Date of Award

Spring 1993

Document Type

Thesis

Degree Name

Master of Science in Environmental Engineering - (M.S.)

Department

Civil and Environmental Engineering

First Advisor

John R. Schuring

Second Advisor

Paul C. Chan

Third Advisor

Dorairaja Raghu

Abstract

This thesis investigates the mechanism of pneumatic fracturing in geologic materials such as soil and rock. Pneumatic fracturing is a recently developed technique for increasing the permeability of geologic formations by the controlled injection of high pressure air. Present applications are focusing on the in situ remediation of contaminated soil and ground water, although pneumatic fracturing has other geotechnical uses such as pumping well enhancement.

A comprehensive literature review of a related technology known as hydraulic fracturing is presented, which serves as background for development of a pneumatic fracturing model. Pressure-time histories from actual pneumatic injections are analyzed in detail to understand the failure mechanism. Several distinct stages of a typical fracture event are identified including: fracture initiation, fracture extension, fracture maintenance, and fracture residual. Reinjection behavior of previously fractured formations is also investigated. The entire fracture event was consistently found to be quite rapid, lasting only several seconds, leading to the conclusion that the formations will respond brittlely.

Based on these pressure-time analyses, an original analytical model is developed for the prediction of fracture initiation pressure and fracture maintenance pressure. The model describes the stress conditions leading to failure in and around a discrete section of borehole during pneumatic injection. The model has a linear form, and assumes the geologic medium is brittle-elastic, uniformly stratified, overconsolidated, horizontally isotropic, and semi-porous. The two dominant terms found to influence fracture pressure are overburden stress and apparent tensile strength of the formation. The effects of pieozometric head are also incorporated, so that the model is applicable to both the vadose zone and saturated zone.

Validation of the model is made with actual field data from several different research test sites. The trends of the data show reasonable agreement with the model, and numerical coefficients are determined by regression. Tentative relationships were developed for two types of geologic media: clayey silt and siltstone/sandstone. Overburden gradients for the clayey silt, siltstone and sandstone ranged from 1.0 to 2.5 psi per foot of depth. Apparent cohesive/tensile strengths for these materials ranged from 5 to 23 psi, 41 to 130 psi and 42 to 52 psi respectively. Sample computations with the model are presented, and the thesis concludes with recommendations for future study.

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