This is quite an impressive trench. In order to prevent the sides of this elongated hole from collapsing, contracting consortium Markus/Van der Veekens has positioned steel sheet piling on both sides, which acts as an earth-retaining structure. There are horizontal steel columns on both sides to prevent the steel piling from falling into the hole. At the bottom of the hole, there are nine neatly lined-up bundles, each containing four sections of sleeve pipe ducting bound together. This ducting is made of thick, black PVC and 16 or 20 cm in diameter. ‘This is the lowest layer’, says project manager Jeroen Plug. ‘We’ll be creating some lasagne here over the next week. On top of this first layer, there will be another five layers of ducting, with layers of sand in-between. All in all, around 90 km of empty ducting will disappear into a trench several hundred metres in length in the space of four weeks. At a later stage, network operators, such as KPN and Liander, will run electricity wires and glass fibre cables through the ducting. These are the main conduits that will be part of a large, new loop system around Zuidas.
Zuidas Energy and Utilities Master Plan
Cables and pipelines in the ground can be a significant cause of disruption and annoyance. Their construction and maintenance causes inconvenience (streets dug up) and there are regular problems during excavation work (gas leaks, power outages). The Land Registry keeps records of the location of all cables and pipelines, but it is expensive and labour-intensive work that can be prone to error. For Zuidas, an alternative solution has been devised: a loop-shaped network in which all the main conduits are positioned neatly alongside each other, with a limited number of gigantic connector blocks (‘coupling fields’) to connect a residential neighbourhood or office complex. This solution is known as the Zuidas Energy and Utilities Master Plan, or MENZ for short in Dutch. If work is being done in an area, such as the Ravel residential neighbourhood, the City Council steps in in order to add another piece to the ‘MENZ loop’. For example, there has been a tunnel of pipes and cables under Gustav Mahlerlaan since 2005. In 2016, it was the turn of Beatrixpark and Prinses Irenestraat. When the loop is eventually complete, it will be relatively easy to connect new construction initiatives, including Zuidasdok. Since there is still empty ducting in the loop, extending or intensifying the network is easy.
Back on the building site, Jeroen Plug discusses the likelihood of leaks. This is because the conduits have been welded or screwed together. Just like when repairing a bicycle puncture, they ‘pump’ them up in order to detect any leaks. An air pressure of two bars is sufficient. Plug also points out that the City Council and the contractor do not have total control of the planning schedule. It also depends on work being done on construction projects and the activities of network operators in the immediate vicinity.
Scratching of heads
The complexities of the substrate also led to some scratching of heads. If you dig into it, you first encounter peat a few metres down, with a layer of clay underneath it. Under that is a layer of sand filled with groundwater. If you remove the peat when digging the trench, you are also taking away a significant amount of weight. The pressure of the groundwater can then cause the layer of clay to burst open, flooding the trench. Hollow tubes have been inserted vertically into the ground in order to reduce that risk. They work like valves, reducing the pressure of the groundwater. So far, everything has run like clockwork, confirms Plug with the ground firm under his feet, but he will be glad when the excavators have refilled the hole. The network operators would agree. In just a few weeks’ time, Liander plans to run all the electricity cables from the A10 to Valley – filling the empty ducting for the very first time.
Text: Henk Leenaers