Soil investigation and soil explorations are essential steps in conducting site investigations to obtain detailed information about the characteristics of the soil and the hydrological conditions at the specific location. These investigations are carried out to gain a comprehensive understanding of the soil properties, such as its composition, moisture content, density, and strength. Additionally, soil explorations provide insights into the hydrological conditions, including groundwater levels, water flow patterns, and drainage characteristics, which are crucial for various engineering and construction projects. By conducting thorough soil investigations and explorations, engineers and geologists can make informed decisions and design appropriate foundations, slopes, and structures that are safe and sustainable for the specific site conditions.
Site Reconnaissance
Site reconnaissance is a crucial activity that involves a thorough inspection of a site, as well as the study of its topography. The main objective of this process is to gather comprehensive information about the soil and groundwater conditions of the site. It is a critical step in any construction project, as the data collected during site reconnaissance is used to design and plan the project accordingly.
During the site reconnaissance process, various aspects of the site are inspected, including the soil composition, topography, and hydrogeology. This helps in identifying potential challenges that could impact the construction project and developing strategies to mitigate them. For instance, if the soil composition is found to be unstable or unsuitable for the proposed construction, the design can be modified to address this issue.
Furthermore, the study of the topography helps in understanding the layout of the site, identifying any natural or man-made obstructions, and planning the project accordingly. It also enables the engineers to determine the optimal location for the construction project, taking into account the existing natural features of the site.
Overall, site reconnaissance is a critical activity that provides essential data that is used to design and plan construction projects. By identifying potential challenges and developing strategies to address them, it ensures that the project is completed safely, efficiently, and within budget.
Purpose of Soil Exploration
Site exploration serves the purpose of obtaining a comprehensive understanding of various aspects related to the site. One of the critical pieces of information gathered during site exploration is the order of occurrences and the extent of soil and rock strata. This data is essential for engineers and geologists to create accurate geological maps and models of the subsurface conditions.
Furthermore, site exploration provides information about the nature and engineering properties of soil and rock formations. Engineers and architects rely on this information to design structures that can withstand the forces and stresses that may be imposed on them. Understanding the characteristics of the soil and rock formations helps in determining the suitability of a site for construction purposes.
Finally, site exploration helps in locating groundwater and determining its variation. Groundwater is a critical resource that must be carefully managed, and the availability and quality of groundwater can vary significantly from one location to another. By identifying the location and variability of groundwater, engineers and geologists can design appropriate measures to protect and manage this vital resource.
Planning of Soil Exploration
The planning of soil exploration is reliant on several factors, including the nature of the subsoil, the type of structure to be constructed, and the importance of the structure. The nature of the subsoil is a crucial aspect to consider since it affects the foundation’s stability and load-bearing capacity. Therefore, soil exploration must be conducted to determine the soil type, its strength, and any potential weaknesses or instabilities.
Another important factor that affects the planning of soil exploration is the type of structure to be constructed. Different types of structures require varying foundation designs, which are determined by the soil exploration findings. For instance, buildings with heavy loads require a strong and stable foundation, while lighter structures may need a less robust foundation.
The importance of the structure is also a significant factor in soil exploration planning. For critical structures such as hospitals, bridges, and high-rise buildings, the foundation must be designed to withstand severe weather conditions, seismic activity, and other external forces. In contrast, less important structures such as storage sheds or small residential buildings may not require such a robust foundation.
In conclusion, soil exploration planning relies on several factors, including the nature of the subsoil, the type of structure to be constructed, and the importance of the structure. These factors guide the determination of the foundation’s design and materials to ensure the structure’s stability and durability.
Methods of Soil Exploration
Soil exploration is the process of studying soil profiles to understand the composition, structure, and other physical properties of the soil. There are several methods available for soil exploration, including open excavation, borings, subsurface soundings, and geographical methods.
Open excavation is a method that involves digging a trench or pit to expose the soil profile. This method is suitable for shallow soil exploration, and it provides a visual representation of the soil layers.
Borings are another commonly used method for soil exploration. In this method, a borehole is drilled into the ground to reach the desired depth. Soil samples are then collected from the borehole and analyzed to determine the soil composition and properties.
Subsurface soundings involve the use of geophysical techniques to study the soil profile. These techniques include seismic surveys, electrical resistivity, and ground-penetrating radar. Subsurface soundings are useful for exploring deeper soil layers and can provide valuable information about soil properties.
Geographical methods involve the use of aerial photographs, satellite imagery, and other geographic information systems (GIS) to study the soil profile. These methods are useful for mapping large areas and can provide information on soil types and distribution.
In summary, soil exploration methods include open excavation, borings, subsurface soundings, and geographical methods. The choice of method will depend on the depth of exploration required, the soil properties being studied, and the size of the study area.
1. Open Excavation
A pit can be dug at a construction site to explore shallower depths, typically ranging from 2 to 5 meters. This can be easily accomplished if the soil at the site has some cohesion. Such a pit can provide an opportunity to visually examine and study different strata, as well as extract soil samples from different depths. The process of chunk sampling can even allow for undisturbed soil samples to be obtained from the pit.
