News & Media

28 July 2009

CAREFUL PLANNING AND TECHNICAL KNOWLEDGE MAKES CONCOR THE PARTNER OF CHOICE ON N12 UPGRADING PROJECT

Concor Roads & Earthworks is currently busy with The South African National Roads Agency Limited (SANRAL) contract to rehabilitate 43.6 km of the single carriageway National Route 12 Section 12. The section of road starts just south of the access to the Beefmaster factory outside Christiana and continues up to the intersection of the N12 with the Provincial Main Road R34 in Bloemhof.

“The last construction to take place on this route was about 30 years ago. Rehabilitation work includes the vertical realignment of the road from its current position to a higher level to facilitate adequate drainage of runoff and ground water from the road,” George Ingram, project manager at Concor Roads & Earthworks, says.

“The vertical alignment has been a problem in the past due to the flatness of the region and vast sections of the old road had deteriorated as a result of this. Of the total 43.6 km of this section of road some 90% is being vertically realigned, with the remaining 10% being constructed in cuttings. This will bring the grade of the road up to the required standard for national roads which carry traffic travelling at speeds of up to 120 km/h,” Ingram says.

The existing road consists of a surface of approximately 7.2 metre wide and unsurfaced shoulders of 2.2.metre on each side of the surfacing.

The existing carriageway is to be widened to a minimum surface width of 12.64 metres comprising two 3.7 metres lanes and two 2.5 metres to 2.7 metres wide surfaced shoulders.

The rehabilitation process involves the reconstruction of the full length of the road including the processing of the existing sub-grade and the supply and processing of fill, selected and sub-base and G1 base materials supplied from cut and borrow pits.

Smooth operator
Work on the contract begins with scarifying the existing road surface. “We then construct the in-situ road bed which is followed by fill material located from borrow pits alongside the road,” Ingram says.

In total there are five borrow pits along the length of the road. These locations were carefully selected in collaboration with SANRAL and will be returned to Department of Minerals & Energy (DME) standards at the end of the contract. The crusher plant at borrow pit 16 is owned and operated by Concor. “This was an economically motivated decision and also ensures the quality of the material being produced,” Ingram says.

The material being used for the fill layer is G7 quality. It is excavated from the borrow pit using hydraulic excavators, loaded into 10 m3 on-highway tippers and dumped on the road construction. This fill layer is compacted to 93% of MODAASHTO using 19 ton padfoot and smooth rollers.

“The padfoot roller is used to break down the oversize material with layers varying from 1 to 5 metres, dependant on the gradient required in the different sections of the road being constructed,” Ingram says. The entire project will use 850 000 m3 of material in total including fill and all layers.

Next is a G6 quality selected layer with material taken from the same borrow pits. “Test holes were done to determine that G6 quality material could be obtained. This was necessary to ensure that we achieve optimum load bearing capacities on the road. The same methodology as for the G7 material is being used, that is, the material is excavated, loaded into tippers and dumped on the compacted G7 material. Two 150 mm layers are being built on top of each other to 95% MODAASHTO respectively,” Ingram says.

“It was initially envisaged that all sub-base layer material would be excavated from the borrow pit and crushed. In some instances, the quality of the material was of such a nature that it could be used without crushing, however, in other instances it is being crushed prior to use,” Ingram says. This layer, designated C4, varies from 150 to 200 mm in thickness and once placed it is compacted to 95% MODAASHTO. This layer is being stabilised with 3 to 4% lime to ensure its integrity.

On top of this is a C3 sub-base layer which is 150 mm thick, compacted to 97% MODAASHTO and stabilised using 2.5% cement for integrity purposes. “All the material for this layer is being excavated at SANRAL’s borrow pit 16 which is an andacite quarry. Blasting is used to liberate the material required whereafter it is crushed to a C3 specification,” Ingram says.

The final layer is a G1 base course which is also from borrow pit 16. “This crushed material is deposited as the final layer to a 150 mm thickness and is being compacted to 88% apparent relative density,” Ingram says.

Following this final layer, the road is primed with E-prime at an application rate of 0.65 litres/m2 to ensure optimum adherence of the 19 mm Cape Seal. This consists of an 80/100 bitumen which is sprayed at an application rate of 1.25 litres/m2 and is then followed with a 19 mm commercially supplied aggregate.

Once this is applied the surface is rolled with a 12 ton three point roller and a 27 ton pneumatic roller whereafter it is covered by a 13 kg/m2 slurry seal and a second fine slurry coating is applied at a 2 kg/m2 rate. On completion of this, the road markings are done and road studs are installed.

Mentrail guardrails will be used on sections which have a gradient or drop of more than 3 metres to ensure optimum safety.

