New soil injection techniques save money and reduce nuisance compared to conventional methods.

By Ir. Patrick Wolfs,
B&W Grondinjectie, Bilthoven.

 
In the Netherlands, if a natural, sealing clay layer is absent in the soil, it has become standard to create construction pits that are below groundwater level using underwater concrete. This may or may not be fitted with tension piles or vertical grouted anchors. In recent times the water glass technique has also been used: a water repelling horizontal soil injection of water glass, mixed with hardener had to replace the missing clay layer. An alternative technique, which offered financial advantages compared to that of underwater concrete.

State of the Art
Construction techniques continued to develop, including that of water glass injection. Not only has the quality of the material improved with time, but also the methods of placing it have been automated and optimised to such a degree that this technique can now successfully compete with more conventional methods in the field of construction pit construction technique.

For example B&W Injectietechniek uses a new injection container that makes it possible to work with 9 pumps simultaneously. With an average of 2 injection points per m², the time required is very short. With respect to the accuracy of the location of the injection hoses (a not unimportant quality aspect): these are no longer measured into place manually, but determined extremely accurately using a GPS receiver. All this and more has resulted in an automated and dynamic process that has clear advantages over other methods. 

That became evident for the construction pit situated on Hollantlaan in Utrecht. The construction site is situated on the very busy Kanaleneiland furniture boulevard. It is the location where a construction pit with an area of 3.200 m² will always lead to problems. The builder first planned a sheet piling wall with an underwater concrete floor with tension piles as a horizontal connection.  For excavating the construction pit to the required depth - wet excavation - it was fitted with grouted anchors to ensure the vertical stability of the sheet piling wall. The grouted anchors had to be partially placed below the adjoining sites, which involved extra risks. And as a result of this: a great deal (a very great deal) of discussions. In consultation with the client B&W Grondinjectie was asked to work out an alternative method. When, a short time later, the various calculations were compared, those for horizontal soil injection were the cheapest solution for sealing the underneath of the construction pit. Also regarding the construction time, this alternative offered clear advantages over underwater concrete: the treatment of the entire construction pit was estimated to be 45 building days.

Finally, the only remaining condition was:  is there an injectable layer of sand present at the equilibrium depth for the future injection layer? A soil survey, consisting of several drillings and making grading curves, showed that this was the case. This was decisive; horizontal water glass-hardener injections were chosen. In practice it became obvious that there was more to gain than just a low cost price. Because of the fact that by using horizontal soil injection the construction pit can be excavated dry, the grouted anchors could be omitted.  A phased construction method made this possible. This proceeded as follows:

The injection lances were first placed from the existing ground level to the depth of the layer to be injected. Two injection lances were placed per m² in each section of the sheet piling wall After a few days injecting started, parallel to the location of the injection lances. For the injection a fully computer-controlled nine-line injection container was used. The mixture of water glass and hardener is mixed on site as required. For each cubic metre of sand approximately 400 liters of injection material was injected, assuming a voids content of approximately 40%. 

After the groundwater was removed, the construction pit was excavated in the centre above the injection layer. A bank was made at the sides of the construction pit to prevent the sheet piling walls having a tendency to move to the inside of the construction pit. In addition the danger of subsidence of adjoining sites was averted. Then the final structural floor can be cast in the centre part of the construction pit. When this is finished, the sheet piling walls can be fitted with diagonal props that apply their load to the new concrete floor. Then the final structural floor can be cast in the remainder of the construction pit

Experience
In the Utrecht construction pit discussed here, at the time of writing (early 2007) work is proceeding on the construction floor of the innermost part of the construction pit. Naturally in dry conditions: the total flow rate of seepage water being pumped out of the construction pit is no more than ± 4 m³ per hour. This is a very small amount for such a large construction pit. Most of this water probably leaked through the sheet piling joints.

Naturally the furniture boulevard contractors were dubious about the construction of the construction pit. Their minds have been put at rest and they now know from experience that this alternative method offers many advantages even in a built-up area. Because the soil is excavated dry and a scour site is not required, shopkeepers and visitors are spared the nuisance caused by muddy access roads (and pavements). Then there is the aspect of space (a smaller building site) and the advantage of a shorter construction time.

In inner city areas such Kanaleneiland in Utrecht, horizontal soil injection has proved a more than satisfactory alternative to underwater concrete and that is without the disadvantages of this latter method.