Road Over Umling-La Pass at 19,024 Feet

Construction of World’s Highest Motorable Road Chismule – Demchok In Eastern Ladakh Using Non-Frost Susceptible Sub-Base

Union Territory of Ladakh is located at approx 11,000 feet above Mean Sea Level (MSL). The area experiences extreme cold climatic conditions coupled with rugged terrains. The region is wedged amongst a series of mountain ranges. The Indus Valley separates the Kailash and Ladakh Ranges and runs along LAC. The passes atop the Ladakh Ranges are gateways to the Indus Valley and onwards into China.

Chismule and Demchok are two strategic locations in the Eastern Region of the road connecting Chismule to Demchok takes off from km 27 at Chisumle on Koyul-Photile-Chismule-Zursar Road and terminates at km 52.000 at Demchok.


Umling La Pass
Umling La Pass is strategically important as it provides a direct link between the forward posts of Demchok (overlooking Indus Valley and beyond) to Chismule. The Pass witnesses very high velocity winds throughout the year and the working conditions are extremely tough due to the wind chill and lack of oxygen.

It is located 15 km from the LAC on the world’s highest motorable pass. The Chismule-Demchok Road traverses through Umling-La Pass at an altitude of 19024 feet. The temperature of this region dips to –250C and experiences heavy snow fall from November to March every year. The road, on an average, remains open for working from April to October (for a period of only five-six months). The road alignment passes through mountainous terrain with the altitude ranging from 4200 m to 5700 m.

The region witnesses the temperature variation of 80C to -250C and receives heavy snowfall up to 45 mm during winters.


Reason for Adopting Non-Frost Susceptible Sub Base (NFSSB)
Due to these extreme cold temperature and exposure of the area to sub zero temperatures for months together, the soil temperature falls below freezing point. Due to the fall in temperature, moisture at subgrade level freezes and results in formation of icy crystals. When water freezes, it expands about 9% of its original volume and is known as frost heave and becomes super cold and remains in liquid state at temperature well below freezing point. The super cold water and icy crystals have a strong affinity; water is drawn to ice crystals that are initially formed and thus continue to grow until ice lenses begin to form.

Frost Heave
The raising of a portion of the ground surface as a result of ice lenses in a frost susceptible soil or base course.

After winter, when atmospheric temperature increases above the freezing point, melting of ice lenses start. As the thermal conductivity of frozen soil is greater than unfrozen soil, melting of ice lenses releases an excess of water within the soil, with a corresponding reduction in load supporting capacity of the subgrade and subsequently pavement as a whole.

Further, since restriction of vertical drainage is a characteristic of frozen ground in seasonal frost areas, the excess water released by melting of ice lenses cannot drain downward through underneath impervious frozen sub soil. The water, therefore, tends to ooze upward and saturate subgrade/sub-base/base course, resulting in very unfavourable conditions, in which, not only the subgrade is in a weakened condition, but the pavement itself may be supported in part by water under traffic loading.

Loss of subgrade supporting capacity can be considerable in magnitude (almost 50%) and may exist for relatively long periods of time after thawing has taken place. The weakening of sub-grade and pavement gets aggravated further during snow melting period because of wide variation in day and night temperatures. During day time, temperature rises and in night it falls even below zero degrees. Due to fall in temperature below freezing point during night, alternate thawing and freezing take place. Each time the soil freezes, it results in loss of density. This, in turn, results in higher potential for moisture absorption. After several cycles of freezing and thawing, a large portion of the subgrade supporting capacity is lost.

In these areas, flexible pavements become brittle and least ductile during the period of sub-zero temperature when the greatest shrinkage tendency and heave occur. Brittleness of the pavement results in cracking of pavement surface. Cracks at pavement surface provide means for ingress of moisture into the base/sub-base/sub-grade. This offers a point where revelling starts and where freezing of moisture in and immediately below the cracks and to further widen and intensify the cracks, as a result of which the life of pavement is affected adversely.

