Civil Engineering April 2021 | Vol 29 No 3

Civil Engineering April 2021 39 using the same installation methodology. This grout interface should be tested to ultimate capacity and subtracted from the capacity derived from suitability tests that include the free length (CIRIA C794). Additionally, grout in direct contact with the piled wall needs to be removed before stressing to prevent strutting of the grout columns against the piles that will result in increased measured capacity. PILE WALLS WITH PASSIVE SOIL NAILS The following question was asked to respondents: If you do not apply prestress to these grouted anchors, do you think this affects the displacement and apparent earth pressure (AEP or triangular) that develops? All respondents indicated that by not ap- plying prestress, increased displacement would result. One respondent indicated that the stiffness of the wall can be increased to circumvent the expected increase in displacements. Another respondent stated that movement needs to occur in the passive system for the pressure to change from triangular to more rectangular. BS8006-2 and EN1992-1-1 provide guidance on determining the design tensile resistance (ULS) of various steel reinforcement elements. This results in similar design resistances compared to those from SANS10162. Table 2 provides design resistances of reinforcement typically used in South Africa as lateral support members. When reviewing Table 2 it should be noted that strand grouted anchors are often preferred when compared to bar grouted anchors. This is due to the increased strength of strands compared to bars of similar diameter and the associ- ated decreased space required inside the small diameter drilled holes. The finite element (FE) models were adjusted, and grouted anchors not pre- stressed, with results provided in Table 3. It should be remembered that the bar cross sectional area could theoretically be reduced when considering the loads attracted to the non-prestressed bars compared to the loads provided in Table 1. This would result in a further increase in predicted displacements. It can however be concluded that the horizontal displacement would increase by roughly 10 mm or 50% using the Hardening Soil Small Strain Constitutive model and when not applying prestress to the same cross-sectional SDAs (compared to Table 1). Occasionally, conventional gravity grouted soil nails are used instead of SDAs or strand anchors to stabilise these walls. The above analysis was therefore adjusted using 25 mm TB500 non-prestressed passive soil nails with results contained in Table 4. Using soil nails results in increased production rates compared to those using grouted anchors, which need to be proof load tested. However, there remains a risk of insufficient bond capacity if not tested. The results of pullout tests on short nails cannot be directly extrapolated to long soil nails and corrections need to be applied as bond shear strength is not a constant and decreases with an increase Table 2 Design tensile resistance of typical bars/ strands used in South Africa according to EN1992-1-1 and BS8006-2 Diameter R tk (kN) EN1992-1-1 Maximum lock- off temporary (kN) Maximum lock-off load permanent (kN) with DCP R td (Transient) (kN) R td (accidental) (kN) Bar 16 (500 MPa min) 100 87 100 69 58 20 (500 MPa min) 157 137 157 109 91 25 (500 MPa min) 245 213 245 170 142 32 (500 MPa min) 402 349 402 279 233 Strand 15.2 (1860 MPa) 228 198 228 158 132 SDA Hollow Bar R25N 150 130 150 104 N/A require double corrosion protection according to EN1537 R32S 280 243 280 194 R38N 400 348 400 278 R51N 630 548 630 438 T30/11 260 226 260 180 T40/16 525 457 525 365 T52/26 730 635 730 508 Table 3 Results from FE using non-prestressed R51 grouted anchors (SLS calculations) Solution 2.5 m c/c with SDAs no prestress 2.5 m c/c with SDAs no prestress Analysis Type Hardening Soil-RS2 Hardening Soil Small-RS2 A1 (kN) 233 234 A2 (kN) 270 270 A3 (kN) 165 168 BM per pile (kN.m) 190 187.5 Dz (mm) maximum all stages 29 30