Sensitive Clays

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 ofsensitive clays occur in some areas, for example, the Champlain clays ofthe St. Lawrence and Ottawa River Valleys. Where such clays have been preconsolidated 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 ofplaces, 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 ofthe 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.Geographic Distribution

Although soft clay deposits are found in many parts of the world, the most extensive areas of highly sensitive clays occur in Scandinavia and eastern Canada. They have also been reported in Alaska, Japan, the former Soviet Union, and New Zealand (Torrance 1987). Small areas of similar deposits may still be discovered, but it appears unlikely that any large areas of such deposits remain undiscovered.

3.Composition and Minerology

Because of their geologic origin, sensitive clays frequently have a characteristic composition and mineralogy. The silt fraction is most often high, typically in the range of 30 to 70 percent. The fraction with grain sizes smaller than 2 ,um generally contains more rock flour than clay minerals, and the clay minerals are predominantly illite.

4.Stability Number

The susceptibility of the clay to flow is not sufficient to induce retrogressive earth-flow failures. The slope geometry after the initial failure must be such that the shear stress in the new slope exceeds the undrained shear strength of the clay. Therefore, the potential distance of retrogression is difficult to evaluate. Nevertheless, the risk of retrogression after the occurrence of an initial failure can be assessed at least qualitatively. From analytical studies and examination of more than 40 case records, Mitchell and Markell (1974) proposed that retrogression can occur only if the stability number, Ns = yH/Cu, is larger than 6 within the depth of potential failure.

5.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.

6. 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.

7.Conclusion

In sensitive clay deposits, landslides are an important component in the development of young river valleys. The primary cause of instability is changes of geometry related to natural erosion or sometimes to human intervention. In slopes in which stability has already become marginal, some
slight seasonal variations in the groundwater regime may trigger a landslide. For the long term, however, the groundwater regime in the vicinity of slopes evolves with the development of valleys in a way that affects the general stability of the valley walls and the morphology of the landslides. Detailed piezometric readings are required to permit a rational interpretation of the groundwater regime. Piezometric data should not be considered isolated data values but a reflection of a groundwater regime controlled by boundary conditions. Approaches to stability analyses of sensitive clay slopes are based largely on experience and the study of many case records. However, consideration of the genesis of clay slopes justifies the type of analysis as well as the approach for determination of the shear-strength parameters. Landslides in sensitive clays are notorious for large and very rapidly enlarging retrogressive earth flows that can be triggered as a result of the initial failure. Expertise has been developed in the evaluation of stability against an initial failure using effective stress analysis, proper pore-pressure data, and post-peak strength parameters. However, it remains difficult to assess whether an initial failure will generate a sequence of retrogressive earth flows. Slopes composed of sensitive clays with a stability number yH/Cu larger than 6 and a liquidity index higher than 1.5 are generally considered as presenting a risk of retrogressive failure following an initial failure.