Construction World December 2021

SPECIALIST CONTRACTORS OR SUPPLIERS

THORNWOOD EMBANKMENT

REHABILITATION

T he embankment at Thornwood Stat ion, located west of Durban, has been progress ively moving over the past several years . The excess ive movement , fi rst reported in the latter part of 2012, prompted concerns of imminent fai lure. In 2013 rock- fi l led gabion baskets were placed in cri t ical areas in an attempt to stabi l i se the slope. Subsequent movement and sett lement of the embankment occurred, and in 2014 a comprehens ive slope stabi l i ty analys i s was carried out . The analys i s cons i sted of a detai led geotechnical invest igat ion which concluded that the embankment slope was marginal ly stable wi th a high potent ial to fai l at any t ime in the future. The invest igat ion ident i fied a series of factors contribut ing to the slope instabi l i ty and proposed remedial act ion. The proposed solut ion included the construct ion of a reinforced concrete bearing wal l and post-tens ioned ground anchors instal led into the underlying bedrock . Kel ler of fered a des ign, supply and instal lat ion solut ion compri s ing 132 no. permanent ground anchors of up to 40 m in length. The dri l l ing rigs used on the contract were two Casagrande C6 machines . A Symmetrix 193 cas ing dri l l ing system was ut i l i sed for dri l l ing of the cased port ion over the f ree length port ion of the anchor holes . The cas ings were advanced unt i l competent rock was encountered, which was up to 40 m deep. The fixed length port ion of the anchor was then dri l led past the cas ing into the competent rock ut i l i s ing a convent ional 6” DTH hammer and a 165 mm dri l l bi t . Due to the high comprehens ive strengths on the competent rock (up to 250 MPa) and highly f ractured in some places , special modi fied dri l l ing bi ts were used. To ass i st wi th the flushing during dri l l ing through the f ractured rock , a mixture of dri l l ing foam was injected into the ai r supply. The dri l l ing foam also acts as a cool ing medium for the dri l l bi ts during hard

dri l l ing. Transnet sent out a des ign and supply tender for permanent lateral support compri s ing 44 no. cable anchors permanent ly fixed into a 132-m- long by 3-m high bearing. The inf rastructure and reinforced concrete bearing wal ls were constructed by others . However, prior to Kel ler ’s involvement , the instal lat ion of the anchors was not successful , and tenders were then sent out again for the instal lat ion of these technical ly chal lenging anchors . Kel ler recogni sed the technical di f ficul t ies and the chal lenging condi t ions that the instal lat ion and stress ing of the anchors posed. The chal lenges were mainly due to t ight tolerances between the anchor and the cas ing ( requi rement for permanent anchors) , l imi ted working and laydown area, the weight of the anchors and the s i ze and weight of the jack requi red to stress the anchors . Af ter several technical meet ings and di scuss ions , the Kel ler team formulated a feas ible and construct ible solut ion. Kel ler ’s proposed solut ion was to replace each of the 44 no. 24 strand anchors wi th 3 no. 8 strand anchors (132 no. anchors in total ) wi th an equivalent combined total capaci ty.

• Company entering: Keller Geotechnics • Project start date: February 2020 • Project end date: 2 March 2021 • Client: Transnet SOC lTD • Main Contractor: Transnet (RME) • Principal Agent: Transnet Freight Rail (RME) • Consulting Engineers: Transnet Group Capital (TGC) • Quantity Surveyor: Transnet (RME) PROJECT INFORMATION

44 20 th Best Project Awards 2021