Question

A plastic bottle contains saturated sand, and water reaching into the neck of the bottle, above the sand. When squeezing the bottle, the water level appears to go down. Explain this phenomenon. Is this sand suitable for the foundation of a bridge pier?

Short answer

Short Answer: When squeezing the bottle, pressure is applied on the saturated sand, causing consolidation and forcing water from the pores, making the water level appear to go down. While this type of sand might not be the most suitable for a bridge pier foundation due to its compressibility and possible high water table, it is crucial to conduct site-specific investigations, such as in-situ and laboratory tests, to accurately determine its properties and behavior before making any conclusive statements on its suitability.

Step-by-step solution

Understand the conditions in the plastic bottle

First, get an idea of the initial conditions inside the bottle. We have saturated sand, which means the sand is completely filled with water. Thus, the water is trapped between the sand grains and occupies the pore spaces in the soil.

Explain the phenomenon of squeezing the bottle

When squeezing the bottle, pressure is applied on the sand. This causes an increase in stress within the soil, leading to a process known as consolidation. Consolidation is the process of water being expelled from the soil (or decrease in the void ratio) due to an increase in stress. As the pressure is applied, water from the pores is forced out, resulting in the water level appearing to go down.

Understand the implications of consolidation

In the context of foundation engineering, consolidation is a critical factor in determining the bearing capacity and settlement of soil under applied loads, such as in the case of a bridge pier. If the soil undergoes significant consolidation under the load, it may cause excessive settlement or failure of the foundation.

Determine the suitability of the sand for bridge pier foundations

To assess if this sand is suitable for a bridge pier, consider some important information: 1. The sand is saturated, which highlights the presence of water, possibly implying a high water table and low bearing capacity. 2. Squeezing the bottle results in consolidation, indicating that the sand is compressible when subjected to loads. Given these observations, it's reasonable to say that this type of sand might not be the most suitable for a bridge pier foundation. It is important, however, to conduct additional site-specific investigations, including in-situ tests (like Standard Penetration tests) and laboratory tests (like consolidation tests) to determine the sand's properties and behavior under load accurately before making any conclusive statements on its suitability for a foundation.

Key concepts

Saturated Sand

Imagine a day at the beach where the sand is drenched from the morning tide. This sand, filled with water, provides an excellent conceptual understanding of what engineers refer to as 'saturated sand'. In our example with the plastic bottle, saturated sand consists of soil particles completely surrounded by water, which occupies all the voids in between them.

Applying pressure to this system, such as squeezing the bottle, forces the water to move out of the sand, and the level of water within the bottle goes down. This illustrates the principle of consolidation, a crucial component in geotechnical engineering. Understanding how saturated sand behaves under pressure is key for determining the suitability of soil for construction projects, as it directly impacts its strength and stability under the foundations of structures.

When construction projects overlook the properties of saturated sand, it can lead to problems like excessive settlement and structural failure. Thus, for a piece of land to support structures like a bridge, it must have favorable conditions that include, but are not limited to, low levels of saturation, or the ability to effectively drain and compact the soil to increase its density and stability.

Foundation Engineering

Foundation engineering is the art and science of designing and constructing structures that transfer the weight of buildings, bridges, and other infrastructures into the ground. It is a branch of civil engineering that ensures the stability and longevity of structures by carefully considering the interaction between the soil and the foundation.

One key aspect of foundation engineering is assessing the soil's properties, such as compressibility, permeability, and shear strength. These properties help engineers determine the soil's behavior under various loads and environmental conditions. Engineers must understand how soil like the saturated sand in our exercise will perform when a load, such as a bridge pier, is placed upon it. Will it compact and settle slowly over time, causing structural issues? Or will it hold firm, ensuring the safety and integrity of the structure?

Engineering decisions must encompass thorough site investigation, including soil testing like the ones mentioned in the provided solution, Standard Penetration Testing (SPT), and lab-based consolidation tests. These tests can help predict how the soil will behave long-term under the weight of the structure and form the bedrock for designing a foundation that will stand the test of time.

Bearing Capacity

Bearing capacity is a fundamental concept in geotechnical engineering that relates to the maximum load per unit area that the ground can carry without experiencing excessive settlement or failure. This concept is intrinsic to the safe design of foundations, and it is directly influenced by soil characteristics like those observed in the saturated sand condition.

Factors determining bearing capacity include soil type, density, depth, and the presence of water. A soil with high bearing capacity means that it can support considerable weights without undergoing significant deformations. When soil has poor bearing capacity, as may be the case with our saturated sand, it requires careful engineering techniques, like reinforcing the soil or opting for a deep foundation, to prevent the possibility of a structural collapse.

To assess bearing capacity, geotechnical engineers perform calculations that consider the soil's ultimate bearing capacity and apply safety factors to prevent overloading. For instance, the compressible nature of saturated sand when under pressure, as highlighted in the exercise, would typically signal lower bearing capacity, cautioning engineers to either improve the soil conditions or choose a more suitable location for weight-bearing structures like bridge piers.