Civil Engineering April 2022 | Vol 30 No 3

18 April 2022 Civil Engineering T he direct shear box is a common device used to determine the internal friction angle of granular materials. ASTM D3080 defines the box dimensions in terms of the maximum particle size (D max ), with the width (W) > 10 × D max , the height (H) > 6 × D max , and the width to heigh ratio (W/H) > 2. Discrete element modelling by Wang and Gutierrez (2010) suggested that results were less sensitive to the width or height compared to the width to height ratio. METHODOLOGY To investigate this, a standard direct shear box was adapted with metal spacers to allow different box dimensions to be set for testing of two saturated granular soils (Table 1). Well graded samples were placed at an average density of 1 658 kg/‌m 3 (standard deviation = 46 kg/‌m 3 ) or relative compaction of 91%. Poorly graded samples were placed at an average density of 1 650 kg/m 3 (standard deviation = 51 kg/m 3 ) or relative compac- tion of 89%. Each sample was tested three times at three different normal stresses (50, 100 and 150 kPa). In total 162 tests were completed. Data labels in Figure 1 show aver- ages (x) and standard deviations (σ) of measured shear ratios (τ max /σ normal ) for all tests at a given box configuration (open symbols). Contours of W/H are overlayed on the graph. While both average values and standard deviations decrease with H/D max and W/D max , the values decrease most consistently with W/H. For well graded samples τ max /σ normal converges to 0.9 (φ ≈ 42°) and poorly graded samples converge to 1.1 (φ ≈ 48°). FINDINGS Results suggest height and width restrictions in ASTM D3080 are adequate, but the width to height ratio should be increased from 2 to 4. Testing thinner samples (provided the height remains within standard) results in more accurate results and lower variability. Testing material larger than the specifications overestimates internal friction angles and results in greater variability.  REFERENCES ASTM 2011. Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions (D3080-11). West Conshohocken, PA: ASTM International. Wang, J. & Gutierrez, M. 2010. Discrete element simulations of direct shear specimen scale effects. Géotechnique, 60, 395-409. Stellenbosch Uni vers i ty geotechni cal research abstracts Particle size and box dimensions in direct shear box testing Dr Charles John MacRobert Pr Eng Senior Lecturer | Department of Civil Engineering Stellenbosch University macrobert@sun.ac.za Shane Teek Geotechnical Geologist/Hydrogeologist GEOSS South Africa shane@geoss.co.za Dr Nanine Fouché Senior Lecturer | Department of Civil Engineering Stellenbosch University naninef@sun.ac.za Stellenbosch University has compiled a series of geotechnical research abstracts outlining research undertaken by the Department of Civil Engineering. The second abstract looks at how the internal friction angle of granular materials can be determined using direct shear box testing. Table 1 Material tested in direct shear box Value Well graded gravel (GW) Poorly graded gravel (GP) D max (mm) 4.00 4.00 D 50 (mm) 2.2 3.1 D min (mm) 0.425 2.0 C u 6.2 1.6 Maximum dry density (kg/m 3 ) 1 815 1 845 Figure 1 Direct shear box results: a) Well graded gravel and b) Poorly graded gravel