Causes of Cave-ins - ACRP · Causes of Cave-ins - 29 - 5 Competent Person - one who is capable of...

Lesson #2

Causes of Cave-insWhat is in this lesson?

1. The four things that impact soil strength

2. The five things that create downward forces that impact soil stability

3. The terminology used to identify soil types

4. The terminology used to describe water conditions

5. The responsibilities of the Competent Person regarding classifying soil types

Key Words• Angular gravel • Competent Person

• Clay • Cemented Soils

• Cohesive Soils • Density

• Fissured Soil • Granular Cohesionless Soil

• Granular Soil • Gravel

• Layered Soil System • Porosity

• Rounded Gravel • Sand

• Silt • Sloped Layered System

• Soil Type A, B or C • Spall

• Spoil Bank • Stable Rock

• Surcharge • UCS

Causes of Cave-ins

- 23 -

Cave-In Protection & Competent Person

- 24 -

Causes of Cave-insIntroductionFrom Lesson #1 As we learned in lesson one, cave-ins can occur when

gravity overcomes the ability of the soil to support itsown weight. Signs that indicate the possibility that thesoil is moving are: Bulging at the bottom or walls ofthe trench, tension cracks parallel to the trench wallsor spalls from the trench walls.

Soil Stability In order for soil to stand when a trench is opened itmust withstand the gravitational forces and theenvironmental factors that may be working to breakdown the soils natural ability to bind together. Thisnatural ability to bind together is called the soilstrength.

Soil Strength The strength of the soil is dependent on the bondingbetween soil particles. This bonding is called cohesion.How cohesive1 a soil is, is dependent upon the type ofsoil, the amount of moisture present in the soil andthe proximity of previously disturbed soil (as in anadjacent trench).

Gravitational Factors Factors that work against the strength of the soilinclude depth of the trench, the weight of the soil andthe weight of the surcharge. Surcharge is the weightplaced on the trench walls by the spoils pile andadjacent equipment. The weight of the soil iscontrolled by the density2 of the soil, the porosity3 ofthe soil and the amount of soil moisture.

Environmental Factors Environmental factors include weather conditions andthe amount of time the trench walls are exposed to theatmosphere.

This Lesson In this lesson we will discuss each of the factorsconcerning soil strength and the forces working tobreak down the soils ability to hold together.

Causes of Cave-ins

- 25 -

STABILITY OF SOIL DEPENDS UPON:

DEPTH�DENSITY�POROSITY�SOIL MOISTURE�SURCHARGE

SOIL TYPE�SOIL MOISTURE�FROZEN SOIL�DISTURBED SOIL��

WEATHER�EXPOSURE�FROST��

ENVIRONMENTAL FACTORSSOIL STRENGTH GRAVITATIONAL FACTORS

1 Cohesive Soil - A clay (fined grained soil), or soil with a high clay content, which has cohesive strength.Cohesive soil does not crumble, can be excavated with vertical side slopes, and is plastic when molded.Cohesive soil is hard to break up when dry, and exhibits significant cohesion when submerged. Cohesive soilsinclude clayey silt, sandy clay, silt clay, clay and organic clay.

2 Density - The weight per unit volume of a substance. (pounds/cubic foot).3 Porosity - The ratio of pore space to total volume. That portion of a cubic foot of soil that is air space and

therefore could contain moisture.

Cave-ins and Cave-in Indicators

Two Forces There are two forces at work on the soil that form thesides of a trench. One is the vertical or downwardforce caused by gravity and the second in thehorizontal or lateral force. Undisturbed soil issupported on all sides by adjacent soil. When soil isexposed by excavation the sides of the trench wall areno longer supported.

When Cave-ins Occur A cave-in occurs anytime the strength of the soil isovercome by the weight of the soil. If a column of soilwere placed under a steadily increasing pressure, atsome point the column would crumble under the load.The amount of force required to cause this failure iscalled the unconfined compressive strength of the soil.

Causes of Failure When a trench is opened, the lateral support for thewall of the trench is removed. This causes the soilnear the bottom of the trench to bulge inward as aresult of the weight of the soil above it. When thishappens the lip of the trench wall will movedownward. This downward movement causes the soilin back of the trench face to hold on to the soil at thetrench face, keeping it from caving into the trench.This creates a tensile stress at right angle to thetrench wall creating tension cracks parallel to thetrench. The trench is now ready for failure.


- 26 -

The Cave-in Under this condition, the first portion of trench to failwill be at the bottom of the trench. This will befollowed by a second failure and finally a third. Themost common failures of trench walls occur in threesteps.

