A clay layer has a thickness of \(3 \mathrm{~m}\), and a volumetric weight of \(18 \mathrm{kN} / \mathrm{m}^{3}\). Above the clay layer the soil consists of a sand layer, of thickness \(3 \mathrm{~m}\), a saturated volumetric weight of \(20 \mathrm{kN} / \mathrm{m}^{3}\), and a dry volumetric weight of \(16 \mathrm{kN} / \mathrm{m}^{3}\). The groundwater level in the sand is at \(1 \mathrm{~m}\) below the soil surface. Below the clay layer, in another sand layer, the groundwater head is variable, due to a connection with a tidal river. What is the maximum head (above the soil surface) that may occur before the clay layer will fail?

Understanding effective stress is crucial for evaluating soil stability and predicting potential failures. It represents the actual pressure transmitted through the soil skeleton. In other words, it's the force that keeps soil particles gripping each other, minus the pressure from water within the pores. To calculate effective stress, we use the total stress, which includes the weights of all the materials above the point of interest, and subtract the pore water pressure. The resulting value is crucial in determining how much load a soil layer can bear before failing.In the text solution for the exercise involving a clay layer and a sand layer, the step-by-step breakdown reveals how to find the effective stress on the clay layer. By combining the weights of dry and saturated sand layers, we get a total load. This total load is then divided by the area to ascertain the effective stress. But remember, this value is only correct if the groundwater does not exert additional pressure – if it does, it's essential to account for this in our calculations.

Volumetric weight, also known as the unit weight of soil, is the weight per unit volume, typically expressed in kN/m³. It's a fundamental parameter in soil mechanics that helps us understand the heaviness and density of soil. There are different types of volumetric weight - dry, when the soil has no moisture; and saturated, when the soil pores are completely filled with water, which is heavier due to the water content.

Practical Use in Calculations

In our exercise, the saturated volumetric weight of the sand layer is used to calculate the load exerted on the clay layer. Saturated volumetric weight is applied to the submerged section of the sand, while dry volumetric weight is used for the portion above the groundwater level. The sum of the loads then contributes to the effective stress on the clay layer, which is significant in assessing the layer's capacity to withstand additional pressures without suffering failure.

The groundwater level, or the water table, is the boundary where the soil pore spaces are completely filled with water. It is a dynamic level that can change with environmental factors such as precipitation or human activities like pumping. The position of the groundwater level in relation to the soil surface can drastically affect the stability of the soil as it influences pore water pressure and, subsequently, the effective stress within the soil.In the provided textbook exercise, the groundwater level is stated to be 1 meter below the surface of the sand layer. This determination is pivotal as we calculate the weights of the dry and saturated parts of the soil differently. Knowing the groundwater level allows us to understand the soil's behavior under different conditions and is indispensable for designing safe structures.

Soil layer failure occurs when the stresses within the soil exceed its strength, resulting in deformation or collapse. This can happen suddenly or over time and is a major concern in civil and geotechnical engineering. Factors leading to soil failure include high pressure or loads, erosion, changes in groundwater level, and earth tremors, among others.

Identifying Risk of Failure

The exercise demonstrates a method to predict potential failure by calculating the maximum head of water the clay layer can bear. This computation uses the effective stress and the volumetric weight of the clay. If the groundwater level rises above the calculated head, the effective stress on the clay layer would increase, which could surpass the layer's strength and lead to a failure. Understanding this threshold is essential for constructing durable structures and ensuring safety.