1. TRENCHING AND EXCAVATION: Safety Principles

2. INTRODUCTION
Trenching and excavation procedures are performed thousands of times a day across the United States . Unfortunately, about 70 people are killed, and thousands more are seriously injured in trenching accidents each year. This presentation is meant to provide an over view of soil mechanics relating to trenching and excavation failures, trench and excavation hazards, general requirements, and hazard controls. This program outlines procedures and guidelines for the protection of employees working in and around trenches and excavations. This program pertains to all Ardent projects that require any trenches or excavations.

3. Cave-ins due to trenching failure
One of the reasons Ardent requires a competent person on – site during excavation and trenching are the numerous potential hazardous that may be encountered or created. Hazards include:
Cave-ins due to trenching failure
Falls from workers too close to the trench edge
Flooding from broken water or sewer mains
Electrical shock from striking electrical cable in the trench.
Toxic liquid or gas leaks from near by facilities or pipes
Auto traffic if the excavation site is near a highway
Falling dirt or rocks from an excavator bucket
Explosion from flammable gases and vapors

4. PROPERTIES OF SOIL
“Soil” is defined as any material removed from the ground to form a hole, trench or cavity for the purpose of working below the earth’s surface. This material is most often weathered rock and humus know as clays, silts and loams, but also can be gravel, sand and rock.

5. PROPERTIES OF SOIL
Soil is comprised of soil particles, air, and water in varying quantities. Soil particles, or grains, consist of chunks, pieces, fragments, and tiny bits of rock that are released by the weathering of parent rocks. Weathering is a natural process of erosion that can be physical or chemical.
Physical Processes include: Freezing and thawing, gravity, and erosion by rivers and rain fall.
Chemical processes include: Oxidation, hydration, and carbonation in which minerals are chemically broken down by the elements.

6. TYPES OF SOIL
The soil that is found on most construction sites is a mixture of many mineral grains coming from several kinds of rocks. Average soils are usually a mixture of two or three materials, such as sand and silt or silt and clay. The type of mixture determines the soil characteristics.
OSHA standards define soil classifications within the Simplified Soil Classification Systems, which consist of four categories: Stable rock, Type A, Type B, and Type C. Stability is greatest in stable rock and decreases through Type A and B to Type C, which is the least stable.

7. STABLE ROCK: Is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.

8. TYPE A SOIL: TYPE B SOIL:
Cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (TSF) or greater.
Cemented soils like caliche and hardpan are considered Type A.
SOIL IS NOT TYPE A IF:
It is fissured.
The soil is subject to vibration from heavy traffic, or similar effects.
The soil has been previously disturbed.
The material is subject to other factors that would require it to be classified as a less stable material.
The exclusions for Type A most generally eliminate it from most construction situations.
Cohesive soil with an unconfined compressive strength greater than .5 TSF, but less than 1.5 TSF.
Granular non – cohesion soil including granular gravel, silt, silt loam, and sandy loam.
The soil has been previously disturbed except soil classified as Type C soil.
Soil that meets the unconfined compressive strength requirements of Type A soil, but is fissured or subject to vibration.
Dry rock that is unstable.

9. TYPE C SOIL:
Cohesive soil with an unconfined compressive strength of .5 TSF or less.
Granular soils including gravel, sand and loamy sand.
Submerged soil or soil from which water is freely seeping.
Submerged rock that is not stable.

10. THERE ARE THREE BASIC DIFFERENT TYPES OF SOIL CLAY SOILS LOAMY SOILS SANDY SOILS

11. SOIL MECHANICS
Soil is an extremely heavy material, and may weigh more than 100 pounds per cubic foot (pcf). A cubic yard of soil ( 3 ft x 3 ft x 3 ft ), which contains 27 cubic feet of material, may weigh more than 2,700 pounds. That is nearly one and a half tons ( the equivalent weight of a car ) in a space less than the size of the average office desk.

