The objective of this project is to conduct a statewide examination of shear strength and other physical properties of naturally occurring sands suitable for use as granular backfill for mechanically stabilized earth (MSE) walls and reinforced soil slopes. Typical ranges of friction angle are being identified and physical, mineralogical, and geological controls on shear strength are being evaluated.

Direct shear testing is being conducted using a traditional small-scale shear box (63 mm x 63 mm x 30 mm) and a large-scale shear box (305 mm x 305 mm x 122 mm) (Insert). Shear strengths from small-scale direct shear (SS-DS) are currently used in geotechnical design. However, a number of granular materials sampled for this project contain significant quantities of gravel. Large-scale direct shear (LS-DS) testing is being conducted to assess the impact that the gravel fraction has on shear strength.

Figure 1 shows comparisons of friction angles from LS-DS versus SS-DS for materials containing less than 5% material by weight retaining on a No. 4 sieve. Figure 1a represents friction angles based on direct shear force measurements in LS-DS. Due to the weight of the top half of the LS-DS box and approximately 8% of the applied normal force being transferred to the LS-DS box interface, a significant component of measured shear force was actually machine friction at the box interface. Figure 1b accounts for a machine friction correction to produce more comparable results between SS-DS and LS-DS.

Figure 1. (a) LS-DS measured friction angle vs. SS-DS and (b) LS-DS machine corrected friction angle vs. SS-DS

Only materials containing less than 5% material retained on a No. 4 sieve were used for initial machine correction comparisons to validate the best overall machine correction method. Samples containing greater than 5% material retained on a No. 4 sieve will be corrected and compared to assess the influence of gravel on shear strength values.

Future Work:
An analysis of geological and regional impacts on physical characteristics and shear strengths is underway to differentiate higher and lower shear strength areas throughout the state of Wisconsin. Triaxial compression tests are being performed on four select samples for comparison with shear strengths obtained from the SS-DS and LS-DS tests.