Trend for Occurrence of Landslide Incidents
Ministry of National DevelopmentSpeakers
Summary
This question concerns Ms Mariam Jaafar’s inquiry into landslide trends and containment measures, where Minister for National Development Desmond Lee noted that annual incidents rose from four between 2017-2019 to 21 in 2020-2021, largely due to high rainfall. He explained that agencies conduct regular slope inspections and the Building and Construction Authority (BCA) requires Qualified Persons to implement safety measures and monitoring for construction-related slopes. Following a 2022 incident at a Clementi BTO site, BCA directed builders to re-inspect adjacent slopes and is investigating the cause to assess the need for additional safety requirements. BCA also issues maintenance advisories to private owners and increases inspection frequencies during wet weather to mitigate landslide risks.
Transcript
62 Ms Mariam Jaafar asked the Minister for National Development (a) whether there has been an increase in landslide incidences in the past five years; (b) if so, what are the drivers behind the increase; and (c) what is being done to contain the number of incidents.
Mr Desmond Lee: The scale and impact of landslides, which are movements of a mass of soil down a slope, vary greatly, depending on the amount of soil movement.
There was an average of four landslide incidents reported to the Building and Construction Authority (BCA) per year between 2017 and 2019, and 21 per year in 2020 and 2021. These were all relatively minor cases involving the movement of shallow layers of soil and did not cause significant damage to infrastructure. Higher-than-normal rainfall was recorded in 2020 and 2021, which could be one factor that contributed to the higher number of incidents in these two years.
As of end-September this year, there has been one landslide incident reported to BCA so far. This was the incident which occurred at the Clementi NorthArc Build-To-Order (BTO) construction site on 2 September 2022. This was caused by slope failure, which refers to the sudden collapse of a slope due to changes in soil strength or the destabilisation of the slope. BCA has inspected the surrounding buildings and found them to be structurally sound. The cause of this incident is, currently, being investigated.
Soil movements may be triggered by various factors, such as heavy and prolonged rainfall, additional loads on the slope or changes to the slope profile due to construction work.
Agencies that manage public land carry out regular inspections on slopes that may pose a risk to public safety and implement appropriate mitigation measures to stabilise the slopes as needed. Our agencies may also conduct inspections more frequently, such as on a weekly or monthly basis, during periods of intense wet weather.
In addition, BCA requires all slopes that are formed or modified by building works to be assessed for risk of failure by a Qualified Person (QP) before construction commences. The QP is required to recommend slope protection measures to be put in place, such as earth retaining walls and stabilising structures, to ensure that the slope remains stable under adverse weather conditions, including extreme rainfall.
The QP is also required to recommend measures to monitor the condition of the slope while works are ongoing. The frequency of monitoring may vary, depending on the complexity of the works. For example, slopes may be monitored twice a week for relatively shallow excavation works, and daily for works in close proximity to other buildings.
As an additional precautionary measure following the slope failure incident on 2 September, BCA has reminded QPs and builders to inspect slopes on or adjacent to construction sites and to take additional measures to ensure that these slopes remain stable, as needed. BCA will conduct checks to ascertain that project parties have taken these measures.
BCA also issues regular advisories to remind land and building owners to implement measures to manage the risk of landslides for their slopes. These include ensuring adequate drainage and proper maintenance of slope- and earth-retaining structures.
BCA will review the cause of the recent slope failure incident after the investigations have been completed and assess if additional measures are required, to ensure the safety of slopes on or near construction sites.