In Situ Testing and its Application to Foundation Analysis in Fine-Grained Unsaturated Soils

The in situ test is an important part of a geotechnical engineer’s arsenal. Currently, there are no established methods for interpreting results of in situ tests in unsaturated soils. The difficulty in interpreting in situ tests in unsaturated soil comes from the presence of suction, which is often unknown and directly influences a soil’s stress state. Unsaturated soils are also subject to fluctuating soil moisture conditions and therefore, fluctuating suction and stress states. For such soils, it is important to establish a methodology to estimate the influence of variable moisture conditions on the interpreted results of in situ tests. This is particularly true when those in situ tests are used as a basis for analysis and design in geotechnical projects. The doctoral study outlined in this dissertation is an important step toward the development of a comprehensive methodology for interpreting in situ tests considering possible variations in soil suction resulting from changes in moisture content. In addition, this study goes one step further by including a laboratory scale investigation of foundation response to loading in unsaturated soil at different moisture conditions that has been also been tested using the pre-bored pressuremeter. The goal is to observe the influence of suction on the results of an in situ test and foundation performance, simultaneously. In this way, the methods of analysis developed for interpreting in situ tests in unsaturated soil can be calibrated and validated against observed foundation performance. The primary objectives of the research work described in this dissertation include collection of data that describes the influence of matric suction on in situ test results including the pressuremeter (PMT), cone penetration test (CPT), and standard penetration test (SPT). In situ test and suction data was analyzed and recommendations for interpreting results of aforementioned in situ tests when soil moisture conditions fluctuate were made. Finally, a method was developed through which bearing capacity and settlement of shallow foundations on unsaturated fine-grained soils can be predicted based on pressuremeter results. To achieve the objectives of this project the following tasks were completed: 1) a detailed literature search was performed to gather information regarding in situ tests in unsaturated soil, model bearing capacity tests on unsaturated soils, and predicting shallow foundation behavior in unsaturated soil with the pressuremeter. 2) Established two field test sites to perform field tests including the pressuremeter, cone penetration test, and standard penetration while monitoring weather conditions and soil volumetric moisture content. Performed in situ tests, collected soil samples, and observe field sites over a two-year period. 3) Performed soil tests to identify engineering properties of the soil for both field sites and developed soil water characteristic curves for soils involved in the research to establish the relationship between the soil moisture content and suction. 4) Conducted extensive analysis of the results of in situ tests to establish empirical relationships. 5) Conducted extensive theoretical analysis of the pressuremeter and CPT data using cavity expansion theory for unsaturated soils. 6) Constructed a testing chamber and load frame such that model bearing capacity tests on unsaturated soils could be performed. Constructed a miniature pressuremeter for testing soil in the chamber. 7) Performed pressuremeter and model bearing capacity tests on unsaturated soil and conducted extensive analysis of the data. The major contributions of this research include: 1) a unique and valuable dataset including the results of PMTs at two fine-grained soil sites, with accompanying measurements of soil moisture content determinations, total suction determinations, and other supporting laboratory test data was developed. Similar datasets were developed for CPTs and SPTs. 2) Empirical relationships between matric suction and pressuremeter parameters including: limit pressure, pressuremeter modulus, and unload-reload modulus were developed for different soil types encountered at the test sites. In addition, relationships including the limited data that could be extracted from the literature, were developed, which expand the knowledge gained through the current study. 3) Empirical relationships between matric suction and CPT parameters including: tip resistance, and sleeve friction, were also developed for different soil types encountered at the test sites. Further empirical relationships between matric suction and standard penetration test N-value and N1(60) were also developed. 4) The use of unsaturated cylindrical cavity expansion theory for prediction of limit pressures under fluctuating soil moisture conditions was further investigated. This involved developing a theoretically based approach to estimating limit pressure as a function of changing suction. In a similar manner to the PMT, a method to apply unsaturated spherical cavity expansion theory for prediction of tip resistance under fluctuating soil moisture conditions was provided. 5) A unique and incredibly valuable dataset was obtained by conducting model scale footing tests and miniature pressuremeter tests in unsaturated soil prepared to different moisture conditions. The dataset includes load-displacement curves from five footing tests, pressuremeter test curves from all five test beds, water content and total suction data from test beds as well as other supporting laboratory data. Observations from this testing were extremely valuable in providing insight into the use of PMT for shallow foundation analysis in unsaturated soil