Section 6

What is soil mechanics? Soil mechanics is the science of understanding and predicting how soil will respond to externally applied forces (or pressures).

Soils are usually cohesionless, cohesive, or organic.
Cohesionless soils have particles that do not tend to stick together. Mostly composed of sand, maybe some silt.

Cohesive soils are characterized by very small particle sizes where surface chemical effects predominate. They are both "sticky" and "plastic".
Organic soils are typically spongy, crumbly, and compressible. They are undesirable for supporting structures.

The grain size distribution (gradation curve) and consistency of a soil are two important physical measurements that are needed to determine the soil's suitability onto which a structure can be built. This "suitability" is usually identified by placing the soil into a classification using the USCS (Unified Soil Classification System).

Most soil classification systems used in construction classify soils based upon two experimental characterizations of soil. These two measurements are (1) a grain-size distribution curve (or gradation curve), and (2) the Atterberg limits (or soil consistency). The grain-size analysis can be either mechanical or with a hydrometer analysis. The mechanical method uses sieves with the standardized openings as shown in table 1.

The percent by weight of soil passing each opening is plotted as a function of the grain diameter (corresponding to a sieve number). The horizontal scale on this plot is logarithmic. The hydrometer method is based on Stokes' Law which indicates that a larger grain size will result in a larger terminal velocity when dropping through a fluid (i.e. the larger size reaches the bottom quicker, assuming uniform density).

Consistency for a particular soil is defined by the water content present when it changes its response to stress. This measurement has been further refined by establishing Atterberg Limits. These limits divide four different "states" of consistency. If a soil is heavily saturated with water and then is dried out, it will move from a liquid state to a plastic state to a semisolid state and then to a solid state. The dividing line between the liquid and plastic states is the liquid limit (LL). The dividing line between the plastic and semisolid states is the plastic limit (PL). And the dividing line between the semisolid and solid state is the shrinkage limit (SL). This is shown in figure 1.

The plastic index (PI) is the range of the plastic region. These limits are expressed as a percent of moisture content. The experimental measurement of these limits requires a "liquid limit device". More specifically, it is a device that measures the water content at which the shear strength of the soil becomes so small that the soil "flows" to close a standard groove cut in a sample of soil when it is jarred in a standard manner. (Come to think of it, this is strikingly similar to how I would characterize my young children's diaper contents!)

The two most widely used classification systems are the American Association of State Highway and Transportation Officials (AASHTO) and the Unified Soil Classification System (USCS). In this class, we will mainly discuss the USCS system that is used by engineering consulting companies and soil-testing laboratories. The following group symbols are used in USCS:

Note: The amount of Fines in a soil sample is the percent by weight that passes a number 200 sieve.

Soil Classification Systems
USCS (Unified Soil Classification System) - classifies soils using grain-size, liquid limit, and plasticity index. Applies these measurements to a chart to determine group symbol.
AASHTO (American Association of State Highway and Transportation Officials) - uses grain-size, liquid limit, and plasticity index. The measurements are plugged into a formula to determine a group index. The lower the GI, the better the soil is for use as a subgrade.

Soil Texture Triangle from Iowa State (includes comparision between AASHTO and USCS)

Soils contain three components, which may be characterized as solid, liquid, and gas. The solid components of soils are weathered rock and (sometimes) organic matter. The liquid component of soils is almost always water (often with dissolved matter), and the gas component is air. The volume of water and gas is referred to as the void. The following are important relationships between these quantities. The notation will follow V_(total), V_a(air), V_w(water), V_s(solid), and V_v(void), where the V stands for volume. The same notation is used for W (weight) and M (mass).

In general, soil properties are simply physical relationships between mass and volume.

In granular soils, compressibility and shear strength are related to the compactness of the soil grains. For a soil in its densest condition, its void ratio is the lowest and it exhibits the highest shear strength and the greatest resistance to compression. The compactness can be evaluated quantitatively by the relative density (D_r) which has the form

The values of g_min or e_maxfor a given soil can be determined by drying, pulverizing, and pouring a sample into a container. The values g_max or e_minfor are found by subjecting this dry sample to prolonged vibrations and loads. The gamma variables above refer to the dry unit weight.

The basic behavior of soil (in fact, the graphs below sums up most of soil mechanics in the qualitative sense).