2. Boring Method
To obtain soil samples from deeper depths, a common method is to use mechanical devices called samplers to drill bore holes. This process involves first drilling a hole into the ground and then visually examining the cuttings that emerge from various depths. These cuttings can provide valuable information about the composition of the soil layers at different depths. Once the desired depths have been reached, the samplers are used to lift soil samples from each depth. This allows for a more thorough analysis of the soil and its properties, which can be useful in a variety of fields such as geology, agriculture, and construction.
Methods of boring
Boring methods are used for exploring the subsurface soil structure. There are several methods available for boring, such as auger boring, auger and shell boring, wash boring, percussion boring, and rotary boring. Auger boring is the simplest method, which is suitable for shallow depths of around 5 m. Auger and shell boring can be used for both soft and stiff soils, while wash boring is a quick method that can be used for all types of soils except for rocks and boulders. Percussion boring is used for making holes in all types of soils, including rocks and boulders, while rotary boring is used for advancing holes in rocks and soils.
Soil samples obtained from the boring methods can be categorized into three types: disturbed, undisturbed, and non-representative. Disturbed samples are obtained by direct excavations using auger and thick wall samplers. In this method, the natural structures of soils may be partly or fully modified or destroyed, although the natural water content can be preserved with suitable precautions. Undisturbed samples preserve the natural structure and properties of soils and are used for tests related to shear, consolidation, and permeability. Non-representative samples consist of a mixture of soil from different soil strata, which might lead to changes in the size of soil grains and mineral constituents. Such samples can help in determining the depths at which major changes are occurring in subsurface soil strata.
The degree of sample disturbance depends on the design of samplers and methods of samplings. The cutting edge of a sampler is one of the design factors that govern the degree of disturbance. Therefore, careful consideration should be given to the design factors to minimize sample disturbance during soil exploration.
The important design features of the cutting edge are a) Area ratio
The given context describes two important parameters for the design of a cutting edge used for soil sampling. The first parameter is the area ratio, which is the ratio between the cross-sectional area of the cutting edge and the cross-sectional area of the soil sample. The context specifies that this ratio should not exceed 25%, except for soft sensitive soils, where it should not exceed 10%. This is an important consideration because if the area ratio is too high, it can lead to excessive disturbance of the soil sample during sampling.
The second parameter described in the context is the inside clearance of the sampling tube. This refers to the amount of space left inside the tube to allow for elastic expansion of the soil sample as it enters the tube. It is important to have sufficient inside clearance to prevent the sample from being compressed or distorted as it is collected. This is especially important for softer soils that are more susceptible to deformation.
Overall, these two parameters are crucial for designing an effective cutting edge for soil sampling. By ensuring that the area ratio and inside clearance are appropriate for the type of soil being sampled, researchers can obtain accurate and representative soil samples that can be used for a variety of applications.
The given context describes the specifications for the clearances of a sample tube used in testing. The inside diameter of the sample tube, referred to as D3, is a critical parameter that must be considered. The inside clearance of the tube should fall between 1 to 10% of the D3, with the ideal range for an undisturbed sample being between 0.5 and 3%. This ensures that the sample is not disturbed during testing, resulting in accurate measurements.
In addition to the inside clearance, the outside clearance of the sample tube is also important. The outside clearance should not be much greater than the inside clearance, with the typical range falling between 0 and 2%. This specification helps to reduce the force required to withdraw the tube, making it easier to handle during testing. By adhering to these clearance specifications, scientists and researchers can ensure that their test results are reliable and accurate.
The following is a description of three different methods of soil testing: the plate load test, the standard penetration test, and subsurface sounding tests.
The plate load test is used to determine the bearing capacity and settlement of a soil layer. A steel plate is placed on the soil surface, and a known load is applied to the plate. The settlement of the plate is measured, and the bearing capacity of the soil can be calculated.
The standard penetration test involves driving a sampler into the soil and counting the number of blows required to drive it a specific distance. The test provides information about the soil’s resistance to penetration, which can be used to estimate its strength and other properties. The sampler should have a smooth outer diameter, and the walls should be kept properly oiled. A non-return valve should also be used to allow for easy escape of water and air during the sampling process.
Subsurface sounding tests are used to explore the soil profile and determine the depth to bedrock or other strata. These tests involve measuring the resistance to penetration of a sampling spoon, cone, or other shaped tool under dynamic or static loading. They can provide an approximate indication of the soil’s strength and other properties.
i. Standard Penetration Test (SPT)
A test is being conducted in a clean hole with a diameter of approximately 55 to 150mm. To ensure stability, the sides of the hole are supported by casing or drilling mud. A split tube sampler with a 50.8mm outer diameter and 38mm inner diameter is used to obtain undisturbed soil samples. The sampler is driven into the soil by a 65kg drive weight with a 75cm free fall. The minimum open length of the sampler is 60cm. The sampling process begins with a seating drive of 15cm, after which the sampler is driven an additional 30cm or until 100 blows have been applied.
