Grounded!: Amazing Classroom Demonstrations in Soil Mechanics

Shale, a solid rock, is used as fill under buildings and roads to raise the building or road to the required elevation. Some types of shale disintegrate after immersion in water—a phenomenon known as slaking. Slaking can be demonstrated by immersing hard, dry clay balls in water. This chapter presents a demonstration of slaking by immersing hard, dry clay balls in water. Slaking shales fool engineers because they are very hard, compact well, and have a high compressive strength. However, when subjected to submergence once or repeatedly, they break down into their component particles. When this happens, the fill settles unevenly beneath whatever is on top of the fill. Roads that start out perfectly flat begin to look like roller coasters as the shale decomposes. Considering that half of the exposed rocks on the earth surface are shale, this is a common problem.

This exercise in pile capacity uses rice and a wooden spoon to demonstrate how soil gains strength when it dilates. A plastic or metal tube is filled with loose, dry rice. When a spoon handle is pushed into the rice, the handle sticks, allowing the tube of rice to be lifted by the handle, which doesn't pull out. Similarly, soil dilatency increases the hold of soil on pile foundations, which are driven into the ground like the spoon handle is driven into the tube. That is, when driving in a pile, some soils dilate, causing increased pile capacity because the soil presses more tightly against the pile. While this effect is not quantifiable, it gives the pile designer more confidence in the structure, because the pile capacity in a dilatant soil will be higher than shown by conventional calculations.

Bentonite, a clay, swells significantly when wetted, exerting high pressure on its surroundings. This demonstration shows how high that pressure can be—high enough to break a concrete block. The block breaks because bentonite attracts a thick layer of water to each clay particle. The layer of water that accumulates around the clay particles gets thicker and thicker, pushing the other clay particles away, resulting in overall swelling of the clay mass. For the purposes of the demonstration, the type of block makes a difference. A lighter-weight concrete block will break sooner than a heavier one. Larger holes in the block, which accommodate more bentonite, will also speed breaking. In the field, swelling clays can move houses, buildings, and walls, upsetting owners the world over.

This chapter presents an overview of the demonstration and setup procedure for a soil retaining wall built from sand with interbedded layers of copier paper. Neither the sand nor the paper is particularly strong, but the resulting composite structure is very strong. This demonstration shows that sand compressive strength is much higher than sand shear strength. Sand may fail either in shear or in compression. Shear occurs when soil particles slide past one another. This does not take much force. Compression occurs when soil particles push directly against each other. This takes a lot of force. One can easily demonstrate this: sliding one soil particle past another hand is very easy. Crushing a soil particle is very difficult. Normally, soils fail in shear because their shear strength is much lower than their compressive strength.

Water makes a difference in soil strength because it affects the "effective stress" between soil particles, which governs how much strength the soil has. This chapter presents a demonstration and setup procedure for illustrating the effective stress of soil. This demonstration uses paper cups to show how water pressure influences soil strength. A cup and a plank represent two soil particles. When the particles are dry, the effective stress is high and hence the cup does not move. But when pore pressure increases in soils, the strength of the soils decreases by as much as 50%. This explains why saturated hillsides can become landslides and why some retaining walls and buildings fall over.

This chapter presents the setup procedure for a demonstration called the magic beaker. When water is added to sandy soil, some of it gets caught in capillary tension, which provides temporary strength. To illustrate, water is added to sand in a beaker. When inverted, the sand doesn't flow out of the beaker. Capillary attraction holds soil particles together as long as the soil is not saturated. This phenomenon can be seen in excavated banks of moist soil that may stand vertically for a long time because the capillary force holds the soil particles together with great force. The smaller the soil particles are, the greater the force is. Hence, gravel won't stand vertically, but silt (small particles) will. The geotechnical engineer or contractor can confidently cut a moist silt bank and have it stand for a while.

Effective stress governs soil behavior. The higher the effective stress is in a soil, the stronger the soil is. Strong soils are important for many civil engineering designs: building foundations, retaining walls, roads, and hillsides. The demonstration in this chapter illustrates how effective stress works. When effective stress in soils is high, soil particles push on each other with a lot of stress, generating a lot of friction and thus strength. Effective stresses can be increased by lowering the pore water pressure term in the effective stress equation. In geotechnical practice, this means draining the soil so the water pressure falls. The lower the water pressure is, the higher the effective stress is and thus the greater the strength is.