Soil exploration and sampling has three basic aspects.
Boring (or drilling, or digging)
Auger produces a disturbed sample. Wash Borings uses water to bring the soil to the surface. Core
Boring can provide a sample of hard material. Cores can also sample soil from large depths.
Sampling (remove soil from the hole, sample should be well marked with date, location, and depth)
Testing (a test pit can be used to obtain an undisturbed sample.)

Disturbed Sample has been "disturbed" and no longer has the same form (i.e. density). The grain size, liquid limit, plastic limits, specific gravity, and some compaction tests can be performed on this sample.
Undisturbed Sample (as close to undisturbed as possible) keeps the same form or condition it had when in the ground. One can perform all the tests as a disturbed sample plus strength, compressibility, and permeability.

General rules: soil should be checked every 75 ft (multi-story), ~150 ft (one story), ~750 ft (highway), reduce distances if non-uniformity is encountered.
For OSHA excavation regulations, soil testing is done mainly to determine the stability of excavation sides.

The depth should be to a soil strata of acceptable bearing capacity (if this is shallow, check sub-strata). In general, in cohesive soils the test should go down to a point where the increase in stress due to foundation loading is < 10% the overburden pressure (overburden pressure is defined below).

The overburden pressure is the effective pressure of the overlaying soil. Such that, if a soil sample has been taken at a depth of 10 ft and the unit weight of the soil is 110 lb/ft³, the overburden pressure is P = gh or P = (110)(10) = 1,100 lb/ft². (Provided this soil has not "seen" or had a history of any higher overburden pressure - which could be encountered for heavily eroded surfaces.)

There are three general ways to induce deformations in solids or semi-solids: tension, compression, and shear.

Soil is not capable of resisting tension, it is capable of resisting compression to some extent. In cases of excessive compression, failure usually occurs in the form of shearing along some internal surface within the soil.

Structural strength of soil is primarily a function of its shear strength, where shear strength refers to the soils ability to resist sliding along internal, 3-dimensional surfaces within a mass of soil.

where s = shear strength, c = cohesion, s = effective intergranular normal (to the shear plane) pressure, and j = angle of internal friction. The quantities s, c, and s have units of pressure.

So what does this equation mean? The shear strength of a heavy clay soil does not increase with increased load because j = 0. The shear strength of a very sandy soil does increase with increasing load because j does not equal 0, but c = 0 for sand. Most soils are a mixture of sand and clay. The following graph illustrates the results of the equation above.

The unconfined compressive strength is, under most conditions, twice the cohesion of clay soils (mathematically: q_u=2c). This will be important to remember when using tools to test a soil's stability to satisfy OSHA requirements.

Three widely used laboratory tests
Unconfined Compression Test - An axial load is placed onto a sample, the load is increased until (a) the soil fails, or (b) 15% strain has occurred. This load is known as the unconfined compressive strength. There is no lateral support on the soil sample for this measurement.	(click to enlarge)
Direct Shear Test - A shear stress is placed on the soil sample. The stress is increased until failure. Several of these tests will provide an experimental measurement for c and j for a given soil. (In the formula s=c+stan(j).)	(click to enlarge)
Triaxial Compression Test - Same as the unconfined compression test but with the addition of lateral pressure.	(click to enlarge)

Two widely used field tests
Pocket Penetrometer - Measures the unconfined compressive strength.
Shearvane (or Torvane) - Measures the cohesion. Here is a close-up view of the dial.

These tools will be talked about in class. They are acceptable ways to measure the strength of the soil to satisfy OSHA regulations when determining proper sloping.

The survivor said "All day he had been asking me, 'If this caves in, where are you gonna go?' I asked him this morning, let's get some boards to shore this thing up and he said, 'We're almost done.' In five more minutes we would have been sitting at the table eating lunch.". . . . . It took firefighters an hour to reach the man's wrist and determine he was dead. It took them another five hours to pull his body from the trench.

As some 50 rescuers worked with buckets and hand shovels to free him, a man buried up to his head talked with them and even joked a little about his predicament. However, after about four hours, the man suddenly quit talking, and died. Officials speculated he may have succumbed to internal injuries and bleeding. He was working in an unshored 15-foot-deep trench to install a sewer line when the accident happened.

