Figure 1. Geological Map of Study Area (After Hanson 1986, 1989)
Quality and stiffness of subgrade soils play a crucial part in performance of the pavements subjected to overwhelming traffic stack. To evaluate the quality of road subgrades, a test known as California Bearing Ratio (CBR) was developed by California State Highways Department. CBR is regularly utilized for flexible highway and airport pavements and occasionally for other development purposes to assess the strength of earth materials. To assess the values of CBR, Standard Proctor Compaction, and Moisture Content and Atterberg Limits, the collected subgrade soil samples were taken to the laboratory. Standard test methods of ASTM were followed for carrying out different tests. The CBR of the samples ranges from 4 – 16% and the plasticity index (PI) values extend from 13-17% which demonstrates the expansive behavior of the subgrade soils. The examination of different results portrays that the subgrade soils from the study area are of destitute quality, thus these soils don't qualify to be utilized as subgrade concurring to the National Highway Authority (NHA) specification. From the study, it can be concluded that the performance of the examined material ought to be upgraded for utilization as subgrade. After checking on the financial contemplations, evacuation of the existing subgrade soil upto 0.6m and stabilization with a natural waste material such as granite cutting waste and fly ash etc. has been proposed as a subgrade soil enhancement strategy. The other suggested soil advancement strategy involves the introduction of an additional base layer.
For strategic and developmental purposes, roads play a critical part, subsequently contributing to all-round progressions. With persuasive advancements in the field of infrastructural improvement, the specialized enhancement in planning and road construction innovation has hustled. Pavement system comprises different layers to carry loads which are constructed and prepared above the natural soils. Sub-grade receives the loads from the traffic and consequently plays a really crucial part contributing to the structural stability of the pavement structure.
The sub-grade comprises of loose soil materials including clay, silt, sand and gravels that have a gigantic impact on roads framework. Weak subgrade soils create damages in roads and buildings construction due to low bearing capacity. Water has the tendency to affect the expansive subgrade soils causing them to swell and shrink when in direct contact with moisture. Clay rich mineral montmorillonite is believed to be responsible for such behavior of clay. In many regions of the world, expansive subgrade soil is one of the major problems for pavement construction.
The evaluation of properties of sub-grade soils counting density, strength, soil stiffness and a few other in-situ parameters are of incredible significance in planning and developing the road pavements for their long term performance. The California Bearing Ratio (CBR) test is carried out concurring to the required conditions for diverse purposes. The CBR value encompasses a direct relation with the quality of a material, in case the other parameters like liquid limit, plastic limit and plasticity index follow the relevant specifications.
The Gilgit-Skardu road is known as Strategic Highway 1 (S-1), which starts on the Karakoram Highway, near Gilgit. Gilgit- Skardu road is deliberately and financially of awesome significance, firstly taking into thought the development of overwhelming traffic utilized for transportation of products and furthermore it is the vital route for country's defence. Assessment of the quality and strength of materials incorporating for the road construction may have an extraordinary impact on the long term life of the road structure.
The primary objective of this study is to conduct an experimental research for analysis of strength parameters including CBR of subgrade soil on one of the most important highways of the country, i.e., Gilgit-Skardu Road, Pakistan and to propose suitable soil stabilization methods for the long-term performance of pavement structures.
Most important infrastructures in transportation are represented by highways which are responsible for controlling the economic growth especially in developing countries (Correia et al., 2016; Mazumder et al., 2016; Mosa et al., 2013). Pavements are known to be the most important parts in highway engineering (Ranadive & Tapase, 2016; Sarsam & Husain, 2016).
Generally, structural design of highway pavements is controlled by the subgrade properties (Shafabakhsh et al., 2014). In most of the cases, subgrade soils with poor engineering properties are responsible for pavements settlement and structures failure which result in huge maintenance costs (Fauziet al., 2016; Mosa, 2017).
Excessive amount of fines in the soil and high values of liquid limit with low values of bearing capacity stimulate the pavements failure (Jegede, 2000).
Traditionally, designs of the flexible pavements were dependent of CBR approach or by taking into thought the elastic distortions. In the late 1980s, CBR got to be prevalent in planning the pavements with utilization of upgraded computing power and speed (Rollings, 2003). To evaluate the strength of road subgrades, a test known as California Bearing Ratio (CBR) was developed by California State Highways Department. CBR is as often as possible utilized for flexible highway and airport pavements and in some cases for other development purposes to assess the quality of earth materials (Garg, 2009).
Despite the latest advances incorporated to pavement design, the California Bearing Ratio (CBR) approach is still one of the most reliable methods for designing pavement, especially in military and civilian aviation pavements design (Semen, 2006) This method is being followed on the basis of more than 60 years of experience throughout the world under a diverse range of conditions. Besides this, the determination of moisture content and in-situ density has been given more importance by the approach to quality assurance of pavement structures.
