Sensitive (Quick) Clay

1.inroduction

Sensitive clay, also known as quick clay, is a type of clay that can undergo rapid and unpredictable changes in strength and behavior when subjected to external forces or disturbances.

This type of clay is formed from the deposition of fine-grained sediments in marine environments with low oxygen levels. Sensitive clay particles are electrically charged and held together by a thin layer of water molecules, which makes them very unstable and prone to collapse when the water content is disturbed or altered.

When sensitive clay is subjected to changes in pressure or loading, such as during construction or excavation activities, it can lose its stability and strength, leading to sudden landslides or other geotechnical hazards. Therefore, it is essential to take appropriate precautions and conduct thorough site investigations before carrying out any construction or development activities in areas with sensitive clay deposits.

Sensitive clays are defined as having a remolded strength of 25 % or less of the undisturbed strength. Some clays are much more sensitive than this, and clays having a remolded to undisturbed strength ratio of I to 20, or even I to 100, are known. Typically, such clays have field water contents equal to or greater than their liquid limits, and such relations may indicate their presence. Extensive deposits of sensitive clays occur in some areas, for example, the Champlain clays of the St. Lawrence and Ottawa River Valleys. Where such clays have been pre-consolidated by partial desiccation, or by the weight of materials subsequently eroded, foundations may be placed on the clays, provided that the foundation load produces shearing stresses under the foundations that are well within the shear strength of the clay, or else excessive settlement and possibly catastrophic failure will result. Disastrous flow slides have developed in the Champlain clays in a number of places, and the hazard must always be considered. Deep excavations in sensitive clays are extremely hazardous, because of possible severe loss in shear strength, resulting from strains within the soil mass beneath and adjacent to the excavation. Determination of the physical properties necessary for evaluating the significance of sensitive clays to a proposed structure requires taking and testing of both undisturbed and remolded samples of the clays, and thorough analysis of the possible hazards involved. Because of the extreme sensitivity of such clays to even minor disturbances, taking and testing undisturbed samples require sophisticated equipment and techniques, and should be attempted only by competent personnel experienced in this type of work.

At present, the risk and extent of retrogression are difficult to evaluate. However, on the basis of studies conducted in eastern Canada, retrogression may be considered to be a consequence of an initial slope failure. In terms of analysis, the first step is to quantitatively examine the initial slope stability. The second step is to evaluate, in a semiquantitative manner, the risk of retrogression that could develop following initial slope failure. In addition to numerical analysis, a certain comprehension of the formation of slopes in sensitive clay deposits is helpful in order to put the processes into a rational framework.

2. Identification

Sensitive clay can be identified in the field by its distinctive physical and geological characteristics, as well as through laboratory testing. Here are some ways to identify sensitive clay in the field:

1. Visual inspection: Sensitive clay has a unique appearance that can be identified visually. It is typically gray or bluish-gray in color, and its surface is smooth and shiny, with no visible cracks or fissures.
2. In-situ testing: Several tests can be performed in the field to identify sensitive clay. One of the most common tests is the ball-and-socket test, which involves compressing a small sample of soil between the fingers to determine its strength and stiffness. If the soil crumbles easily and feels soft and sticky, it may be sensitive clay.
3. Geophysical surveys: Geophysical surveys can be used to identify sensitive clay deposits by measuring the electrical resistivity of the soil. Sensitive clay has a low electrical resistivity, which can be detected using specialized equipment.
4. Laboratory testing: To confirm the presence of sensitive clay, laboratory testing can be performed on soil samples collected from the field. These tests typically involve measuring the soil’s plasticity, water content, and sensitivity to changes in pressure and loading.

It is important to note that the identification of sensitive clay requires a thorough understanding of the local geology and soil characteristics. If you suspect that sensitive clay may be present in an area, it is recommended to consult a geotechnical engineer or other qualified professional to perform the necessary testing and analysis.

2.1 ball and Socket Test

The ball and socket test is a simple in-situ test that is commonly used to identify sensitive clay in the field. It involves compressing a small sample of soil between the thumb and forefinger to determine its strength and stiffness.

To perform the ball and socket test, follow these steps:

1. Take a small sample of soil (about the size of a walnut) and remove any large stones or debris.
2. Roll the soil sample into a ball between your palms, using light pressure.
3. Gently press your thumb into the center of the soil ball, creating a small depression or “socket”.
4. Place your index finger on top of the soil ball, and press down with gentle, even pressure.
5. Observe the behavior of the soil. If it crumbles easily, feels soft and sticky, and does not maintain its shape, it may be sensitive clay.

Sensitive clay will typically crumble and lose its shape when pressure is applied, indicating its low strength and high sensitivity to disturbance. This test can be useful in identifying sensitive clay in the field, but it should be supplemented by other methods such as laboratory testing and geophysical surveys for a more accurate assessment.

Amir Hejazi