The objective of the research is to investigate the dynamic behavior and liquefaction potential of inorganic and organic silts commonly found in Rhode Island. This will involve a detailed sampling and laboratory testing program to determine the environmental conditions (stress state, cycles of loading) and compositional factors (void ratio, plasticity, fines content) necessary for liquefaction and cyclic mobility in these soils. Specific objectives of the research include the following:
1. Obtain high quality piston and block samples of inorganic and organic silts for laboratory testing. Sampling locations will be chosen after consultation with RIDOT personnel and local geotechnical engineering firms such as GZA Geoenvironmental, Inc., The Maguire Group, Aldinger and Associates, and others. Different commercially available samplers will be used to determine what is the best equipment for sampling these very difficult soils.
2. Evaluate the quality of the samples for advanced geotechnical testing us-ing established techniques from the literature (Lunne et al., 1997; Terzaghi et al., 1997).
3. Perform cyclic triaxial tests on both ‘undisturbed’ and reconstituted samples of inorganic and organic silts. Funds are requested to obtain a state-of-the-art triaxial testing apparatus and a fixed piston sampler.
4. Develop relationships between the cyclic stress level required for liquefac-tion and the number of cycles of loading for these soils. This information can then be related to relative density and (N1)60 values to quantify the cy-clic resistance ratio of the silt.
Highway
This is to study the liquefaction potential of inorganic and organic silts under earthquake loads. These soils are prevalent in Rhode Island and in many urban areas, and there is no clear evidence about how these soils will behave during a design earthquake. This research will involve a detailed laboratory test-ing program involving cyclic triaxial tests and a sampling program involving fixed piston and block samples. A careful sampling program is critical for this type of study because of the extreme difficulty in obtaining undisturbed samples of satu-rated silts. The results of this research will have a direct impact on the seismic design of surface transportation infrastructure, such as highway bridges and em-bankments, built on the organic and inorganic silts of Rhode Island.
The research will be accomplished through the following tasks.
Task 1 Literature Review
A literature review of the current understanding of the liquefaction potential of silts will be performed. Sources of information will include the geotechnical literature, the research of the late Dr. Vito Nacci of the Civil Engineering De-partment at URI, and the geotechnical data reports associated with the Route 195 realignment project. Detailed Standard Penetration Test data will also be collected from the URITC sponsored digital GIS catalog of borehole data for the future development of correlations with the laboratory testing data.
Task 2 Identify Sampling Locations
Locations for sampling will be chosen based on a review of existing soils data in Rhode Island and consultation with various sources. Existing soils data will be obtained from RIDOT projects such as the relocation of Route 195 (GZA 2000; Haley and Aldrich 2000) and the Wellington Avenue Bridge project, and from the borehole database. Meetings will be held with RIDOT personnel as well as area geotechnical consultants to identify potential sites. A tentative list of companies includes Paul B. Aldinger and Associates, Inc., Guild Drilling Co., Inc., GZA Geoenvironmental, Inc., Maguire Group, Inc., and Pare Engi-neering Corporation. The state geologist, Dr. Jon Boothroyd, will also be consulted. It is estimated that 3 to 5 locations will be chosen for piston sam-pling and 1 site will be chosen for block sampling.
Task 3 Acquire Cyclic Triaxial Apparatus and Fixed Piston Sampler
The acquisition of a fully automatic cyclic triaxial apparatus will allow for a direct measure of the dynamic behavior and lique-faction potential of the Rhode Island silts. This state-of-the-art equipment in-cludes automated back pressure saturation of samples, anisotropic consolida-tion, full stress-path testing, and cyclic loading.
Special sampling equipment is also required because of the extreme difficulty in obtaining “undisturbed” samples of silt. It is very important to have high quality samples to capture the in situ structure and behavior of the material. A fixed piston sampler, also known as an Osterberg or Hvorslev sampler, is a thin-wall tube sampler with a piston that immobilizes the top of the sample as the tube is pushed into the soil. This type of sampler provides a better vacuum on the sample during removal than a standard Shelby tube and results in significantly higher quality samples.
Task 4 Obtain “Undisturbed” Samples of Inorganic and Organic Silts
At each location identified in Task 2, at least one fixed piston tube will be pushed into both the organic and inorganic silts. Traditional Shelby tubes will also be used at these locations to assess sample quality with the different sampling techniques.
A block sample of the inorganic silts will be obtained if an appropriate construction project can be identified in Task 2. A project is needed where an excavation will be made into the silts. Personnel from URI will travel to the site and excavate by hand a 1 ft3 block sample. This type of sampling yields very high quality samples. Samples obtained from both blocks and thin-walled tubes will be used for the cyclic triaxial testing program and for physical property determination.
Task 5 Perform Cyclic Triaxial Testing on “Undisturbed” and Reconstituted Samples
Cyclic triaxial tests will be performed to determine the stress levels and cycles of loading that will cause liquefaction or amplification of ground motions. Sample quality will be assessed by testing “undisturbed” samples obtained from the proposed fixed piston sampler, standard thin-walled samplers (Shelby tubes), block sample, and samples reconstituted in the laboratory. It is estimated that, depending on the extent of the field sampling program, ap-proximately 20 to 35 triaxial tests will need to be performed.
Task 6 Analysis of Test Results
The results of the laboratory tests will be analyzed to determine the dynamic properties of the silts and quantify their cyclic resistance. Relationships of cyclic stress ratio vs. the number of cycles of loading to cause liquefaction will be developed.
Task 7 Progress reports and final report
Progress reports will be submitted to the URITC and RIDOT on a quarterly basis that detail the tasks accomplished throughout the research project. A final report will be presented at the end of the project.
This project will begin in the summer, 2003.
$315,475.00 ($180,973.00 Yearly)
There will be significant student involvement in this research both through graduate research and undergraduate employment. The research is intended to fulfill the requirements of a Ph.D. for a graduate student in either the Ocean Engineering or Civil and Environmental Engineering Depart-ment. In addition, the sampling portion of this research can be incorporated into field trips for both Civil and Ocean Engineering courses that involve geotechnical engineering.
There is currently no research underway in Rhode Island to study the liquefaction potential of inorganic and organic silts. This is despite the fact that these deposits underlie significant areas of the city of Providence and other urban areas in the United States. An important aspect of the proposed research that differs from most studies of this type is the focus on high quality piston and block sampling to capture the in situ structure of the soil. This can be extremely difficult in silts, and most comparable research projects perform tests on samples reconstituted in the laboratory.
The results of this research will be compiled into a report that will be presented to the URITC and RIDOT at the end of the study. Findings will also be published in the geotechnical literature such as the TRB Journal and Conference Proceed-ings, ASCE Journal of Geotechnical and Geoenvironmental Engineering, and the ASTM Geotechnical Testing Journal.
The results of this research will provide valuable information for Providence and the broader geotechnical community about the seismic properties of inorganic and organic silts and their cyclic resistance to earthquake loading. Specifically, this research will attempt to answer the question of whether or not the silts will liquefy or experience significant losses in strength during a design earthquake.
Liquefaction, silts, earthquakes, dynamic properties of soils, cyclic triaxial testing, in situ sampling