Consolidation Testing

ERDC TN-DOER-N5

July 2000

Ideally, the vane shear test would be conducted in the field to prevent any change in strength that

might be associated with any sampling procedure used in this soft material; however, if relatively

undisturbed core samples are collected, laboratory vane shear tests can be run to provide an

indication of shear strength. Laboratory tests are often more feasible because of logistics and cost

considerations. ASTM D 2573 (ASTM 1999a) and D 4648 (ASTM 1999b) give the procedures for

field and laboratory vane shear tests, respectively.

A vane strength is determined on the undisturbed material. The test is performed by inserting a

vane of known dimensions into the sample so that the top of the vane is at least one vane length

deep in the soil and rotating the vane at the prescribed rate of rotation; the rotation rate is then

correlated to soil strength. A remolded strength is subsequently determined on each sample. The

remolded strength is obtained by performing the original test to failure, and then rotating the blade

through five revolutions and re-performing the test at that same testing location. By so doing, the

material tested is the remolded sediment at the edges of the vane.

For a recent project for New York District, a laboratory vane shear test device was used to obtain

shear strength values for the materials collected at the Mud Dump, New York District's former

offshore disposal site. Two vanes were used in the testing device for this project; both vanes had

12.7-mm- (0.5-in.-) diam blades but the lengths of the blades were different (12.7 and 25.4 mm (0.5

and 1 in.)). The size of the blade used was determined by the consistency of the material to be

tested; the larger blade was used for softer materials. The rate of shearing was set in accordance

with standard specifications. To run the vane shear test, sections from the cores were selected for

testing at specified locations. Sections 76.2 to 101.6 mm (3 to 4 in.) long were cut from the cores,

and the material was tested while in the coring tube. By testing the material in the tube, additional

disturbance of the material was minimized and potential problems with the material not being able

to stand under its own weight were eliminated.

Consolidation Testing. Laboratory consolidation testing of soft soils often requires use of two

types of consolidation tests. Both a modified version of the standard oedometer (consolidation) test

and a self-weight test must often be conducted; these tests provide data for the low and high ends

of the anticipated range of void ratios, respectively. However, on relatively firm dredged materials

that are mechanically dredged, use of oedometer testing alone may suffice.

For consolidation testing of soft materials, a modification to the ASTM D 2435 (standard oedometer

test) (ASTM 1999c) loading sequence is required, as outlined in Appendix D of Engineer Manual

(EM) 1110-2-5027 (Headquarters, USACE, 1987). A lighter loading sequence is necessary on soft

sediments because the material cannot sustain the normal set of loads; i.e., the bearing capacity of

the dredged material is too low. Maximum void ratios that can be tested in the oedometer are 5 to 6.

If the void ratios in the field deposit will exceed those that can be simulated in the oedometer test,

then a self-weight test will be required to provide compressibility data at the higher void ratios. This

test allows a slurry of dredged material to undergo self-weight consolidation in the 152.4-mm-

(6-in.-) diam, 304.8-mm- (12-in.-) high consolidometer. Deformation measurements are taken over

time, and the device is then disassembled for incremental sampling of the specimen. Typical void

ratios encountered in the specimen after completion of consolidation range from 5 to 12 (from