Buxton, N.C. (1998)
A man died on a beach when an 8-foot-deep hole he had dug into the sand caved in as he sat inside it. Beachgoers said Daniel Jones, 21, dug the hole for fun, or protection from the wind, and had been sitting in a beach chair at the bottom Thursday afternoon when it collapsed, burying him beneath 5 feet of sand. People on the beach on the Outer Banks used their hands and shovels, trying to claw their way to Jones, a resident of Woodbridge, VA., but could not reach him. It took rescue workers using heavy equipment almost an hour to free him while about 200 people looked on. Jones was pronounced dead at a hospital. You just wouldn't believe the outpouring of concern, people digging with their hands, using pails from kids," Dare County Sheriff Bert Austin said.

The Antlion - An insect that has evolved an instinctual sense of slope stability and excavation failure. It uses this sense to catch prey in the bottom of a sand pit. If the field of excavation needed a mascot, this insect is a grotesque looking selection that would be appropriate.

How dangerous is soil? Soil is heavy! It can exert a very large amount of pressure and be extremely forceful when moving!

A reasonable unit weight of soil is 120 lb/ft³. This corresponds to 3,240 lb in every cubic yard - most cars don't weigh this much! Would you be willing to have a Chevy car dropped 4 feet onto your body? How about 1 foot? How about if it is gently placed onto your chest?

The column of soil marked 3 begins getting subjected to an unconfined compression strength "test" at its base. This is where the soil, many times, fails first and is marked with a 1. The next section to fail is 2 then 3. Cracks (or tension cracks) appearing in the ground next to an excavation are indications that the sides may not be stable and are pulling away! (But cracks don't have to appear for a cave-in to occur.)

Note: This description of a soil cave-in follows a common sequence of events. But some cave-ins may not follow this particular sequence. In fact, parts 1, 2, and 3 could all fail at once.

Often a worker can be trapped by a first cave-in and fellow workers will jump, willy nilly, into the trench to help! This may put the rescuers in harms way. There is at least one cave-in on record where 2nd and 3rd cave-ins have occurred "catching" separate groups of rescuers.

Statistics: 50% of all excavation fatalities are rescuers, an excavation accident is 15 times more likely to result in death than any other construction accident, 8/10 of all deaths occur in < 15ft, 4/10 of all deaths occur in < 10ft, between 100-400 people are killed per year in excavations, 1,000-4,000 are injured every year

So why do these "accidents" occur? Possible reasons include:
1. Attempting to save $$ (and time) by not properly sloping or shoring.
2. Boss has requested you get down into an unsafe trench. You don't want to "rock the boat" or get your boss mad by refusing.
3. It is "wimpy" to be afraid of dirt. This is the so-called "cowboy-ish" effect. This is closely related to peer pressure to do the job and not worry about the safety aspects.
4. Not being educated on the hazards of a potential cave-in.

Co-workers may be consulted or assist professional emergency response personnel during a rescue. A problem arises when co-workers are emotionally connected to the victim and become rash and irresponsible when trying to rescue them.

A good bottom-line philosophy on excavation safety:
It is very risky to cut corners on excavation safety. One accident and there will be law suits, fines, penalties (possible prison time) not to mention personal grief and trauma of losing a co-worker or getting one seriously injured. One accident can put you out of business. For the long-term financial and emotional health of your business and co-workers, it is best to follow safety regulations.

Is this a safe situation? Notice the huge tension cracks developing in the soil behind the worker. Why is the worker putting themselves into harms way? There is no sloping or retaining structure for the soil behind the worker.

Here is another risky situation. This picture is from the Pioneer Press, 9/9/01.

Actual Accident Site Picture (1956)

*This document is not intended as a complete and comprehensive statement of all regulations. It is only an abbreviated summary of selected sections. Click here for the complete OSHA description of excavation regulations.

The Bureau of Labor Statistics reports (based upon claims made to workers compensation) that between 1976 and 1981 the deaths associated with work in excavations accounts for nearly 1% of all annual work related deaths. These statistics also indicate that excavation accidents caused about 1,000 work-related injuries each year and about 140 result in permanent disabilities and 75 in death. These statistics are rather old and have probably increased. If one takes this figure of deaths and assumes they are evenly distributed about the 50 states with about 50 excavation companies per state, then approximately one of your co-workers will die from an excavation accident in a 30 year construction career and many more will get injured.

