Consolidation
Consolidation is the gradual reduction of volume of soil due to drainage of some of its pore fluid (generally water). Consolidation continues until all the excess pore water pressure that was generated by an increase in total stress is completely dissipated. Swelling is the reverse of consolidation where the volume of soil gradually increases due to a negative pore water pressure. Consolidation settlement can occur due to reasons such as the construction of a structure with over-saturated clay layers. Heaving, on the other hand, will occur due to reasons such as swelling of the clay layer on the bottom of an excavation. Calculating the rate and magnitude of consolidation settlement and swell heave is critical to ensure the safety of geo-structures.
The simplest way of studying consolidation and swell is through one-dimensional a consolidation test where the soil is laterally constrained (zero lateral deformation is allowed). This way the volume change is equal to the vertical deformation of the soil’s surface only. The Oedometer test is the most commonly used test to determine the consolidation and swell parameters of soil specimens during one-dimensional consolidation.
In an oedometer, disc shape specimen is held inside a rigid ring with a smooth surface to provide zero lateral strain condition (where the ratio of lateral effective stress to vertical effective stress is K0- the coefficient of earth pressure at rest) with porous stones on top and bottom of it providing drainage paths on vertical direction. The top porous stone has a slightly smaller diameter than the specimen and can move inside or outside the ring with the specimen. The bottom porous stone is situated on the pedestal which is also very rigid. The ring can be fixed (on the pedestal) or floating with the specimen. The more widely used oedometer setup is the one with a fixed ring.
The specimen is generally submerged in water during the test. In order to determine the consolidation and swell characteristics of soil specimen vertical load is applied incrementally with specific amplitudes and duration and the resulting displacement is measured and recorded in specific time intervals (ASTM D2435). The test can be conducted following two methods (methods A and B as described in ASTM D2435).
There are other types of oedometers that support backpressure saturation of the specimen or constant rate of consolidation instead of incremental loading but the cells are similar in principle.
In order to run a one-dimensional consolidation test the following devices and accessories are needed:
A cell made out of rigid material that is resistant to corrosion (e.g. Stainless steel) with minimal compression, porous stones with appropriate permeability, a cutting ring to hold the specimen, a top cap, and a solid clamping system to hold the ring in place in case of fixed ring consolidation.
● Orvalis (TM) Oedometer Accessories and consumables:
Includes caps and pedestals, Porous Stones made from different materials and different porosity and permeability, alignment screws, volume change measurement accessories and etc.
● Specimen preparation kits:
Consists of a Cutting Ring, extruder, lathe and etc.
● Load/Displacement/Pressure/volume measurement sensors/gages and data logging devices.
● Load frame or dead-weight consolidation frame.
● Pressure and volume control units in case of back-pressure saturation and constant rate of consolidation test.
To inquire about Load frames and Sensors click here.