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How To . . .

The following information is useful for determining the proper trench shielding for your projects.

How to Size a Trench Shield

Depth of cut* ____________
Soil conditions*
Type A (25#) ____________
Type B (45#) ____________
Type C (60#) ____________
Hydrostatic ____________

* see depth certification chart

Outside Pipe Diameter___________
(Shield must be a minimum of 12”
wider than the pipe.)

Pipe Length ___________
(Shield must be 2’ to 4’ longer than
the pipe.)

Bucket Width ___________
(Inside shield: 12” less than shield.)

Machine Lift Capacity ___________
(1.5 times shield weight at 20’ radius.)

The following will explain how to determine the specifications listed above.

How to determine the depth of cut:
g  Slope must start 18” or 1.5’ feet below the top of the Shield.

How to determine the width of a trench shield:
g  Trench Shield width determined by O.D. of the pipe or the O.D. of the excavator bucket.
g  Allow 6” of clearance on each side of pipe bell.
g  Shield should be a minimum of 12” wider than the excavator bucket O.D.

How to Determine the length of a trench shield:
g  Inside length clearance of pipe is approximately 2’ ft. less than the overall length of the shield.
g  Shield must be 2’ to 4’ longer than the pipe.

How to Determine the machine lift capacity:
g  1.5 times the shield weight at 20’ ft. radius at grade.

Calculating Depth Ratings
g  PSF ratings and depths are based on short term exposure with excavation open a period of time equal to 24 hours or less.

Lateral pressure per foot of depth:
g  Type “A” Soil = 25 lbs.
g  Type “B” Soil = 45 lbs.
g  Type “C” Soil = 60 lbs.

Depth rating using shield capacity (lbs.):
g  1200 / 25 = 48 ft.
g  1200 / 45 = 27 ft.
g  1200 / 60 = 20 ft.

Soil Definitions
g  Apparent cohesion
g  Cemented soil
g  Layered soil
g  Plastic
g  Saturated soil
g  Submerged soil

No soil is Type “A” if:
g  it is fissured.
g  it is subject to vibration.
g  it has been previously disturbed.
g  it is part of a sloped, layered system which dips into the excavation on a slope of 4 horizontal to 1 vertical [4H:1V] or greater.
g  it is subject to other factors requiring classification as less stable.

Type “B” Soil
g  Cohesive soil with unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf.
g  Granular cohesionless soils, e.g., gravel, silt, silt loam, sandy loam.
g  Type “A” fissured or subject to vibration.
g  Unstable dry rock.
g  It is part of a sloped, layered system which dips into the excavation on a slope less steep than [4H:1V], but only if the material would otherwise be classified as type “B”.

Type “C” Soil
g  Cohesive soils with unconfined compressive strength of 0.5 tsf or less.
g  Granular soils including gravel, sand, and loamy sand.
g  Submerged soil or soil from which water is freely seeping.
g  Submerged rock that is not stable.
g  Sloped, layered system which slopes into an excavation at an angle of 4H:1V or steeper.


Sloping vs. Trench Shields

Items having a high potential for cost over runs are:

g  Excessive excavation due to trench sloping.
g  Trucking excavated material from the job site.
g  Importing specified trench backfill material.
g  Excessive crew “down time” due to waiting for bedding stone or repair of damaged existing utility.
g  Roadway and lawn restoration.
g  Pipe testing and leak repair.
g  Documenting sewer and lead locations.

Unnecessary excavation due to sloping results in:

g  Higher trucking costs for removing excavated material.
g  Higher cost of importing backfill.
g  Higher street and lawn restoration.
g  More excavator cycles per cut.
g  Higher crew costs due to increased time of waiting for excavator to cut sloped trench.
g  Lower pipe production.
g  Possible liquidated damages due to inefficiency.


The High Cost of Restoration

Street and lawn restoration costs for projects in cities and subdivisions can be higher than the cost of laying the pipe. This illustration shows how the proper use of trench shields eliminates unnecessary excavation thus reducing the damage to streets, lawns, sidewalks, and existing utilities.