541. What are the sinkhole testing methods.
Electrical Resistivity (ER)
Imaging Principles Methods
Electrical resistivity imaging (ERl) is a geophysical method of determining the subsurface resistivity distribution of the soil. The resistivity measurements are made by injecting current into the ground through two current electrodes and measuring the resulting voltage difference at two potential electrodes. The equipment has a computerized switching program that automatically changes the spacing of the electrodes along the array. The apparent resistivity is measured and processed using computer inversion software to obtain a cross section of estimated true resistivity values. As part of the modeling process, data points with high error values may be eliminated and spikes in the data reduced. Soil types can be interpreted using known resistivity values.
Ground Penetrating Radar (GPR)
The ground penetrating radar equipment used was a RAMAC X3M manufactured by MALA GeoScience in Mala, Sweden. The setup used consisted of the RAMAC monitor control unit coupled to the X3M, which was attached to a 250 MHz antenna mounted on a self-contained cart and/or a 500 MHz antenna. The cart has a calibrated measuring wheel electronically coupled to the control unit which allows precise linear measurements to be obtained along the transect line during the fieldwork. These measurements and distances can be translated into accurate locating of subsurface features when they are identified. Ground Penetrating Radar (GPR) is also known as subsurface interface radar or impulse radar and can provide a nearly continuous profile of the subsurface conditions. High electromagnetic frequencies (typically between 40 MHz - 2.5 GHz) are transmitted from an antenna that is usually located on or near the ground surface. The electromagnetic pulse is transmitted into the ground and reflections return where there is an intelface between subsurface materials of differing dielectric properties. The antenna has electronics that function as a transmitter and a receiver. The reflections returned from the subsurface are relayed from the receiver to the digital control unit and are recorded on internal memory. Depths to the reflected interfaces can be calculated using depths from boring data or can be estimated by using known propagation rates for similar materials. GPR does not specifically identify the subsurface materials, but it does identify when a change in the subsurface strata has taken place. This can be very useful in documenting the continuity of subsurface layers such as a confining layer for ponds and landfills, identifying potential areas of karst erosion and locating underground pipes or other utilities.
Geotechnical Test Methods
The deep borings were completed using sampling intervals required by ASTM Method D-1586 describing the Standard Penetration Test (SPT) or "split spoon" method of sampling. The penetration resistance testing and sampling were accomplished with the use of a 2" outside diameter sampler, seated six inches into the bottom of the borehole, and advanced an additional one-foot under the effort of a 140 pound hammer faHing freely thirty inches. The number of blows, required of the hammer to advance the sampler one foot into undisturbed material, was noted as the blow count (N) of that particular stratum. Portions of each sample taken were classified, sealed in moisture-proof, labeled containers and returned to our laboratories for verification of field classification.
Borings were advanced using a rotary rig, utilizing are-circulating attapulgite or bentonite drill fluid to maintain the open bore hole in non-cohesive soils and to remove cuttings created by the drill bit. Upon completion, the deep borings were grouted closed in accordance with water management district regulations. The hand auger borings were completed using a bucket type of auger, which allows samples to be taken and visually classified at approximately six-inch increments. Static hand cone penetrometer data was also gathered from the hand auger borings at I-foot intervals.
The penetrometer consists of a measuring instrument, a probing rod and a cone tip. The penetrometer is pushed perpendicular into the soil at a constant speed of approximately 2 centimeters per second by applying equal pressure on both grips. The resistance measured by the cone is read on a pressure gauge at the surface with the maximum resistance being recorded by a dragging pointer.
No direct correlation between penetration resistance and "N" value can be made, however, a factor of 1/4 is general used to relate penetrometer resistance (kg/cm2) Classification of soil samples for both the standard penetration test (SPT) borings and the hand auger borings were also made using the Munsell color number and common name.