GTI Trenching & Shoring

7/26/2019 GTI Trenching & Shoring

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Trench ing &Shoring Safety

Susan Miller

Safety Training Coordinator

Murray State University

Soil is Heavy

• A cube of soil measuring 1

ft. on a side weighs at

least 100 lb (more in many

cases). A cubic yard of soil(3 ft. on a side) contains

27 of these, or 2,700 lbs.

total.

• This weighs about as

much as a mid-sized

automobile.

Dangers

• On average, 50 workers are

killed at excavation sites yearly.

• Cave-ins account for 76% of

fatalities.

• Being struck by or crushed by

equipment, dropped loads,

equipment rollovers,electrocutions, and other events

account for the remainder.

Excavation or Trench? What’s the difference?

• Excavation – any man cut, cavity, trench, etc.,

formed by earth removal. Excavations can

include a building basement, roadbed or trench.

• Trench – narrow underground excavation that is

deeper than it is wide and no wider than 15 feet.

• All trenches are excavations, but not allexcavations are trenches. Trenches are usually

more dangerous.

Overview of OSHA’s rule

• 29 CFR 1926.650-.652

– Assign a competent person

– Keep spoil piles and heavy equipment awayfrom the edge of trench

– Use adequate protective systems

– Train on hazard recognition and avoiding

unsafe conditions

Notifying the Authorities

Before starting work, OSHA requires:

• Determine the approximate location of

underground utilities.

• Contact the utility company to inform them of

proposed work and have them mark location

of lines.

Competent Person

• One who can identify existing and predictable

hazards in the surroundings, or unsanitary,

hazardous, or dangerous working conditions,

and who has the authority to stop work untilthey are fixed.

• Does not have to be at the

excavation jobsite at alltimes.

How Deep is the Excavation?

Four-foot rule-

• OSHA requires that workers

have a means to get in and out

of a trench if it is four or more

feet deep.

• You must not have to travel

more than 25 feet to reach themeans of egress.

How Deep is the Excavation?

Five-foot rule-

• OSHA requires protection from cave-ins by

protective systems unless the excavation is:

– Entirely in stable rock, or

– Less than five feet and the competent person

inspects the excavation and determines there is no

indication of a potential cave-in.

Testing & Classifying the Soil

• If your jobsite trench is not in stable rock or is

five-feet or more deep, it must be protected

from a cave-in.

• Testing and classifying soil is a criticalcomponent to protecting employees from a

cave-in. The competent person is responsible

for testing and classifying the soil.

• If there is ever a doubt about the soil type, it

must be treated as type C.

Stable Rock

• Natural solid mineral matter that can be

excavated with vertical sides and remain intact

while exposed.

• Usually identified as granite or sandstone.

• Probably not going to be on MSU campus.

Type A Soil

• TYPE A SOILS are cohesive soils with an unconfined

compressive strength of 1.5 tons per square foot (tsf)

or greater.

• Examples of Type A cohesive soils are often: clay, siltyclay, sandy clay, clay loam and, in some cases, silty

clay loam and sandy clay loam.

• No soil is Type A if it is fissured, is subject to vibration

of any type, has previously been disturbed, is part of a

sloped, layered system where the layers dip into the

excavation on a slope of 4 horizontal to 1 vertical

(4H:1V) or greater, or has seeping water.

Type B Soil

• TYPE B SOILS are cohesive soils with an unconfined

compressive strength greater than 0.5 tsf but less than

1.5 tsf.

• Examples of Type B soils are: angular gravel; silt; siltloam; previously disturbed soils unless otherwise

classified as Type C; soils that meet the unconfined

compressive strength or cementation requirements of

Type A soils but are fissured or subject to vibration; dry

unstable rock; and layered systems sloping into thetrench at a slope less than 4H:1V (only if the material

would be classified as a Type B soil).

Type C Soil

• TYPE C SOILS are cohesive soils with an unconfined

compressive strength of 0.5 tsf or less.

• Type C soils include granular soils such as gravel,

sand and loamy sand, submerged soil, soil from whichwater is freely seeping, and submerged rock that is not

stable.

• Also included in this classification is material in a

sloped, layered system where the layers dip into the

excavation or have a slope of four horizontal to one

vertical (4H:1V) or greater.

Why bother classifying?

• The most important reason for classifying soil

is that the results can be used to determine

what type of protective system can be used.

