Road Pavement Design in Frost Prone Areas

High altitude areas which are subjected to heavy snowfall and frost action necessitate special consideration for design and construction of pavements. U.K.Guruvittal, Chief Scientist & Dr G Bharath, Scientist, CSIR-Central Road Research Institute (CSIR-CRRI), New Delhi

The Himalayas situated along India’s northern border rise to considerable heights. While the lower reaches of these mountains experience intense rains, in the higher altitudes heavy snowfall is common. Areas experiencing snowfall can be grouped into two categories:
  • Areas having altitudes between 2500 m to 3000 m where snowfall is not heavy and snow clearance operations are relatively easy
  • igh altitude areas above 3000 m where intensity and periodicity of snowfall is more, and avalanches, frost, blizzards and icing are also encountered
Due to the high availability of water from rains and snow in the Himalayan region, many of India’s perennial rivers originate here. 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, high altitude areas which are subjected to heavy 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. The Himalayan region, being vast, considerable variations occur in soil types, it’s minerology, origin and deposition, 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:
  • Soil which is susceptible to frost formation
  • Temperature in soil should be below freezing point of water
  • Moisture (water) in the soil
If these conditions occur uniformly, heaving will be uniform. However, under actual field conditions, variations in these three factors from place to place results 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 period trapped water above frozen soil saturates pavement layers and drastically decreases bearing capacity of subgrade soil.

Sources of Water for Frost Action
Water which contributes towards frost action in pavements can come from two sources (a) Surface water which enters the pavement through cracks and joints (b) 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 into subgrade soil from three sources, (i) Ground water (ii) Capillary Water and (iii) 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 pervious strata is fairly common when road is located in a side hill cut which intersects such pervious strata.

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. Casagrade provided following rule for identification of frost susceptible soils:
  • In case of non-uniform soils (soils having coefficient of uniformity ‘Cu’ value greater than 5), any soil having more than 3 per cent of particles finer than 0.02 mm will be frost susceptible
  • In case of uniform soils (soils having coefficient of uniformity ‘Cu’ value lesser than 5), any soil having more than 10 per cent of particles finer than 0.02 mm will be frost susceptible
The concept given by Casagrande was further modified by US Corps of Engineers and they proposed four groups (F1 to F4) of soils to identify frost susceptibility of different soils. This classification is given in IRC SP:48 Hill Roads Manual also (Table 1).

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 given below:

    • Removing and replacing frost susceptible subgrade

      Soils which are very much prone for frost action (as per Table 1) can be removed from subgrade layer at least up to typical frost penetration depth, and replaced by non frost susceptible soil

Table - 1 Grouping of Soils Based on Frost Susceptibility
Group Description Characteristics
F1 Gravelly soils containing between 3 and 20 per cent finer than 0.02 mm by weight Least frost susceptible and least thaw weakening
F2 Sands containing between 3 and 15 per cent particles finer than 0.02 mm by weight Increased frost susceptibility and thaw weakening
F3 a. Gravelly soils containing more than 20 per cent finer than 0.02 mm by weight
b. Sands, except fine silty sands containing more than 15 per cent finer than 0.02 mm by weight
c. Clays with Plasticity Index (PI) value more than 12
Frost susceptible and high thaw weakening
F4 a. All silts including silty clays
b. Fine silty sands containing more than 15 per cent finer than 0.02 mm by weight
c. Lean clays with PI less than 12
Frost susceptible and high thaw weakening
  • Design the 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
Designing Road Pavement in Frost Prone Areas
This case study road is in Himalayan region at an average elevation of 4250 m. This road gets covered by snow during winter. Snow collection and melting on the hill slopes and road section keeps repeating, especially during beginning and end of winter season. As a result, water seeps into subgrade soil as well as hill slopes, saturating it (Photos 1 to 3). Hence, frost formation in subgrade is expected. Before undertaking pavement design, subgrade soil collected from site was subjected to various tests and cement stabilisation.

Test results on subgrade (in-situ) soil
In-situ soil obtained from hill cut was being used as subgrade. It was noticed that subgrade soil in different parts of this road stretch was uniform and comprised of sandy soil having varying percentage of gravel and a small amount of silt (Photo 4). The results of the tests conducted on this soil are given in Table 2. From the results, it can be seen that subgrade soil can be classified as ‘Gravelly sand’, which is non-plastic. The grain size distribution of soil is given in Fig 1. From the data presented in Table 1 and Figure 1, it may be seen that subgrade soil in the project road belongs to F2 category having moderate susceptibility to frost action.

Road Pavement Design in Frost Prone Areas

Road Pavement Design in Frost Prone Areas

Table - 2 Results of Engineering Tests on Subgrade (in-situ) Soil
Property Result
Liquid Limit (%) 24
Plasticity Index Non Plastic
Particle size distribution Gravel (%) 26
Sand (%) 58
Silt (%) 13
Clay (%) 3
Modified Proctor Test – gd kN/m3
OMC (%)
22.1
7.5
CBR (%) at 97% of Modified Proctor Density More than 20%

Road Pavement Design in Frost Prone AreasFigure 1: Grain Size Distribution of In-situ Soil

Cement stabilisation of in-situ soil
The in-situ soil meets the gradation and plasticity requirements for cement stabilised materials as per MORTH Section 403 (Table 3). In-situ soil was stabilised using 43 grade Ordinary Portland cement (OPC). Strength development after stabilisation was excellent. IRC SP:72 which is used for design of road pavements for traffic up to 2 msa, specifies 1.7 MPa compressive strength at 7 days for sub-base course and 3.0 MPa compressive strength at 7 days for base course, when cement stabilisation is adopted. IRC SP:89, which is adopted for highway works, specifies compressive strength at 7 days to be from 1.5 MPa to 12 MPa for sub-base / base. Unconfined compressive strength test results of cement stabilised in-situ soil after 7 days of humid curing are given in Table 4. It can be seen that due to well graded nature of in-situ soil, cement stabilisation results in very high strength development.

