Settling for stability

Settling for stability

In the area of land reclamation, size matters – and so does money, of course. The downturn and uneven economic recovery has led to the withdrawal of a good number of smaller ships, fuelling the relentless drive for size.

At the same time, ports in some areas face more difficulty than ever in securing finance for – or even justifying – major capital projects involving land reclamation and the construction of deepwater quays capable of handling ever larger vessels.

Further, if you want a quay suited to the loading and unloading of the larger ship sizes, it can mean building a structure capable of supporting 1,000 tons on each crane leg.

But these factors are also mixed with growing environmental awareness, explains Colin Spinley of Royal Haskoning. “Recent changes in legislation have meant increased pressure to leave poor quality soil in situ. While this reduces dredge and dump quantities and reduces the amount of fill needed, it also creates its own problems as the poor material needs to be improved and stabilised in some way,” he says.

The need for deeper berths coupled with heavier cranes has also created an engineering issue, he says. “It is very difficult to be certain that increased loads and deeper dredging can be applied safely to existing structures.” Existing quay structures were often designed economically and have little extra margin against deepening or increased loading, he explains.

For part of its resistance to sliding, a quay wall is dependent on the dredged or “passive” side of the wall. Increasing the crane loads on top of the wall, while at the same time removing passive resistance, can reduce the factor of safety against failure of the wall, says Mr Spinley. Further, for quay walls that have been operational for many years, it can be difficult to determine the existing structural condition. Although it is possible to make visual inspections, there is a limit to what you can see underwater, making it difficult to ascertain much about the structure’s integrity as a whole.

Owing to the limitations of existing quay walls and because of uncertainties, it is often necessary to reclaim land either directly in front of the original quay or build out into a completely different area of the port to gain the required deepwater.

Another development for Royal Haskoning has been a greater emphasis on seismic design issues, from the inclusion of seismic structural design to undertaking liquefaction assessments for reclamation fills. This increase has been driven by a shift towards large port developments in locations subject to earthquake loading and from a heightened awareness of the potentially devastating effect on structures. Mr Spinley adds: “You can see this in its most dramatic form by looking at the quay and attendant crane collapse at Haiti.”

At the other end of the scale, there are major challenges for engineering projects as emerging economies show explosive growth.

Cai Mep International Terminal (CMIT), one of a handful of new deepsea terminal developments at Cai Mep, 80 km from Ho Chi Minh, Vietnam, has needed large-scale reclamation involving 3-3.5m cu m of earth. “However, fast tracking a development like this brings up a few issues,” says Paul van Weert, head of civil engineering at APM Terminals.

“Ideally you want all of the settlement to happen before you start on the infrastructure building. This isn’t always possible in an economic way, so in projects like these we set up a number of vertical dewatering drains together with a six to nine metre high surcharge load to speed the process.” Although these measures are effective in places like Cai Mep, which is a low-lying tidal area, some 15% of the settlement process is still left to take place while the terminal is in operation.

This, on a project the size of Cai Mep, makes for a huge amount of extra soil. Fortunately, most of this could be pumped in hydraulically from the dredge, which was also sizeable.

To optimise the required quantity of surcharge material, the site was split into two areas. The first has to be completed six months prior to the second. The surcharge material from the first area is used as fill and surcharge in the second. The issue of surcharge loading in an area with a very soft subsoil is trickier than many realise. When a site is completely covered with a surcharge mound (which can require an enormous amount of earth in one go), there is a serious risk of landslides. To minimise this risk, the contractor had to increase the surcharge in three to five steps, adding some two metres of material each time. Along the waterfront it was decided to use vacuum drainage in order to speed up the consolidation, limit the total surcharge height and thus limit the risk of landslides.

As Mr van Weert explains, you cannot always be sure that the effect will be even. “We measure the differential settlement with care. If the reclamation settles unevenly, it could later cause a number of serious problems, including undermining things like the stability of the box stacks – so if we see it begin to happen, we put extra loading on to that part of the construction.”

Mr Spinley agrees: “The ground is often highly variable, meaning that there is a risk of differential settlements on any site, even where surcharging has occurred. Where fill is placed against existing structures in port expansion projects, you have to pre-load the interface, just to avoid getting a step.”

It is essential to think about stability issues during this part of the build, he adds, because if the surcharge is loaded up too high too quickly, there is a possibility that a failure could occur during construction – with disastrous consequences for programme and budget. The decision is often taken to work in stepped phases to minimise the risk as well as the amount of earth needed.

Both Mr Spinley and Mr Weert say it is a finely tuned balancing act between trying to achieve settlement as quickly as possible in order to get the terminal operational, and the need to maintain stability throughout the works.

Other measures in consideration of a minor differential settlement have been taken up by the Cai Mep facility. Taking into account future maintenance and repair cost, a flexible pavement system consisting of a cement-based mixture with concrete pavers on top was selected.

However, the client often has its own settlement operational limits which are related to pavement levels and controlling slopes for handling equipment. This can be down to a number of millimetres over three decades.

Colin Spinley concludes that along with the growing environmental awareness, projects will just have to plan around the issues that are thrown up when putting down foundations on a softer base.

“Increasingly ports will recognise that you can’t always construct new ports economically on very short timelines,” he says. “It is becoming ever more important to understand the programme and cost risks posed by poor ground conditions and to identify ways in which these risks can be managed. This may mean lengthening construction times, increasing the operational tolerance to total and differential settlements, or innovative approaches that blend all of these risks together.”

‘Recent changes in legislation have meant increased pressure to leave poor quality soil in situ’

Colin Spinley, Royal Haskoning

‘Fast tracking a development like this brings up a few issues’

Paul van Weert, APM Terminals