## 4 Experimental Methods
### Impact Penetration Based Soil Investigation  
Conventional Techniques for soil investigations, particularly for large civil projects in remote areas, are quite expensive in both time and money. In recent years, interest has been developed to explore the feasibility to use the high-speed earth-penetrating projectiles for application to soil investigations. [PERSON] et al. [145] have suggested that if the impact velocity and properties of the soil medium are known, the decelerations and depth of a given projectile can be predicted. This also means that if the decelerations and depth are known for a given projectile and impact velocity, it is possible to determine the properties of the soil media (unrealistic material) being penetrated with more reliability and to provide the comparative evaluation of the earth subsurface. In-situ testing methods, such as static velocity \"Cone Penetration Testing (CPT)\", have been developed to determine the geotechnical properties of soils where the main parameters, sleeve friction, cone resistance, and pore pressure at (or behind) the cone are measured during the penetration of a conical tip into the soil. The interpretation of CPT data is well supported by a theoretical and empirical background [146]. The dynamic penetration-based impact penetrometer (lance insertion retardation meter, LIR meter) with lower handling requirements than CPT, and ballistic penetrator and deep penetrating anchors (DPA) have been recently developed for assessing the geotechnical properties. The impact penetrometer only uses acceleration sensors to determine the geotechnical properties based on the acceleration data measured. There are two types of impact penetrometers available, i.e., lightweight devices for near-surface investigations [147] and huge penetrometers [148] for anchor performance evaluation. From field testing and laboratory observations, [PERSON] et al. [146] and [PERSON] [149] show that quantitative measures, such as cone resistance, bearing capacity, shear strength, or void ratio, can be extracted through further processing of qualitative penetrometer data. \"[PERSON] and [PERSON] [150] present an empirical model based on model penetrometer tests and reference triaxial tests to determine undrained shear strength of clayey soil targets and the influence of penetrator mass, penetrator tip shape, impact velocity, and the strain rate on undrained shear strength\". The authors conclude that projectile mass and tip shape have a negligible influence on the strain rate effects during the tests. The experimental methods are used to assess the effect of projectile shape on penetration depth [151; 152; 153] and curve fitting of experimental results to existing theoretical models [153; 154]. Such methods avoid study of the physical properties/processes of the phenomena of penetration. In shallow penetration, the granular medium loses most of its energy in the initial stage of penetration with transient interaction of the projectile with the target medium while in deep penetration, the compression and shear of the granular medium result in energy dissipation [41]. The energy absorption in the shear is primarily due to friction and changes in volume as grains \"slip, roll and climb\" over each other ([PERSON] et al., 2011). [PERSON] et al. [155] provide a summary of the recommendations for approximate empirical models and penetration depth relationships suggested for the description of high-speed penetration into geological shields. The summary also includes the empirical studies not available in the English language. The penetration depth in the projectile impact into the soil media reaches its maximum value at critical speed, i.e., velocity at which the maximum value of the penetration depth is reached [156]. The study by [PERSON] et al. [123] inferred that the maximum achievable depth of penetration in granular media appeared proportional to the density of the projectile material.