A Brief Report on Preliminary Understanding of the Building Collapse Mechanism at Mulpani, Kathmandu
On 3 September 2019, an under-construction RC-framed 3-story residential building collapsed possibly under excessive earth pressure buildup on the building back following a heavy rainfall. As the building was erected on the side of a near vertical natural soil deposit and the back side of the building was built of an RC retaining wall (which could also act as a shear wall of the building), the earth fill between the natural deposit and the retaining wall seems to have exerted the excessive earth pressure onto the whole building leading to its collapse. The real cause of the collapse can only be understood after a detailed investigation of the site, which may need all building drawings, study of earth pressure evaluation before design, stability calculations, foundation design details, etc., but about an hour of visual inspection by a Nepal Geotechnical Society team on 5 September 2019 has led the team to prepare a preliminary report based on what the team members inspected and saw at the building collapse site and its periphery. A 7-page report has been prepared for the reference of all interested engineers and can be downloaded as a PDF file (size: 2.1 MB) from here.
With limited time and resources and most importantly, with very limited/restricted cooperation from the residential area builder, it was not very easy to find out what exactly happened in the collapse site on that day, but about an hour of visual inspection has led the investigation team to understand the following points.
- While designing structural parts of the building, stability of the backside slope and backfill earth pressure were probably not appropriately and adequately evaluated.
- The RC retaining wall, which was probably also designed to serve as backside shear wall of the building was not properly designed and constructed. Based on the nature of the wall failure, there are enough rooms to suspect the quality of this wall construction
- Although the foundation system is still unconfirmed, there are chances that the foundation footings might have also settled or slid. If the foundation failure, either in sliding or in settlement did not take place, it is for sure that the building collapsed due to ground floor and first floor column failure in bending due to earth pressure push on the RC retaining wall.
- Compared to other adjacent buildings (Figure 3), the backfill material width (i.e., the distance between the retaining wall and the natural ground) at the collapsed building site is greater, which might have exerted greater pressure on the retaining wall at this location only. The risk of failure of other buildings on the side and at similar sites in future is also high!
- This geotechnical-cum-structural risk seems very high in this area, especially because the builders seem to have developed this area by massive cutting and filling work, which might have led to poor ground conditions at many locations especially due to possible poor compaction and underground erosion. So, all ground stability-related (slope instability and subsidence/settlement) investigations and analyses must be carried out appropriately before designing the structural system.
- Moreover, all completed buildings in the similar risk of failure/collapse must be evaluated for stability by trained geotechnical, earthquake, and structural engineers.
- Government authorities need to interfere with the builder’s design details and make a mechanism to conduct detailed investigation of the building collapse so as to save people’s life and money and to promote engineers’ professional credit and the government’s existence itself.
Download the full report in PDF (size: 2.1 MB).
The Visual Inspection Team Members:
- Netra Prakash Bhandary, President, Nepal Geotechnical Society
- Indra Prasad Acharya, Vice President, Nepal Geotechnical Society
- Mandip Subedi, General Secretary, Nepal Geotechnical Society
- Udaya Raj Neupane, Executive Member, Nepal Geotechnical Society
- Mabin Dahal, President, Nepal Geotechnical Students Society
- Sanjaya Panday, Nepal Geotechnical Students Society
- Harish Paneru, Nepal Geotechnical Students Society
- Pawan Babu Bastola, Nepal Geotechnical Students Society