Soil arching refers to the ability of a hole in cohesionless soils, or loose rocks, to remain open when it should collapse. When civil engineers drive a tunnel, soil (or rock) arching relieves some of the stress caused by the soil (or rock) on the top of the tunnel. This chapter presents the setup procedure for a demonstration that illustrates soil arching with foams balls in a pipe. Arching allows engineers to cut a tunnel in soil without lining the tunnel. Furthermore, arching explains why the soil in front of some tunneling machines stands up, at least for a while, and why tunnel lining doesn't have to hold the full weight of soil or rock above the tunnel. Roman arches are an example of the arching phenomenon applied to structural engineering.

Draining soil is important in civil engineering. Drains buried in the ground are supposed to attract groundwater and carry it away in the drain. This chapter presents the setup procedure for the geotechnical Rorschach test—a demonstration that shows that draining doesn't happen in unsaturated flow. Civil engineering soil drains, which are often made of gravel, are supposed to carry groundwater away from the structure of interest, such as a retaining wall. However, this only works in saturated flow. With unsaturated flow, the water may flow around the drain and into or against the structure that is supposed to be protected by the drain. The resulting moisture problems include formation of mold or mildew, weakening of the soil, and increased unit weight of the soil. This same principle is used in creating “capillary breaks” in geotechnical structures, such as roads.

Several ways to strengthen soil exist, ensuring it forms an adequate foundation for a building or that it has enough strength to stand as a slope. Compaction is one commonly used method. Geofibers, a type of geosynthetic, are another method with the added advantages of increasing permeability and reducing unit weight. Geosynthetics are plastics embedded in soil that enhance some property of the soil, including strength. This chapter presents the setup procedure for the demonstration called hairy soil. This demonstration uses plastic fibers, called geofibers, to dramatically strengthen cohesionless soil. Geofibers have interesting collateral benefits. Use of geofibers, while increasing strength, also increases permeability and reduces unit weight. Compaction, by comparison, increases strength, but decreases permeability and increases unit weight, which can increase settlement. Geofibers are used when building fills and repairing landslides and for underfooting reinforcement.

Dense soils dilate when they are compressed or sheared. If the soil is confined, the dilation causes an increase in strength. This chapter presents the setup procedure for a demonstration called sand cylinder hoisting. Soil is placed in a cylindrical mold and then compressed with a special apparatus with no mechanical connection to the cylinder. The cylindrical mold can then be lifted off the lab bench, thus illustrating the increase in strength due to dilation. Geotechnical engineers, given the choice, prefer to keep sand used in foundations confined, so that when it is sheared and dilates, it will generate more friction. Pile capacity, for example, is increased by dilation.

This chapter presents a demonstration showing that water does not always pass through a sieve. In the demonstration, "magic" water stands on the surface of the sieve, while other "water" passes right through. This occurs because the molecules on the surface of a liquid are arranged differently than those below the surface. The surface molecules form a "skin" that has some strength. Water has a much stronger skin than alcohol. The geotechnical significance of this phenomenon includes the "threshold gradient" principle for geotextile filters. If the geotextile filter has small holes, water will not go through until some minimum hydraulic gradient is reached. The minimum hydraulic gradient to get the flow started is greater than that required after flow starts. That is, a larger water pressure is needed to start the flow through the geotextile than is needed to maintain existing flow through the geotextile.

Soil identification is very important to geotechnical engineers because soil type determines soil behavior. Although silts and clays behave very differently in civil engineering practice, they are hard to distinguish visually because both soil types comprise very small particles. A very simple manual test to determine if a soil is silt is the dilation test presented in this chapter, which allows an engineer to distinguish silts from clays. While they can be distinguished with expensive analytical equipment, this is much simpler and quicker. In terms of civil engineering uses, clays behave fundamentally differently than silts do. Clays compress more slowly, drain more slowly, are harder to compact, and can be more difficult to excavate.

This chapter presents a demonstration to explain the phenomenon of piping under a sheetpile dam. More than 90% of all dams are made of soil (earthen dams). When water flows through these dams too energetically, the soil can wash away. This is called piping, or internal erosion. Piping or internal erosion occurs rapidly, is very hard to stop, and often leads to complete failure. The chapter also discusses the case history of the Teton Dam in Idaho, one of the most famous piping failures. The dam was built on porous rock abutments that allowed water to seep into the dam with little loss of energy. The high-energy water entering the dam easily caused the silty soil to pipe, leading to complete loss of the dam.