12. HYPOTHETICAL COLUMN
From a soil mechanics point of view, one can visualize the soil as a series of multiple columns of solid blocks, with the blocks piled one on top of the other.
In the soil column shown, each solid block measures one foot square, weighs approximately 100 lbs, and supports the weight of all of the blocks above.
This means that a block sitting at a five foot depth supports its own weight and the combined weight of the four blocks resting on it. The combined weight of this column is 500 lbs spread over one square foot area; 500 pounds per square foot ( psf ).
This five – block column constitutes a 500 pound force exerted vertically on whatever lies below.
A column of soil exerts not only vertical force, but also a horizontal force in all outward directions. The outward force is equal to one – half the vertical force.

13. In undisturbed soil conditions, these hypothetical columns press against each other, maintaining an equilibrium. Therefore, the horizontal pressures of all the columns are balanced, producing a stable relation ship.

14. A. When a trench is excavated, the stable relationship no longer exists. The horizontal pressure on the soil blocks along the trench wall is no longer in equilibrium, and a block may not be able to support its weight and the weight of any blocks above. At the point where the soil can no longer withstand the pressure, the wall will shear and break away from its stable position.
B. The first failure occurs as the bottom of the wall moves into the trench .
C. This movement creates an undercut area at the base of the trench as soil material along the wall falls into the trench. Often there is a second movement in which more of the wall erodes. Finally, the erosion at the base of the trench leaves the upper part of the column supported only by cohesion to the columns around it.
D. More soil from the column will soon fall into the excavation.

15. SOIL BEHAVIOR SANDY SOIL: Tends to collapse straight down.
WET CLAYS AND LOAMS: Tend to slab off the side of the trench.
FIRM DRY CLAYS AND LOAMS: Tend to crack.
WET SAND AND GRAVEL: Tend to slide

16. INDICATIONS OF AN UNSTABLE TRENCH
A number of stresses and weaknesses can occur in an open trench or excavation.
Increases or decreases in moisture content can affect the stability of a trench or excavation.
Tension cracks usually form one – quarter to one – half of the way down from the top of a trench.
Sliding May occur as a result of tension cracks.
In addition Tension cracks can cause toppling.
Subsidence and bulging of the vertical face of the trench can cause wall failure if unbalanced stress is uncorrected.
Heaving or squeezing is caused by downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut.
Boiling is when water flows upward into the bottom of the cut. A high water table is one of the causes of boiling

17. TRENCH FAILURE
The following conditions will likely lead to a trench cave – in. These conditions are listed in order of seriousness.
Disturbed soil from previously excavated ground.
Trench intersections where large corners of earth can break away.
A narrow right – of – way causing heavy equipment to be too close to the edge of the trench.
Vibrations from construction equipment, near by traffic, or trains.
Increased subsurface water that causes soil to become saturated, and therefore unstable.
Drying of exposed trench walls that cause the natural moisture that binds together soil particles to be lost.
Inclined layers of soil dipping into the trench, causing layers of different types of soil to slide one upon the other and cause the trench walls to collapse.

18. MAKING THE TRENCH SAFE PROTECTIVE SYSTEM – A method of protecting employees from cave – ins, from material that could fall or roll from an excavation, or from the collapse of adjacent structures. Protective systems include:
SUPPORT SYSTEM
SLOPING AND BENCHING SYSTEM
SHIELDING SYSTEM
ANY OTHER SYSTEM THAT PROVIDES NECESSARY PROTECTIONS

19. SHORING – Shoring a trench supports the walls of the excavation and prevents their movements and collapse. Shoring does more than provide a safe environment for workers in a trench. Because it restrains the movement of trench walls, shoring also stops the shifting of adjacent soil formations containing buried utilities or on which sidewalks, streets, building foundations, or other structures are built.

20. TRENCH SHIELDS – Trench shields, also called trench boxes, are placed in un- shored excavations to protect personnel from excavation wall collapse. They provide no support to trench walls or surrounding soil, but for specific depths and soil conditions, will withstand the side weight of a collapsing trench wall.

