be used in determining the percent passing onto the pavement. The input parameters shall fall within the ranges presented as follows:

Hard Bedrock: Type 1 and 2
CRSP-2D Normal Coefficient (Rn) 0.29 - 0.31
CRSP-2D Tangential Coefficient (Rt) 0.91 - 0.93
CRSP-2D & 3D Surface Roughness (S) 0.8
CRSP-3D Hardness Coefficient Parameter Range 0.80 - 0.82
Soft Bedrock: Type 3
CRSP-2D Normal Coefficient (Rn) 0.25 - 0.27
CRSP-2D Tangential Coefficient (Rt) 0.90 - 0.93
CRSP-2D & 3D Surface Roughness (S) 0.8
CRSP-3D Hardness Coefficient Parameter Range 0.52 - 0.54

In addition, for very soft rock (Type 4), a global stability analysis shall be performed for the extended slope height above 50 feet, that results in a factor of safety of 1.5 or greater. The stability analysis program shall consider both planar and curvilinear failure surfaces that simulate the jointing, weathering, and rupture anticipated in the rock unit under consideration. The Surface Roughness (S) presented above shall be used for modeling new pre-split slopes and benches; however, the S value presented has been increased to represent the slope after ageing to the end of the typical warranty period.

Instead of a cut bench constructed 5 feet above the ditch grade, as is the general design for slopes with ratios of 1H:1V or steeper per WVDOH Design Directive 403, a Rockfall Catchment Area Ditch (RCAD) shall be designed and constructed. The RCAD ditch shall be designed as follows:

RCAD with a minimum 4H:1V Backslope B and a maximum height of 80 feet for Backslope A.

When the Backslope A above the RCAD ditch is 25 feet or less, a minimum 15-foot wide ditch shall be constructed; otherwise a minimum 25-foot bench shall be constructed. When a 3/4H:1V or flatter slope is planned immediately above the RCAD ditch, the engineer shall analyze the RCAD ditch using the CRSP program to demonstrate that rocks do not outrun the ditch.

In any event, hydraulics may dictate the minimum ditch width. Flatter Backslope B up to 6H:1V and different widths other than those stipulated above may be allowed, provided a CRSP analysis is performed demonstrating that the 90% retention criteria is met.

 

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