“The contract also calls for the construction of new precast pipe culverts and precast portal culverts as well as the necessary inlet and outlet structures including lengthening of existing reinforced concrete box culverts and construction of headwalls and wing walls, box culverts and concrete lined side drains. We are also undertaking slope protection and establishment of vegetation,” Ingram says.

Included in the contract are two road-over-rail bridges, spanning the main rail line from Johannesburg to Cape Town via Kimberley, that need to be widened to the same width as the road. “The railway line carries up to 14 locomotives with carriages and wagons in a single day and because construction of the railway structures will be undertaken over live electrified rail way lines this places very obvious physical and working restrictions on the site,” Ingram says. “Fortunately, good communication with Transnet has made this challenge somewhat easier and mitigated against risks.”

Construction began with the building of two pier walls on either side of the railway line. The span over the railway lines was done with precast beams which were cast on site outside Christiana. “These were placed from pier to pier and then from pier to abutment on two different Sundays when all railway traffic was purposely halted for five hours at a time to accommodate this construction work.

Challenges
The biggest challenge faced on the project is the need to accommodate traffic while constructing the road. “This is particularly challenging given the working condition limitations and the short contractual period of 34 months. Added to this is the fact that it is not possible to work at night on this contract,” Ingram says.

The restrictions on the actual road site mean that there is limited space within which to operate machinery. “The area on one side is 6.4 metres and on the other it is only 6.2 metres which does not allow a large turning circle for machinery,” Ingram says. When material is being dumped by the tipper trucks on required sections, it is necessary for these vehicles to reverse for the full extent of that section. Normally the vehicle would drive to the point where the material needs to be deposited, and then turn around and reverse a very short distance to dump.

“The right hand side of the road is being constructed first and once completed we will move over to work on the left hand side. We are able to plan construction to the nth degree because the volume of traffic on this national route is not seasonally affected. So, although there are some higher peak periods than others it is generally very consistent with the majority of the traffic being heavy haulage and amounting to between 850 and 900 heavy vehicles per day,” Ingram explains.

The centre line of the carriageway is being adjusted where necessary to accommodate the construction on the right hand side.

“Stop and go closures operate during the day with traffic lights at night to regulate the traffic flow along the route and to ensure safety wherever possible. Overhead lamp posts are sited leading up to each traffic light/stop go section to ensure optimum visibility,” Ingram says.

Where necessary, temporary construction measures are being undertaken on the left hand side of the road to provide single lane traffic in each direction while construction of the widened fills and layerworks on the right hand side of the road is being completed.

Construction is only permitted on four 4 km sections of the road at any one time and each construction zone has to be separated by at least 4 km. A maximum of two construction zones are controlled by the stop/go traffic light at any one time and these are installed at each end of the construction zone.

“Another challenge being faced is on the bridge approaches which have fill sections of 100 000 m3 per bridge. These need to be constructed in half sections because of the need to accommodate the traffic 24 hours a day and there is no other access apart from the bridge over the railway lines,” Ingram says.

The bridge approaches are on average 5 metres above ground level but in some cases higher. “We channelled traffic on the existing road while widening of the sides was taking place and the fill was built in benches so we were able to shift the traffic over to a temporary bypass section,” Ingram adds.

Three cuttings will be constructed where 100 000 m3 of material will be drilled and blasted. “It was necessary to construct a 3 km two lane bypass across private land to facilitate the sub-contractor’s drill and blast operation under safe working conditions,” Ingram says.

“Services such as Telkom overhead lines, Eskom overhead lines, and water mains cross the road at various places and it has been necessary in the case of Telkom and Eskom lines to liaise with all the appropriate parties to remove the lines prior to construction work taking place. The water pipes have had to be accommodated during the construction of the roadworks by the extension of the existing portal culverts that house the water mains,” Ingram says.

Requisite on the contract is the use of local unskilled labour and training has formed an important element of this contract. “The training centred around environmental issues, safety, casting of in-situ culverts and guardrail installation.

“Close cooperation between all team members, the ability to carefully plan all logistical matters, and attention to technical details will be the hallmarks of success on this project,” Ingram concludes.

90% of the existing road has had to be vertically realigned to facilitate adequate drainage of runoff and groundwater from the road. This is as a result of the flatness of the region.

The existing carriageway is to be widened to a minimum surface width of 12.4 metres, comprising two 3.7 metre lanes and two 2.5 metre to 2.7 metre wide surfaced shoulders.

When material is being dumped by the tipper trucks it is necessary for these vehicles to reverse for the full extent of the section as a result of the limited space in which operations are taking place.

Widening of two road-over-rail bridges is included in the contract and has required careful planning as this rail line carries up to 14 locomotives with carriages and wagon daily.

 

SUE UPTON
CONCOR GROUP
TEL : +27 011 495 2288