By knowing and adhering to OSHA regulations, the risks can be greatly reduced. The OSHA standards regulate the use of support systems, sloping and benching systems and other systems of protection as a means of protection against excavation cave-ins. In addition, the standards regulate the means of access to and egress from excavations, and employee exposure to vehicular traffic, falling loads, hazardous atmospheres, water accumulation, and unstable structures in and adjacent to excavations.

"Cave-ins are not the only excavation danger. Undetected underground utilities, water accumulation, hazardous atmospheres, loose rock and soil, and even creatures, such as snakes, are a threat. In all these situations, prevention is the key."

Prevention: The Ultimate Solution To Excavation Safety
by Jerry Woodson, J.J. Keller and Associates, Inc., Neenah, WI
(This quote first appeared in the February 1998 Issue of Utility Safety Magazine)

Basic Terminology

Excavation: Any artificial (man made) cut, cavity, trench, or depression in an earth surface, formed by earth removal.

Trench: A narrow excavation in which the depth is greater than the width, but the width of a trench is not greater than 15 feet.

Shoring: Is a structure or system (usually made of metal or timber) that supports the sides of an excavation and which is designed to prevent cave-ins. It is sometimes a pre-engineered shoring system comprised of aluminum hydraulic cylinders (crossbraces) used in conjunction with vertical rails (uprights) or horizontal rails (walers). Used to prevent cave-ins.

Failure: This term refers to the breakage, displacement, or permanent deformation of a structural member or connection so as to reduce its structural integrity and its supportive capabilities.

Competent Person: Is a person who is capable of identifying existing and predictable hazards in the surroundings, or working conditions which are unsanitary, hazardous, or dangerous to employees, and who has the authorization to take prompt corrective measures to eliminate them.

Tabulated Data are tables and charts approved by a registered professional engineer and used to design and construct a protective system.

Soil Terminology

Cemented soil: Is a soil in which the particles are held together by a chemical agent, such as calcium carbonate, such that a hand-size sample cannot be crushed into powder or individual soil particles by finger pressure.

Cohesive soils: Is a clay, or a soil with a high clay content, which has cohesive strength. Cohesive soil does not crumble, can be (note: "can be" is not the same as "should be") excavated with vertical sideslopes, and is plastic when moist. Cohesive soil is hard to break up when dry, and exhibits significant cohesion when submerged. Cohesive soils include clayey silt, sandy clay, silty clay, clay and organic clay.

Fissured: Is a soil material that has a tendency to break along definite planes of fracture with little resistance, or a material that exhibits open cracks, such as tension cracks, in an exposed surface.

Granular: Is a soil that is mainly composed of gravel, sand, or silt with little or no clay content. Granular soil has no cohesive strength. Some moist granular soils exhibit apparent cohesion. Granular soil cannot be molded when moist and crumbles easily when dry.

Soil Type

Stable Rock: Is a natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.

Type A: Is a cohesive soil with an unconfined compressive strength of 1.5 ton per square foot (tsf) - in SI units, 144 kPa (1 Pa = 1N/m2), or greater.
Examples: clay, silty clay, sandy clay, clay loam, hardpan, cemented soils. No soil will be considered Type A if: the soil is fissured, subjected to vibration, was previously disturbed, is part of a sloped layered system sloping into the trench, or is seeping water.

Type B: Cohesive soil with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa).
Examples: angular gravel (similar to crushed rock), silt, silt loam, previously disturbed soils unless otherwise classified as C, dry unstable rock, some sloped layered systems.

Type C: Cohesive soil with an unconfined compressive strength of 0.5 tsf (48 kPa) or less.
Examples: granular soils including gravel, sand, and loamy sand; any submerged or soil with freely seeping water, and any soil not otherwise classified.

Where soils are configured in layers, i.e. they have different geological structures, the soil must be classified on the basis of the soil classification of the weakest soil layer. Each layer may be classified individually if a more stable layer lies below a less stable layer.

