School of PE Blog

Those taking the FE Civil exam should be familiar with the basic concepts in soil mechanics related to the phase relationships of soil. Phase relationships are also covered on the PE exam, regardless of which depth version of the exam is taken. This blog aims to provide an overview of soil phase relationship concepts, terminology, and the basic calculations involved in solving for soil component volumes and weights.

Soil can be understood to have three "phases." Specifically, these are the solids within a soil, the voids between these solids which are occupied by air, and the voids between the solids which are filled with water. The various proportions of these phases within a given soil contribute to its behavior and properties.

Before discussing concepts and calculations related to the soil phases themselves, it is necessary to understand the specific weight of water and the specific gravity of the solids portion of soils. The constant value known as the specific weight of water can be understood as the weight of water in pounds within a cubic foot of volume of water. Specifically, the value of water's specific weight is 62.4 lb/cubic ft. The constant value known as the specific gravity of soil solids is a dimensionless unit which can be understood as the ratio of the typical density of the soil solids within a soil to the density of water within a unit volume. It is not actually a consistent number but varies by the type of soil being considered, and it is also an average value based on the different types of particles within the soil (it being assumed that the soil is homogenous). It can be taken, however, as typically ranging between 2.6 and 2.85. For some organic soils, however, it can be substantially lower. The value for a particular soil can be determined by performing a specific gravity test on the soil. For the purposes of the exam, if a calculation problem involving soil phases is given, this value would likely be given so that some other value related to the soil composition can be determined.

In terms of phase relationships, soil can be understood to involve five potential variables. Namely, these are the individual volumes of the solids, air (empty voids), and water (filled voids), as well as the weights of the solids and the water. Note that the weight of the air-filled voids is considered negligible. Given the information we have about the specific weight of water and the specific gravity of soil solids, any of the five variables could be solved for if three are given. The exam taker should therefore be familiar with how to solve for any of the variables by understanding the relationships between them, as follows.

The volume of water can be calculated by dividing the weight of water by the specific weight of water. The volume of solids can be determined by dividing the weight of solids by the product of the specific gravity of soil solids and the specific weight of water. Likewise, the weight of water can be determined by multiplying the volume of water by the specific gravity of water. The weight of solids can be determined by multiplying the volume of solids by the product of the specific gravity of soil solids and the specific weight of water. Any of the volumes can be found by subtracting two other volume components from the overall volume. For example, the air volume is the difference between the overall volume and the sum of the soil and water volumes. Similarly, the weight of an unknown component (water or solid) can be found by subtracting the overall weight by the known component (water or solid). A phase diagram is a useful tool for visualizing the content of the soil (solids, empty voids, and water-filled voids) and their associated variables by separating them into different regions of the diagram. The above-described calculations can then be done as needed to determine the unknown variables of the diagram.

There are a number of other terms related to soil content that the exam-taker should be familiar with which are all defined by particular ratios involving the above-described weights and volumes of a soil. A soil's degree of saturation is defined as the ratio of the volume of water to the overall volume of voids within a soil (filled or not). It is sometimes expressed as a percentage, with 100% being a fully saturated soil containing no air voids. A soil's porosity is defined as the ratio of its volume of voids (filled or not) to its total volume (including solids and voids). A soil's water content can be found by dividing the weight of a soil's water by the weight of its solids. Finally, what is known as a soil's void ratio can be found by dividing the total volume of voids (filled or not) by the volume of solids.

One should be familiar also with the concept of a soil's dry unit weight as well as its saturated unit weight. A soil's dry unit weight can be understood, in terms of phase relationships, as the unit weight of a soil when there is no water present within the void spaces of the soil. The dry weight can be determined by laboratory test after oven-drying the soil. A saturated unit weight, by contrast, is the unit weight of a soil when the void spaces are entirely occupied by water. Completely dry soil as well as completely saturated soil are sometimes referred to as "two-phased" soils since they are lacking in the water component and the air component, respectively. Because of the necessary arrangement of solids within soils, all soils have at least some number of voids within them. Therefore, it is not possible to have a single-phased soil.

In summary, FE exam-takers should be familiar with the basic concepts of soil mechanics including those related to phase relationships for soils, how various unknown variables may be solved for given particular information about the soil content, the various terminology related to the ratios of these variables, and the concepts of dry unit weight and saturated unit weight.