Excavatability is defined as the ability to select the most effective surface excavation method for excavating rock and rock mass that ultimately results in smaller, manageable rock sizes. The main methods involved in the surface excavation are digging, ripping and blasting. (Tsiambaos & Saroglou, 2010; Mohamad et al., 2005; Mohamad et al. 2019). Direct digging and ripping are referred to as a mechanical excavation method, while digging is defined as the process of cutting rock material with a penetration blade and then putting the result into a bucket (Hadjigeorgiou & Poulin, 1998). While ripping refers to loosening the soil by dragging one or more steel tynes of a bulldozer. This method is suitable for excavations involving weak blast rocks but too strong to be removed by an excavator. Ripping is an economical method of breaking soft rock masses to remove fragmented material (F.G.Bell, 2008). The mechanical excavation method provides more advantages compared to the blasting method. It is safer, environment-friendly and has minimum ground disturbance, uniform muck size, selective excavation capability, continuous operation, and higher production rates in complex ground conditions (Bilgin et al., 2014). While blasting is the most common method to break rocks and produce a fragment size distribution, explosives and other methods have a similar and very strong force. However, it has an environmental impact such as ground vibration, airblast, dust, fumes and flyrock (Mohamad et al., 2013). The ripping is usually inexpensive compared to blasting but it is rather challenging to decide the need for ripping on a particular rock and estimate excavation cost.
In tropical countries like Malaysia, there is a problem of disputes in excavation work involving weathered rock, especially for moderately weathered to highly weathered rock. Before starting excavation work, it is a practice of engineering and geology in this country to carry out methods such as geophysics to provide a solution to these difficulties and disputes. Although geological and geotechnical assessment methods are the best in obtaining rock mass and rock material properties, geophysical methods such as resistivity and seismic refraction methods are alternative methods for determining bedrock depth and are very useful. This includes defining the terms hard mass, rock mass and complex rock characteristics. This situation may result from the engineer being not familiar with the technique and the interpretation of the results is quite challenging to understand and can be disputed. Nevertheless, it is seen that surface excavation performance can be improved with geophysical methods, with a combination of geological and geotechnical mapping methods. Therefore, by implementing the geophysical methods with conventional methods, the site investigation problems can be reduced. However, the cost and variation order (VO) faced by the government on earthwork is bound to be the highest compared to other VO. This has caused many losses to the government and delays in many projects.
Seismic refraction is a very good site investigation method and is suitable for implementation at the initial investigation stage, which is before starting a construction project. This is because seismic velocity depends on several rock mass qualities including weathering, density, porosity, strength and overall rock mass characteristics. (Olona et. al, 2010) compared geophysical methods with traditional methods, and found that geophysics can provide a good characterization, especially for the heterogeneous rocks in the area. It was well explained that spacing, orientation, condition of discontinuities and the other essential parameters used in describing rock masses specifically for surface excavation. Although the rock engineering properties are relatively complicated to determine, it still needs attempts to assess the required rock properties and provide reliable value in solving problems related to rock engineering. In addition, geophysics provides other detailed information such as cavities, dissolution features in carbonate rocks and boulders, and locating possible potential groundwater accumulation (Al-Garni et al., 2002; Saad & Mohamad, 2016; Azrief Azahar et al., 2019), as well as can provide volumetric measurements and subsurface conditions (Mehidi et al., 2019). The seismic refraction survey can produce stratigraphy, geomaterials features, and subsurface information in two-dimensional (2D) (Hazreek et al., 2013). At present, there are many other benefits of using geophysics and it is widely used for engineering purposes and so on. (Haryati et al., 2019) has studied overburdened material on top of a fresh rock in the quarry area which will be removed for its material. Therefore, the seismic refraction method has been used to obtain the profile and depth of the rock before removing the thick overburden.
Currently, the study conducted by (Muztaza et al., 2022) has shown that geophysical methods are very essential to be carried out in earthworks to obtain more economical and effective cost and time. They use the seismic refraction method to assess excavations in the site area before developing the area into a quarry. Therefore, through tomography 2D the subsurface layer is obtained and subsequently, the estimated volume of rippability can be determined. Accordingly, the excavatability of the site studied had been determined with minimal time and cost. Likewise, the study conducted by (Kausarian et al., 2014) conducted the seismic refraction method in several locations in the granite quarry area. From the studies made, Vp shows a certain range of velocities even if the rock mass of the same type is at the same depth. This indicates several factors such as inhomogeneous rock layers due to the non-uniform content and distribution of rock minerals. In granite rocks, there are also issues such as porosity, groundwater conditions, and also the existence of cracks in the rock mass. Vp shows a decreasing trend as the weathering grade decreases in the deeper parts even though the grade sequences are incomplete. Then, the suitable excavation method for the quarry is determined by using the Caterpillar seismic velocity chart.
