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Excavation
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Excavation is one of the most hazardous construction operations. Refer OSHA Subpart P, Excavations, 29 CFR 1926.650,651 and 652.

 

CONFINED SPACE is a space that is limited openings for entry and exit, unfavorable natural ventilation, may contain or produce hazardous substances, and is not intended for continuous employee occupancy.

 

EXCAVATION is any man-made cut, cavity, trench or depression in an earth surface that is formed by earth removal. A TRENCH is a narrow excavation made below the surface of the ground. In general, the depth of a trench is greater than its width, and the width (measured at the bottom) is not greater than 15ft (4.6 m). if a form or other structure installed or constructed in an excavation reduces the distance between the form and the side of the excavation to 15ft or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.

 

UNCONFINED COMPRESSIVE STRENGTH is the load per unit area at which soil will fail in compression. This measure can be determined by laboratory testing, or it can be estimated in the field using using a pocket penetrometer, by thumb penetration test, or by other methods.

 

Causes of Trench Failures

 

Tension cracks: Tension cracks usually form at a horizontal distance form at a horizontal distance of 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench.

 

Sliding: It is also called as sluffing may occur as a result of tension cracks.

 

Toppling: In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s vertical face shears along the tension crack line and topples in to the excavation.

 

Subsidence and Bulging: An un supported excavation can create an unbalanced stress in the soil, which in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause failure and entrapment of workers in the trench.

 

Heaving or Squeezing: Bottom heaving or squeezing is caused by the downward pressure created by the downward pressure created by the weight of the soil. This pressure causes a bulge in the bottom of the cut.

 

Boiling: It causes by the upward water flow into the bottom of the cut. A high water table is one of the causes of boiling

 

Determination of Soil type.

Stable rock: Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.

 

Type A soils: It’s a cohesive soils with an unconfined compressive strength of 1.5 tons per sq.ft (144 kPa) or greater. Clay, silty clay, sandy clay, clay loam. No soil is Type A if 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 soils: It’s a cohesive soil with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa). E.g. Angular gravel, slit loam, 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 subject to vibration, dry unstable rock and layered systems sloping into the trench at a slope less than 4H:1V (only if the material would be classified as a Type B soil).

 

Type C soils: It’s a cohesive soil with an UCCS of 0.5 tsf (48kPa) or less. Other Type C soils include granular soils such as gravel, sand and loamy sand, submerged soil, soil from which water is freely seeping and submerged rock that is not stable. Also included in the classification is material in a slopped, layered system where the layers dip into the excavation or have a slope of four horizontal to one vertical (4H:1V) or greater.

 

Layered geological strata: where soils are configured in layers, i.e., where a layered geological structure exists, 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, i.e., where a type C soil rests on tope of stable rock.

 

Test Equipment and Methods for Evaluating Soil type:

Pocket Penetrometer: Penetrometer are direct-reading, spring operated instruments used to determine the unconfined compressive strength of saturated cohesive soils.

 

Shearvane (Torvane): To determine the UCCS of the soil with a Shearvane, the blades are pressed into a level section of undisturbed soil and the torsional knob is slowly turned until soil failure occurs. The reading must be multiplied by 2 to get result in tons/sqft.

 

Thumb Penetration Test: This involves an attempt to press the thumb firmly into the soil in question. If the thumb makes an indentation in the soil only with great difficulty, the soil is probably Type A. if the thumb penetrates no further than the length of the thumb nail, it is probably Type B soil, and if the thumb penetrates the full length of the thumb, it is Type C soil. This test is less accurate of the three methods.

 

Dry Strength Test: Dry soil that crumbles freely or with moderate pressure into individual grains is granular. Dry soil that falls into clumps that subsequently break into smaller clumps (smaller clumps can be broken only with difficulty) is probably clay in combination with gravel, sand or slit.

 

Plasticity or Wet Thread Test: This test is conducted by molding a moist sample of the soil into a ball and attempting to roll it into a thin thread approximately 1/8 inch (3mm) in dia (thick) by 2 inches (50 mm) length. The soil sample is held by one end. If the sample does not break or tear, the soil is considered cohesive.