Three Types of Cave-ins and their Indicators

Introduction There are several ways that cave-ins occur. The threeprimary ones are:

Bulging at Bottom of Trench (1) When the soil sags and forms a bulge at thebottom, it is getting too heavy to hold itself up. Thisoften occurs with wet soil. The weight of the addedwater causes the soil to exceed its holding capacityand the bulge folds into the trench.

Tension Cracks (2) Soil will develop tension cracks at the top thatallow the top to lean toward the center of theexcavation. When this happens, the wall can tumbledown into the trench. When you see these conditions,even in shallow excavations, you should stop workand install some form of protection.

Causes of Cave-ins

- 27 -

1

2

3

1

2

3

Subsidence

Profile of excavation�showing subsidence�and bulging

Formation of�tension cracks

Tension Crack

Classic Failure (3) The wedge described earlier in this lesson isconsidered the classic failure. This failure occurswhen a wedge of the soil slides into the trench. Thistype of failure supports the reasoning behind the useof sloping as a means of controlling cave-ins.

Other Signs Other signs of impending trench failure are; spalling4

of materials from the sides of the trench, the sliding ofportions of layered soils, and the falling of fracturedsections of rock. All of these signs are described ingreater detail in the next lesson.


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4 Spall - Chips of soil that break off the walls of a trench.

Responsibility of Competent PersonClassifying Soil The Competent Person5 must be able to classify each

soil and rock deposit associated with the trench as toStable Rock6, Type A7, Type B 8or Type C9 soil.

Basis of Classification The classification of soil type must include at least onevisual and one manual test. While several tests areavailable it is recommended that the CompetentPerson primarily rely on those tests described in theregulations. This testing must be done by theCompetent Person and performed prior to and duringthe job.

Reclassification If after the soil has been classified conditions change,the Competent Person must reevaluate the situationand if necessary change the soil classification.

Dynamic Situation As you can see the classification of soil is a dynamicsituation. For this reason the Competent Person mustbe on or near the job at all times and besystematically evaluating the soil conditions as well asother hazards that may be encountered by the crew orpublic. In some cases one Competent Person may beable to evaluate the conditions at two or more locationsif they are close together.

Causes of Cave-ins

- 29 -

5 Competent Person - one who is capable of identifying existing and predictable hazards in the surroundings, orworking conditions which are unsanitary, hazardous, or dangerous to employees, and who has authorization totake prompt corrective measures to eliminate them.

6 Stable Rock - Natural solid mineral material that can be excavated with vertical sides and will remain intact whileexposed. Unstable rock is considered to be stable when the rock material on the side or sides of the excavationis secured against caving-in or movement by rock bolts or by another protective system that has been designedby a registered professional engineer.

7 Type A Soils - cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) orgreater. Examples of cohesive soils are: clay, silty clay, sandy clay, clay loam and in some cases, silty clay loamand sandy clay loam. Cemented soils such as caliche and hardpan are also considered Type A.

8 Type B Soils - cohesive soils with an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsfor, granular cohesionless soils including; angular gravel (similar to crushed rock), silt, silt loam, sandy loam and,in some cases, silty clay loam and sandy clay loam. Previously disturbed soils except those which wouldotherwise be classed as Type C soil. Soil that meets the unconfined compressive strength or cementationrequirements for Type A, but is fissured or subject to vibration or; dry rock that is not stable. Material that is partof a sloped, layered system where the layers dip into the excavation on a slope less steep than 4H:1V, but only ifthe material would otherwise be classified was Type B.

9 Type C Soil - cohesive soil with an unconfined compressive strength of 0.5 tsf or less, or granular soils includinggravel, sand and loamy sand, or submerged soil or soil from which water is freely seeping, or submerged rockthat is not stable, or material in a sloped, layered system where the layers dip into the excavation or slope of4H:1V or steeper.

Soil Strength Factors

Introduction

Terms Used to Describe Soil Texture One of the functions of the Competent Person is toidentify soil type by observation. This requires anunderstanding of how soil is typed and classified. Oneof the common methods of typing soil is by texture.Texture is the basis for the OSHA soil classificationsystem. Texture is based on the size of the particles.

Soil Particle Size There are three common soil particle size groupings.They are:

Clay Clay is composed of mineral particles less than 0.002mm in diameter

Silt Silt is composed of individual mineral fragments thatrange from 0.002 to 0.05 mm in diameter.

Sand Sand is individual rock or mineral fragments thatrange in diameter from 0.05 to 2.0 mm in diameter.