Due to the economic, technical and environmental considerations, the utilization of various types of natural materials in subgrade construction of highway works cannot be avoided (Ferber et al., 2009). Hence, pavements system can be improved by the modification of weak subgrade soils which results in the reduction of initial and maintenance costs (Al-Haddad &Abed, 2016; Sarsam et al., 2016).
The study was carried out at 3 locations on Gilgit-Skardu road (Figure 1), to be specific, Baghicha, a town in Skardu, 47 km away from Skardu city which coordinates 35°35'21" N and 75°19'59" E and is at an elevation of 1975 meters above sea level. Tungush, a small town in Skardu, 40 km away from Skardu city on Gilgit-Skardu road which coordinates 35°33'04" N and 75°20'49" E and is at the height of 2015 meters above sea level. Kachura, a town 20 km away from Skardu city which coordinates 35°25′35″N 75°27′18″E and is at an elevation of 2150 metres above sea level.
Figure 1. Geological Map of Study Area (After Hanson 1986, 1989)
Alluvial, fluvial, glacial and lacustrine depositional environments are reflected by terrigenous clastic sediments of the Skardu Basin. Deosai Mountains, the Deosai Plateau, the Masherbrum Range, and the Marshakala Group of the Haramosh Range, as well as the catchment regions of the upper Indus and Shyok Rivers, upstream from Skardu are the noticeable sources for deposition of this clastic material (Burgisser et al., 1982).
A total number of 9 disturbed soil samples were collected from 3 locations (Figure 1), which were then taken to the laboratory for carrying out various tests. ASTM D4318 (2010) standard test procedure was used for the determination of liquid limit, plastic limit and plasticity index. Soaked California bearing ratio (CBR) was determined by the ASTM D1883 (2016) standard test method. ASTM D698 (2012) standard method was employed for carrying out proctor compaction test for the determination of maximum dry density and optimum moisture content. The CBR test was applied to the compacted soaked samples. A plunger of standard area was permitted to penetrate into the sample and the load required for the penetration of plunger was plotted on a chart against the recorded penetration with the readings observed at regular time intervals.
Various results of the performed tests are shown in Table 1. The liquid limit of the 9 tests ranges between 36 to 44%. Plastic limit was observed between 19 to 31% whereas the corresponding values of Plasticity Index were observed between 13 to 17%. Such high values of these limits demonstrate the expansive and problematic behavior of the subgrade soils. The optimum moisture content (OMC) values are recorded between 11 to 14.5% with the maximum dry density (MDD) values recorded as 1.85 to 1.94 (g/cc). These OMC and MDD values suggest that the subgrade soil samples have excessive amount of fines with weak strength. The soaked CBR values were demonstrated as 5 to 17%.
Table 1. Results of Experimental Study
The higher values of liquid limit, plastic limit, optimum moisture content and the lower value of maximum dry density (MDD) conform that the samples are expansive and clayey in nature with problematic behavior. The CBR values [5-12%] suggest that the samples are poor in quality and soil may result in pavement failure within the study area.
The plasticity index values [13-17%] of the samples surpass the value indicated by National Highway Authority [10%]. 6 out of 9 samples have CBR values less than that specified by National Highway Authority [7%]. The performed tests investigation illustrates that the quality of the materials ought to be improved in order to withstand the overwhelming traffic effectively.
The enhancement within the subgrade material may be carried out by either one or a combination of diverse procedures, such as expulsion of the weak strength material and then stabilizing with a high quality material, such as granite cutting waste (Shah et al., 2019), fly ash (Zumrawi, 2015), marble dust (Saygili, 2015), copper slag waste (Ravi et al., 2016), waste glass (Nirmala & Shanmugapriya, 2017) or introducing an additional base layer. These improvement techniques may result in upgradation and enhancement of poor quality subgrade soils.
On the premise of assessment from this study it is determined that the subgrade soil samples are of poor quality; therefore advancement to these problematic soils must be introduced for utilization as subgrade material. Distinctive improvement strategies are considered by taking into account, the financial considerations. Removal of weak soils and then stabilization with higher quality material is recommended as the foremost valuable advancement strategy for the subgrade soils. Soil evacuation strategy for improvement of subgrade soil quality will require the excavation of the weak subgrade soil upto 0.6m depth and subsequently replacement with higher quality stabilized material. The proposed stabilization procedure requires addition of natural waste material such as granite cutting waste, fly ash, marble dust, copper slag waste, waste glass from local factories or introduction of an additional base layer. The strategy may be embraced after taking into thought, the financial achievability of the material to be used in stabilization. Further studies can be carried out in terms of correlation between index properties, unconfined compressive strength and CBR approach of these subgrade soils.