Table - 3 Grading Limits of Material for Cement Stabilisation
Sieve Size in mm Percentage by mass passing Sub-Base / Base Gradation obtained for In-situ soil
53.00 100 100
37.50 95 - 100 100
19.0 45 - 100 97
9.5 35 - 100 80
4.75 25 - 100 75
0.600 8 - 65 48
0.300 5 - 40 30
0.075 0 - 10 5

Table - 4 Unconfined Compressive Strength Test Results of Cement Stabilised In-Situ Soil after 7 days Curing
Cement Added Unconfined Compressive Strength
3% 5.85 MPa
6% 9.12 MPa

Pavement Design for Proposed Road - Alternative 1
Subgrade soil belongs to F2 category (Table 1), so, there is no need to replace the subgrade soil. Providing adequate thickness of road pavement can alleviate the frost action. The in-situ soil which will be used for subgrade is having very good CBR value (more than 20 per cent). Design traffic is expected to quite low, since it is located in a remote area. Design traffic was assumed to be 5 msa. IRC SP:48 proposes that design of pavement should be related to actual depth of frost penetration. Since daily temperature variations data was not available for the site, empirical design proposed in IRC SP:48, can be adopted. IRC 37 states that in frost prone areas, minimum thickness should be 450 mm even when subgrade CBR warrants a lesser thickness. IRC SP:48, states that structurally strong layer like dense bituminous macadam (DBM) and bituminous concrete (BC) would be needed since heavy machinery for snow clearance operations are to be used. Further to prevent percolation of water in the pavement layer, pavement cross section suggested in Table 5 should be adopted for full formation width, leaving no gap between edge of the pavement and drain or parapet.

Table – 5 Pavement Composition (Alternative 1)
Layers Layer Thickness Material Specification
Wearing Course BC 40 mm MoRTH Specifications Section 507
Binder Course DBM 75 mm MoRTH Specifications Section 505
Crushed Stone Base CRM 250 mm MoRTH Specifications Section 407
Non Frost Susceptible Sub-Base GSB (Fines content to be less than 3%) 200 mm MoRTH Specifications Section 401
Subgrade Local in-situ soil (Compacted) 500 mm MoRTH Specifications Section 300
To be laid and compacted in three layers
Total 565 mm + 500 mm Subgrade

Pavement Design for Proposed Road - Alternative 2
The in-situ soil is well graded gravelly sand. After stabilising with cement, in-situ soil develops very good strength. Hence, cement stabilised pavement section as per IRC 37 is proposed as an alternative (Table 5). As suggested in Alternative 1, to prevent percolation of water in the pavement layer, the pavement cross section suggested in Table 5 should be adopted for full formation width, leaving no gap between edge of the pavement and drain or parapet.

Table – 5 Pavement Composition (Alternative 2)
Layers Layer Thickness Remarks
Wearing Course BC 40 mm MoRTH Specifications Section 507
Binder Course DBM 75 mm MoRTH Specifications Section 505
Single coat SAMI  MoRTH Specifications Section 517
6 per cent cement stabilised in-situ soil (Base and Sub-base combined) 350 mm - To be constructed in two layers of 150 mm (Top layer) and 200 mm (Bottom layer) compacted thickness MoRTH Specifications Section 403
Subgrade Local soil (Compacted) 500 mm MoRTH Specifications Section 300
To be laid and compacted in three layers
Total 415 mm + 500 mm Subgrade

Additional Issues Pertaining to Black Top Layers
  • Warm mix additive is recommended to be used in DBM and BC mixes to ensure adequate compaction even at lower temperature, during the compaction process of bituminous mixes. IRC SP:101 ‘Interim Guidelines for Warm Mix Asphalt’, provides details of warm mix modification.
  • Polymer modified bitumen (PMB) is recommended as binder for the wearing course. Due to very low temperature conditions expected at site, only elastomeric modifiers (viz. SBS) are preferred and recommended. Mixing and laying temperatures shall be strictly adhered to as per specifications (IRC SP:53).
  • In binder course as well as wearing course, use of anti stripping agent is recommended to prevent damage of bituminous layers due to the moisture ingress.
  • Construction activities such as bituminous work and cement stabilisation should be completed before onset of winter season.
Acknowledgements
The authors are thankful to Mr M.N.Nagabhushana, former Senior Principal Scientist, CSIR-CRRI for his valuable contributions and association during the study.

References
  • IRC SP:48, ‘Hill Roads Manual’, Published by Indian Roads Congress, New Delhi (1998)
  • IRC 37, ‘Guidelines for the Design of Flexible Pavements’, Published by Indian Roads Congress, New Delhi (2018)
  • IRC SP:101, ‘Interim Guidelines for Warm Mix Asphalt’, Published by Indian Roads Congress, New Delhi (2014)
  • IRC SP:53, ‘Guidelines on Use of Modified Bitumen in Road Construction’, Published by Indian Roads Congress, New Delhi (2010)
  • MoRTH Specifications for Road and Bridge Works, Published by Indian Roads Congress, New Delhi (2013)
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