Slopes and Embankments

Soil Core Borings.
 Obtain soil core borings for cuts greater than 10 ft. or embankments taller than 15 ft. in areas with suspect foundation soils (less than or equal to 10 blows/ft.). TCP testing alone may be adequate.
The exploration should include the following:
  • The soil under future embankments. Advance borings to a depth equal to the height of the embankment or 20 ft., whichever is greater. Conduct TCP testing at 5-ft. intervals.
  • Soil in proposed cuts. Advance borings to a depth of 15 ft. below the bottom of the proposed cut. Conduct TCP testing at 5-ft. intervals.
  • Ground water elevation measurements. Include ground water elevation measurements (including date of measurement) as part of the data acquisition for slopes and embankments. Obtain the groundwater elevation minimum 15 minutes after the initial encounter with ground water. Site conditions may require installation of piezometers to establish a true ground water surface elevation and method of monitoring surface fluctuations.
Soil Testing.
Perform the appropriate field and laboratory tests necessary to determine the soil shear strength for proper soil evaluation of the structure being designed. Consider both the short-term and long-term conditions:
  • Short-term conditions. Use the Texas Cone Penetration test, in-place vane shear tests, unconsolidated undrained (UU) triaxial tests, and or direct shear tests.
  • Long-term conditions. Use consolidated undrained (CU) triaxial tests with pore pressure measurement and/or drained direct shear tests.
Estimate long-term strengths of clay soils based on the index properties of the soil. Use the following figure to correlate Texas Cone Penetration test results to angle of internal friction for cohesionless soil.
TCP vs. Angle of Internal Friction for Cohesionless Soils (click in image to see full-size image)
Figure 2-2. TCP vs. Angle of Internal Friction for Cohesionless Soils