SEALING SANDBOILS

The formation of sand boils

In large parts of the Netherlands, soil layers and peat layers form a seal for ground and/or water pressure in the soil. If there is a leak in these sealing layers, this is also known as a sand boil.

The west of the Netherlands is particularly sensitive to sand boils at ground level as the hydraulic head of the groundwater (pressure water) under the sealing layers, is higher than the ground level.

Sand boils are regularly caused by external influences, such as:

• The installing or drawing of sheet piling or piling;
• The carrying out of controlled borings;
• The installing or removal of filters for pump drainage;
• The removal (excavation) of the natural ballast layer on a sealing clay layer can result in ruptures in the clay layer.

The party which causes the sand boil, or its client, is often in the above cases responsible for sealing the sand boil again. Injection techniques offer a solution for this.

Locating the source of the sand boil

A targeted injection can be carried out using the geotechnical soil test. However, this is dependent on the depth of the water sealing layer and where the sand boil occurred. If the sealing layer contains peat layers then this hampers locating the leak, as water chiefly travels horizontally in peat layers. Thus, the exit point of the groundwater can lie far from the leak in the sealing layer.

If it is a thick layer of peat we can perform an additional test of the soil to locate the actual leak. We perform this using a thermal imaging camera. For more information please see the page on thermal imaging detection or our informative website: www.warmtebeelddetectie.nl .

Injecting the sand boil

We will first drill below the sealing layer using a small crawler drilling rig. As we are wet-drilling, we can reach great depths with these relatively small rigs.

Once the drilling has reached the desired depth we start injecting the selected injection fluid. The injection fluid shall, after some time has passed, flow along with the leak-flow of the sand boil and then react. The leak slowly becomes blocked-up, which in turn results in the surface leak-flow visibly getting weaker or stopping completely. If the sand boil continues to flow then a follow-up injection is carried out at another location and in this way a definite sealing of the sand boil is built up.