 

Visual Test: Vibrations, crack-line openings along the failure zone that would indicate tension cracks, look for existing soil disturbance, signs of bulging, boiling or sluffing as well as for surface water seeping.

 

Shoring Types:

Shoring is the provision of a support system for trench used to prevent movement of soil, or any structures near by. Shoring or shielding is used when the location or depth makes sloping back to the maximum allowable slope impracticable. Shoring systems consist of posts, wale’s, struts and sheeting. Timber, aluminum, pneumatic and hydraulic are the major type of shoring.

 

Hydraulic shoring: Used often today. All shoring should be installed from the top down and removed for the bottom up. Hydraulic shoring should be checked at least once per shit fro leaking hose and or cylinders, broken connections, cracked nipples, bent bases and any other damaged or defective parts.

 

Pneumatic shoring: It works in a manner similar to hydraulic shoring. The primary difference is that pneumatic shoring uses air pressure in place of hydraulic pressure. A disadvantage to the use of pneumatic shoring is that an air compressor must be on site.

 

Trench Boxes: These are primarily to prevent 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. The space between the trench boxes and the excavation side are back filled to prevent lateral movement of the box.

 

Sloping and Benching:

Sloping: Maximum allowable slopes for excavations less than 20 ft (6.09 n) based on soil type and angle to the horizontal are as follows:

Allowable slopes – for stable rock the Height/Depth ratio is vertical and slope angle is 90 deg.

       for Type A the Height/Depth ratio is :1 and slope angle is 53 deg.

– for Type B the Height/Depth ratio is 1:1 and slope angle is 45 deg.

– for Type C the Height/Depth ratio is 1.5/1 and slope angle is 34 deg.

– for Type A (short term)  the Height/Depth ratio is 0.5:1 and slope angle is 63 deg.

Pls note that for a maximum excavation depth of 12 ft.

 

Benching: There are two basic types of benching, simple and multiple. The type of soil determines the horizontal to vertical ratio of the benched side.

As a general rule, the bottom vertical height of the trench must not exceed 4 ft(1.2 m) for the first bench. Subsequent benches may be up to a maximum of 5ft (1.5 m) vertical in Type A soil and 4 ft (1.2 m) in Type B to a total trench depth of 20 ft (6.0 m). All subsequent benches must be below the maximum allowable slope of that soil type. For Type B soil the trench excavation is permitted in cohesive soil only.

 

Spoil:

Temporary Spoil: Not to be placed closer than 2ft (0.61 m) from the surface edge of the excavation, measured from the nearest base of the spoil to the cut.

Permanent Spoil: Should be placed at some distance away from excavation.

 

Site Assessment Question

Is the cut, cavity, or depression a trench or an excavation?

Is the cut, cavity, or depression more than 4ft (1.2 m) in depth?

Is there water in the cut, cavity or depression?

Are there adequate means of egress?

Are there any surface encumbrances?

Its there exposure to vehicular traffic?

Are adjacent structures stabilized?

Does mobile equipment have a warning system?

Is a competent person in charge of the operation?

Is equipment operating in or around the cut, cavity, or depression?

Are procedures required to monitor, test, and control hazardous atmosphere?

Does a competent person determine soil type?

Was a soil testing device used to determine the soil?

Is the spoil placed 2ft or more from the edge of the cut, cavity or depression?

Is the depth 20 ft (6.1 m) or more for the cut, cavity, or depression?

Does the procedure require benching or multiple benching? Shoring? Shielding?

If provided, do shields extend at least 18 in (0.5 m) above the surrounding area if it is sloped toward the excavation?

If shields are used, is the depth of the cut more than 2 ft (0.6m) below the bottom of the shield?

Are any required surface crossing of the cut, cavity, or depression the proper width and fitted with hand rails?

Are means of egress from the cut, cavity, or depression no more than 25 ft (7.6m) from the work?

Is emergency rescue equipment required?

Is there documentation of the minimum daily excavation inspection?

 

 

 

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