While the lower reaches of Ladakh ranges experience intense rains, in the higher altitudes heavy snowfall is a common phenomenon. Ladakh ranges have many glaciers and experience snowfall at times though being a dry region. On the flip side, large amount of moisture in sub-soil leads to slope instability and road pavement distress. Pavement performance in the region is affected by freezing and thawing, thermal stresses induced by temperature variations, and use of studded tyres/track wheeled vehicles. Hence, these areas which are subjected to snowfall and frost action necessitate special consideration for design and construction of pavements.


Frost Action in Road Pavements
The distress caused to road pavements due to freezing and thawing of water which is trapped within subgrade or subsoil immediately below the subgrade is called frost action. Frost action refers to two separate but related processes: frost heave which results in upward movement of pavement layers and subgrade due to expansion of accumulated moisture as it freezes and thaw weakening resulting from soil saturation as ice melts.

In the vast Himalayan region, considerable variations occur in soil types, in the mineralogy, origin, and deposition, in ground water conditions, climatic conditions, etc. Certain combinations of these conditions make subgrade unsuitable for road works. Frost action in subgrade soil is one such issue. Moisture in subgrade soil can lead to formation of frost during winter months depending upon altitude of the road. Frost heaving can result in cracks and poor riding quality in the pavement, while frost thawing leads to pavement failure due to loss of subgrade support. Three conditions are required for frost occurrence:-

1. Frost susceptible soil (significant amount of fines)
2. Subfreezing temperatures (freezing temperatures must penetrate the soil and, in general, the thickness of an ice lens will be thicker with slower rates of freezing)
3. Water (must be available from the groundwater table, infiltration, or held within the voids of fine-grained soil).

If these conditions occur uniformly, heaving will be uniform. However, under actual field conditions, variations in these three factors from place-to-place result in differential heaving. In some locations, more than one cycle of freezing and thawing may occur within a year, and this would be more damaging to road pavement than one single but longer cycle of freezing and thawing. Thawing generally happens from top downwards. Hence, during thawing, trapped water above the frozen soil saturates pavement layers and drastically decreases the bearing capacity of subgrade soil. Sources of Water for Frost Action

Frost Action
A general term for freezing and thawing of moisture in materials and the resultant effects on these materials and on structures of which they are a part, or with which they are in contact.

Water which contributes towards frost action in pavements can come from two sources:

• Surface water which enters the pavement through cracks and joints
• Subsurface water.

Road pavements are not entirely impermeable and moisture infiltration can occur during rain or snow melting. Pavements which have thin bituminous surface course are especially vulnerable in this regard. Subsurface water can enter subgrade soil from three sources.

• Ground water
• Capillary water and
• Lateral movement of moisture within sub-soil.

A groundwater level within 1.5 m of the proposed subgrade top level usually results in availability of sufficient water for frost action. In case of clayey subgrades, this water table depth even in excess of 3 m can result in frost action since capillary rise of water is very high in clayey soils. Sometimes it is seen that, even though normal water table may be at considerable depth below, ‘perched water table’ if present, can contribute considerably towards frost action. When water is drawn upwards from ground water through capillary action, it results in moisture entry into soil layers much above water table. In hilly areas, lateral movement of moisture from a previous strata is fairly common when road is located in a side hill cut which intersects such pervious strata. The road to Umling la at many locations is already along a water stream susceptible to capillary action.

Frost Boil
“The breaking of a small section of highway or airfield pavement under traffic with ejection of soft, semi-liquid subgrade soil. This is caused by the melting of the segregated ice formed by frost action.”

Soils Prone to Frost Action
Frost action generally does not happen in clean, well-drained sands, gravels, crushed rock, and other similar granular materials. They have high permeability, which allows trapped water to drain out quickly. Additionally, considerable amount of voids present in such soils permit water to freeze without segregation into ice lenses.

