Obtain soil borings for walls carrying a traffic surcharge or any wall taller than 5 ft. Evaluate need for soil borings for walls shorter than 5 ft. on a case-by-case basis. For short-term conditions in cohesive soilsuse undrained shear strength determined using laboratory strength tests on undisturbed Shelby tube samples for design analysis. Strength measurement from pocket penetrometer tests and hand torvanes should not be solely used to evaluate undrained shear strength except as supplement to other laboratory strength tests on undisturbed tube samples. Within cohesionless material, SPT can be used to evaluate the internal friction angle but is not intended to supersede results from direct shear testing. A more rigorous sampling and testing program may be required for long-term evaluation of walls founded on cohesive soils.

Boring Locations and Spacing

. Obtain borings at 200-ft. maximum spacing unless history or variability of site conditions warrant tighter spacing.

Boring Depth for Fill Walls

. For Mechanically Stabilized Earth (MSE) walls, spread footing walls, temporary earth walls, and block walls, bore to a depth as deep as the height of the wall depending on wall type and existing and proposed ground lines. The minimum boring depth is 15 ft. below the bottom of the wall unless rock is encountered. Extending borings 5 ft. into rock for fill walls is usually adequate.

Boring Depth for Cut Walls

. For tied-back walls, and soil and rock nail walls, always base the depth of boring on the final grade lines. Advance borings for soil nail and rock nail walls through the material that is to be nailed. Extend borings a minimum of 20-ft. below the bottom of the proposed wall. Borings for cut walls may need to penetrate rock significant distances depending on the depth of the cut and height of the wall.

Cantilever walls, including drilled shaft walls and sheet pile walls, require the depth of borings to extend beyond the anticipated depth of the shaft below the cut, which is typically between one and two times the height of the wall.

Soil Samples and Testing

. Provide additional testing for taller walls, walls on slopes, or walls on soft foundations as necessary for complete evaluation of wall stability and settlement characteristics. Additional testing includes but is not limited to obtaining samples for applicable index testing, consolidation testing, triaxial testing, or in-place shear testing to determine soil strength. Consult with the wall designer for development of the complete soil exploration plan.

Ground Water

. Include ground water elevation measurements (including date of measurement) as part of the data acquisition for retaining walls. Obtain an additional groundwater elevation at a minimum of 15 minutes after the initial measurement. Site conditions or the design objective may require the installation of piezometers to establish a long-term or steady state ground water conditions.