Slopes and Embankments

Soil Borings
. Obtain soil 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.). Additional laboratory testing may be required to determine soil parameters for short-term and long-term stability analysis, and consolidation settlement analysis (for embankments). Consult with the wall or roadway designer for development of the complete soil exploration plan.
The exploration should include the following:
  • The soil under future embankments. Advance borings to a minimum depth below existing grade equal to the height of the embankment or 20 ft., whichever is greater. If compressible soils exist extend the foundation soil boring considering height, width, and loading of embankment. Conduct Standard Penetration tests (SPT) in accordance with AASHTO T 206 or ASTM D1586 every 5-ft. interval beginning at 5-ft depth from the surface. Where cohesive soils are observed collect thin-walled Shelby Tube samples in accordance with AASHTO T 207 or ASTM D1587 at the intermediate locations between SPT samples.
  • Soil in proposed cuts. Advance borings to a minimum depth of 15 ft. below the bottom of the proposed cut. Conduct Standard Penetration tests (SPT) in accordance with AASHTO T 206 or ASTM D1586 every 5-ft. interval beginning at 5-ft depth from the surface. Where cohesive soils are observed collect thin-walled Shelby Tube samples in accordance with AASHTO T 207 or ASTM D1587 at the intermediate locations between SPT samples.
  • Ground water elevation measurements. Include ground water elevation measurements (including date of measurement) as part of the data acquisition for slopes and embankments. Obtain an additional groundwater elevation at a minimum of 15 minutes after the initial encounter. Site conditions or the design objective may require the installation of piezometers to establish a longterm or steady state ground water conditions.
Soil Testing
. Perform the appropriate field and laboratory tests, in accordance with Chapter 3 field testing, and Chapter 4 Section 2 laboratory testing, 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. In cohesive soils, use undrained shear strength determined using laboratory strength tests on undisturbed Shelby tube samples for design. Strength measurement from pocket penetrometer tests, hand torvanes, or field vane shear tests should not be solely used to evaluate undrained shear strength except as a supplement to laboratory strength tests on undisturbed tube samples. Avoid correlations of undrained shear strength based on SPT tests. Use unconfined compression tests, unconsolidated undrained (UU) triaxial tests, consolidated undrained (CU) and/or direct shear tests.
  • Long-term conditions. Use consolidated undrained (CU) triaxial tests with pore pressure measurement and/or drained direct shear tests.
Estimation of long-term effective stress friction angle of clay soils based on published correlations with index properties of the soil is acceptable. Correlations between corrected SPT blow counts to drained angle of internal friction for granular soils presented in AASHTO LRFD Bridge Design Specifications Table 10.4.6.2.4-1 may be used, and a graphical representation is presented in Figure 2-2. However, the selection of specific values in the range may require experience and care. For cohesive soils, published PI correlations from Atterberg results proven to yield reasonable estimates of effective friction angle may be used.
For cuts with high plasticity clays exposed to weathering or cyclic wetting and drying, long-term shear strength reduction due to soil relaxation may be possible. For such instances a reduction in shear strength as appropriate to account for shear strength loss due to weathering and long-term relaxation should be considered in the evaluation of long-term stability of embankments or slopes.
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Figure 2–2. SPT vs. Angle of Internal Friction for Cohesionless Soils