On the other hand, silts are highly frost susceptible. Relatively smaller sized voids found in silty soils, high potential for capillary action, and better permeability than clayey soils, accentuate frost action problem in silty soils. Clays have a high potential for capillary rise of water. However very low permeability of clays results in lower capillary action. Consequently, frost may occur in clayey soils, but its impact will not be as severe as silty soils.

Mitigating Frost Action
Alleviation of frost action and its harmful effects on road pavement involves structural design considerations as well as other techniques. These methods can be categorised as:

• Removing and Replacing Frost Susceptible Subgrade: Soils which are very prone to frost action can be removed from subgrade layer at least up to typical frost penetration depth and replaced by non-frost susceptible soil.
• Design Pavement Structure based on Reduced Subgrade Support: In this method, pavement thickness is increased to account for the damage and loss of support caused by frost action.
• Providing a Capillary Break/ Encapsulation of Frost Susceptible Layer: By breaking the capillary flow of water from ground water table, frost action can be reduced since frost heaving requires substantial supply of moisture than whatever may be available in the soil pores.

Laying of Non-Frost Susceptible Sub Base (NFSSB) and Base Layers
In order to ensure that the road remains strong and retains its smooth driveability throughout its design life, it had to be ensured that the pavement is free of the effects of heaving and thawing. Coarse aggregate with all particles about the same size is best suited for this purpose as it has pore size that are too open and porous to promote capillary flow. It should be free of fines which might fill voids and provide capillary channels around larger particles. Accordingly, layer composition here was chosen as NFSSB (larger particles with no sand as shown in table 1) rather than GSB which is normally used at lesser altitudes. This has prevented the uneven settlement on Chisumle-Demchok Road due to frost heaving and freeze thawing.

Size of Aggregates(mm)	GSB (%)	NFSSB (%)
63-45	-	82.50
26.50	34.87	-
13.20	16.84	-
11.20	-	17.50
6.70	19.68	-
Sand	28.61	-

Challenges of Cold Weather Concreting and Preventive Techniques
Undertaking pavement surfacing activity in the cold climate is challenging which demands adoption of special precautions when compaction, placing and rolling of the mix and understanding of the various factors that contributes to the peculiarity of cold weather pavement works.


Frost Heaving and Thaw Weakening
Frost heaving and subsequent thawing is extremely detrimental to pavements which occur primarily in two ways. Heaving in winters as the water in pores freezes and expands and collapsing of surface in summers as the ice lens thaws. The soil on Chusumle – Demchok Road is extremely prone to frost heaving. This is due to a combination of following factors:

Soil Composition: In addition to rocks and boulders, the soil composition around Umling La has a mix of silt and fine sand. They have high permeability and exhibit strong capillary forces (3-8 feet rise). As a result, their frost heave potential is very high.

Round the Year Availability of Water: Though the water table is not high in this region; due to its altitude, Umling La witnesses snowfall for approximately six-seven months a year and the snow accumulated during this period keeps melting gradually for the balance duration. This makes sufficient water available for frost heaving through surface infiltration.

Extreme Temperatures: The temperature variation between winters -25oC and summers 8oC is very high which creates suitable conditions for frost heaving.


Construction of Road at Umling-La Pass using NFSSB
By adopting of NFSSB layer in Umling-La the chances of failure are reduced considerably. The NFSSB layer which has aggregate size of 63-45 mm creates numerous voids which inhibits the movement of water to upper layers as it acts as a barrier and stops the capillary action of water completely. As no water from lower layers of sub grade reach the top layers, limiting the formation of ice or thawing of upper layers leading to reduction in failure of pavement and in turn increasing the stability and life of road.


The major challenge of the Chismule-Demchok Road was overcome by using the NFSSB methodology. Today, the road is the pride of BRO as it traverses through Umling-La Pass at an altitude of 19024 feet, becoming the highest motorable pass in the world.