Soil piping – Should it be a concern for our beautiful islands in its peak of its development era
What is soil piping?
Piping occurs when water erodes beneath the surface of the ground creating an underground tunnel known as soil pipe. This usually begins as small pores underground and are enlaged with increase erosion, in some instance these hole may be even large enough for a person to crawl through.
In areas where there are craks in the soil or areas of less resistance , water will start to move through creating a void. Eventually after constant erosion the surface layer of the ground will not have any support beneath and thus collapse creating a depression. In some other instances the soil pipe can be formed from openings in the ground that has been left behind when plants died or trees have been uprooted. Animals can also help create soil pipes by burrowing and tunneling in the soil. These voids provide an opening for moving water and create ideal situations for soil pipe formation.
Soil piping is a common feature along side river bank leading to river bank failure. As water seeps beneath the river bank it creates an alternate route, this is eroded and shaped by the water forming a channel (soil pipe). As more water seeps into the bank, the soil becomes heavier and more likely to break apart making it prone to erosion and failure. Soil piping has been related to earth dams failure, dike failures and formation of sink holes.
How can you identify soil piping?
Since it occurs beneath the soil it makes it difficult to identify the soil pipe up until the ground has collapsed. Small openings known as flute holes connect the soil pipe to the surface. These are often found in the river bank, though they are not easy to spot they help researchers to locate soil pipes before massive soil failures occurs.
Can soil piping be regarded as a Risk?
Soil piping is a natural process, but often human induce activities may result in change in surface and underground water flow and result in increased subsurface erosion and making soil pipe a potential risk. Soil pipe collapse may become a threat to farming and can threatened the stability of a building.
In Seychelles there already had been a few sightings of depressions that can be identified as a result of soil pipe collapse. Each of these depressions varies in diameter and often runs deep in the ground. In most common cases these depression are found around the house that has been built on underground water sources or next to rivers. These depressions are only noticeable after several years later. In these areas soil subsidence may also be observed. In worst case scenarios several depressions have been located in the same area which makes the area unsafe to walk on. Luckily so far we have not experience any massive failures. However, in areas where the depressions have been found, houses have been rendered unstable as the soil subsidence causes the walls of the houses to crack. Collapsed embankments or retaining walls next to rivers have also been as a result of soil piping.
What is the way forward? Are there any solutions we can adopt?
Understanding more about soil piping is a way forward to avoid massive soil failure in the future. Firstly, new developments need to take into account past history of the area in terms of underground water flow and not only focus on the terrain and surface flow of an area.
There’s also need to emphasize more on integrating our natural environment in development instead diverting, backfilling and drying up our rivers and marshlands. Proper mitigation measures should be put in place when building next to the river including the implementation of buffer zones to maintain the various riverine processes and also protect its natural resources.
When considering flood defence, there are always new engineering solution that can be adopted while constructing embankments next to a river to reduce the effect of soil piping. As illustrated on Figure 4 below, a cut-off is added at the base of the wall, this increase seepage length, therefore reduces exit gradient and also stabilise the base of the wall.