Deltares studies the strength of sand in Groningen

Published: 11 October 2017

How strong will the sand in the Groningen subsurface be during an earthquake? That is the question we want to answer and that is why we are taking samples at three locations in Groningen from a layer that will be frozen first. We will conduct a range of tests on the samples to see how strong the different layers of sand are and how the subsurface behaves.

Earthquakes can disturb the balance in sand and groundwater, which can suddenly change into a sort of temporary quicksand in a process known as ‘liquefaction’. When sand liquefies, it loses its load-bearing capacity, resulting in possible damage to buildings, dikes, gas pipes and electricity pylons. The aim of our study is to establish a clearer picture of the risk of liquefaction in Groningen.

Knowing more about the risk of liquefaction

‘No liquefaction has been seen in the past after earthquakes in Groningen but experience in other countries has shown that this is a risk in areas affected by earthquakes,’ explains Mandy Korff, a geo-engineering expert at Deltares. ‘On behalf of the Nederlandse Aardolie Maatschappij (NAM), we are looking at the conditions in which sand liquefies. We are looking at different types of sand, and particularly sand with different ages. We expect, for example, old sand from the Pleistocene to be stronger than is currently assumed in our numerical models, and in any case stronger than more recently deposited, younger sand. On the basis of these tests, we can arrive at firmer conclusions about the risk of liquefaction associated with a given earthquake magnitude. That knowledge will allow us to make better decisions about whether, and if so which, measures are needed to prevent or mitigate damage.’

Ice sample trials for the first time in the Netherlands

An area of ground measuring approximately 2 x 2 m and down a depth of up to 20 m will be frozen with liquid nitrogen in three locations so that the soil samples can be taken to our laboratory in Delft with as little disturbance as possible.

In the laboratory, we thaw the samples in a controlled way and simulate an earthquake by loading the samples cyclically. That allows us to see how the sand from Groningen behaves and when liquefaction occurs. Taking frozen samples is new to the Netherlands and even Europe: the technique has never been used previously here. Engineering company Wiertsema has taken on this job and is executing the drilling operations. In addition, they have called in specialists from Canada and the United States who have experience with drilling in frozen ground. To monitor the process in the ground during freezing and drilling, we are using new glass-fibre techniques so that we can be sure the samples will not be disturbed.