Challenges
ON SITE
Challenges
The major challenge of cramped space experienced on the Illovo Central site was felt more than any others by subcontractors Bamboo Rock ( for bulk earthworks ) and Terra Strata ( lateral support ), who were required to work cheek by jowl .
“ At just 3 800m 3 , the site has a small footprint , which created a challenge for the sequencing of work . Working alongside each other were our own workers and those of Terra Strata , within a confined space , as well as working adjacent to neighbouring sites . The major requirement was for the bulk earthworks contractor to do our work while giving the lateral support contractor the space to do their work . Fortunately , we have worked with Terra Strata previously and have a good working relationship , so everything worked out fine ,” says Bamboo Rock managing director , Nic Chapman .
There had previously been a house on the site , leaving concrete and bricks in the earth . With a Cat 336 excavator on site having a crushing bucket , the approximately 30 000m 3 concrete and brick spoil was crushed on site and that material transported to a quarry in Crown Mines , which Bamboo Rock was in the process of rehabilitating . This was a large piece of land that a property developer was preparing for a future residential development .
“ We had an impact compaction team on site to accept the crushed rubble and compact it ,” says Chapman .
The geotechnical report was another cause of concern , but one that Chapman says they were prepared for . While the geotechnical report was inaccurate , given the circumstances of a previous development on the site that Chapman says was not expected to be . Bamboo Rock had worked on the neighbouring site , Illovo Point , and had encountered unexpected rock there . “ The information we had regarding the basement was very limited . The report said the rock level was -9 to -10 , so on paper it looked as though rock lay below the earthworks level . Having previously done Illovo Point , we knew there was rock at a higher level there .
“ You could say the report was misleading , but we expected it and in fact budgeted for it . Even so , we still experienced about 20 % more rock than we had measured and expected ,” says Chapman . His advice to others to avoid subsequent disputes around ground conditions is to do comprehensive drilling of the site to get a clearer picture .
As Concor pointed out , the site is in a highly developed location and with fronting on busy streets , and in this environment noise and dust suppression is vital — especially with a school and conference centre abutting .
Bamboo Rock and Terra Strata went to extremes to contain noise , dust , and rubble within the site confines . “ We had a water truck permanently on site to suppress dust , and water dampened the adjacent road . We also had two people cleaning trucks before they left the site to limit the amount of dirt falling onto Melville Road ( in front of Summer Place ). Melville Road is busy , especially during rush hours , so parking was also severely limited ,” says Chapman .
In this environment , and with a Gautrain tunnel running deep below , the blasting methodology also required careful planning : no rig blasting could be undertaken because of the proximity of lateral walls and neighbouring developments . “ There were 28 blasts on the site — all small , contained blasts to a limit of 25 peak particle velocity ( ppv ) to 30ppv per blast . We were required to keep them below a limit of 50ppv , so we were well within that limit . We used jack hammers and hydraulic hammers for the removal .
“ The tricky part was operating right up against Illovo Point ,” concludes Chapman .
In addition to the Cat 336 excavating machine in the 40-tonne class , the contractor had 16 Mercedes tipper trucks , one site foreman , a site manager , and six workers . nn
List of professionals
Project Type Client Architect Project developer Civil / structural engineer Bulk earthworks Lateral support Mechanical engineer Quantity surveyor Electrical engineer Fire consultants Traffic engineer
Illovo Central Multi-purpose and primarily residential Johannesburg 4 Properties FWJK Architecture FWJK Developments MVW Consulting Engineers Bamboo Rock as selected subcontractor to Concor Terra Strata as selected subcontractor to Concor Arup FWJK Quantity Surveyors Quad Africa Consulting Engineers MDB Fire Protection Engineers Arup
20 - CEC September 2018