Discussion of “New Earth Retention System with Prestressed Wales in an Urban Excavation” by Jong-Sik Park, Yong-Sun Joo, and Nak-Kyung Kim

Abstract

The authors should be congratulated for the innovative retention system presented in the paper. The search for structural solutions permitting support to deep and wide excavations without using ground anchors has been a challenge for engineers, requiring a proper combination of geotechnical and structural expertise. This discussion aims to describe a solution with strong similarities to the one presented in the paper, which has been applied to support an excavation in Lisbon, Portugal, in 1982. Fig. 1 shows a plan and a cross section of the excavation, which was square in plan (39 × 39 m) and 24–29 m deep, for the construction of seven basements of a new building. The ground comprised a fill layer and Miocene sedimentary marine soils covering basalt rocks. As shown in Fig. 1(b), the retaining structure consisted of an anchored diaphragm wall supporting the upper part of the cut (corresponding to the fill and the decompressed Miocene soils) and nailing for the depths at which stiffer soils are encountered. After the construction of the first anchor level at the face adjacent to the masonry building, its owner obtained a court order prohibiting further installation of any anchors or nails under his property. Several solutions to overcome this unexpected problem were then discussed. The adopted one was conceived by Edgar Cardoso, an expert on bridge design, and was developed by the staff of the contractor, Teixeira Duarte (Lousada Soares 2003). The solution, denoted in Fig. 1 as a prestressed arch, is represented in more detail by the schemes in Fig. 2 and by the general view in Fig. 3. It consists of a polygonal tendon of 14 high-strength steel strands prestressed to 2,100 kN, coupled to a system of five steel-framed struts applying to the wall forces ranging from 300 to 350 kN. The tendon is anchored at the two corners of the cut by steel anchor plates inserted in concrete blocks linked to the diaphragm wall. The solution’s sequence of construction is described in the following: 1. Partial demolition (from the interior of the cut) of the diaphragm wall at the two corners to insert the anchor plates connected to the reinforcement of the wall;