In civil engineering, it is very important to ensure that the soil under a building can handle the weight safely.
One of the most significant laboratory techniques employed to assess the strength and deformation behavior of the soil is the Triaxial Test. From constructing foundations for buildings, roads, and embankments to areas prone to earthquakes, this test assists engineers in making vital design decisions.
What is a Triaxial Test in soil testing?
Triaxial test is a laboratory test that is conducted on soil to determine its mechanical response in a controlled environment of stress and drainage.
In the test, pressure is applied on a cylindrical sample from the sides (confining pressure) and subsequently loading vertically (axial stress) until the sample fails.
There are two primary types of triaxial tests:
The principal parts of a triaxial testing equipment are:
Step-by-Step Description of Static Triaxial Test of Soil Mechanics
The static triaxial test is commonly employed to find out the soil's shear strength. It replicates the way soil is under all directions of pressure deep in the ground and even a heavier load from the top — exactly like in actual construction work.
Step 1: Sample Preparation
Take a representative sample of soil from the location (undisturbed or remoulded).
Cut and trim it to a standard cylindrical shape, commonly with a diameter of 38 mm and height of 76 mm.
Seal the cylinder with a thin rubber membrane with the help of a membrane stretcher so that there is no entry or loss of water during the test.
Preparation of the sample ensures equal distribution of stress during the test and keeps the sample intact.
The rubber membrane serves as a seal, keeping water out of the sample in the chamber.
Step 2: Mounting in Triaxial Cell
Put the soil sample prepared vertically within the chamber of triaxial testing.
Insert porous stones at the bottom and top to provide drainage if necessary.
Lock the setup and start filling the chamber with water to provide confining pressure evenly around the sample.
The confining pressure mimics the in-situ earth pressure that the soil would undergo under natural ground conditions. It aids in replicating the way the soil would behave when surrounded by other layers of soil underground.
Step 3: Saturation of the Soil
Slowly add water pressure to the soil sample to displace any air trapped inside.
Utilize a method referred to as back pressure saturation, where water is added from both ends of the sample.
Watch the B-value; if it goes to 0.95 or higher, the soil is saturated.
Air in the soil affects test results. Saturation guarantees pore water pressure measurements are valid, particularly relevant in undrained and consolidated tests where water movement is critical.
Step 4: Soil Consolidation
After saturation, hold confining pressure without loading any vertical (axial) load.
Let the soil consolidate naturally under this stress. Check for volume change — when it's steady, the sample is consolidated.
This is an imitation of long-term conditions under which soil may settle naturally beneath structures. It's particularly significant in clay soils, which are slow to adapt to imposed stress.
Step 5: Axial Load Application
Begin applying vertical load from the top with a motorized piston.
Apply load at a constant rate of strain (typically 0.5–1% per minute).
Keep adding load until the soil fails — reflected in bulging, cracking, or sudden strain increase.
This step is used to determine the ultimate strength of the soil under compression. It's critical in designing foundations and forecasting how the soil will perform under bearing heavy loads.
Step 6: Recording Test Data
During loading, record:
One of the most significant laboratory techniques employed to assess the strength and deformation behavior of the soil is the Triaxial Test. From constructing foundations for buildings, roads, and embankments to areas prone to earthquakes, this test assists engineers in making vital design decisions.
What is a Triaxial Test in soil testing?
Triaxial test is a laboratory test that is conducted on soil to determine its mechanical response in a controlled environment of stress and drainage.
In the test, pressure is applied on a cylindrical sample from the sides (confining pressure) and subsequently loading vertically (axial stress) until the sample fails.
There are two primary types of triaxial tests:
- Static Triaxial Test – Extends slow and continuous vertical loading to measure the strength of the soil.
- Cyclic Triaxial Test – Subjects reacted loading to replicate earthquakes or traffic vibrations.
The principal parts of a triaxial testing equipment are:
- Soil specimen: Cylindrical sample, usually 38 mm × 76 mm.
- Triaxial cell: Chamber that induces confining pressure.\
- Loading frame: Impresses axial load by way of a piston.
- Pore pressure system: Measures water pressure within the soil.
- Data logger: Stores stress, strain, and pore pressure during the test.
Step-by-Step Description of Static Triaxial Test of Soil Mechanics
The static triaxial test is commonly employed to find out the soil's shear strength. It replicates the way soil is under all directions of pressure deep in the ground and even a heavier load from the top — exactly like in actual construction work.
Step 1: Sample Preparation
Take a representative sample of soil from the location (undisturbed or remoulded).
Cut and trim it to a standard cylindrical shape, commonly with a diameter of 38 mm and height of 76 mm.
Seal the cylinder with a thin rubber membrane with the help of a membrane stretcher so that there is no entry or loss of water during the test.
Preparation of the sample ensures equal distribution of stress during the test and keeps the sample intact.
The rubber membrane serves as a seal, keeping water out of the sample in the chamber.
Step 2: Mounting in Triaxial Cell
Put the soil sample prepared vertically within the chamber of triaxial testing.
Insert porous stones at the bottom and top to provide drainage if necessary.
Lock the setup and start filling the chamber with water to provide confining pressure evenly around the sample.
The confining pressure mimics the in-situ earth pressure that the soil would undergo under natural ground conditions. It aids in replicating the way the soil would behave when surrounded by other layers of soil underground.
Step 3: Saturation of the Soil
Slowly add water pressure to the soil sample to displace any air trapped inside.
Utilize a method referred to as back pressure saturation, where water is added from both ends of the sample.
Watch the B-value; if it goes to 0.95 or higher, the soil is saturated.
Air in the soil affects test results. Saturation guarantees pore water pressure measurements are valid, particularly relevant in undrained and consolidated tests where water movement is critical.
Step 4: Soil Consolidation
After saturation, hold confining pressure without loading any vertical (axial) load.
Let the soil consolidate naturally under this stress. Check for volume change — when it's steady, the sample is consolidated.
This is an imitation of long-term conditions under which soil may settle naturally beneath structures. It's particularly significant in clay soils, which are slow to adapt to imposed stress.
Step 5: Axial Load Application
Begin applying vertical load from the top with a motorized piston.
Apply load at a constant rate of strain (typically 0.5–1% per minute).
Keep adding load until the soil fails — reflected in bulging, cracking, or sudden strain increase.
This step is used to determine the ultimate strength of the soil under compression. It's critical in designing foundations and forecasting how the soil will perform under bearing heavy loads.
Step 6: Recording Test Data
During loading, record:
- Axial strain (vertical deformation of the sample).
- Deviator stress (axial-confined pressure difference).
- Pore water pressure (particularly in tests undrained).
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