Combining the Particle Sizes These three particle size groupings are used to definefour textures, Clay, Sand, Silt and Loam andcombination classes such as clayey silt, sandy clay,etc (see Lesson 3 for more details). We realize that theuse of clay, sand and silt to describe particle size andtexture appears to be in conflict. However, we are notresponsible for the methods used over the years todescribe soils. The use of the terms to describe bothconditions is common and true, even if it does lead to alevel of confusion. For that reason OSHA has devisedits own texture system, the Type A, B and C soils.OSHA also refers to soil texture as either fine grained orcoarse grained. OSHA soil classification system isbased on visual observation and performanceinformation gained by performing manual testing. Whileknowledge of texture is important, Type A, B, and Cclassifications are related to performance andobservation.

Clarification In the next lesson on Soil Classification we will go intomore depth about these four textures and how they areused to determine the three OSHA soil classifications.

Other Terms Besides these basic terms, the excavation regulationsdescribe the following terms and conditions inclassifying soils.


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Sand ClaySilt

Four Soil Classifications The OSHA excavation rules categorize soil as stablerock, or Types: A, B and C soil. These classificationsare based on the Unconfined CompressiveStrength10 (UCS) of the soil. (The amount of pressurein tons per square foot required to cause the soil tofail.) Even though it primarily is the responsibility ofthe Competent Person, each person on the job shouldbe able to identify soil in these three categories.

Unconfined Compressive Strength The load that a soil will handle before failure isdependent upon the combination of gravitational forces,environmental conditions and soil strength. The abilityof the soil to support a load can be estimated bydetermining the tons per square foot required to causethe soil to fail. The UCS of the soil can be estimated inthe field accurately by one of two common testinstruments. In the lesson on soil testing and visualobservation, use of the test instruments will bediscussed in length.

Comparing UCS to Soil Classification Type A soil is the strongest with an UCS of 1.5 tsf orgreater. Type B soil ranges between 1.5 tsf and 0.5 tsf.Type C soil is any soil that has a UCS of less than 0.5 tsf.

Soil TypesCemented Soils Cemented soil is soil

in which the particlesare held together by achemical agent, suchas calcium carbonate.Finger pressurecannot crush hand-size sample ofcemented soils into powder or individual soil particles.

Cohesive Soils Cohesive soils arecomposed of clay(fined grained soil), orsoil with a high claycontent, which hascohesive strength.Cohesive soil does notcrumble, can beexcavated with vertical side slopes, and is plastic11

when molded. Cohesive soil is hard to break up whendry, and exhibits significant cohesion even whensubmerged. Cohesive soils include clayey silt, sandyclay, silty clay, clay and organic clay.

Causes of Cave-ins

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10 Unconfined Compressive Strength - the load per unit area at which a soil will fail in compression. It can bedetermined by laboratory testing, or estimated in the field using a pocket penetrometer, or thumb penetration test,and other methods.

11 Plastic - as it applies to a soil means a property of the soil which allows the soil to be deformed or moldedwithout cracking or changing is volume appreciably.

B CA

1.5 tsf 0.5 tsf

Granular Soil Granular soil includes gravel, sand, or silt, (coarsegrained soil) with little or no clay content. Granularsoil has no cohesive strength. Some moist granularsoils exhibit apparent cohesion. Granular soil cannotbe molded when moist and crumbles easily when dry.

Granular Cohesionless Soil Granular cohesionless soils include angular gravel(similar to crushed rock), and the soil textures of silt,silt loam, sandy loam and in come cases, silty clayloam and sandy clay loam. Generally, granularcohesionless soils are very unstable and may requireprotection from cave-ins due to the high lateralpressure exerted by angular gravel.

Gravel A soil is said to be gravel any time that more than onehalf of the visible grains are larger than 3/16”. It isimportant to identify if these individual grains arerounded or angular. Typically soils with gravelcontaining rounded grains are less stable than soilswith angular grains. Angular gravel is commonly TypeB soil and rounded is Type C soil.


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Soil Conditions

Fissured Soil Condition A fissured soil condition exists when a soil has atendency to break along definite planes of fracturewith little resistance. It includes conditions where soilexhibits open cracks, such as tension cracks, on anexposed surface. These cracks can be observed in thetrench wall and in the area adjacent to the trench.

Fractured Rock Condition Fractured rock is similar to fissured soil, in that it hasa tendency to break along definite planes of fracture.Fractured rock may or may not be unstable dependingon the slope of the fractures.

Layered Soil Condition A layered soil system is two or more distinctly differentsoil or rock types arranged in layers. Micaceous seamsor weakened planes in rock or shale are consideredlayered. Depending on soil type, layered systems maybe less stable than unlayered systems.

