The maximum and minimum dry density tests were performed on BOF slag samples according to ASTM D4253 and ASTM D4254, respectively. The maximum and minimum void ratio for each BOF slag sample was calculated using the average specific gravity (G ) values obtained for each sample, as reported in Table 5.5. Table 5.8 presents s the maximum and minimum dry unit weights and the corresponding void ratios of the BOF slag samples. The maximum dry unit weights of BOF slag obtained from the vibratory compaction method was similar (in the ±5% range) to those obtained from the standard Proctor compaction test procedure (ASTM D698) (see Table 5.7). The vibratory compaction method tends to lead to slightly higher dry unit weights for coarse-grained soils than those obtained with the standard Proctor compaction method. This trend was not observed for all BOF slag samples. The slightly lower maximum dry unit weight values obtained with the vibratory compaction method were attributed to the effects of segregation and loss of fines that inevitably occurs during vibratory compaction of materials that contain a wide range of particle sizes.
Table 5.8 Maximum and minimum dry unit weight test results
5.5.3. Large-Scale Direct Shear Tests
Large-scale direct shear (LDS) tests were performed on BOF slag samples to determine its shear strength parameters. The Mohr-Coulomb criterion has been traditionally used to represent the shear strength of soils:
tan
S = +c σ φ Eq. 5.3 where S=shear strength; c=cohesive intercept; φ =angle of internal friction; and σ = effective normal stress acting on the shear plane. The shear strength corresponding to the peak states of the fresh and aged BOF slag samples were used to obtain the c− fitting φ parameters appearing in Eq. 5.3. The critical-state friction angles for both the fresh and aged BOF slag samples were also determined from the large-scale direct shear test results.
Table 5.9 shows the LDS testing matrix for the BOF slag samples considered in this study. Samples of Batch-1 Fresh BOF slag were also tested at two different moisture contents (corresponding to R values of 95 and 100%). Figure 5.15 (a) and (b) show the horizontal displacement versus the horizontal shear stress curves for samples of Batch-1 Fresh BOF slag prepared at R=95% and R=100%, respectively. The horizontal displacement versus shear stress curves for dense Batch-1 Fresh BOF slag show a distinct peak, which is indicative of dilative behavior. Dilation is observed in dense frictional materials tested at low confining stresses. The peak states were observed at horizontal
displacements of approximately 15-20 mm. Critical state was attained at approximately 40 mm of horizontal displacement. Figure 5.16 (a) and (b) show the shear strength envelopes corresponding to critical and peak states for Batch-1 Fresh BOF slag samples prepared at R=95% and R=100%. From the critical-state shear strength envelope, a critical-state friction angle of 45.5° was calculated for Batch-1 Fresh BOF slag.c− φ fitting parameters equal to 73kPa and 52.3o were obtained for Batch-1 Fresh BOF slag compacted at R=100%; these values were equal to 29kPa and 52.3o when R=95%. The highest shear strength (corresponding to critical and peak states) values were measured for R=100%.
Figure 5.17 (a) and (b) shows the horizontal displacement versus horizontal shear stress curves for Batch-2 Fresh and Batch-2 Aged BOF slag samples compacted at R=95%, respectively. Similar to Batch-1 Fresh BOF slag samples, the horizontal displacement vs. horizontal shear stress curves of Batch-2 Fresh BOF and Batch-2 Aged BOF slag samples showed that these samples are dilative. Peak states were observed at displacements of approximately 10-20 mm. Critical state was reached at horizontal displacements of approximately 30-40 mm. Figure 5.18 (a) and (b) provide the shear strength envelopes corresponding to the critical and peak states for Batch-2 Fresh and Batch-2 Aged BOF slag samples prepared at R=95%. The critical-state friction angles for Batch-2 Fresh and Batch-2 Aged BOF slags samples were equal to 48.1°and 45.3°, respectively. c− fitting parameters equal to 48kPa and 49.8φ o were obtained for Batch-2 Fresh BOF slag compacted at R=95%; these values were equal to 41kPa and 52.8o or Batch-2 Fresh BOF slag compacted at R=95%. Table 5.10 provides a summary of all shear strength parameters obtained from the large-scale direct shear tests performed on BOF slag samples.
Table 5.9 Test matrix for direct shear testing of BOF slag samples
0 20 40 60 80 Horizontal Displacement (mm)
0 100 200 300 400 500
Horizontal Shear Stress (kPa)
50 kPa 100 kPa 200 kPa 300 kPa
(a)
0 20 40 60 80
Horizontal Displacement (mm) 0
100 200 300 400 500
Horizontal Shear Stress (kPa)
50 kPa 100 kPa 200 kPa 300 kPa
(b)
Figure 5.15 Horizontal shear stress versus horizontal displacement for Batch-1 Fresh BOF slag samples prepared at a) R=95% and b) R= 100%
0 100 200 300 400
Figure 5.16 Shear strength parameters for Batch-1 Fresh BOF slag samples prepared at R=95% and R=100%: (a) critical-state and (b) peak-state
0 20 40 60 80 Horizontal Displacement (mm)
0 100 200 300 400 500
Horizontal Shear Stress (kPa)
50 kPa 100 kPa 200 kPa 300 kPa
(a)
0 20 40 60 80
Horizontal Displacement (mm) 0
100 200 300 400 500
Horizontal Shear Stress (kPa)
50 kPa 100 kPa 200 kPa 250 kPa
(b)
Figure 5.17 Horizontal shear stress versus horizontal displacement graphs for: (a) Batch-2 Fresh BOF slag prepared at R=95% (b) Batch-Batch-2 Aged BOF slag prepared at R=95%
0 100 200 300 400
Batch-2 Fresh BOF slag @ w=8%, R=95%
Batch-2 Aged BOF slag @ w=8%, R=95%
Batch-2 Fresh BOF slag @ w=8%, R=95%
Batch-2 Aged BOF slag @ w=8%, R=95%
Batch-2 Aged BOF slag @ w=8%, R=95%
c-φ fitting parameters
Batch-2 Aged BOF slag @ w=8%, R=95%
c-φ fitting parameters 48 kPa - 49.8o B-2 Fresh 41 kPa - 52.8o B-2 Aged
(b)
Figure 5.18 Shear strength parameters for Batch-2 Fresh BOF slag at R=95% and Batch-2 Aged BOF slag at R=96%: (a) critical-state, and (b) peak state
Table 5.10 Summary of shear strength parameters obtained from large-scale direct shear tests for BOF slag
Sample ID R φc c− fitting parameters φ a Batch-1 Fresh BOF slag 95%
45.5° 29 kPa-52.3°
Batch-1 Fresh BOF slag 100% 73 kPa-52.3°
Batch-2 Fresh BOF slag 95% 48.1° 48kPa-49.8° Batch-2 Aged BOF slag 96 % 45.3° 41 kPa-52.8° R = Relative compaction; φc=critical-state friction angle
afitting parameters correspond to the peak shear strength envelopes
BOF slag exhibits much higher shear strength than natural soils typically used in the construction of geotechnical structures. The sources of the high shear strength observed for BOF slag are: i) particle angularity and surface texture, ii) presence of gravel-size particles and iii) presence of a wide range of particle sizes. These factors contribute to high inter-particle friction and particle interlocking.