Module 17: Excavations

Tension in the soil causes fractures

Fissured Soil Fissured soil is soil that tends to break up. Lines of fracture, which may further develop into open cracks, can form along the surface of fissured soil due to tension in the soil.

Little or no clay content

Granular Soil Granular soil is gravel, sand, or silt (coarse-grained soil), with little or no clay content.

Easily disintegrates when dry

Granular Soil These types of soils do not hold any form and are not pliable or flexible when wet. Granular soils easily disintegrate when dry.

Shoring system can successfully resist expected load.

Required Protective systems shall have the capacity to resist without failure all loads that are intended or could reasonably be expected to be applied or transmitted to the system.

Clay

Type A Soil Type A soils with an unconfined compressive strength of 1.5 tons per square foot can be readily indented by the thumb; however, they can be penetrated by the thumb only with very great effort.

Great effort to penetrate with thumb

Type A Soil Type A soils with an unconfined compressive strength of 1.5 tons per square foot can be readily indented by the thumb; however, they can be penetrated by the thumb only with very great effort.

Granular soils lacking cohesion

Type B Soil Granular soils lacking cohesion, including angular gravel and silt loam, are all examples of Type B soil. Cohesive soil with an indefinite strength of more than 0.5 tons per square foot, but less than 1.5 tons per square foot is called Type B soil.

Sandy loam

Type B Soil Granular soils lacking cohesion, including angular gravel, silt, silt loam, sandy loam—and in some cases, silty clay loam and sandy clay loam—are all examples of Type B soil.

Silt

Type B Soil Granular soils lacking cohesion, including angular gravel, silt, silt loam, sandy loam—and in some cases, silty clay loam and sandy clay loam—are all examples of Type B soil.

Submerged soil

Type C Soil Submerged soil, soil from which water is freely oozing, and submerged rock that is not stable may be classified as Type C soils.

Easily penetrated several inches by the thumb

Type C Soil Type C soils with an unconfined compressive strength of 0.5 tons per square foot can be easily penetrated several inches by the thumb and can be molded by light finger pressure.

Sand

Type C Soil Type C soils with an unconfined compressive strength of 0.5 tons per square foot can be molded by light finger pressure. Gravel, sand, and loamy sand are examples of Type C soils.

Shielding System

A constructed box inserted into the trench to provide additional support to the trench face

Materials used in protective systems must be approved by whom?

A professional engineer Any equipment or material used must be evaluated and approved or rejected by a registered professional engineer.

What factors must workers keep in mind when they are told to construct a surface crossing?

All of the above Surface crossings over trenches are not allowed unless conditions dictate such crossings are necessary. If surface crossings are necessary, such crossings must be constructed under the supervision of a registered professional engineer. Other crossing requirements include a minimum width of 20 inches, equipped with standard rails, and must extend a minimum of 24 inches past the surface edge of the trench.

High clay content

Cohesive Soil Cohesive soil has high clay content and is very durable and strong. It doesn't break up or decay and can be excavated using vertical side slopes.

Doesn't disintegrate when dry

Cohesive Soil When cohesive soil is moist, it is said to be in plastic form. Cohesive soil doesn't disintegrate when dry and is very solid when submerged.

When performing an excavation, employees are not permitted to work in atmospheres where oxygen is less than _______.

19.5%

Workers assigned the job must know not to install spoils within _____ feet of an excavation's edge.

2 Never place spoils within two feet of an excavation's edge.

Surface crossings over trenches must have a minimum width of _______.

20 inches

A stairway, ladder, or ramp is provided in trench excavations that are deeper than _______.

4 feet

Because they have an unconfined compressive strength equal to or greater than 1.5 tsf, the most stable excavations are cut in type _____ soils.

A Type A soil is a type of cohesive soil with an unconfined compression strength value of 1.5 ton per square foot (tsf), or greater.

The formation of one or a series of horizontal levels or steps along the side walls of an excavation refers to _____.

Benching Benching is a method of protecting employees from cave-ins by excavating the sides of an excavation such that they form a series of horizontal levels or steps.

Thumb penetration is used to test the compressive strength of which soils?

Cohesive The thumb penetration test can be used to evaluate the unlimited compressive strength of cohesive soils.