Causes of Cave-ins

- 33 -

STRONG

WEAK

WEAKSTRONG

STABLE UNSTABLE

Sloped Layered Condition A Sloped Layered System exists any time the layersare at an angle. This is of special importance when thelayers slope into the trench. If the upper layer is theweakest it may slide into the trench. If the lower layeris the weakest it may collapse and allow the upperlayer to fall into the trench.

Soil Moisture

As A Stability Factor Each soil can go from being very crumbly to runnymud depending upon the amount of moisture in thesoil. Soil at either end of the moisture spectrum couldcreate an unsafe trench. However, for any soil there isa critical amount of moisture that gives the soil itsmaximum bonding power or cohesion.

OSHA Soil Moisture Terms OSHA has defined five soil conditions associated withthe amount of moisture in the soil. They are:

• Dry soil

• Moist soil

• Wet soil

• Saturated soil

• Submerged soil

These conditions and their impact on the soilclassification system are described in detail in thelesson on soil classification.

Frozen Soil

Stability Frozen soil and permafrost can be very stable. Inconditions where the soil is frozen well below thetrench bottom the stability of the trench walls may beequal to that provided by stable rock.

Instability The major safety considerations with frozen soil arethe presence of unfrozen soil at the bottom or justbelow the bottom of the trench and thawing of thetrench walls. These conditions can turn a stable soilinto a liquid like material that is at the low end of thestability table.


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Causes of Cave-ins

- 35 -

Presence or Absence of Adjacent Disturbed Soil

Impact of Disturbed Soil Undisturbed soil is almost always stronger thandisturbed soil, regardless of the tamping methodsused and the degree of compaction. When replacingan old water line, it is common practice to digadjacent to the oldline and otherundergroundutilities and thusadjacent to an oldtrench. The prismof soil between thetwo trenches isvery unstable.Likewise at anypoint that youcross a previouslydug trench theside walls of thenew trench will be weakened.

Gravitational Factors Five Factors These are the factors that work to overcome the

stability of soil. There are five of them and we shalltake a quick glance at each.

Depth of Trench

Double Depth Increases Weight by 4 As was mentioned in the introduction, soil has atendency to slide from the wall of the trench in awedgelike fashion. Therefore, the volume and theweight of the soil will increase as the trench isdeepened. A rule of thumb that can be used is that ifyou double the depth of a trench you increase thevolume of the soil that could fall into the trench by afactor of four. A five foot deep trench could drop 625pounds of soil for each foot of length. As a safetyfactor, OSHA has established that a trench more thanfive feet deep, regardless of the soil type, must beshored orotherwiseprotectedagainstcave-in.

State Specific SomeStateshave setthisminimumdepth at4 feet. Itisimportantthat you determine the minimum depth for your State.

OLD TRENCHES

5 ft

. +

PROTECTION�

REQUIRED


- 36 -

Soil weight

Material Type, Weight, Moisture The density of the soil, that is, the weight of eachcubic foot of soil, is dependent upon the nature ofmaterials that make up the soil particles and thus theweight of each particle. It is also dependent upon theporosity of the soil. The greater the porosity, the moremoisture the soil can hold and therefore the more itwill weigh for each cubic foot. For example, dry claymay weigh as little as 65 pounds per cubic foot andwet clay as much as 110 pounds per cubic foot.

Impact of Moisture Notice that soil moisture affects both the weight andthe cohesiveness of the soil. Therefore, soil moisture isone of the key elements we need to be aware of whenevaluating soil stability.

Responsibility of the CP The Competent Person is responsible for identifyingthe soil moisture condition and classifying the soilmoisture into one of the five OSHA soil moisturecategories. This will be accomplished during the visualand manual testing. A detailed discussion of thevarious moisture conditions is found in lesson 3.

Clay

Loose Earth

Packed Earth

Sand & Gravel

Dry Wet

WEIGHT of SOIL in lbs./cu. ft.

85 110

75 105

95 115

90-120 125

SO

IL T

YP

E

Surcharge

Spoil Bank

Additional Weight When the spoils are placed too close to the trench wallthey place additional load on the trench wall thatcould contribute to a cave-in. This added weight andthe weight of any equipment placed next to the trenchwall are referred to as surcharge12 loads.

Reduction of Surcharge To reduce the impact of the spoils bank13 the spoilsmust be placed so that the toe of the slope of thespoils bank is at least two feet from the trench edge.This is a minimum distance. To determine the bestplacement of the spoil bank you must first identify thesoil type and depth of the trench. Then run animaginary line from the bottom of the trench to the topof the ground. The center of the spoil bank should bebeyond this line. This places the least amount of loadon the trench walls.

