The paper deals with the special structure, physiochemical characteristics, water-absorbing property, engineering characteristics and characteristics of deformation. Based on the in-depth probes into the relationship of the above mentioned factors and the stability of rock mass, the paper points out the link between stand-up time of engineering rock mass and rock support in red clay rock of a tunnel.


According to the engineering needs, a tunnel has to go through a 700-meter long red clay rock mass. The red clay rock mass belongs to P2h and was formed in the oxygenation environment of the dry climate. Because the red clay rock has special engineering mechanics property, the stability of its rock also has special properties. To ensure the safety and the timely completion of the engineering construction, it is necessary to conduct a comprehensive study on the engineering property of the red clay rock mass and it is also a must to make an analysis of constringency and measurement of rock deformation while the digging of the tunnel is going on.

Physiochemical property

The red clay rock is brick red, structure of soil. Its mineral components are mixture of Illite and Kaolinite (d < 0.005 mrn), Illite is the most mineral components. The surface area of comparative is 100–350 m2/g. The amount of exchange eation is 20–50 mmol/100 g. The amount of water in the red clay rock varies with the distance between 0 and 9 meter. It can be seen from the following table that the original water component in the red clay rock is 6.5% and the amount of water increases, usually from 6.5–16% with the distance to the space narrowing, because the influence of water in air increases, too.

Water-absorbing property

The water-absorbing property of red clay rock is shown in its absorption of water, softening and swelling. The rate of its absorption of water is around 40%. The rock swells after taking in water and its strength decreases gradually until loses the strength completely.


Compared with other rocks, the red clay rock has special structure constitution, physiochemical property, water-taking property, so it has unique features in terms of engineering mechanics.

Mechanics index

Research shows that the number of Rc is between 1–3 MPa, C is 0.5–1 MPa, φ is about 35°, E0 is 100- 160 MPa and μ is between 0.3–0.45. Part of the testing data is shown in Table 2. According to the table, the red clay rock's mechanics index is greatly influenced by the amount of water contained inside. The more water the rock contains, the lower the mechanics index is.

(Table in full paper)

Long-term strength trend

When the red clay rock is exposed to air, it absorbs water and become softer, swells and its strength decreases, usually around 50%. In the underground areas where there is plenty of water, the red clay rock loses it strength completely because it absorbs too much water.

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