A geotechnical investigation is known as an investigation of the physical characteristics of the soil (Sushma, 2009). The majority of the soil’s characteristics are largely dependent on the type of soil, and it plays a significant role in foundation planning. The fine grained soils are usually very difficult to treat and require more care than coarse grained soils. Fine grained soils with a high clay content are commonly referred to as soft soils and create severe problems in the designing of geotechnical structures (Garala and Madabhush, 2018). These soils have a poor compressive strength and high consolidation, and they are usually stiff when they are moisture free and lose their stiffness when they interact with water. This results in a reduction in allowable bearing capacity and increased settlement, causing significant damage to buildings and foundations (McDowell, 1959; Bell, 1996; Venkaramuthyalu et al., 2012; Firozi et al., 2016; Shah et al., 2022a). In earthquake-prone areas, these soils become more challenging because they can increase seismic activity and can significantly enhance the amplitude, frequency, and duration of ground movement that hits the foundations and damages the superstructures (Garala and Madabhushi, 2018). Before starting any construction project on them, it is necessary to identify and stabilize them by conducting site investigations. Nowadays, the stabilization of soft soils through various admixtures including Polysaccharide, Xanthan Gum (XG), Guar Gum (GG), Granite Sand, Calcium Lignosulphonate, Lime, Cement, and Silica Fume, etc., is in practice (Alrubaye et al., 2016a; Alrubaye et al., 2016b; Fattah et al., 2016; Vydehi and Moghal, 2021; Amulya et al., 2022; Al-Gharbawi et al., 2023; Rasheed et al., 2023). This process can increase soil strength, durability and stiffness, and reduces soil plasticity and swelling/shrinkage potential (Hausmann, 1990; Sherwood, 1993; Prusinski and Bhattacharja, 1999; Naeini et al., 2012; Das, 2015).

In recent decades, Pakistan, a South Asian country, has seen so many earthquakes (Sajjad et al., 2014). The reason for this is that Pakistan is located in one of the world’s most seismically active tectonic zones, which includes collision boundaries between the Arabian, Indian, and Eurasian plates. The active and complex tectonic system has caused more than 12 earthquakes of magnitude greater than 6.0–7.6 in the last 30years (Qadri and Malik, 2021). The Kashmir earthquake of magnitude 7.6 in 2005, the Ziarat earthquake of magnitude 6.4 in 2008, the Awaran earthquake of magnitude 7.7 in 2013 and the 2015 Himalayan earthquake of magnitude 7.5, are only a few of the notable recent events (Qadri et al., 2022). These incidents resulted in many deaths and a burgeoning financial loss of billions of dollars, which is very worrying for any developing country (Khaliq et al., 2019; Qadri and Malik, 2021). Following the 2005 Kashmir earthquake, leading scientists and government departments updated the building codes model of Pakistan (BCP-SP 07, 2007).

Abbottabad, a city in the Khyber Pakhtunkhwa (KPK), Province of Pakistan, that is known as a hub of educational institutions and industries was also devastated by the 2005 Kashmir earthquake. The study area is in the red zone with high earthquake chances, according to the Pakistan’s Building Codes. It is also one of the KPK’s most densely populated areas, with rapidly increasing population (Shah et al., 2022b). Scientists have analyzed the seismicity and rocks of this area in detail. The area includes the Jurrasic (Samana suk formation), Cretaceous (Chichali, Lumshiwal, and Kawagarh formations), Palaeocene (Hangu and Lockhart formations), and Eocene sequences (the Nammal and Sakessar formations) (Umar et al., 2015; Akhtar et al., 2019). The area is a part of the fold-and-thrust belt, and has undergone significant deformation, leading to the construction of a large thrust faulting system. The Bhagnotar and Nathiagali faults are the main thrust faults in the area. The Bhagnotar thrust is the major one, where the Hazara formation of the Pre-cambrian age is pushed over the Samana suk formation of the Jurassic period (Akhtar et al., 2019). However, the detailed investigation regarding the soils of the study area is still missing, which is one of the major gaps in the literature. The classification of the soils and the identification of clays can be extremely helpful in understanding the geology of the area in more detail. A detailed soils classification information provides useful insight to the residents of the area to properly plan their building foundations. This is particularly significant in a high seismic zone due to the presence of many faults, which can severely affect the clay soils. The residents of the study area are mainly small-scale business persons, farmers, and self-employed workers, those cannot afford the hefty cost of the heavy foundations made up of huge amounts of steel bars, etc. Therefore it is imperative to identify the clay soil zones in the study area and provide a cost-effective way of stabilizing them to reduce the heavy foundation expenditures and also the extent of earthquake-related structural damage. Keeping this in view, the present study is designed to; (a) classify the soils of all major areas of Abbottabad city with proper foundation analysis and, (b) to explore the potential of limestone powder in stabilization of the weak soils of the area.

