S2845 Controlled Gradient Consolidation
ASTM D2435, D4186; AASHTO T216
Geotest offers 2 systems for closed loop consolidation testing. The first, S2845, is similar to S2843 (see related items) but has the additional capability of performing controlled pore pressure gradient tests. In this procedure, any excess pore pressure starting from extreme low values can be selected. An electronic comparator, by
regulating the normal load, automatically maintains a constant excess pore pressure for the duration of the test. Strain and pore pressure, each with its own output, allow visual monitoring of all parameters and automatic data acquisition. The controlled gradient (CG) test offers advantages over conventional staged or controlled rate of
strain (CS) tests: Tests are performed faster; in-situ conditions can be more easily realized and approximated. The main reason for running a controlled rate of strain (CS) test is to keep a low pore pressure gradient.
S2845 does not require a computer for control. If real time data acquisition is desired, a computer is needed. Staged tests with the same capabilities of S2843 can be performed with S2845. In addition, this apparatus can do Ko tests using S2864 Ko consolidation system.
Incremental Loading Tests
The conventional multistage loading regime is supported by the COMPUCON software package. The operator creates a test file in the “Create Test” mode, containing specific information regarding the number of load increments, drainage conditions, the back pressure and load pressure requirements, and end of increment criteria (maximum deformation or length of time of increment). Single or double drainage conditions may be specified. Under single drainage conditions, excess pore pressure at the base of the sample may be measured. Unloading increments may also be specified. Plots of percent axial strain versus log of time, and percent axial strain versus square root of time, in addition to current EDDI readings, load pressure, back and/or excess pore pressure, elapsed time, and increment number are displayed on the monitor during the course of testing. At the end of a test, selection of the “Print Options” menu allows for various outputs of test data. Plots of percent axial strain versus log of time may be selected for printing. Data files containing measurements converted to axial strain along with the pressure and time data presented in tabular form may be also selected for printing.
Controlled Normal Stress Tests
The operator selects a rate of normal stress application and end of increment criteria based on time or deformation. Load pressure transducer readings allow the pressure regulator to constantly adjust the applied load to meet the desired rate of load increase (or decrease). Run time plots and current data are displayed on the monitor. Data is continually saved to disk for retrieval in the form of plots and tables as with the incremental loading tests.
Controlled Rate of Strain Tests
A rate of axial strain is selected by the operator. After application of a small seating load and back pressure (if desired), EDDI readings are monitored and the computer controls the pressure source to maintain a constant rate of axial strain. A maximum value for the ratio of pore pressure to applied axial stress is selected by the operator. Should this value be exceeded or the induced excess pore pressures at the base becomes immeasurable, a warning will be displayed, and the operator may select to interrupt the test and select another strain rate. The software supports controlled pressures measured at the base of the sample in accordance with ASTM D4186 (Standard Test Method for One-Dimensional Consolidation Properties of Soils Using Controlled-Strain Loading). The operator may select an option for secondary compression assessment. In this case, the constant strain test is interrupted at a pre-selected load. The load is held constant and excess pore pressure at the base, percent axial strain and elapsed time are recorded. Furthermore, rebound behavior may be assessed. An axial strain rate equivalent to that during the loading portion of the test is maintained by the COMPUCON unit. When rebound is complete, excess pore pressure is allowed to dissipate at constant load or deformation.
Outputs from this test include percent axial strain versus average effective stress, percent axial strain versus log of time for secondary compression increments, and coefficient of compression versus log of average effective stress plots. Run time plots are displayed on the monitor in addition to current data (EDDI reading, percent axial strain, and pressure readings).
Controlled Gradient Tests
Three types of controlled gradient tests are supported by the COMPUCON software. The operator may select to maintain a constant pore pressure gradient or maintain the excess pore pressure measured at the base of the sample at a constant value, or maintain the ratio of excess pore pressure to applied normal load. As with the other loading regimes supported by the COMPUCON software, a number of increments may be selected, with end criteria based on time, load or deformation. Run time plots and current data are displayed on the monitor and test data may be plotted or printed in tabular form at the end of the test via the “Print Options” menu.
The standard apparatus comes with accessories including teflon lined ring to prepare and test 2.5" diameter by 1” high specimens. Maximum load with a 100 psi pressure source is 40 tons per square foot. Higher loads are attainable if pressure source is increased. Samples up to 4" in diameter can be tested. PC and interface cards, although not included, can be supplied as an option.
Specifications for S2843, S2845 and S2847:
|Sample size: 2.5" *|
|Load capacity (100 psi supply): 40 tsf**
|Minimum load (S2843): 1/64 tsf|
|Minimum load (S2845, S2847): 1/4 tsf
|Load readout precision: 1 part in 5,000 full scale|
|Strain readout: 0.5x0.0001" or metric
|Back pressure readout: 0-100 psi|
|Supply pressure readout: 0-200 psi|
|Pore pressure readout: 0-200 psi|
|Electrical requirements: 115VAC (230VAC available)|
|Size: 20x14x24" high (without sliding reservoirs)|
* Other sizes available: 60 and 100mm, 4"
** 80 tsf load capacity is an available option
S2843 Digicon Consolidation Apparatus
S2900 CRS Consolidation Cell