• If it is decided that the soil will not be classified,then:

– Excavations must have a slope of 1 ½ horizontal to

1 vertical (34 degrees).

What’s Next?

• Once the soil type at an excavation is

determined, the next task is to select what

protective system will be used.

– Sloping or benching of the sides

– Supporting the sides with shoring

– Placing a trench shield between the sides

• MSU is free to choose the most practical

design approach depending on the jobsite

circumstances.

Sloping & Benching

• If the soil has been classified, there are more options

for sloping and benching.

Soil Type Height/DepthRatio

Slope Angle(in degrees)

Stable Rock Vertical Vertical 90o

Type A ¾:1 53o

Type B 1:1 45o

Type C 1½:1 34o

Type A Soil Slope – ¾:1

Type A Bench

Simple

MultipleBench

Type B Soil Slope – 1:1

Type B Bench

Simple

Multiple

Type C Soil Slope – 1½:1

Type C Soil – Sloping & Shielding

Shoring & Shielding

• Shoring and shielding is used when the

location or depth of the cut makes sloping back

to the maximum allowable slope impractical.

• Shoring provides a support system for trenchfaces to prevent movement of soil,

underground utilities, roadways and

foundations.

Shoring Systems

• Shoring systems consist of posts, wales, struts

and sheeting.

• Two basic types of shoring:

– Timber

– Aluminum hydraulic

Hydraulic Shoring

• Hydraulic shoring is a prefabricated strut

and/or wale system made of aluminum or

steel.

• Hydraulic shoring offers a critical advantageover timber shoring because you do not have

to enter the trench to install or remove it. Most

systems are light enough to be installed by one

worker.

Example of Hydraulic Shoring

Shielding

• Trench boxes are different from shoring

because, instead of shoring the trench face,

they are intended primarily to protect workers

from cave-ins and similar incidents.• The excavated area between the

outside of the trench box and

the face of the trench should

be as small as possible.

• Box should extend 18 inches

above surrounding area if

sloping toward excavation.

Example of Trench Box

Personal Protective Equipment

• Hard hats are required for trench work as there

are always overhead hazards whether working

alongside the trench or in the trench.

• Ventilation equipment may be required if thereis a possibility of an atmospheric hazard.

Installing & Removing Protective Structures

• Connect support system members securely.

• Avoid overloading system members.

• Install other structural members to carry loads

imposed on the support system when you

need to remove an individual member.

• Remove from the bottom up.

• Backfill the excavation as soon as possible.

Getting In & Out of the Trench

• Structural ramps

• Ladders

• Earthen ramps

Jobsite Hazards

• Vehicle exposure – wear warning/reflective vest

• Spoil pile – 2 feet away minimum

• Falling loads – do not work under equipment

• Water – leave the trench during rainstorms, divert

surface water away from trench

• Crossing over – never a good idea

Hazardous Atmospheres

• Oxygen levels <19.5% or >23.5%

• Combustible gas >20% of LEL

• High concentrations of hazardous substances

• When testing for contaminants or oxygen levels-

– Testing must be conducted before employees enter

trench and regularly to ensure safe atmosphere.

– Frequency of testing should be increased if equipmentis operating in the trench.

– Testing frequency should also be increased if welding,

cutting, or burning is done in the trench.

Emergency Rescues

• Respirators may be required, you must be

trained and in the MSU respirator program.

• Lifelines must be provided and attended to at all

times if you enter bell-bottom pier holes, deepconfined spaces or other similar hazards.

• If you enter a confined space, you must be

trained to recognize and deal with the hazards.Normally, trenches are not considered confined

spaces because they are well ventilated.

Inspections

• Daily and before the start of work

• As work conditions change

• After every rainstorm

• When fissures, cracks, undercutting, water

seepage, bulging at bottom, etc. occur

• When there is a change in the size, location or

placement of the spoil pile• When there is any indication of change or

movement in adjacent structures

What’s wrong with this trench?

• No means of egress

• Spoil pile too close to edge

• Shoring not complete

• Missing backfill

• No edge stabilization

• No hard hats

• No air monitoring

Summary

• Cave-ins account for most fatalities

• Four-foot rule – must have means of egress

within 25 feet of all workers in trench

• Five-foot rule – protection from cave-ins must beprovided by using shoring, sloping or trench box

• Soil classifications – Solid rock, A, B, C

• Hazards

GTI Trenching & Shoring

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