Soil Mechanics Lab: Unconfined Compression Test

Experiment: Unconfined Compression Test on Undisturbed Sample

1. OBJECTIVE

The objective of this experiment is to determine the unconfined compressive strength (q_u) of an undisturbed cohesive soil sample, which is then used to estimate the undrained shear strength (c_u) of the soil.

2. APPARATUS REQUIRED

• Unconfined compression test apparatus (loading frame)
• Proving ring with a dial gauge
• Deformation dial gauge
• Undisturbed soil sample (cylindrical)
• Vernier calipers
• Weighing balance
• Oven
• Moisture cans

3. THEORY

The unconfined compression test is a specific type of unconsolidated-undrained triaxial test where the confining pressure (σ₃) is zero. This test is applicable only to cohesive soils, such as saturated clays, that can stand unsupported without confinement.

In this test, a cylindrical soil sample is subjected to an increasing axial load until it fails. The axial stress at the point of failure is defined as the unconfined compressive strength (q_u).

For a purely cohesive soil, the shear strength is considered to be independent of the normal stress (ϕ_u=0). The Mohr-Coulomb failure envelope is a horizontal line. From the Mohr’s circle for the unconfined compression test, the major principal stress (σ₁) is the axial stress at failure (q_u), and the minor principal stress (σ₃) is zero.

The radius of the Mohr’s circle represents the shear strength (c_u). Therefore, the relationship between the undrained shear strength and the unconfined compressive strength is:

This test provides a quick and simple method for determining the undrained shear strength of cohesive soils.

4. PROCEDURE

1. An undisturbed cylindrical soil sample was obtained. Its initial height (h₀) and diameter (D₀) were measured using Vernier calipers. The initial weight of the sample was also recorded.
2. The initial cross-sectional area (A₀) and volume (V₀) of the sample were calculated.
3. The soil sample was placed centrally on the bottom plate of the compression testing machine.
4. The upper plate was adjusted to make contact with the top of the sample. The readings of the proving ring dial gauge and the deformation dial gauge were set to zero.
5. A compressive load was applied to the sample at a constant rate of strain.
6. Simultaneous readings of the deformation dial gauge and the proving ring dial gauge were taken at regular intervals of deformation.
7. The load was applied until the sample failed, indicated by the development of failure cracks or a decrease in the proving ring reading.
8. After the test, a small sample of the soil was taken from the failure zone to determine its water content.

5. OBSERVATION AND CALCULATIONS

1. Initial Sample Properties:

• Initial Height of sample (h₀) = 7.5 cm = 75 mm
• Initial Diameter of sample (D₀) = 3.8 cm = 38 mm
• Initial Area of sample (A₀) = π/4 D₀² = π/4 (3.8)² = 11.34 cm² = 1.134×10^-3 m²
• Initial Volume of sample (V₀) = A₀ × h₀ = 11.34 × 7.5 = 85.05 cm³

2. Water Content Determination:

• Weight of empty can (W₁) = 7.407 g
• Weight of can + wet soil (W₂) = 167.084 g
• Weight of can + dry soil (W₃) = 137.5 g
• Weight of water (W_w) = W₂ – W₃ = 167.084 – 137.5 = 29.584 g
• Weight of solids (W_s) = W₃ – W₁ = 137.5 – 7.407 = 130.093 g
• Water Content (w) = (W_w/W_s) × 100 = (29.584/130.093) × 100 = 22.74%

3. Load and Deformation Data:

The following table summarizes the key readings from the test. The axial stress is calculated on the corrected area (A’) at each strain level.

Axial Strain (ε) = Δh / h₀

Corrected Area (A’) = A₀ / (1 – ε)

Axial Stress (σ) = Load (P) / A’

4. Determination of Unconfined Compressive Strength (q_u):

From the stress-strain curve and the data table, the maximum axial stress sustained by the sample is the unconfined compressive strength.

q_u = 35.75 kPa

5. Determination of Undrained Shear Strength (c_u):

The undrained shear strength is half of the unconfined compressive strength.

c_u = q_u / 2 = 35.75 / 2 = 17.88 kPa

6. RESULT

The unconfined compressive strength (q_u) of the soil sample is 35.75 kPa.

The undrained shear strength (c_u) of the soil sample is 17.88 kPa.

7. DISCUSSION AND CONCLUSION

The experiment was successfully conducted to determine the unconfined compressive strength and the undrained shear strength of the given undisturbed cohesive soil sample. The stress-strain curve shows that the axial stress increased with strain to a peak value, after which it started to decrease, indicating a plastic failure characteristic of a cohesive soil.

The value of undrained shear strength (c_u) obtained is a crucial parameter in geotechnical engineering for assessing the short-term stability of foundations, slopes, and embankments on clayey soils.

Potential sources of error in this experiment could include:

• Disturbance of the “undisturbed” sample during sampling and handling.
• The sample ends not being perfectly parallel, leading to non-uniform stress distribution.
• Errors in measuring the dimensions of the sample.
• Not maintaining a constant rate of strain during the application of the load.

In conclusion, the unconfined compression test provides a simple and rapid method for evaluating the shear strength of cohesive soils. The results obtained are valuable for preliminary design and analysis in geotechnical projects.