Loch Thom: Ground Investigation and Preliminary Results from Jet Erosion Testing (JET)

Loch Thom reservoir was formed in 1825 by the construction of an earth embankment approximately 20m high, making it one of the highest embankment dams in Scotland at that time. In 1875, to increase storage capacity, the Main Embankment dam was raised by approximately 2.4m and a second 10m high dam (Garvocks Embankment) was built across a saddle along with three other smaller embankments. A need to improve the installed drawdown capacity was identified following a statutory inspection in 2013. The dams have been described as having puddle clay cores, however, there are no surviving records and a contemporary account given at the ICE in 1843, by Robert Thom, who designed the original Main Embankment, states that “the bank was built in layers of puddle peat or alluvial earth alternating with gravel beaten together with wooden dumpers until completely mixed, on the principle that the whole embankment should be made water-tight.” A ground investigation was undertaken at both dams to establish the internal composition of each dam using intrusive and geophysical methods. The vulnerability of each dam to internal erosion is currently being assessed to inform drawdown requirements, and includes Hole Erosion Tests (HET) and Jet Erosion Tests (JET). It is believed to be the first time such tests have been undertaken on soils from a UK dam. This paper describes the approach to the investigations, details of the fieldworks and testing undertaken and preliminary JET results.