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Adopting Geophysical Testing Techniques in Geotechnical Investigation and Shallow Foundation Design
Amer, Mohammad
Amer, Mohammad
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Description
A Master of Science thesis in Civil Engineering by Mohammad Amer entitled, “Adopting Geophysical Testing Techniques in Geotechnical Investigation and Shallow Foundation Design”, submitted in Spring 2018. Thesis advisor is Dr. Magdi El-Emam. Soft and hard copy available.
Abstract
The purpose of the current research is to implement Geophysical Techniques (GT) in measuring small strain wave velocity at different soil properties. Soil properties that are considered include void ratio (degree of compaction), water content (degree of saturation), particle sizes (gradation), clay content, and cement content. Furthermore, to develop a soil wave velocity-stress relationship, wave velocity through externally loaded soil is measured under a shallow foundation applying different vertical stresses. To fulfill the investigation purpose, local sand was collected and subjected to in-depth laboratory tests, such as sieve analysis, compaction tests, shear tests, and full soil classification. For index tests, twenty 6-in-diameter specimens were prepared with different soil properties. In addition, two identical 1/3-scale strip footing model tests are constructed and instrumented with geophones, accelerometers, and load cells. The purpose of these tests is to establish correlations between wave velocities measured on granular materials at different applied vertical stresses. These relationships can be utilized to predict, from in situ velocity measurements, the velocity expected under a shallow foundation required for numerical modeling of various soil materials. Index test results indicate that the P-wave velocity decreases by 40 to 70% as the soil water content increases up to certain thresholds of 3.5 to 4%, which itself increases with compaction effort. In addition, the wave velocity increases by 30 to 80% when sand gradation tends to be fine. A 10% clay content increases the sand wave velocity by 40% in dry condition and 200% in wet condition. Results also show that the addition of 3% Portland cement resulted in a 5 time increase in soil wave velocity, especially after three-days of curing time. The scaled footing tests indicate that the P-wave velocity increases nonlinearly as the footing applied stress increases, and the largest value was measured directly at the bottom of footing. For practical implementation, a nonlinear relationship has been developed to calculate the increase in the P-wave velocity due to footing external applied stress. This increase can be used together with measured in situ velocity, which can be measured using any suitable geophysical method, to estimate the change in soil modulus at different depths below the footing.