Recent studies show that limestone can enhance the geotechnical parameters of clay soils. Muhiddin et al., 2019; Ibrahim et al., 2020; Abdulrahman et al., 2022; Agrawal and Gupta, 2011; Pastor et al., 2019; Tanzadeh et al., 2021, utilized the limestone powder in their studies to stabilize the weak soils. Results showed that limestone powder effectively improves the Atterberg Limits, Compaction parameters, UCS, CBR, swelling potential, and Activity, etc., at significant levels. However, these studies have not analyzed the effect of limestone powder on some of the key segments in foundation design like Ultimate and Allowable bearing capacities of clay soils for various foundations, including strip footing, square footing and rectangular footing. The investigation of other parameters like cohesion, angle of internal friction, Liquidity Index, Consistency Index are also lacking in the literature. The deep bearing capacity and foundation analysis by considering all the factors, including 1) shear strength parameters, 2) compressive strength, 3) shape, depth and inclined load factors, were also limited in stabilization studies, in which other admixtures including lime, cement, fly ash, egg shells, rock powders, etc., were utilized for clay stabilization. Keeping in view these gaps, the limestone powder was chosen for stabilization, and we analyzed the suitability of limestone powder on least investigated parameters. Apart from that, various limestones were exposed in the Abbottabad region, including Samana Suk, Lockhart, Kawagarh, etc. (Umar et al., 2016), and are heavily used in the construction industry in the form of coarse aggregate and also as a decorative stone. During processing, a huge quantity of limestone waste powder is dumped into the Dor River, posing a serious environmental concern. In the present study we explored the application of limestone powder in soils stabilization as it is readily available and requires sustainable management. This work will be a great addition to the existing literature, and will help the engineers to understand the effect of limestone powder on clay’s behavior. The methodology adopted for this work will also be very beneficial for the researchers who are actively working on ground improvements through sustainable and cost-effective techniques.

Geotechnical investigations are really necessary to analyze the underlying ground soil before starting any mission to construct on it. Knowledge about the index and engineering properties of soil plays a vital role in understanding the behavior of soil and its use as a building, road, or different construction material (Sushma, 2009; Garala and Madabhushi, 2019; Rehman et al., 2021; Shah et al., 2021; Zaheer et al., 2021; Shah et al., 2022a; Shah et al., 2022c). Most of the soils of the Abbottabad city besides the Kalapul location belong to group A 6 (clay soils of poor strength), whilst the Kalapul soils fall into class A-1, which is known as stone fragments, gravels and sand of excellent to good strength by the AASHTO classification system (Figure 15). On the basis of the Unified classification (Figure 14), Abbottabad clay is categorized as CL soil, which is known as low to medium plastic clay, which means that the chances of swelling of this soil are less. However, it includes smaller grain dimensional particles, i.e., more than 60 percentage of the material is smaller than seventy-five microns. This size can lead to extreme settlement/consolidation problems due to heavy construction loads. The values of liquid limit, plasticity index, and optimum moisture contents were notably excessive in Abbottabad clay (Figure 13), which in turn increase the Liquidity Index, and diminished the Consistency Index. Both of these parameters are very important to understand the clay behavior. The LI is a measure of the extent of closeness of the soil moisture content to its LL. Where the soil is at its LL, the value of its LI is 100% and the soil acts as a liquid. In contrast, zero LI means that the soil is at its PL, while negative values of LI indicate that the water content of soil is lower than its PL and so the soil is hard. The soil with zero CI means that it is in the liquid state and thus very soft and weak, whereas soil possessing high (e.g., 100) CI indicates that it is in the plastic state, that is, it is firmer/stronger. The soil becomes strong when its CI exceeds 100; increases in CI beyond 100 increases the strength of the soil significantly (Arora, 2000; Budhi, 2010). The obtained results of LI and CI indicated that the Abbottabad clays are currently in the hard state, but these values are very nearer to the plastic stage, which means that any small increase in moisture content may cause the excessive settlements of the structures build on it. On the other hand, the CBR, UCS, density, angle of internal friction, bearing capacity, and penetration resistance are lower in these clays due to the presence of smaller grain-sized particles in large proportions. It is regarded that soil with such grain size absorbs a giant quantity of water, which increased plasticity parameters, diminished CBR, UCS, shear strength, dry density, angle of internal friction, and overall bearing ability of various vertical and inclined loads. On the basis of the outcomes obtained, Abbottabad soils require substantial enhancements before they can be used for building, development, and sub-base materials. The area is also a part of an excessive seismic zone (Umar et al., 2015; Akhtar et al., 2019), which is another challenging task to construct on the soils of this region, because clays can enhance earthquake activity due to its higher potential to increase amplitude, frequency, and duration of ground movement that badly effects the foundations and buildings (Garala and Madabhushi, 2018). In addition, water drainage issues are also very frequent in the city. In the rainy season, water in the location starts to flow onto the roads and grounds, which may also cause massive losses to the buildings erected on them, as the soil has higher plasticity and lower strength parameters. As exists in Pakistan, the Geotechnical engineers normally recommend raft footing for masonry in areas like Abbottabad, which has many active faults and higher clay contents to prevent differential settlement of individual feet. These risks have made the study area unsuitable for other shallow foundations, including strip, square and rectangular, etc. The raft base is designed as a single mat (or common base) for all the load-bearing elements of the structure. It includes a huge amount of steel bars, which makes it very expensive and unaffordable for the people of the area due to their low economic conditions.