The penetration resistance of the sampler, denoted by N, is determined by counting the number of blows required to drive the sampler 30cm beyond the seating drive. However, if the value of N exceeds 15, Terzaghi and Peck recommend using an equivalent penetration resistance, Ne, in place of the actual N value.
Gibbs and Holtz conducted an experimental study to investigate the impact of overburden pressure on the value of N, a parameter commonly used in geotechnical engineering. They specifically examined how this parameter is modified for air dry or moist sand. The results of their study can be represented by a mathematical relationship.
The term “Ne” refers to the corrected value of the overburden effect, which is a factor that affects the results of the Standard Penetration Test (SPT). The SPT is a widely used method for measuring the resistance of soil to penetration by a standard sampler driven by a standard weight dropped from a standard height. However, the actual number of blows recorded during the test can be affected by the weight of the drill string, the length of the sampler, and the weight of the hammer, among other factors. These factors can cause an overestimation of the soil resistance, and therefore a correction factor is applied to obtain the corrected value of the SPT N value. This corrected value is represented by the symbol “Ne”.
The given context provides a formula or equation where the effective overburden pressure is defined as a function of two corrections, namely the overburden correction and the dilatancy correction. It is important to note that the overburden correction is applied first, followed by the dilatancy correction.
The effective overburden pressure is a critical parameter that is used in various fields such as geotechnical engineering, soil mechanics, and rock mechanics. It is a measure of the pressure that is exerted on the underlying soil or rock layer due to the weight of the overlying material.
In the formula provided, the effective overburden pressure is expressed as a function of two corrections. The overburden correction takes into account the reduction in effective stress caused by the weight of the overlying material. This correction is applied first, and the resulting value is then used to calculate the dilatancy correction.
The dilatancy correction takes into account the tendency of the soil or rock to expand or contract under stress. This correction is applied after the overburden correction, and the resulting value is used to calculate the effective overburden pressure.
It is important to note that both corrections are essential for accurate calculations of the effective overburden pressure. The overburden correction is applied first, as it has a significant impact on the value of the effective stress. The dilatancy correction is then applied to account for the deformation characteristics of the soil or rock under stress.
ii. Cone penetration test or Dutch cone test
A type of test is conducted to obtain a continuous record of the soil’s resistance by penetrating a cone steadily under static pressure. The cone has a base of 10 square centimeters and an angle of 60 degrees at the vortex. The purpose of the test is to determine the cone resistance by forcing the cone down for a distance of 8 centimeters and recording the maximum value of resistance. This test is particularly useful in determining the bearing capacity of pits in cohesionless soil.
To calculate the cone resistance, it is important to note that the cone resistance qc (measured in kg/sq.cm) is roughly equivalent to 10 times the penetration resistance N. By conducting this test, valuable information can be gathered about the soil’s ability to support heavy loads and structures. Overall, this test is an important tool for assessing the strength and stability of soil, particularly in situations where the soil may be subjected to heavy loads or other external forces.
4. Geographical Methods of Soil Exploration
i. Electrical resistivity method
The method being referred to involves measuring and documenting alterations in the average resistivity or apparent specific resistance of different types of soil. To carry out the test, four metal spikes are inserted into the ground at equal intervals along a straight line to serve as electrodes. The process entails applying a direct voltage to the two outer potentiometer electrodes, followed by computing the average potential drop between the inner electrodes. The setup of this process can be observed in the accompanying figure.
Mean resistivity (ohm-cm)
Resistivity mapping and resistivity sounding are two methods used in geophysics to study the subsoil. The resistivity mapping method is used to determine horizontal changes in the subsoil. It involves keeping the electrodes at a constant spacing and moving them as a group along the line of tests. The parameters used in resistivity mapping are the distance between the electrodes (D), potential drop between outer electrodes (E), current flowing between outer electrodes (I), and resistance (R).
On the other hand, the resistivity sounding method is used to study the vertical changes in the subsoil. This method involves expanding the electrode system about a fixed central point by gradually increasing the spacing from an initial small value to a distance approximately equal to the desired depth of exploration. The parameters used in resistivity sounding are also the distance between the electrodes (D), potential drop between outer electrodes (E), current flowing between outer electrodes (I), and resistance (R). By using both resistivity mapping and resistivity sounding methods, geophysicists can gain a better understanding of the subsoil and its properties.
ii. Seismic refraction method
The given method is highly efficient and dependable when it comes to creating profiles of various layers, as long as the lower layers exhibit a consistent pattern of increasing density, velocity, and thickness. In other words, the reliability of the method depends on the clear distinction between different layers, and the ability to identify their unique characteristics. The method is particularly effective when the deeper layers exhibit a clear trend of increasing density, velocity, and thickness, allowing for an accurate and efficient profiling process. By utilizing this method, researchers and scientists can obtain valuable insights into the composition and structure of different strata, contributing to our understanding of the earth’s geology and its evolution over time.