Unit weight, void ratio, and porosity are basic descriptors of soil. These calculated numbers are used in geotechnical engineering to reflect the engineering behavior of soils. This chapter presents a demonstration called “jar of rocks” to bring these numbers to life. For example, the strength of a soil is related to its unit weight, while the void ratio is a measure of soil permeability. Soils must be able to drain to maintain their strength. High permeability means the soil will drain quickly, which is good. Void ratio is also a measure of compressibility: how much a soil will compress under a building like the Leaning Tower of Pisa. The strength of a clay soil is also related to its water content. A geotechnical engineer uses the water content to estimate the behavior of clay.

Upward groundwater flow occurs naturally and artificially. Artesian springs are naturally occurring upward flows that may be seasonal, or continuous, depending on the local geology and rainfall. Artificial upward flows can be caused by installing a levee or dam. As water seeps under the dam, it flows upward on the downstream side of the dam, perhaps forming sand "volcanoes." These upward water flows can cause liquefaction, similar to that which occurs during earthquakes or significant ground shaking when water flows through loose, saturated sands. Although the effect is the same, it is more dramatic during an earthquake. This chapter presents a demonstration to explain the phenomenon of liquefaction by upward gradient.

Water flows through soils, under dams, and into basements. Civil engineers must predict where water will flow and how much to avoid potentially severe consequences. For example, water flows through earthen dams. If the difference between the upstream and downstream water elevations is too large and little or no effective internal seepage control is in place, piping occurs, the soil washes away, and the dam may fail completely. Engineers use flownets to estimate how the water will flow. This chapter presents two demonstrations: critical hydraulic gradient and peaceful flownet. In the first demonstration, water flows through soil in a purpose-built tank. When the water flows too quickly, the soil turns to quicksand and pipes and flows directly through the soil. In the second demonstration, the water flow path can be estimated and the flownet drawn, which allows predictions of the quantity and direction of flow.

The unit weight of a soil suggests its strength, compressibility, and permeability. Higher unit weights suggest higher strengths and lower compressibilities and permeabilities. These properties are commonly used in geotechnical engineering design. However, different soils with the same unit weights have different strengths, compressibilities, and permeabilities. Hence, the unit weight, while useful and often used to estimate these properties, is not directly transferrable across soil types. To resolve this, relative density can be used. Relative density is a measure of how dense a cohesionless soil is compared with its maximum density. Calculation of relative density requires determining the maximum and minimum densities of the soil. This chapter presents a demonstration of relative density of soil in a graduated cylinder.

In geotechnical engineering, drains and filters are used behind basement walls, retaining walls, in wells, in soil embankments, in landfills, and under roadways. They convey water away from the structure, which has two effects: reducing water pressure on the structure and increasing the strength of the soil. These drains require a filter to prevent soil from entering the drain. The filter may be made of a special soil or a geotextile. This chapter demonstrates a soil filter using marbles. This demonstration shows how a graded granular filter works. Graded granular filters work because of their structures. They have large particles on bottom, successively grading to smaller particles on the top, allowing them to filter out different grades of soil.

Cohesionless soils (silt, sand, and gravel) develop their strength from friction among the soil particles. The strength of such soils may be increased by increasing the confining pressure on the particles. This is useful in shallow foundation design. Column footings are placed below grade instead of on the ground surface, so the soils under the footing experience greater confinement and hence have greater strength. Not only does this make the building foundation safer, it allows the use of smaller footings, which saves money. This chapter presents a demonstration called “sand in a jar” that illustrates this principle. A sandy soil is tested, qualitatively, for strength in two conditions: confined and unconfined. The materials used make a little difference. Any jar will do, as long as it's deep enough to show the effect. Also, any cohesionless material will work.

Earthquakes cause the ground to shake. The shaking motion is basically characterized as combinations of waves, primarily shear and compression waves. These waves encounter civil engineering structures (buildings, bridges, and canals) and cause damage. Civil engineers need to understand the waves to reduce the effect the waves will have on the structures they design. This chapter presents two demonstrations. The first demonstration deals with compression waves, and the second demonstration focuses on the shear waves. The compression waves are generated by energy imparted in the longitudinal direction. The shear waves are generated by energy imparted in the transverse direction.

