SOILS EXPLORATION PROGRAM
A soils exploration program usually involves test pits and/or soil borings (boreholes). During the site visit (Phase II), you should work out most of the soils exploration program. A detailed soils exploration consists of:
1. Determining the need for and extent of geophysical exploration.
2. Preliminary location of each borehole and/or test pit.
3. Numbering of the boreholes or test pits.
4. Planned depth of each borehole or test pit.
5. Methods and procedures for advancing the boreholes.
6. Sampling instructions for at least the fi rst borehole. The sampling instructions must include the number of samples and possible locations. Changes in the sampling instructions often occur after the fi rst borehole.
7. Determining the need for and types of in situ tests.
8. Requirements for groundwater observations.
Soils Exploration Methods:
The soils at a site can be explored using one or more of the following methods.
• Geophysical methods—nondestructive techniques used to provide spatial information on soils, rocks, and hydrological and environmental conditions. Popular methods are:
1. Ground-penetrating radar (GPR):
GPR, also called georadar, is a high-resolution, high-frequency (10 MHz to 1000 MHz) electromagnetic wave technique for imaging soils and ground structures. An antenna is used to transmit and recover radar pulses generated by a pulse generator. The returned pulse is then processed to produce images of the soil profi le. The key geotechnical uses are soil profi le imaging and location of buried objects. GPR produces continuous-resolution images of the soil profi le with very little soil disturbance. GPR is not suitable for highly conductive (.15 milliohms/m) wet clays and silts. GPR resolution decreases with depth.
2. Seismic surveys :
Seismic investigations utilize the fact that surface waves travel with different velocities through different materials. The subsurface interfaces are determined by recording the magnitude and travel time of the seismic waves, essentially compression waves (P waves), at a point some distance from the source of the wave. The velocity of propagation is the most important parameter in the application of seismic methods. The densities and elastic properties of the
geological materials control the velocity of propagation. When a seismic wave encounters a boundary between two elastic media, the wave energy is transmitted by refl ection, refraction, and diffraction. Seismic refl ection and refraction are used in geotechnical site characterization.
In seismic refl ection tests, the travel times of waves refl ected from subsurface interfaces are measured by geophones. Geophones are motion-sensitive transducers that convert ground motion to electric signals. The travel times are correlated to depth, size, and shape of the interfaces. The angle of refl ection of the waves is a function of the material density contrast. Seismic refl ection is used when high resolution of soil profi le is required, especially at large depths (.50 m).
4. Other geophysical methods of geotechnical engineering interests:
(a) Gamma density, or gamma-gamma, measures electron density and can be used to estimate the total soil density or porosity.
(b) Neutron porosity measures hydrogen density. It is used for porosity estimation below the groundwater level.
(c) Sonic-VDL measures the seismic velocity. It is useful to measure soil stiffnesses and to detect bedrock elevation.
(d) Microgravity is used to detect changes in subsurface densities and is particularly good at detecting cavities. A gravimeter is used at discrete points on the earth’s surface to detect small changes in gravity. These changes are called gravity anomalies and are related to density changes