In situ testing of soil with the cone penetrometer and pre-bored pressuremeter are becoming more commonplace in geotechnical engineering practice in the United States. The cone penetration test (CPT) in particular is seeing more widespread use among consulting engineers, and is becoming increasingly prevalent as a topic of interest at conferences and meetings. Pressuremeter (PMT) testing is often used for foundation design in stiff soil deposits, and also is becoming more prevalent in US practice. During a subsurface exploration, a zone of unsaturated soil is usually encountered, sometimes extending to considerable depth. While it is generally understood that the behavior of unsaturated soils is different from the commonly assumed saturated-undrained or drained soil behavior; there are no methods for interpreting CPT and PMT results that account for these differences. A great deal of research has been devoted to the CPT and PMT; however, there are currently no reliable comprehensive methods for interpreting in situ test results from unsaturated soils. The research proposed will focus on the development of such a method.=20 The CPT and the PMT will be investigated through a research program that includes laboratory soil testing, calibration chamber testing, and analytical/numerical modeling. This research proposal represents the first step in a long-range research plan. The proposed research includes three major components: 1) design and development of a calibration chamber and testing procedure for unsaturated silt; 2) CPTs and PMTs conducted in an unsaturated silt within the chamber; and 3) development of a cavity expansion based method of interpretation for the CPT and PMT in unsaturated soil. The first research component involves several important considerations including: chamber geometry; top cap and pedestal details; control and measurement of pore air and pore water pressure; pressure control system details; soil bed preparation methods; soil bed instrumentation; CPT and PMT testing apparatus and procedures; and others. Research proposed will utilize a silty soil because it is easier, compared to clay, to handle in terms of soil bed preparation, and because matric suction values expected for the test silt can be measured with tensiometers. The proposed research will benefit from the fact that the Co-PI has successfully developed a technique for rapid measurement of matric suction with small pressure transducers equipped with a high-air entry porous stone. Results of matric suction measurements using this technique during dynamic loading in a centrifuge indicate that the response time of these devices is excellent. Development of methods for preparing soil test beds in the chamber will involve a trial and error process with soil sampling for analyzing homogeneity and repeatability. In addition, samples from soil beds will be tested with a triaxial system equipped to define the unsaturated soil behavior. Second and third components of this research include a series of CPT and PMT tests in the chamber under different matric suctions, and interpretation of the data in terms of soil properties and outcome of modeling studies. Finite element modeling of an expanding cylinder and an expanding sphere in an unsaturated soil will be performed to simulate the PMT and CPT. This modeling will be performed using a fully-coupled finite element analysis method that utilizes an elastoplastic constitutive model for unsaturated soil, developed through the Co-PI=EDs CAREER research. The results of this analysis will be calibrated using unsaturated triaxial test and PMT calibration chamber test results, and will also be used to conduct a parametric study of the variables of interest. An analytical cavity expansion based method of interpretation will be developed for interpretation of CPT and PMT results. This will allow engineers to make reasonable predictions of soil properties at moisture conditions other than those that exist when the in situ tests are performed.=20 A primary goal of this research is to gain important knowledge about the influence of matric suction on in situ test results. In accomplishing this goal a valuable set of experimental data will be established in an area of soil mechanics where information is scarce. A unique calibration chamber will be constructed and methods for preparing and testing unsaturated soil beds will be developed.=20

Agency
National Science Foundation (NSF)
Institute
Division of Civil, Mechanical, and Manufacturing Innovation (CMMI)
Application #
9813137
Program Officer
Richard J. Fragaszy
Project Start
Project End
Budget Start
1998-09-01
Budget End
2002-08-31
Support Year
Fiscal Year
1998
Total Cost
$206,870
Indirect Cost
Name
University of Oklahoma
Department
Type
DUNS #
City
Norman
State
OK
Country
United States
Zip Code
73019