**Introduction**

In a consolidated undrained (CU) triaxial test, the volume of the specimen can either decrease, increase, or remain unchanged during the shearing phase. The behavior of the volume change depends on the initial state of the sand, the applied stress conditions, and the characteristics of the sand particles.

**Factors influencing volume change**

Several factors influence the volume change during the shearing phase of a consolidated undrained triaxial test on loose sand:

1. **Initial void ratio**: The initial void ratio of the sand specimen plays a significant role in determining the volume change. If the sand is initially loosely packed with a high void ratio, it is more likely to undergo volume decrease during shearing. On the other hand, if the sand is initially densely packed with a low void ratio, it may experience volume increase during shearing.

2. **Applied confining pressure**: The applied confining pressure also affects the volume change. If the confining pressure is low, the sand particles have more freedom to rearrange and settle, leading to volume decrease. Conversely, if the confining pressure is high, the sand particles are forced closer together, causing volume increase.

3. **Particle shape and size distribution**: The shape and size distribution of the sand particles can also influence the volume change. Irregularly shaped particles with a wide range of sizes tend to interlock and create a denser packing, resulting in volume increase during shearing. Conversely, uniformly sized and rounded particles may allow for volume decrease due to particle rearrangement.

**Volume change behavior**

Based on the above factors, the volume change during shearing in a consolidated undrained triaxial test on loose sand can be categorized as follows:

1. **Decrease in volume**: If the sand specimen is initially loosely packed with a high void ratio and/or subjected to low confining pressures, it is more likely to undergo volume decrease during shearing. This volume decrease is mainly due to particle rearrangement and settling.

2. **Increase in volume**: If the sand specimen is initially densely packed with a low void ratio and/or subjected to high confining pressures, it may experience volume increase during shearing. This volume increase is primarily caused by particle reorientation and dilation.

3. **Unchanged volume**: In some cases, the volume of the sand specimen may remain relatively unchanged during the shearing phase. This typically occurs when the initial void ratio and confining pressure are such that there is an equilibrium between particle rearrangement and dilation.

**Conclusion**

In a consolidated undrained triaxial test on loose sand, the volume of the specimen during the shearing phase can either decrease, increase, or remain unchanged. The volume change behavior depends on various factors such as the initial void ratio, applied confining pressure, and particle characteristics. It is essential to consider these factors when interpreting the results of a consolidated undrained triaxial test on loose sand.