CONSTRUCTING THE GAUTRAIN UNDERNEATH THE N3 HIGHWAY

Posted on Mar 5, 2009 in Media Releases

Background

Between Marlboro Station and Linbro Park the Gautrain alignment passes underneath the busy N3 highway. It is at this point that the route splits into two – one leg travelling northwards towards Midrand and Pretoria, and the other eastwards towards Rhodesfield and the OR Tambo International Airport.

The construction underneath the N3 freeway utilizes the “cut-and-cover” technique. This involves excavating (or “cutting”) open “boxes” into the ground, constructing concrete culvert-type structures in these open cuts (through which the trains will eventually pass) and then backfilling over the structures with soil. This technique is analogous to tunnelling but is used where the “tunnel” is relatively shallow and where existing surface structures can be temporarily relocated. The N3 cut-and-cover structures are being excavated and constructed sequentially in conjunction with temporary lane deviations to enable the freeway to remain open at all times.

Since July 2008 traffic has been deviated along the N3 northbound and southbound carriageways to accommodate the complex process of constructing the cut-and-cover structures below the main carriageway and adjacent access ramps without affecting traffic flow. Once the underpass has been completed, the surface will be covered up, while traffic flow will be reinstated to its original position. Construction works at the N3 crossing are expected to be completed by July 2009.

Why under the N3 and not over?

Marlboro station, which lies immediately adjacent to the N3 freeway on it’s western flank, is an “at-grade” station i.e. constructed at ground level. Due to the natural topography, this station is at a lower level than the N3 off and on-ramps.  It would thus require an impossibly steep incline for the trains to leave Marlboro station at ground level and then to climb up onto a bridge or viaduct structure to cross over the N3. An overhead structure was therefore not a practical solution.

Challenges in constructing the N3 Crossing

Constructing the N3 underpass is a significant engineering challenge that is being achieved through careful planning and design and the implementation of modern construction techniques. Specific challenges that the Bombela construction team has faced in this section include:

Close-up of the cut-and-cover structures under construction

• Implementing and controlling a multi-phase construction programme at five different work areas simultaneously.
• The coordination of construction labour, plant and equipment as well as blasting activities under live traffic conditions.
• Major utility diversions including Eskom transmission lines, storm-water, sewers and water over, under and alongside a busy strip of road in Linbro Park.
• The excavation and stabilization of the northern part of the existing Linbro Landfill site.
• The combination of successfully diverting utilities while ensuring continuous traffic flow prompted the implementation of multiple road diversions around the various construction areas (between the N3 on and off-ramps, the Marlboro Bridge and the Linbro Park entrance and exits).