Causes of Cave-ins

- 37 -

SPOILS

AT LEAST 2ft.AT LEAST 2ft.

Surcharge Load

Surcharge Load

12 Surcharge - The weight contributed to the walls of a trench by the load adjacent to the trench. Typically this loadis composed of the spoil bank and construction equipment.

13 Spoil bank - The bank of materials removed from a trench or excavation.

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TRENCH�LESS THAN�

20 FEET�DEEP

TYP

E A

(3/

4 : 1

)

TYPE B

(1

: 1)

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TYPE C (1.5 : 1

)


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Rocks & Chunks When the spoils contain larger chunks or bouldersthat may roll back into the trench, the spoils shouldbe placed at a greater distance from the trench. Inaddition, extra protection, such as a berm, may berequired to prevent the material from rolling andfalling into the trench.

Surcharge - Equipment

Besidesthe weightof thespoils, thetrench wallcould beoverloadedbyequipmentthat isusedalongsideof thetrench. In some cases it is necessary to moveequipment along the trench edge. The equipmentshould be kept back of the point that would representthe center of the spoils bank, at least two feet from thetrench, or the trench wall should be protected in somemanner. When it is necessary to work in a trenchwhere equipment is operating adjacent to the trench,the protection system must be designed for thisadditional load.

Surcharge-Vibration

Equipment & Traffic As important as the surcharge from the weight of theequipment are the vibrations caused by the presenceof the equipment. Vibrationcaused by any equipmentsuch as the backhoe,loader, boom truck, ortamping equipment cancause the soil to loosen andresult in a cave-in. Theequipment could be in, oralongside the trench andhave the same impact.Large pieces of equipmentor high volume traffic somedistance from the trench can have the same impact onbank stability due to vibration.

Environmental FactorsWeather Conditions

Rain There is a critical balance that exists between toomuch and too little moisture. There are various waysthat this balance can be disturbed. For instance, letssay you open a trench in the morning and the wallsare stable. A rain storm during the day could increasethe soil moisture beyond the critical stage and causethe walls to collapse.

Time When a trench is opened the walls are exposed to theair. Moisture in the soil will evaporate into theatmosphere. This causes the soil to dry and couldcause the trench walls to crumble.

How Frost Weakens the Trench Frozen ground may thaw and weaken and crumble.On the other hand an unfrozen open trench may befrozen and the expansion of the soil from the freezingcould cause the walls to cave-in. Another factor toconsider is the depth of the frost penetration. Shouldyou dig below the frost line the soil may be mud andthus unstable causing the soil above it to cave-in.

Causes of Cave-ins

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RAIN

GROUND WATER

SURFACE RUNOFF

SOFT SATURATED ZONE

WATER�ACCUMULATION

Indicators of FailureObservations The Competent Person must frequently observe the

trench floor, walls, and the area adjacent to the trenchfor early signs of conditions that can lead to a cave-in.Some of the more common conditions are:

• Tension cracks parallel to the trench wall

• Changes in moisture conditions of the trench wall orfloor

• Spalling of the trench walls

• Crumbling of the trench walls

• Bulging of the walls near the trench floor

ConclusionThe strength of the soil, environmental andgravitational forces on the soil compete against eachother and affect soil stability. The Competent Personmust know about each set of factors contributing tosoil instability and observe the effects produced bychanges in the work site environment. They must beable to predict probable hazards and beknowledgeable in ways to increase the strength of theprotective system to accommodate surcharge loadsand changing conditions.


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Causes of Cave-ins Worksheet

1. What are the three major items that impact soil stability?

a.

b.

c.

2. The ability of a soil to bind together is called?

3. Soil strength is measured in tons per square foot, the term which describes this is called?

4. What is the name given to soil that has a tendency to break along a definite plane?

5. At what size does soil become gravel?

6. Which is stronger, (a) disturbed soil or (b) undisturbed soil?

7. Name one positive and one negative impact moisture can have on a soil condition?

Causes of Cave-ins

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8. A Competent Person must be able to identify and classify soil into four OSHAclassifications. These classifications are:

a.

b.

c.

d.

9. What do each of the following signs of distress indicate

a. Spalling __________________________________________

b. Tension cracks _____________________________________

c. Slumping at the base of the trench wall __________________

d. Bulging trench walls _________________________________


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Causes of Cave-ins - ACRP · Causes of Cave-ins - 29 - 5 Competent Person - one who is capable of...

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