Civil Engineering April 2021 | Vol 29 No 3

Civil Engineering April 2021 41 in length due to non-uniform exten- sion of the nail (see van der Merwe & Hoffman, 2020). In all the calculations, the bond strength and bond shear stiffness were kept constant, although this will vary based on the installation methodology (gravity grouting, injection grouting (IGU) and post injection grouting (IRS)). The attracted loads exceeded those that can reasonably be carried by a 25 mm TB500 using appropriate factors of safety (see Table 2). The displacements of the same system were predicted to be 41 mm to 50 mm when using 25 mm soil nails. This exceeds the permissible tolerance of 35 mm indicated in the SAICE Lateral Support in Surface Excavation Code of Practice (1989). The piles were moved closer to one another at 1 m c/c, ultimately increasing the flexural stiffness of the wall (EI). The increase in the wall stiffness brought the displacements down to comparable values when compared to that using prestressed grouted anchors. Decreasing the pile spacing from 2.5 m c/c to 1 m c/c and placing soil nails at a 1 m c/c spacing would increase the cost of the wall by an estimated 30% to 50%. Careful consideration needs to be given to group effects of nails at such close spacing. This reduced spacing should result in reduced vertical settlement of the crest of the wall due to increased frictional resistance (Day, 1994). If displacements of 41 mm to 50 mm were however acceptable, it would be difficult for a soil nail to resist 245 kN in bond between the soil and grout using gravity grouting in a Berea Red profile. The spacing of the nails should therefore be reduced in any event, unless equipped with Tube-á-Manchette (TáM), to ensure bond capacity can reasonably be achieved. Figure 6 provides the output from the elastoplastic-exponential analyses showing that a more triangular pressure will develop behind the wall compared to a more rectangular pressure when applying prestress (Figure 4 in Part 2). Figure 6 Elastoplastic-exponential beam on elastic analyses of pile wall with soil nails 2.7 m Wall 1 Reinforced concrete piles (lag.) Pile diameter: 60 cm, 6 rebars Φ25, @ 2.5 m O.C. FykRebars = 413.8 MPa, Fck = 35 MPa Supports beam analysis Wall δ = 50% Φ Wall δ.drive = 66% Φ 2.7 m 2.7 m 2.9 m 11 m 4 m El. –11 m Deflection (cm) Pressures (kPa) –4 69.38 kN/m 103.1 kN/m 10° 10° 10° El. 0 m F γt = 19.625 kN/m 3 φ’ = 30 ° E = 45 000 kPa rEur = 3 e =0.4 Effective horizontal soil pressures Wall bending Seismic pressures –2 0 2 Moment (kN-m/m) –150 50 –200 200 85.26 kN/m Boring 1 Table 4 Results using passive soil nails to retain the soldier/ contiguous pile wall (SLS calculations) Solution 2.5 m c/c with soil nails 2.5 m c/c with soil nails 1 m c/c with soil nails 1 m c/c with soil nails 1 m c/c with soil nails Analysis type Elastoplastic – Exponential Mohr Coulomb – RS2 Elastoplastic – Exponential Mohr Coulomb – RS2 Hardening Soil -RS2 A1 (kN) 192.1 245 73.6 80 100 A2 (kN) 233.3 237 91.6 100 110 A3 (kN) 168.5 158 64.5 67 70 BM per pile (kN.m) 357.5 245 153 166 136 Dz (mm) maximum all stages 41 50 19 38 38