Redevelopment of a major Moroccan port has called for some complex analysis of the diaphragm wall and pile designs.
Designing a scheme to rebuild and expand a port while the facility remains in full operation is never going to be a simple task. But when the port’s existing quay structures also need to be upgraded at the same time, the construction and logistical challenge becomes even more complex.
Although construction and remediation work on Morocco’s Port of Jorf Lasfar is still underway, the progress made so far suggests that Ramboll’s approach to the design is proving effective. Work on redeveloping the port on the Atlantic Ocean involves adding four new quays to meet anticipated demand, while the existing three quays are being repaired and upgraded.
“The existing port opened in 1982 but the concrete caisson structures have deteriorated due to the combination of relatively high ambient temperature and exposure to the marine environment,” says Ramboll principal geotechnical engineer Ian Lewis. “Reinforcement corrosion has been the main cause of structures, however, chemical attack has also affected this.”
The Port of Jorf Lasfar is a busy industrial port that deals in the shipment of coal, phosphate, fertiliser, sulphur, and ammonium. Demand is forecast to increase so new quays are needed and rehabilitation of the existing structures is also vital.
“The client’s original design was to use precast concrete caissons throughout the project but Archirodon, the contractor for the scheme, submitted an alternative reference design at the tender stage which introduced the concept of diaphragm walls in combination with caissons to meet the timescales for the scheme,” explains Lewis.
Archirodon appointed Ramboll in July 2012 to develop the design. “At this point the thickness of the diaphragm walls and the spacing was set, but the depth and the detail of the work was still to be determined,” says Lewis.
“The current project involves work on seven quays. Extension works on P1 and P5 is being undertaken using caissons, while the work on P2, P3 and P4 is being carried out using diaphragm walls and tie back piles. The upgrade work on P6 and P7 is limited to rehabilitation of the existing structures and deepening of the berths.”
Before starting on the detailed design further ground investigation was necessary to develop a deeper ground model of the layered marls, sandstone and limestone geology. According to Lewis, although a gypsum band was found at depth there was nothing that significantly changed the design approach other than some soft material below where the caisson construction was planned.
The 15m diameter precast concrete caissons are being fabricated in a specially developed dry dock on site and floated out into position before being backfilled to remove the water.
“We are using 20m and 22.2m high caissons which are being butted up to each other to form a large gravity wall structure,” says Lewis.
Design of the caissons was carried out by Lewis’ colleagues in Denmark where Ramboll has a centre of expertise on such structures. The design was analysed by the Danish team using Plaxis.
“During the design stage there were concerns about the caissons overturning due to soft areas in the underlying marls and the high bearing pressures exerted by the caissons and high mooring loads expected,” says Lewis. “This issue has been overcome by ground improvement using the dig and replace approach.”
The diaphragm walls are up to 400m long and 34m deep to ensure they toe into the more competent marl or limestone beds. “The geology varies along the length of the wall but rather than change the depth of the wall, it is the reinforcement that alters,” says Lewis.
The wall is formed using a hydrofraise from 1.2m thick panels with the primary panels measuring 2.4m wide and secondary panels 2.6m. The centre of each of the panels will be tied by an inclined anchor to a 1.2m diameter, 25m long bored cast insitu pile installed behind the diaphragm wall.
“The steel tie rod goes from 1m below the quay level on the diaphragm wall panel to 4m below ground,” says Lewis. “The inclination helps us to gain lateral restraint and reduce the bending moments in the pile. It does make the connection challenging to construct on site as the connection is below sea level but if it was any higher the bending moments in the pile would be too great.”
The position of the bored piles is fixed by the fact that they and the diaphragm wall have a dual purpose and will support the rails for the dock cranes once the port development is completed. The spacing on P2 is 12.5m, on P4 it is 11m, while on P3 the spacing is 20m.
“The design of the tie rod was further complicated by the differential lateral movement between the diaphragm wall and the piles being limited to less than 25mm due to the tolerances of the dock cranes,” explains Lewis.
Initially the design was analysed in Plaxis 2D, but because it was considered important that the load sharing of the mooring forces into the tie back piles was understood, a Plaxis 3D model was created.
“During the analysis work a range of geotechnical properties were investigated,” explains Lewis. “The results provided information on the most effective design of the diaphragm walls and piles from a point of view of cost and performance and this gave rise to the variation in reinforcement cage rather than the depth of the wall.
“The loading conditions considered in the design are mooring, berthing, crane (vertical/horizontal), 1m water lag at low tide, 1m water lag at high tide, seismic, and 40kPa quay surcharge.”
Another complication for the work is the fact that most of the quay extensions are being carried out on the landward side of the existing quays and will replace earth embankment access roads, so the work is being partly carried out through new fill and pre-existing material. “There was very little as-built information for the existing earthworks so we have had to use our engineering judgement on the nature of the fill,” says Lewis. “The reality is that the older fill is formed mostly from large rocks and some areas have consolidated more than others and as a result the tie back piles are more embedded in some areas than others.”
Throughout the work P2, P3 and P4 had to be kept operational so the work has been phased to allow the port to have access to sufficient quay wall that could be used for operational purposes.
Work on site has been underway for just over a year and, so far, has progressed well. Lewis says he expects the port upgrade to be completed in early 2015.