Robust road infrastructure along the borders is a sign of a nation’s true might. In order to ensure the operational readiness of its armed forces and augment the pace of growth and development of the generally remote border regions, the Border Roads Organisation is deployed to undertake road construction. The border areas are generally located in varied terrain and climatic conditions such as high altitudes and mountains, deserts, jungles, marshes or plains, each with its own challenges.
India’s Ladakh region consists of difficult and rugged terrain with inclement weather and harsh working conditions. The Border Roads Organisation is constantly evolving innovative techniques to overcome the challenges faced due to harsh weather, and the climatic and terrain conditions resulting from the high altitude, particularly glaciated and moraine-predominant terrain. In high altitude mountainous terrain, permafrost (defined as soil in which temperatures remain continuously at or below freezing point for, at least, two consecutive years) and the active la-yer (which thaws seasonally) are the primary subsurface components of the land-atmosphere system. Permafrost restricts the mobility of soil-water and infiltration. Regions with permafrost are characterised by low air temperatures during winter, low saturation pressure of water vapour and frequently stable stratification of the atmospheric surface layer. All these conditions lead to low evaporation. The characteristics of permafrost, permafrost dynamics, soil-water freezing and soil-ice melting together pose an enormous challenge for construction activities of any type on such surfaces.
Analysis of challenges
Due to high altitudes, mountainous terrain, snowfall, subzero temperature for prolonged periods and glaciated terrain, construction activities in the Ladakh region face enormous challenges. The construction of a durable surface fit for traffic movement on a permafrost layer is extremely challenging due to the unstable subsurface. Constructing in mountainous regions using permafrost as a foundation poses slope instability hazards.
The severe weather conditions often shorten the construction season and simply slow down any construction activity. Cold temperatures are tough on machinery, and increased abrasion and wear must be considered. These facts are often not accounted for deeply enough, but rapidly increase construction costs. The steep terrain and rugged conditions existing in permafrost regions also limit access options. Generally, construction sites are in far-flung and remote areas, with limited local resources and population. The optimal time for construction in these areas also has to be determined. Extreme working conditions during winters and the melting of the snow layer of the permafrost during summers can cause challenges with respect to site access and operation of machinery.
The most problematic aspect of construction in permafrost regions is the introduction of concrete or grout into the
ground and the resulting hydration heat given off during the curing process, which lasts at least a month. When smaller amounts of concrete or grout are involved, such as micro piles, subzero ground temperatures can cause it to freeze before attaining the appropriate bearing capacity. The concrete is severely weakened if it freezes before it can reach its full strength, which limits the applicability of concrete structures in these regions. The frozen nature of the soil requires special equipment for excavation that is capable of functioning at subzero temperatures. The curing of grout in the permafrost layer is also a major challenge due to the low temperatures and freezing of the water required for curing.
Meanwhile, foundations in permafrost have special design considerations, as the bearing capacity of permafrost soil is very low and the underlayers do not provide any stable base strata due to continuous thawing, freezing and differential settlement.
Bituminous paved surfaces are not an option for this terrain configuration, as bituminous works require ambient temperatures of a minimum of 16 °C and laying temperatures ranging from 120 °C to 150 °C. Achieving these temperatures in permafrost regions is very difficult. Permafrost regions demand additional logistics support due to their remote location and lack of natural resources. They require detailed planning and timely deployment of resources and manpower, along with means of sustenance in such harsh weather and climatic conditions.
Mitigation measures
In order to avoid subsurface instability, the permafrost should be allowed to breathe by building road embankments using large stones of 150-250 mm size, with sizeable gaps between them. The gaps act as pores, allowing heat to escape to the surface. These large stones reduce the effect of insulating the ground below, causing the permafrost layer underneath to be well ventilated, thereby maintaining low temperatures, reducing thawing and stabilising the terrain.
The equipment being deployed for the task uses high speed diesel, anti-freeze coolants and turbochargers in order to operate in extreme cold temperatures. Insulated shelters, extreme cold climate clothing and special rations should be provided to the operators and labourers working in permafrost regions. Vehicle and plant repair workshop detachments with highly trained manpower should be established as close to the worksite as possible, and provided with enough fast-moving spares to keep the equipment functional for the maximum duration. Forward stocking of construction stores, rations, fuel, oil and lubricants will allow optimum utilisation of the construction season.
In order to facilitate access to the construction sites, special transport such as aerial ropeways or cable cars need to be installed for the duration of the construction period. In certain locations, access by air might be the only way to get to the construction site, limiting the weight of the equipment used. Access by air is also highly weather dependent and very costly.
In order to minimise the thermal disturbances caused by concrete work such as the construction of avalanche protection or bridge structures in permafrost regions, there is a need to delay freezing by using warm water for the grout mixture injected into pile boreholes. One way of keeping the snow layer intact is to install tubes called thermosyphons, which allow heat to escape from the ground, thus keeping it cold, so that the soil supporting the piles does not shift enough to endanger their integrity. Currently, thermosyphons for permafrost stabilisation are manufactured in the US, Canada, Russia and China.
The curing process can be successfully carried out by using anti-freeze additives. Internal curing is another technique that can be used to boost the curing process in cold weather. The main thing to be avoided is letting the concrete freeze. To this end, warm water can be circulated around the concrete to keep temperatures above 5 °C, facilitating curing. Yet another method of ensuring structural stability is by keeping the ground frozen by cooling it. This method utilises thermal-physical and strength calculations to ensure perennially frozen ground for the foundations. Curing of grout to achieve the required strength can be done by artificially maintaining grout temperature above 0 °C using external heat sources or warm water injection. Varieties of cement with rapid rates of hydration can also be used. Other means include the use of salts or other cement additives, closed cell spray foam for creating polymer piles, and grouts with very low water content.
Increased design considerations help reduce construction challenges. While designing the pile foundation, only side friction needs to be accounted for, as these foundations in permafrost derive resistance mainly from side friction rather than end bearing. Load transfer between the pile and the surrounding ground is, therefore, crucial. Thermal disturbances must be minimised and the layers should be kept cool while casting of foundation piles. Changes in the soil strength due to thawing of excavation slopes must be considered, in particular if the excavated frozen slope is exposed to warm summer temperatures. Even though a larger number of explosives are needed to excavate frozen soils with ice, care should be taken to reduce the bearing capacity of adjoining areas by suitably applying incremental quantities of explosive until optimal effects are achieved.
In order to achieve the necessary ambient temperature for bituminous work, geocell technology for surfacing work is recommended. Geocells are filled with aggregate, which allows for ventilation of the moraine and underlying active layers, thereby keeping the temperature of these layers low and preventing thawing.
Conclusion
The construction and subsequent maintenance of roads are affected by the peculiar geological and climatic characteristics of permafrost regions. These unique conditions make construction and maintenance activities in these remote and harsh conditions a formidable proposition. A number of technical, resource management, equipment management and other challenges need to be surmounted for the successful execution of these tasks. These challenges have to be consistently defied through mitigation measures in order to create an enviable engineering marvel.