Cohesionless soils (silts, sands, and gravels) need strength to hold up civil engineering loads, such as bridges, houses, and towers. However, the strength of the soil is not an inherent property of the soil. Strength depends on the stress that is applied to the soil. This a demonstration shows how cohesionless soils’ strength may be improved by increasing the confining pressure on the soil particles, which is useful in shallow foundation design. In this case, column footings are placed below grade instead of on the ground surface, so the soils under the footing experience greater confinement and hence greater strength. Not only does this make the building foundation safer, it allows the use of smaller footings, which saves money.

Capillarity plays a part in soil behavior. Capillary attraction holds soil particles together as long as the soil is not saturated. For example, cut banks of moist soil can stand vertically for a long time, because capillary action can hold the soil particles together with great force. This chapter presents two demonstrations that show the effect of capillarity. The first demonstration illustrates capillarity using cotton balls. The second demonstration illustrates nonwoven geotextile strength. Geotechnical engineers embed nonwoven geotextiles in the ground to strengthen soil. Nonwoven geotextiles comprise many tiny, tangled fibers. The geotextile adds tensile strength to the soil, which, by itself, does not have tensile strength. The geotextile's tensile strength in air may be moderate. However, once the nonwoven geotextile is buried in the ground, the strength increases significantly, just as with the cotton balls in the demonstration.

Soils subjected to excessive loads from buildings may fail if their strength is exceeded. The strength of soil, however, is not a constant during loading or failure. Failure occurs after an initial “threshold” strength is exceeded. After that, the force (or stress) needed to keep the soil moving in failure decreases for some soils. This behavior is known as stick-slip behavior. This chapter presents a demonstration of the stick-slip behavior of soil, which involves two kinds of friction: static and kinetic. When a rubber band holding a block is pulled in the demonstration, static friction builds up until failure occurs. Once the block is in motion, kinetic friction is invoked. The block may then slide so much that the static friction is recovered and the block stops, only to be reactivated by continued pulling of the rubber band.

In some civil engineering structures, soils are sometimes in contact with nonsoils. For example, concrete retaining walls, plastic geomembrane liners, and piles have soil-nonsoil interfaces. The designer must know how much friction the soil and the other material can develop to complete the design. The demonstration in this chapter illustrates the interface friction angle, a measure of this friction. In the demonstration, a soil is placed over another material on a table. The table is tilted until the soil slides off. The angle of tilt, at the commencement of sliding, is measured. Specific engineering applications of this concept include the capacity of a pile driven in sand depending on the soil friction developed on the sides of the pile; the pressure sand puts on a retaining wall depending on the amount of friction at the soil-wall interface; and the stability of a slope of municipal solid waste in a landfill depending on the waste-geosynthetic interface friction angle.

The strength of a soil is critical to civil engineering work. Almost all built structures rest on soil foundations, making strong soils as foundations very important. Cohesionless soil strength depends on the soil's mineral type, particle size, and particle shape. This chapter demonstrates the role of particle shape in the soil's strength. When evaluating a building site for a project, the geotechnical engineer considers the shape of the soil particles. More angular soil particles develop higher stresses at the points of contact with each other, thus generating higher strength. Smoother particles produce less stress. Higher stress makes it harder for the soil particles to move around and slide downhill.

Clay soil permeability measures how easily water flows through soil. Geotechnical engineers typically measure this in the laboratory using the very slow falling head test. However, finding a soil that produces data in this test in a few hours, the usual time for an undergraduate laboratory, is difficult. This chapter explains how to use corn starch to produce a “soil” that gives reasonable falling head permeability data in minutes, instead of days, thus allowing a demonstration of soil permeability. Soil permeability is important in the design of dams, roads, foundations, walls, slopes, ponds, sediment control structures, and landfills. Soils must drain quickly to maintain their strength. The coefficient of permeability is used to calculate how fast a soil will drain. However, low drainage is needed when the soil is used for, say, a canal lining, or a landfill liner, where water retention is desired.

This chapter demonstrates bulking of soils. After soil is purchased, it is excavated, placed in dump trucks, and taken to the site to be used. Twenty cubic yards of soil, as measured in the ground, will need a truck that carries more than 20 cubic yards to carry it away because the soil fluffs up during excavation. This increase in total volume is termed bulking and is demonstrated here in a beaker. Dry and moist sand are put in separate beakers. The cylinders are emptied and then refilled. The moist sand exhibits a large increase in volume, illustrating the soil bulking up when excavated. In some cases, dry, extremely loose soils may decrease in volume when excavated. That is, the effect may be either positive or negative.

