Construction World December 2022

SPECIALIST CONTRACTORS OR SUPPLIERS

T he communi ty of Khutsong i s located on the West Rand of South Af rica near the mining town of Carletonvi l le. Given the dolomi t ic geology, s inkhole format ion i s a common occurrence in Khutsong. When s inkholes form, the local municipal i ty are of ten requi red to repai r, rebui ld or relocate inf rastructure to ensure i ts res idents are safe and that ser vice del iver y cont inues . The relocat ion of the 30 M ℓ Khutsong reser voi r i s one such project undertaken by the Merafong Ci ty Local Municipal i ty. The exi st ing reser voi r ser vicing Khutsong Extens ion 3 was previously unable to retain water due to the format ion of a large s inkhole under the reser voi r floor. The relocat ion project involved, construct ing a new thi rty mega- l i tre reser voi r and support ing inf rastructure several hundred metres away f rom the exi st ing, unused reser voi r. The support ing inf rastructure includes connect ing pipel ines for both the supply to and di stribut ion f rom the reser voi r, a pump stat ion and general faci l i t ies for access ing, maintaining and securing the inf rastructure. As a resul t of the dolomi t ic geology, one of the major des ign requi rements for the reser voi r floor was the capaci ty to span a s ink hole five metres in diameter whi lst ful ly loaded to over flow height . Thi s requi rement led to the fi rst round, prestressed reser voi r raf t floor in Af rica being des igned and constructed at the Khutsong 30 M ℓ reser voi r. The bas ic phi losophy behind the prestress ing opt ion for the floor involves applying pre-compress ion to the raf t floor to provide addi t ional res i stance to crack- inducing tens i le stresses . The t iming of the stress ing, the profi le of the cables and the concrete mix des ign were the three cri t ical factors in ensuring the per formance of the 450 mm thick raf t floor slab. The concrete mix des ign for the raf t floor requi red a compromi se between an ideal water retaining concrete mix and PT slab mix. A ver y del icate balance needed to be achieved between reducing shrinkage, increas ing the impermeabi l i ty and ensuring suf ficient early strength gain before shrinkage occurs . A further RELOCATION OF 30 M ℓ KHUTSONG RESERVOIR

compl icat ion was ensuring that the mix was st i l l workable af ter being on the road for just over an hour given the vicini ty of the nearest ready-mix batch plants . Several trial mix des igns were conducted as finding the balance between acceptable shrinkage, permeabi l i ty, workabi l i ty and early strength gain proved a sens i t ive process . The opt imum mix was however found by balancing a sui table water cement rat io for permeabi l i ty wi th Chr yso’s shrinkage compensat ing admixture, CHRYSO® Sereni s . CHRYSO® Plast ZA 1559 also proved an ef fect ive plast ici s ing admixture and ensured workabi l i ty af ter 80 minutes whi lst also providing adequate early strength gain. An Af ri sam CEM I I /A-M 42.5R type cement was selected for both product per formance and rel iabi l i ty of del iver y during what has been a tough period to secure cement del iveries post lockdown. To test the des ign’s per formance, a sample beam measuring 59,25 m long, 0,210 m wide and 0,450 m deep was des igned and cast on-s i te wi th a s ingle mono strand cable to s imulate the influence of one cable on the raf t floor. In terms of concrete placement , the des ign engineer requi red the pour to be completed wi thin a 24-hour window to ensure the homogenei ty of the concrete under early stress ing. At the work face i tsel f, ef ficiency and adequate resourcing planning were crucial for successful execut ion. 

PROJECT INFORMATION

• Company entering: CHRYSO • Client: Merafong City Local Municipality • Start date: 10 February 2021 • End date: 20 March 2022 • Main Contractor: Quantibuild • Consulting Engineer: Morad Consulting • Readymix: RMX WG Wearne Readymix

48 21 st Best Project Awards 2022