Obtain soil core borings for walls taller than 10 ft. Evaluate walls shorter than 10 ft. on a case-by-case basis. TCP testing alone may be adequate to design walls and evaluate wall stability for short-term loading conditions in cohesive profiles and short-term and long-term loading conditions in cohesionless and rock profiles. A more rigorous sampling and testing program may be required for long-term evaluation of walls founded on cohesive soil.

Soil Borings.

Obtain soil borings at 200-ft. spacing unless site conditions or the wall designer requires tighter or coarser spacing.

Boring Depth for Fill Walls.

For 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 drilled shaft walls, tied-back walls, and soil and rock nail walls, always base the depth of boring on the final grade lines. Cantilever drilled shaft walls require the depth of boring to extend the anticipated depth of the shaft below the cut, which is typically between one and two times the height of the wall. Advance borings for soil nail and rock nailed 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.

Soil Samples and Testing.

Provide additional testing for taller walls, walls on slopes, or walls on soft foundations as necessary to completely evaluate wall stability. Additional testing includes but is not limited to obtaining samples for 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 the groundwater elevation minimum 15 minutes after the initial encounter with ground water.

 Site conditions may require the installation of piezometers to establish a true ground water surface elevation and method of monitoring water surface fluctuations.