Neath Port Talbot Council has said that 50 more properties could be at high risk from ground movement in the Welsh village of Ystalyfera.
The council ordered the evacuation of 10 properties last month and revealed the wider risk to other homes at a public meeting attended by more than 200 people last night.
A new risk map which was developed by the Earth Science Partnership as a result of monitoring work showed the higher risk.
The council said that residents in the affected properties will not be forced to leave at this stage as they do not believe there is an imminent risk of landslide. However, surveys of individual properties are now planned to look at the effect heavy rainfall could have on localised ground movement.
Fact file: The Pant-teg landslip
The Pant-teg landslip is part of a wider landslip system present on the slopes of Mynydd Allt-y-grug. Historical ground movements have occurred above and below a slope toe that follows the main road through Ystalyfera at Cyfyng Road where residents have been evacuated.
At least 26 ground movements have been recorded in the area since 1897, seven of which were sizeable events. The current movement appears to be the result of material falling from the crest of the slope which was regarded following a landslide in 2012.
Earth Science Partnership (ESP) has been working with Neath Port Talbot Council since 2015 and is using a series of borehole installations and ground surveys to monitor the area.
According to a document from ESP released by the council, there is unlikely to be a viable overall engineering solution to stop further landslides from occurring.
In the document, ESP said: “In terms of the terrace of 10 properties affected at Cyfyng Road, based on our site reconnaissance, knowledge of the wider landslip, geomorphology of the slopes to the east of Cyfyng Road and recent groundwater monitoring, we consider it likely that additional ground movement and slope regression will occur, possibly de-stabilising the structures and adjacent areas.”