The addition of up to 25% limestone powder changed the Abbottabad clays category to A4-type and MH type from A6 and CL type respectively. The strength of A4 and MH soils is considered to be fair according to AASHTO and Unified standards. This addition results in a reduction in the Plasticity Index (from 16.9%–18.6% to 3.6%–8.3%), Liquid Limit (from 30.21%–34.9% to 21.0%–26.6%), Liquidity Index (from −47.33%–−23.07% to −202.7%–−63.8%), and enhancement in soil UCS (from 79 kPa–121 kPa to 101 kPa–145 kPa), soaked CBR (from 3%–6% to 9%–13%), Consistency Index (from 123.07%–147.33% to 163.8%–308.3%), angle of internal friction (from 15°–20° to 19°–25°), density (from 15.13 kN/m³–17.66 kN/m³ to 16.85 kN/m³–19.40 kN/m³) and bearing capacities for various footings and loads including strip, square and rectangular. Similar findings were also achieved by Pastor et al., 2019, Ibrahim et al., 2020; Muhiddin et al., 2019 etc. According to Pastor et al., 2019, the addition of limestone up to 25%, significantly reduces the Liquid Limit, Plastic Limit, Swelling potential and Compressibility, and it can enhance the unconfined compressive strength very effectively. The study conducted by Ibrahim et al., 2020, indicated that the 18% limestone percentage is the most effective one to reduce the Atterberg Limits, Optimum moisture contents, void ratio and swelling potential of clay soil, and increases the UCS and Maximum Dry Density (MDD). The results of study conducted by Muhiddin et al., 2019, shows that the 30% limestone addition enhances the UCS, CBR and MDD and diminishes the OMC, and Atterberg limits. All of these scientists have not determined the effect of limestone addition on the clays liquidity index, consistency index, shear strength, and bearing capacities for various foundations.

This overall improvement and conversion of clay soil class to silt class due to the addition of limestone (Figures 14, 15) will help to reduce the strength and speed of the seismic waves to reach the surface to impact the foundations in earthquakes, which in turn will allow people of the area to build square/rectangular foundations instead of mat/raft foundation. Each of these supports is based on columns reinforced with steel bars and concrete. The quantity of steel billets required in these is much less compared to a raft, which makes them inexpensive, because steel billets per ton are currently priced in Pakistan at around 1071 USD. This will make the construction process very cost-effective in the city.

The main reasons for the reduction of clay plasticity parameters with the limestone addition is its low water adoring minerals, including calcite, etc. The activity value of calcite is 0.33 (Moos, 1938; Shah et al., 2022b), which shows their reduced water absorption capacity. The increase in soil strength parameters (Compressive and Shear), CBR, consistency index, etc., can be attributed to the higher density of limestone than clay or the reaction between clay and limestone minerals or grain size variations between admixture and the clay soil.

This article discusses the Geotechnical parameters and stability of soils in Abbottabad, Pakistan. Limestone powder is used as a strengthening additive. The conclusions are drawn as follows.

1. Most of the soils of Abbottabad belong to the A6 and CL group except for the Kalapul area, which is in the A1 and GP category.

In general, this research will be useful for the local community to format the foundations of buildings, and to explore other inexpensive foundations instead of Raft after significant limestone treatment. Likewise, the use of limestone waste will also decrease the negative environmental impacts and gain monetary and environmental advantages from its reuse. Moreover, this work will benefit engineers, people and the scientific neighborhood around the world to gain in depth understanding of the impacts of limestone powder application on the clay parameters.