The collapse of two retaining walls in a span of four days in Pune, crushing several people to death, has raised questions about the sturdiness of such structures, especially those constructed on the outskirts of the city.
According to an expert in the field, rampant encroachment of natural streams and lack of rules about retaining walls have made such constructions vulnerable. During the course of his research, geomorphologist Shrikant Gabale has identified 68 ‘flash flood’-prone zones in the city. Most of these zones are in the villages that were merged in the Pune Municipal Corporation’s limits as the city expanded over the years.
“Almost 80 percent of these zones are in the merged villages which, over the last few decades, have seen the maximum construction boom,” said Gabale, whose firm Unity Geospatial works in the field of geoinformatics.
After the merger with the civic body, the farm areas in these villages soon made way for concrete structures as the demand for new houses skyrocketed. The escalation of the land cost in these areas also meant that almost every square feet of available area was used for construction and in many cases, builders simply ignored the rules while undertaking construction work.
Geographically, Pune lies in a valley, with many small streams-crisscrossing its area. Researchers have identified six major river basins which are active in the region. First order streams are the smallest tributaries which feed into the river network.
Gabale’s research has focussed more on the ‘vanishing streams’ in these areas. “Our research has shown that almost 25 percent of the first order streams have vanished in the city in the last 30 years as concrete structures have taken their place,” he said.
The seasonal streams, for the flow of rainwater, have given way to concrete structures and in many cases, retaining walls have been constructed on these streams. “Such areas are especially vulnerable during heavy rains, as they are weakened by flash floods,” said Gabale.
The rules on the construction and maintenance of retaining walls seem few and far between. The Development Control and Promotion Rules — the handbook on construction norms within the PMC limit — doesn’t say much about how such walls are supposed to be built. While norms on the construction of a boundary wall are well defined, no such norms are available for retaining walls.
These walls need to be constructed after taking into consideration the natural water sources and bedrock of the area, said Gabale. The topography of the area also needs to be taken into account before such walls are constructed but such norms are hardly followed, he said.