Generally, the evaluation of surface excavation can be divided into grading, graphical method and approximation by seismic velocity. Seismic methods are widely used in rock excavation, but the accurate result cannot be obtained by this method alone. This is because several factors are not taken into account in this method. For example, embedded boulders and massive formations consisting of large blocks or very soft rocks are undetectable, often occurring in sedimentary, igneous, and metamorphic rocks such as basalt, gabbro, and granite (Weaver, 1975). Besides, this seismic method is also undetectable on abrasive rock potential. (Hadjigeorgiou & Poulin, 1998) have provided a good summary of the possible sources of errors in the seismic velocity of the data that result in misleading estimates.
In 1958, Caterpillar developed a chart related to seismic velocity and rock type, whether the rock is rippable, marginal and unrippable for each bulldozer model. Seismic velocity values for the marginal to non-rippable category are higher for larger-size dozers. Komatsu has also produced a performance chart for each of its dozers, as has Caterpillar which also shows rippable, marginal and non-rippable velocity zones for various rocks. (Macgregor et al., 1994) commented that the Caterpillar ripper performance chart is the most widely used method. It is complete for every bulldozer model, even though Komatsu has also produced charts for each bulldozer model. They suggested that the Caterpillar chart is too optimistic by predicting rock mass should be ripped economically, while in reality, it is difficult to achieve its production value. (Koczanowski et al., 1991) also commented that the Caterpillar chart is highly dependent on seismic velocity, and contributes to unpredictable efficiency predictions when certain conditions such as boulder formation, thin layers, or water tables are within the depth to rip.
The indirect method is a method that is commonly carried out by using the seismic refraction method, referring to the standard table shown in the Caterpillar Performance Handbook 2017. Caterpillar Tractor Company (2001) has proposed the use of seismic velocity charts on the ripper performance. Caterpillar made a comparison of the wave velocities for several case studies that used their equipment, finding that there were good indications of ripper performance. Through this chart, rippability is measured qualitatively and quantitatively in terms of rippability i.e rippable, marginal, and/or non-rippable, as well as a rating scale from 0 to 100. The physical principle used in determining rippability is that seismic waves move faster in rocks that have a higher density, on the contrary, in a less consolidated rock mass. The study conducted by (Mohamad et al., 2011) on sedimentary rocks also commented that Caterpillar charts only provide information related to seismic velocity whereas other factors such as discontinuity spacing and strength affect the excavation of the sedimentary rocks were carried out in this study. Rock masses that have a lower wave velocity are more easily ripped. Currently, many researchers have used it in studies related to the usage of geophysics method in surface excavation (Ismail et al., 2018; Aziman et al., 2019; Muztaza et al., 2022; Akingboye & Bery, 2022; Jug et al., 2020; Ndiaye et al., 2020). Most scholars support the idea that other parameters affect surface excavation performance other than the seismic velocity method. This includes geomechanical parameters of the rock mass, machine parameters and rock material parameters
The first serious discussions and analyses of geological factors influencing surface excavation emerged during the 1970s. (Weaver, 1975) defined rock type, seismic wave velocity, rock hardness, rock weathering, rock structure and rock fabric. The researchers also argued that although the assessment of rippability can be obtained through seismic wave velocities, the geological condition is still crucial as a guide for engineers and geologists to face troubles that may arise. Therefore, the assessment of rippability chart/ graph to be proposed is advised to consider each of these factors, which is beneficial for estimating the cost and method of excavation. Similarly (J.Smith,1987) described several factors that are significant in evaluating rippability other than seismic velocity. Among them depend on the type, structure, and weathering of rocks, as well as rock fabric. This statement is agreed by (Bozdag,1988), in addition, suggests that production requirements, mine geometry and excavating machinery are significant for rippability. (Macgregor et al.,1994) conducted a study using ripping and geological databases for bedded and non-bedded rock to determine the factors affecting the ripping productivity of bulldozers. The factors that influence productivity are uncontrolled rock compression strength, weathering rate, seismic velocity, roughness, joint strength, bedding spacing in unripped rock and bulldozer mass. This selection is necessary to produce effective excavation and optimum production. The study conducted by (R. N. Singh et al.,1987) studies the effect of rock mass characteristics on ripper performance and ripper selection process based on its function, which produced the correlation between the tractor ripper’s design capabilities and performance.
Rock excavation assessments conducted in the past have been critically studied, using various approaches and methods. Recently, there is an increase in the amount of literature on excavation assessments in tropical countries (Liang, 2016, Mohamad et al., 2017, Ismail et al., 2018a, Mohamed et al., 2006, Tating et al., 2015, Md Dan et al., 2016a, Alavi Nezhad Khalil Abad et al., 2014, Siti Norsalkini, 2019, Mohamad, Abad, et al., 2011a). Comparison between assessments completed by considering various factors such as rock type, geological, geotechnical and geophysics parameters, excavation methods, machine characteristics, etc. However, there are differences between the accessor. Together these studies provide important insights into the factors affecting excavatability in a tropical region, it is important to consider producing an excavation classification system that takes into account technological factors that are always appropriate and rapidly increasing. As developing technology is increasing in our country, there is an urgent need for reliable and simplified excavation assessments to consider the unique factors in a tropical region. Therefore, a study needs to be conducted to carry out rippability assessment in tropical regions using both seismic velocity and graphical methods