The Determination of the Preconsoli'dation Load and Its Practical Significance

I References, J Hamilton, C Crawford, Iii Lowe, J Zaccheo, P Feldman, H Smith, R Wahis, H Wissa, A Christian, J Davis, E Heiberg (+3 others)
1959 Journal of the Soil Mechanics and Foundations Division   unpublished
ABSTR ACT: Controlled-gradient (CG), constant-rate-of-strain (CRS), an d conventional incremental-l oading (STD) consolidation testing are compared and evaluated. Undisturbed samples of tbree soils common to Kentucky were used in the testing program. Results of I 5 CG, 14 CRS, and 32 STD consolidation tests are evaluated. Feasibility of the new test methods for routine testing is briefly discussed and recommendations are made for refi nements in testing procedures. The foundation design of a
more » ... tion design of a structure requires a reliable estimate of the magnitude and rate of settlement. Buildings and bridges must be designed to withstand such estimated differential and total settlements. Highway embankments must be designed to minimize settlements that produce uneven road surfaces and pavement distress. Information needed to estimate magnitude and rate of settlement is obtained by laboratory consolidation tests of soil samples taken from the proposed site. For tbe past 40 years, specimens from such samples have been subjected to standard incremental-l oading tests where increasingly large increments of load, and the resulting measured deformations, have been used to estimate settlement. This laboratory test requires approximately two weeks and yields information which requires much interpretation. These shortcomings eventually led to the development of controlled-gradient (CG) and constant-rate-of-strain (CRS) consolidation tests. Soil is not a homogeneous material having easily defi ned engineering properties, and different methods of testing may yield different values of soil properties. The objectives of this research are (I) to compare test data obtained from CG and CRS tests with those obtained from conventional incremental-loading (STD) tests, and (2) to determine the feasibility of using CG and ( or) CRS testing in routine investigations. The CRS test was first described in 1959 by Hamilton and Crawford (I) as a rapid means of determining tbe preconsolidation pressure, P c · In the CRS test, imposed boundary conditions are similar to those in the STD test, but with one-way drainage. The specimen is confi ned laterally by the same type of ring used in the conventional test apparatus (oedometer), and drainage of pore water is permitted at the top only. In the original CRS test, however, the specimen is loaded at a constant rate of strain instead of incrementally. The strain rate is chosen such that " signifi cant" pore pressure does not develop in the specimen; thus, effective stress is assumed equal to the applied stress. Continuous stress-strain points provided a well-defmed stress-strain curve; this is not possible in the STD test. Hamilton and Crawford pointed out that, in the STD test, gas bubbles in test specimens accounted for most of tbe initial compression observed at loads below P c · Lowe, et a!., (2} agreed with their fmdings