Civil Engineering March 2021 | Vol 29 No 2

60 March 2021 Civil Engineering GEOSTRUC TURAL Eng i neer i ng The first part of this article (van der Merwe & Konstantakos, 2020), published in the April 2020 issue of Civil Engineering (Vol. 28 No. 3), provided feedback on the design practice followed by geotechnical engineers in the design of deep excavations in South Africa. Various considerations such as site investigations, choosing the appropriate lateral support systems, the use of and spacing of a soldier piled solution, deformation due to groundwater extrac- tion, the use of EN1997-1, apparent earth pressures (AEP) and prestress loads applied to grouted anchors, were discussed. The BS 8006-2 code of practice for strengthened/reinforced soils and soil nail design will be adopted in South Africa for the design of soil nail walls. The revised SAICE Lateral Support in Surface Excavation Code of Practice will therefore focus predominantly on the design of multi-anchored pile walls. The second part of this article will discuss beam-on-elastic (BOE) analysis methods, appropriate constitutive models, reduced passive resistance due to 3D effects, interface friction, reliability-based design, estimation of displacements, and the struc- tural design of shotcrete infill panels. A comprehensive questionnaire was developed and forwarded to respondents as part of updating the SAICE Lateral Support in Surface Excavations Code of Practice. Damon Clark from Damon Clark & Associates provided feedback to the questionnaire subsequent to the publica- tion of Part 1 of the article. GEOTECHNICAL BOE AND FINITE ELEMENT SOFTWARE PACKAGES The following question was asked: Do you use BOE Winkler Spring methods and geotechnical finite ele- ment (FE) software packages to model multi-anchored pile walls? Three respondents indicated that they use Prokon’s BOE module as a quick check to confirm structural forces, while two re- spondents indicated that they use DeepEx, and a third PY Wall. Other software pack- ages such as Wallap are commonly used in the UK and Australia for the design of embedded pile walls. Prokon’s BOE module provides only for linear springs and not elastic-plastic springs. Using only linear springs could result in pas- sive resistance values exceeding those that can actually be developed by the retaining soils. The Prokon BOE module also does not consider staged excavations, with displace- ment cumulatively increasing with each excavation stage. Additionally, the spring stiffness value cannot be easily adjusted to increase with depth (for sandy or gravelly profile) with Prokon’s BOE unless one uses the frame FE analysis suite. Software that incorporates non-linear springs and staged excavation analyses is more appropriate to the analysis of embedded pile walls. DeepEx, PY Wall, and Wallap make allowance for the use of non-linear springs, staged excavation analyses and spring stiff- ness values that increase with depth and are therefore better suited to analyses of anchored pile walls. Six respondents indicated that they use Phase2/RS2 (Rocscience) geotechnical FE software, while a seventh respondent uses Plaxis. One user further indicated that Digging deeper – a review of South African practice on deep excavation design in urban areas | Part 2 Frans van der Merwe Pr Eng Principal Geotechnical Engineer, City of Cape Town Chair, Working Group updating SAICE Lateral Support in Surface Excavation Code of Practice frans.vandermerwe@capetown.gov.za Alan Parrock Pr Eng Consultant/Specialist Engineer ARQ Consulting Engineers alan@arq.com Figure 1 Deep basement excavation at King Shaka International Airport, Durban (Credit: S. Naidoo)