Geotextiles are plastic cloths used in civil engineering applications, such as to strengthen soils or to filter water and soils. Geotextiles are used in retaining walls, embankments, and drains, and for underfooting reinforcement. A geotextile must be strong enough to withstand field installation, where it may be subject to forces or stresses from construction equipment. Typically, these forces or stresses are unknown to the designer. The geotextile profession has adopted a set of semi-empirical criteria to help evaluate whether the geotextile will withstand certain stresses. This chapter presents the California bearing ratio (CBR) puncture test, which is a simple measure of the geotextile's ability to resist installation forces. AASHTO specification M288 gives this and other geotextile survivability specifications.

Dense soils dilate when sheared, whereas loose, cohesionless soils collapse when vibrated. The simple apparatus in this demonstration uses ping pong balls to illustrate both behaviors. Soil dilation in saturated soils causes a negative pore pressure, which increases effective stress. When this happens, the strength of the soil increases. So, for example, when a hillside is excavated, a dense soil will dilate a bit. If water exists in the soil, the pore pressure goes negative, and the strength increases. Hence the ability of some soils to stand vertically if they are in a dilated condition. However, as time passes, other water is sucked into the soil and dissipates the negative pore pressure, thus weakening the soil and leading to a landslide.

Geologic materials have long been used to build civil engineering facilities. Their availability, price, and durability tend to be attractive to facility owners. This chapter presents an exercise that helps people recognize the use of geologic materials and gain an appreciation for it. Students are sent out to discover geologic materials used in civil engineering. They take pictures and report back. The equipment used includes a camera and good observational skills. Students return with pictures or video of geologic materials used in civil engineering projects, including limestone, granite, marble, slate, and miscellaneous stone. This exercise is best done in conjunction with lectures on geology and a laboratory exercise on rock identification. Different rock classes and types are identified in class and are then found in the field.

Soil erodes and finds its way into streams and lakes. If the soil stays in suspension, it causes harm to wildlife. This chapter illustrates the settlement rates of soils in jars, showing that soils have different suspension times. In the demonstration, different soils are put in water-filled jars and shaken. The jars are left on a table, and the soil settles in the jars. Silts and sands settle quickly, while clays tend to take more time. Because finer soils take more time to settle, keeping these from entering creeks and lakes is particularly important because they will stay in suspension for a long time, troubling wildlife. This is the reason that silt fences are designed to catch very fine soil particles running off construction sites during rain, before the soil particles reach a creek or lake.

Buildings are often held up by footings underneath columns. If the soils are too weak, or the column load is too big, the footing plunges into the earth in a bearing capacity failure. This chapter presents a demonstration for examining the bearing capacity of soil. This exercise shows bearing capacity by identifying the three principal modes of shear failure using various arrangements of piles, foundations, loads, and the factors that contribute to increasing bearing capacity. Bearing capacity of soil is related to the soil itself and to the size of the bearing area. A larger bearing area confines the soil under it, making it stronger. Cohesionless soils, in this case represented by dowels, gain strength when pressed together under a load, such as a footing. A larger area load presses more soil together, thus generating higher strength.

Retaining walls hold soil in a vertical position, creating space in front of the wall that would otherwise be occupied by sloping soil. In the demonstration presented in this chapter, a model retaining wall is placed in model soil and moved from side to side to show the failure surface that forms behind the wall. The demonstration is useful for geotechnical students to show the formation of the failure surface in cohesionless soil, so that they may better understand the formation of the failure surface and see that retaining wall theory conforms with reality. In terms of geotechnical theory, the failure surface is used to help predict the earth pressure on the wall, so the wall can be built strong enough to withstand that pressure

Rocks that fall toward civil engineering structures must be prevented from damaging those structures. Civil engineers analyze rock slopes and cliffs to prevent unwanted rockfalls and mitigate potential damage from them when they do occur. This chapter presents a demonstration in which various rocks are rolled down particle board or plywood sheets overlying stairs onto various soil types. The exercise allows engineering students to get a feel for the rockfall process and to understand some mitigation measures civil engineers take to keep the rocks from damaging property. The mass and shape of a rock and the material type used in the landing zone are examined to see the effect on the rock's rollout distance during a rockfall. Many methods of arresting rocks exist, including rockfall fences, steel cable nets, trenches, solid barriers or walls, and sand pits. The most common of these is the sand-filled trench.