General Excavation Area Safety

Daily inspections of an excavation area shall be done by a competent person. This should be done prior to work and after a rainstorm, and as needed throughout the shift. The atmosphere shall not be (1) oxygen deficient, (2) Explosive/flammable/oxidizing, or (3) toxic (poisonous, corrosive, irritating). There are many situations where hazardous gases can build within an excavation (e.g. welding/burning, chemical usage)

Surface Encumbrances: All hazards shall be removed, secured, or safeguarded. This includes, but is not limited to, sharp, blunt, and heavy objects. Also included are holes, wells, pits, shafts, cables, and any equipment that could pose a hazard.

Underground Installations: Utilities must be located prior to excavations. Utility companies shall be contacted in advance. If work proceeds near the utility, the installation shall be located by a safe means. Unearthed utilities shall be supported.
Newspaper articles about underground utilities being damaged.

Access and Egress: A ladder, ramp, or stairway shall be provided in trench excavations that are 4 feet or more in depth, so as to allow no more than 25 feet of lateral travel. Walkways/bridges that cross over excavations shall have standard guardrails. Ladders must be secured and extent at least 36 inches above the landing.

Water Accumulation: Surface water shall be diverted away from trench. Employees shall be removed from a trench during a rain storm.

All employees that are exposed to vehicular traffic shall wear warning vests. No one shall work underneath a suspended load.

Mobile Equipment Approaching Edge of Excavations: Warning signals (logs, hands or mechanical signals, barricades, etc.) must be used when the operator does not have a clear and direct view of the edge.

Loose Rock or Soil: The placement of excavated materials (spoil) shall be a minimum of 2 feet from the edge of excavation or have a sufficient retaining device.

Soil Classification and Sloping

Each employee in an excavation shall be protected from cave-ins by an adequate protective system. One has the following options to provide this protection: sloping and benching, sloping with supports and shields in lower portion, timber shoring, aluminum hydraulic shoring, trench shields.

What must be done to select the proper protection depends on the depth of the excavation and the soil type.

Here is an outline of the steps needing to be followed to meet OSHA guidelines:

I. A competent person must make one visual and one manual analysis of the soil. Layered systems should be classified according to their weakest layer. Reclassification must be done if conditions change.

Visual Tests
Excavated soil and soil in excavation sides: fine-grained soil is cohesive, sand or gravel is granular.
Soil as it is excavated: clumps indicate cohesive soils, easily broken soil is granular.
Sides of excavation and adjacent area: fissured material, layered systems, surface water or seepage, sources of vibration, previously disturbed soil, etc.

Manual Tests (Detailed Description)
Plasticity (or ribbon test): Cohesive soils stick together.
Dry strength: dry, granular soil crumbles easily; dry soil which is difficult to break is probably clay. A drying test is used to determine if soil is fissured, unfissured, or granular.
Thumb penetration: Type A soil is readily indented by thumb with great effort; Type B if the only the thumbnail penetrates; Type C soil is easily penetrated several inches by thumb and can be molded by light finger pressure.
Pocket penetrometer: Determines unconfined compressive strength.
Shearvane: Determines soil cohesion

II. Determine the Sloping and Benching
Diagram of proper sloping and benching ( Type A, Type B)

Classify Soil (Type A, B, or C)
Determine Maximum Allowable Slope

UW-Stout Pocket Guide to Excavation Safety
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Excavation Safety Seminar (VHS tapes) - UW-Stout Library, Call Number TA730 .E93x 1995 main stacks

For questions or comments regarding these pages contact Dr. Alan Scott / scotta@uwstout.edu / this page was last updated September 14, 2006

Name	Equation	Description
Void Ratio		Ratio of the void volume to the solid volume.
Porosity		Percent of the total volume that is taken up by the void.
Degree of Saturation		Percent of the void that is taken up by the water.
Water Content		Percent of the weight of water to the weight of solids.
Unit Weight		Total density of the soil. Includes solids and the void.
Dry Unit Weight		Density of the soil when it is completely dried out.
Unit Mass, Dry Unit Mass		Density of the soil and the dry density of the soil.
Specific Gravity of Solids		Ratio of the density of solids to the density of water. Note:
	Other Important Relationships: W=W_s+W_w V=V_s+V_v=V_s+V_w+V_a

Soil Type	Maximum Allowable Slope (Horizontal Distance:Vertical Distance)
Solid Rock	vertical sides
A*	3/4H:1V
B	1H:1V
C	3/2H:1V