---
title: "Triaxial test pros, cons, procedure"
subject: "Soil Mechanics"
chapter: 10
key: "semester_4/soil_mechanics:triaxial-advantages-procedure-parameters"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/triaxial-advantages-procedure-parameters
site: BCT Engineering
---

# Triaxial test pros, cons, procedure

Soil Mechanics · Chapter 10: Shear Strength of Soil

- Asked in: 2078 Poush
- Video: [Triaxial Shear Test | Elementary Engineering](https://www.youtube.com/watch?v=Vu160KZbNxw) — Elementary Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/triaxial-advantages-procedure-parameters
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10

## Question

What are the advantages and disadvantages of a triaxial compression test? Briefly explain how you conduct the triaxial test and compute the shear parameters for the soil from the test data.

## Short answer

Advantages include controlled drainage, weakest-plane failure, and uniform stress; disadvantages are high cost, long duration for clays, and axisymmetric stress. The test consolidates a membrane-covered specimen under cell pressure, then shears axially; shear parameters c and φ are found from the common tangent to Mohr circles from multiple tests at different confining pressures.

## Answer

The **triaxial compression test** applies an all-round cell pressure $\sigma_3$ to a cylindrical specimen and then increases the axial stress until the sample fails.

#### Advantages

- Drainage conditions can be controlled and pore pressure measured.
- Failure takes place on the weakest plane; stresses are fairly uniform.
- Suitable for all soil types and for different stress paths.
- Effective and total stress parameters can both be obtained.
- Volume change and strain can be recorded.

#### Disadvantages

- Equipment is costly, and the test needs a skilled operator.
- CD and CU tests take a long time (days for clays).
- Stress state is axisymmetric ($\sigma_2 = \sigma_3$), which is not the real condition for plane-strain problems such as strip footings and slopes.
- Sample disturbance during preparation, and end restraint from the platens.
- Preparation of an undisturbed sample is difficult in sand.

#### Procedure

1. Prepare a cylindrical specimen (typically 38 mm diameter, 76 mm long), cover it with a rubber membrane and place it between porous discs on the pedestal.
2. Fill the cell with water and apply the cell pressure $\sigma_3$.
3. For CU and CD, allow consolidation under $\sigma_3$ with drainage open; for UU, keep drainage closed.
4. Apply axial load at a constant rate of strain until failure (or 15 to 20% strain), recording load, deformation and, if required, pore pressure $u$.
5. Repeat on at least three identical specimens at different cell pressures.

#### Computing the shear parameters

For each test, the deviator stress at failure is

$$
\sigma_d = \sigma_1 - \sigma_3 = \frac{P}{A_c}, \qquad A_c = \frac{A_0}{1-\varepsilon}
$$

where $P$ is the axial load and $A_c$ is the corrected area.

1. Calculate $\sigma_1 = \sigma_3 + \sigma_d$ for each test (use $\sigma' = \sigma - u$ for effective stress).
2. Draw a Mohr circle for each test on the $\sigma$-$\tau$ plane with diameter from $\sigma_3$ to $\sigma_1$.
3. Draw the common tangent to the circles, which is the failure envelope.
4. Read the intercept on the $\tau$-axis as $c$ and the slope as $\phi$ (or $c'$, $\phi'$ for effective stress).

Alternatively, $\sin\phi = \dfrac{\sigma_1 - \sigma_3}{\sigma_1+\sigma_3+2c\cot\phi}$, and for $c=0$, $\sin\phi = \dfrac{\sigma_1-\sigma_3}{\sigma_1+\sigma_3}$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is a disadvantage of the triaxial test?
   A. It cannot control drainage
   B. The stress state is axisymmetric (sigma2 = sigma3), unlike plane-strain field problems
   C. It cannot measure pore pressure
   D. It is cheap and quick
   Answer: B. Real strip footings and slopes are plane strain, which the cylindrical specimen does not reproduce. (Key point: Disadvantage)

2. What is the typical procedure?
   A. Apply axial load first then cell pressure
   B. Apply cell pressure sigma3, then increase axial load at a constant strain rate until failure
   C. Apply constant shear force on a fixed plane
   D. Use only one specimen and one cell pressure
   Answer: B. At least three identical specimens at different cell pressures are tested. (Key point: Procedure)

3. How is deviator stress at failure computed?
   A. sigma_d = P x A0
   B. sigma_d = P / A0 without correction
   C. sigma_d = P / A_c, with A_c = A0/(1 - strain)
   D. sigma_d = sigma3 / P
   Answer: C. The cross-section area increases as the sample shortens, so a corrected area is used. (Key point: Deviator stress)

4. How are c and phi found?
   A. Average the sigma1 values
   B. Divide sigma1 by sigma3
   C. Draw Mohr circles from sigma3 to sigma1 and the common tangent; intercept is c and slope is phi
   D. Use the largest circle only
   Answer: C. The common tangent to circles from several tests is the failure envelope. (Key point: Getting c and phi)

## Related in Soil Mechanics

- [UCS test soil type and parameters](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aunconfined-compression-soil-type-mohr) — Soil Mechanics, chapter 10
- [Triaxial advantage over direct shear](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-advantages-over-direct-shear) — Soil Mechanics, chapter 10
- [Triaxial tests by drainage; UCS vs cu](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-tests-by-drainage-ucs) — Soil Mechanics, chapter 10
- [Triaxial drainage conditions in the field](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-drainage-field-conditions) — Soil Mechanics, chapter 10
- [Limitations of direct shear test](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adirect-shear-limitations) — Soil Mechanics, chapter 10
- [Draw UCS and direct shear results](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aucs-direct-shear-results-plot) — Soil Mechanics, chapter 10

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