Inspections of excavations and adjacent areas must take place _______ by a competent person.

Daily

You're working on a big construction site and there's a lot of stuff going on. Which project is the most dangerous?

Digging a 10-foot-deep trench for a pipe Excavating is considered the most hazardous operation in the field of construction. An excavation is any human-made cut, cavity, trench, or depression in the earth's surface formed by earth removal.

Combustible gas concentrations are 30% of the LFL.

Don't Work Employees must not be allowed to work in hazardous or toxic atmospheres where combustible gas concentrations are greater than 20 percent of the lower flammable limit.

The atmosphere in the trench is 18 percent oxygen.

Don't Work Employees must not be allowed to work in hazardous or toxic atmospheres where oxygen is less than 19.5 percent or higher than 23.5 percent.

A competent person at an excavation site should have specific training and knowledge in all these areas EXCEPT:

Employee job assignments Employee job assignments are not part of a competent person's training of an excavation site; it should include specific training and knowledge about soil classification, requirements of the standards, and use of protective systems at an excavation site.

Who must ensure that materials and equipment are in good working condition?

Employers According to OSHA employers must ensure that materials and equipment are in good working condition.

When designing an effective protective system, all these factors MUST be considered EXCEPT:

Environmental impact When designing an effective excavation protective system, consider soil classification, depth of cut, water content of soil, changes due to weather and climate, and other operations in the vicinity.

Excavation entirely in stable rock.

Exempt Exceptions to the requirement for excavation protective systems include circumstances where excavations are made entirely in stable rock.

After completing the work on excavations:

Fill excavations back to their normal state Excavations must be immediately filled back to their normal state after completing the work.

In the event that an emergency rescue is necessary at an excavation site, all equipment should be present at all times EXCEPT:

Fire blankets Fire blankets are only used around fire hazards. Excavation sites normally only deal with cave-ins, caught-between, and hazardous atmosphere hazards.

What situation is indicated by small spalls along the vertical walls of an excavation?

If clods of soil spall (def. spall: flaking or otherwise breaking off from the main portion) off a vertical side, the soil could be fissured. Small spalls indicate moving ground and can pose potentially dangerous situations.

A trench that is six feet deep and 100 feet long with ingress and egress points on each end should have at least _____ other points of ingress or egress.

It is essential that a stairway, ladder, or ramp be provided in trench excavations 4 feet or more in depth so as to require no more than 25 feet of lateral travel for employees.

The unconfined compressive strength of soils can be tested by all these means EXCEPT:

Laboratory hygrometer Estimates of unconfined compressive strength of soils can be made by using a pocket penetrometer or a hand-operated shearvane. Other tests include drying and sedimentation. A hygrometer is an instrument for measuring relative humidity.

Hazards directly related to excavations Include all these types EXCEPT:

Lack of water—dehydration Lack of water, or dehydration, is a body hazard but is not considered a hazard directly related to excavations.

Shoring System

Providing the support back to the excavation walls by

When choosing a protective system for an excavation, all of these conditions should be considered EXCEPT:

Proximity of hospitals to worksite Proximity to a hospital from a worksite is not a condition considered when choosing a protective system for an excavation.

A properly designed protective system includes all these factors EXCEPT:

Material manufacturer's name The material manufacturer's name is not required in the construction of a properly designed protective system.

A 4-foot-deep trench that an inspector said provides no indication of a potential cave-in

No Exceptions to the requirement for excavation protective systems include circumstances where excavations are less than 5 feet (1.52 meters) in depth and where examination of the ground by a competent person provides no indication of a potential cave-in.

An excavation done entirely in stable rock

No Exceptions to the requirement for excavation protective systems include circumstances where excavations are made entirely in stable rock.

For trench collapse prevention, OSHA has approved all these systems EXCEPT:

Railing Systems that are used to keep trenches from collapsing include sloping, shoring, benching, and trench shield systems.

Surface crossings must be installed by a _______.

Registered Engineer

Backfilling progresses together with the removal of support systems.

Required Backfilling shall progress together with the removal of support systems from excavations.

