Quicksand condition refers to the phenomenon where particles of cohesionless soil, such as fine gravel and sand, become buoyant due to vertical upward seepage flow. This leads to sand boiling, which causes a decrease in both the bearing capacity and shear strength of the soil. The resulting agitation of the soil particles can be visibly observed.
It’s important to note that quicksand condition is not a type of soil, but rather a flow condition that occurs in cohesionless soils. This condition commonly occurs in excavation pits that are below the water table and require pumping to keep the area dry. When water is removed from the excavation pit, it can lead to quicksand condition and the associated reduction in soil strength.
How Quicksand Condition Occurs?
Quicksand condition arises when the upward seepage pressure exceeds the downward pressure exerted by the submerged weight of soil, causing the sand grains to separate. This results in the soil being unable to provide any load-bearing capacity as it loses shear strength. The shear strength of cohesionless soil is directly influenced by the effective stress.
The given expression provides the calculation for effective stress.
Fig. 1: Quick Sand Condition
Fig. 2 provides a visual representation and explanation of the terminologies used in equation 2. When the components of equation 2 are substituted, the resulting expressions are as follows: [Please note that without specific information about the context and the equation being referred to, I am unable to provide a more accurate and detailed explanation.]
Equation 5 can be expressed in a different way based on the given context.
The substitution of the value of submerged unit weight in terms of void ratio involves replacing the original expression of submerged unit weight with an equivalent expression that is in the form of void ratio. This substitution allows for a different representation of the same information, which may be useful in certain calculations or analyses.
By substituting the value of submerged unit weight in terms of void ratio, we can express the weight of a unit volume of submerged material in relation to the void ratio, which is the ratio of the volume of voids to the volume of solids in a given soil sample. This substitution can be mathematically derived using relevant equations and relationships between submerged unit weight and void ratio.
The substituted expression of submerged unit weight in terms of void ratio provides an alternative way of expressing the same concept, allowing for different interpretations or analyses of the given data. It may also facilitate comparisons or correlations with other soil properties that are related to void ratio. Overall, the substitution of the value of submerged unit weight in terms of void ratio offers a different perspective on the behavior and characteristics of the soil, which can be valuable in various geotechnical applications.
The effective stress of soil can become zero when the hydraulic gradient ‘i’ is unity, meaning the head causing the flow is equal to the length of the specimen, based on the given values of G=2.67 and e=0.67, resulting in the equation 9 yielding a result of one. This condition is known as the boiling condition, where the soil appears to be boiling with violent and visible agitation of particles, and the discharge suddenly increases due to an increase in the coefficient of permeability. The soil behaves like a liquid with no shear strength, and if a weight is placed on its surface, it sinks down.
Quicksand, which has a unit weight about twice that of water, cannot support the weight of a person or animal, and behaves like a viscous liquid. A person can float in it with about one-third of their body above the quicksand, but movement requires great effort and energy. Suffocation can occur if a person gets tired and their head falls into the quicksand in panic.
If caught in quicksand, it is important to keep the head above the soil surface and move slowly towards the bank. Trying to catch a tree on the bank or pulling oneself out of the quicksand should be attempted. It is noted that when there is surcharge on cohesionless soils, the head required to cause quicksand increases.
Quicksand Condition at Construction Site
the soil’s ability to support weight, resulting in sinking or shifting ground. Certain construction sites are particularly vulnerable to quicksand conditions. One such example is excavations in granular materials behind cofferdams alongside rivers. These areas are prone to quicksand due to the nature of the soil and the presence of water.
Another situation where quicksand can occur is in areas where artesian pressures exist. Artesian pressures refer to cases where the head of water is greater than the usual static water pressure. This can result in an unstable environment that is vulnerable to quicksand conditions.
Additionally, construction sites located behind river embankments that are designed to protect against floods are also susceptible to quicksand. The presence of water combined with the structure of the embankment can create conditions that allow quicksand to form.
It is important for construction workers and engineers to be aware of the risk of quicksand and take appropriate measures to prevent accidents and injuries. This can include conducting soil tests to determine the stability of the ground, using appropriate equipment and safety measures, and monitoring the site closely for any signs of quicksand or other hazards.
How to Avoid Quicksand Condition?
Boiling during excavation can be avoided through two methods. The first one involves reducing the water table at the site before the excavation process starts. The second method involves increasing the length of upward flow. These steps will help prevent the formation of boiling conditions during excavation.
Another situation where boiling may occur is when a pervious sand stratum is situated beneath a clay soil in an artesian pressure condition. In such cases, the water pressure may build up and lead to boiling. It is important to be aware of these conditions and take necessary measures to prevent them from occurring during excavation.