Shear strength measurements are performed to test sediments and rocks to determine their stress-strain-time behavior. Some materials are brittle and exhibit little stress when strained (rocks). Others are work-hardening (e.g., compacted clays and loose sands) or work-softening. In the clayey, soft, saturated marine sediments often measured for strength, stress decreases as the sediment is strained beyond a peak stress. The sediment yields (fails) at the peak stress, which can be defined as the sediment's strength. Shear strength or shear resistance of sediments is the most important aspect of slope stability. However, the shear strength values obtained by ODP on the JOIDES Resolution do not alone allow any slope stability analysis. They represent a relative strength profile.
During ODP, several different instruments were used to collect shear strength data: Motorized Torque Transducer, AVS, Wykeham-Farrance spring-type device, hand-held Torvane, and SoilTest Pocket Penetrometer. The tests were run on whole-core samples (in the end of the core), split-core samples (in the split-core surface), or remolded sediment samples.
Two types of strength tests were conducted on board during ODP: vane shear and penetrometer tests. Instruments used to conduct vane shear tests include the Motorized Torque Transducer, AVS, hand-held Torvane, and the Wykeham-Farrance device. Undrained shear strength was determined using a vane inserted into soft sediment and rotated until the sediment failed. The torque required to shear the sediment was related to the shear strength of the material. The pocket penetrometer measured compressive strength. The vertical strain measurement taken by the penetrometer can be related to shear strength; however, the strain value must be divided by 2 to obtain shear strength.
The AVS was the only shear data type that was collected by a personal computer–based data acquisition program. The first documented program for the AVS system was deployed during Leg 154. The next major revision of the AVS data acquisition program was used to collect data beginning with Leg 170. This revision was in preparation for deployment of the Janus database. Measurements made with the Torvane and the penetrometer had to be documented by the scientists or technicians doing the test.
There was no general calibration of the AVS system. However, vane dimensions and spring constants were important coefficients that were used to calculate strength. Those items were occasionally calibrated after initial calibration at purchase. The spring constant was a value relating the deflection angle to torque.
Additional information about ODP shear strength measurements can also be found in Chapter 9 of Technical Note 26 (Blum, 1997).
Early in ODP, shear strength data were collected on log sheets which were sent to ODP/TAMU at the end of each cruise. The data were entered into the S1032 database and the log sheets were microfilmed for archival storage. Data entry routines were implemented so that data entry could be done on the ship and the practice of collecting data on log sheets ended. Strength data were stored in the S1032 database through Leg 146. Starting around Leg 144, strength data were also saved in spreadsheet files that were brought back for archival on the ODP/TAMU servers. The AVS data acquisition program generated files with the vane shear data starting around Leg 154, but Torvane and penetrometer data documentation was up the to scientists collecting those data.
The data model for shear strength data can be found in "Janus Shear Strength Data Model" in "Appendix N." Because the data collected on the different instruments were very different, there are three sets of tables that contain strength data: Automated Vane Shear (AVS), Torvane (TOR), and Penetrometer (PEN). Included are the relational diagram and the list of the tables that contain data pertinent to each of these data types, column names, and the definition of each column attribute. ODP Information Services Database Group was responsible for the migration of pre-Leg 171 data to Janus.
AVS, PEN, and TOR analyses can be retrieved from Janus Web using predefined queries. The shear strength (AVS/PEN/TOR) query Web page allows the user to extract data using the following variables to restrict the amount of data retrieved: leg, site, hole, core, section, run numbers, depth ranges, or latitude and longitude ranges. In addition, the Shear Strength query gives the user options to retrieve data by instrument and output the raw data. Raw data could only be retrieved if the raw data were saved during data acquisition and consists of the individual torque and strain values.
Table T32 contains the data fields retrieved from the Janus database using the Janus Web predefined query with the output raw data option. The first column contains the data item, the second column indicates the Janus table or tables where the data are stored, and the third column is the Janus column name or the calculations used to produce the value. "Description of Data Items from Shear Strength Query with Output Raw Data Option" in "Appendix N" contains additional information about the fields retrieved and the data format for the archived ASCII files.
Verification of all shear strength data sets was not completed because of time constraints. The variety of instruments and the lack of data acquisition programs for data collection made it more difficult to verify the measurements. To complete the verification, more research would be necessary to find the documentation about equipment and procedures used to collect these data and verify the data using log sheets and comparison with the reported data in the Initial Reports volumes.
Most data collected after the Janus database was operational on Leg 171 were verified as part of the Janus data management and verification procedures (see "Janus Data Management and Verification"). Occasionally, Torvane and penetrometer data were collected, but data files were not transferred to ODP/TAMU at the end of the leg. Some verification was done on the pre-Leg 171 data; however, if there is a discrepancy between the database and data in the Initial Reports volumes, the published data should be considered more reliable.