21. SLOPING - Sloping an excavation means cutting the walls of the excavation back to an angle to its floor. This angle must be cut at least to the angle of repose for the type of soil being used. The angle of repose is the greatest angle above the horizontal plane at which a material will rest without sliding.

22. BENCHING OR STEPPING - method of protecting employees from cave – ins by excavating the sides of an excavation to form one or a series of horizontal levels or steps, usually with vertical or near vertical surfaces between levels.

23. GUIDELINES FOR WORKING IN AND NEAR A TRENCH
Before any excavation, underground installations must be identified. All underground utility locations must be documented on the proper forms. All overhead hazards (surface encumbrances) that create a hazard to employees must be removed or supported to eliminate the hazard.
Means of access / egress from trench excavations. A stairway, ladder, ramp or other safe means of egress shall be located in trench excavations that are 2 feet or more in depth so as to require no more that 25 feet of lateral travel for employees.
All spoil piles, tools, and equipment will be stored a minimum of 2 feet from the sides of the excavation. The spoil pile must not block the safe means of egress.
A competent person will inspect all excavations and trenches 3 feet or deeper daily, prior to employee entry, and after any rain fall, soil change, or any other time needed during the shift.
Adequate protective systems will be utilized to protect employees working in trenches and excavations 5 feet or deeper. This can be accomplished through sloping, shoring, or shielding.
All Excavations and trenches that have the potential for toxic substances or hazardous atmospheres must be tested at least daily. If the atmosphere is inadequate, protective systems will be utilized.

24. GUIDELINES FOR WORKING IN AND NEAR A TRENCH
Never enter an excavation until the excavation has been inspected.
Do not walk under loads being handled by back hoes, derricks, or hoists.
Stay clear of any vehicle that is being loaded.
Barricade all excavations to protect pedestrians and vehicles.
Wear protective clothing and equipment, such as hard hats, safety glasses, work boots, and gloves. Use respirator equipment if necessary.
No employee will work in an excavation and trenches where water is accumulating. Get out of the trench immediately if water starts to accumulate in the trench.
Do not work above or below a co – worker on a sloped or benched excavation wall.
If the excavation is to be over 20 feet deep, it must be designed by a registered professional engineer who is registered in the state where work will be performed.
If work is in or around traffic, employees must be supplied with and wear orange reflective vest. Signs and barricades must be utilized to ensure the safety of employees, vehicular traffic, and pedestrians.

25. GUIDELINES FOR WORKING IN AND NEAR A TRENCH
Make sure you are never alone in a trench. Two people can cover each other’s blind spots.
Stop work immediately if there is any indications of a cave – in. Make sure any problems are corrected before starting work again.
Never enter an excavation before a protective system is in place when required.
Do not install the shoring while you are inside the trench. All shoring must be installed from the top of the trench.
All materials used for shoring must be thoroughly inspected before use and must be in good condition.
Every excavation must be backfilled immediately after the support system is removed.

26. CAVE –IN- The separation of a mass of soil or rock material from the side of an excavation, or loss of soil from under a trench shield or support system, and its sudden movement into the excavation, either by failing or sliding, in sufficient quantity so that it could entrap, bury, or otherwise injure and immobilize a person.

27. EXCAVATION – Any man made cut, trench, or depression in an earth surface, formed by earth removal.

28. HAZARDOUS ATMOSPHERE – An atmosphere which by reasons of being explosive, flammable, poisonous, corrosive, oxidizing, irritating, oxygen deficient, toxic or otherwise harmful, may cause death, illness, or injury.

29. SURFACE LOADS – Generated by the weight of anything in proximity to the excavation, push starts for a cave – in ( anything up top pushing down ). Common surface loads: weight of spoil pile weight of nearby buildings, poles, pavement, or other structural objects weight of material and equipment

30. TRENCH – A narrow excavation ( in relation to its length ) made below the surface of the ground. The depth is greater than the with, but the width of a trench ( measured at the bottom ) is not greater that 15 feet.

32. COMPETENT PERSON – One 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 authorization to take prompt corrective measures to eliminate them.