The safe planning of an excavation should include all these preparation steps EXCEPT:

PPE costs When planning a safe excavation site, PPE costs do not come into the equation. Using proper PPE is an essential part of planning a safe excavation site.

Excavation filled back the day after work is completed.

Prohibit Excavations must be immediately filled back to their normal state after completing the work.

Workers remove the protective system, starting from the top and working down.

Prohibit Once the excavation has been cleared, workers should carefully remove the protective system, starting from the bottom and working up, while taking care in releasing all supporting members.

You're a supervisor on a crew digging a 12-foot-deep trench. Which of the following practices are safe?

Spoils are placed so that water is diverted away from the excavation, While workers are in the trench, a competent person monitors the excavation project until it is completed, Spoils have been placed four feet away from the trench, Workers are using water removal equipment to keep the water level in the trench low. Appropriate retaining devices must be used to protect workers from things that could fall into excavations.

Construction vehicles can be kept from rolling into an excavation by implementing all these precautions EXCEPT:

Surface crossings Surface crossings cannot prevent a construction vehicle from rolling into an excavation.

Warning System

Temporary device, such as a barricade, used when mobile equipment is operated near the excavation site

Cemented soils

Type A Soil Type A soil is a type of cohesive soil with an unconfined compression strength value of 1.5 tons per square foot or greater. Examples of Type A soil include cemented soils, such as caliche and hardpan.

Benching System

The formation of one or a series of horizontal levels or steps along the side walls of an excavation

When overhead loads are being lowered into trenches, trench workers can be best protected by _____.

Trench box and shoring systems Both trench boxes and shoring serve to protect workers from cave-ins and from materials being lowered into the trench. Trench boxes differ from shoring, however, in that trench boxes are shields used in trenches to support the trench face.

Excavations of more than four feet in depth must be tested for all these hazardous accumulations EXCEPT:

Vegetation Vegetation is not a hazardous accumulation. When testing at a excavation site, a gas monitor is used to determine hazardous conditions.

Workers should be kept out of trenches where _____ has accumulated in the floor.

Water Employees shall not work in excavations in which there is accumulated water, or in excavations in which water is accumulating, unless adequate precautions have been taken to protect employees against the hazards posed by water accumulation.

You're working on the crew that's digging a 10-foot-deep trench for a pipe. You know that the main danger with excavations is the possibility of cave-ins, but what's another hazard you'll have to watch out for?

Water accumulation The dangers of excavations come from the possibility of cave-ins, in addition to the possibility of the lack of oxygen (asphyxiation), fire, accidental break of underground utility lines (such as gas, electricity), collapse due to moving machinery near the edge of the excavations, inhalation of toxic materials, and water accumulation.

Using protective systems in excavation work can protect against all of these undesired events EXCEPT:

Water accumulation Using protective systems in excavation work cannot protect against water accumulation.

In layered soil systems, the system is categorized by analysis of the _____ layer.

Weakest In a layered soil system, the weakest layer is used for analysis to categorize the system.

Atmospheric contaminants in a trench are tested regularly.

Work Atmospheric contaminants in a trench must be tested regularly while performing work, to ensure a hazard-free environment.

Diversion ditches are used.

Work Diversion ditches, dikes, or any other suitable method can be used to prevent surface water from entering an excavation. Furthermore, a competent person must inspect excavations after heavy rains.

The atmosphere in the trench is 20 percent oxygen.

Work Employees must not be allowed to work in hazardous or toxic atmospheres where oxygen is less than 19.5 percent or higher than 23.5 percent.

An excavation near two buildings

Yes Excavations near adjacent structures may create surcharges, changes in soil conditions, or other disruptions that could lead to an excavation accident

An 11-foot deep trench that an inspector said is not likely to cave in

Yes Exceptions to the requirement for excavation protective systems include circumstances where excavations are less than 5 feet (1.52 meters) in depth and where examination of the ground by a competent person provides no indication of a potential cave-in.

An excavation for a house basement that is nine feet deep and not benched

Yes House foundation/basement excavations are exempt from the OSHA requirements for protective systems if they meet certain conditions, such as the house foundation/basement excavation is less than seven and one-half feet in depth or is benched for at least two feet horizontally for every five feet or less of vertical height.