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The walls-roof construction method1 is a method of underground construction of buildings, tunnels (more often stations between tunnels) and other underground spaces. This method is often used to build underground constructions where the available space above ground is limited, either physically or in time.

This method of construction consists of a number of steps.

The walls

The first step is to build walls in the ground, partitioning off the future underground space from its surroundings. These walls can be constructed using a number of different techniques, like diaphragm walls (also called slurry walls), bored pile walls or sheet piling.

The type of wall depends on a number of criteria, including:

The roof

Once the walls are in place the roof is constructed at ground level, usually with reinforced concrete. Once the roof is finished, most of the rest of the construction can be carried out out of sight and interference from ground level.

Excavation

The next step after constructing the roof is to excavate the space under it. Of course, there need to be one or more openings in the roof to get the excavated material out of the enclosed space. Excavation can be done under heightened air pressure if keeping groundwater at bay is an issue.

The rest

The following steps in this construction method depend on the local circumstances and on what is being constructed. Basement constructions are often excavated one floor at a time, so that each floor acts as shoring, instead of struts, for the walls.

Shoring of the walls is almost always needed, if the excavation is down to any kind of depth. The walls have to bear an enormous load generated by the surrounding ground and possibly present groundwater.

In cases where the construction is (partly) below groundwater level a watertight layer is needed at the lower end of the construction, to prevent flooding of the underground space. A number of methods can be used.

In areas where impermeable ground layers are found, it is sometimes sufficient to construct the walls to beyond the depth of one of these layers. This is by far the most inexpensive method, unless the impermeable layers are only found at very great depth. Then a cost analysis should point out whether to build the walls a bit longer than needed or to construct an artificial impermeable layer. If the layer is not completely impermeable (which is almost always true), one or more pumps in the lowest reaches of the underground space will often be sufficient to keep flooding at bay.

Methods to create an artificial impermeable layer almost always depend on getting a layer of concrete or some other binding agent at the right depth. The method to get it there can vary, though. One method uses high air pressure during excavation to keep the groundwater out, followed by construction of an impermeable floor when excavation reaches the desired depth. Other possibilities include grouting of an underground layer before excavation, or cementing the ground into an impermeable layer by injecting certain chemicals.

Real world example

So, where is this method of underground construction used? A number of better examples will soon be found in Amsterdam, where construction of the North/South metro line is just starting. Amsterdam is a city (the capital in fact) in the Netherlands. I mention this, because that fact brings with it a number of specific problems, including weak ground material, high groundwater levels and very limited above ground space, requiring the construction designers to jump through a number of hoops to pull the project off.

One of the solutions they have come up with is to build tunnel sections using a just recently pioneered technique using an underground mole to dig out the tunnel. This poses a number of problems, because the ground through which it is digging is non-cohesive, weak and saturated. Ever dig a hole near the surf on a beach? Remember that no matter how hard you dig the hole keeps filling up again? That is close to the same problem the engineers faced when designing the digging machine.

Three of the stations in the projected metro line are situated in densely populated areas, making normal construction methods for these stations impossible. They will be built using the walls-roof method, so that - in theory - the population will encounter minimal discomfort from the construction works. See the site of the project for more details:

    http://www.noordzuidlijn.amsterdam.nl/pages/startpagina_e.php

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1 A rather bad translation of the Dutch term "wanden-dak methode"

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