IN A major effort to reduce landslide incidents ahead of the monsoon season, the National Highways Authority of India (NHAI) has introduced a slope drainage pipe system at 83 vulnerable points along the 85 km-long Kalka-Shimla NH-5, one of the most landslide-prone road corridors in Himachal Pradesh.

The intervention, which is nearing completion, forms part of the ongoing slope stabilisation project being carried out between Parwanoo and Kathlighat. Officials said the drainage system was conceived after slope protection measures undertaken earlier failed to fully address the problem of rainwater runoff during intense monsoon spells.

The drainage system involves construction of small water collection points at the top of hillsides. These collection points are connected to a network of pipes and cement-lined drains that channel rainwater directly to the bottom of the slope. The objective is to prevent water from flowing freely over the slope surface, which often triggers soil erosion, damages protective structures and leads to landslides.

“The initial slope protection work primarily consisted of green mesh netting, standard drilling and anchoring, most of which was completed last year. Subsequently, experts from IIT Bombay and IIT Ropar, who have been advising NHAI on slope stabilisation measures, recommended the installation of drainage pipes as an additional safety measure for better protection of the slopes and to minimise landslide risks,” Yogesh Verma, Site Engineer for the Kaithlighat-Dhalli section of NH-5, told The Indian Express.

Officials said the idea emerged after the 2025 monsoon exposed limitations in the existing protection system. Although netting and anchoring had been completed at several sites, heavy rainfall caused large volumes of water to flow down the slopes, resulting in slope failures and damage to mesh structures, anchors and retaining measures.

“During the 2025 monsoon, water flowing from the hilltops to the valley in large quantities damaged some of the protected slopes. Experts observed that uncontrolled surface runoff was a key reason behind these failures. The drainage pipe system was subsequently recommen-ded to divert rainwater in a controlled manner,” said sources associated with the project.

NHAI project director (Kalka-Shimla) Anand Dahiya said the system has already shown encouraging results during recent pre-monsoon showers. “As the pre-monsoon rain began, we observed positive outcomes at sites where the drainage system has been installed. Cement-lined drains ranging from one to three feet in width have been constructed along both sides of the protected slopes. These drains are connected to plastic pipes that carry water safely to the bottom, reducing pressure on the slope surface,” Dahiya said.

He said highly vulnerable stretches, including Parwanoo-Chakki Mod, Solan-Datyar and Kandaghat-Kaithlighat, were prioritised in the first phase of the project. “The drainage system is now being incorporated as an integral part of slope protection works being executed at all 83 identified sites between Kalka and Shimla,” Dahia said.

The drainage initiative complements the broader slope stabilisation programme launched by NHAI for the first time on the Kalka-Shimla highway. The project covers 83 vulnerable locations, including 81 hill-side slopes and two valley-side sites at Chakki Mod and Datyar. The works involve green mesh netting, soil nailing, drilling, anchoring and vegetation-based stabilisation measures aimed at preserving the natural landscape while reducing landslide risks.

The project represents a departure from the earlier shotcrete technique, under which concrete was sprayed directly onto hill slopes. While effective in controlling slope failures, the method left behind large barren concrete surfaces, particularly visible along the Solan bypass.

The Kalka-Shimla highway has witnessed repeated disruptions during monsoon seasons. More than 128 landslides were recorded along the route in 2019 and 2023 alone, leading to prolonged road blockages and significant economic losses.

According to NHAI estimates, natural disasters caused losses of nearly Rs 200 crore on national highway projects in Himachal Pradesh between January 2021 and July 2025. This includes losses of Rs 26.86 crore on the Parwanoo-Solan section of NH-5 and around Rs 171 crore on the Mandi-Palampur-Kiratpur highway corridor.