Apr 11, 2018
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Welcome, in this episode I want to talk about trenching and excavation safety. I know here locally we just had another trench collapse that trapped 2 workers. And I am sure you may have heard about incidents near you or reading about them on a national level. Managing safety for these operations is quite simple once you know the requirements and understand some of the nuances that go along with the different soils and protective systems we have.
Cave-ins pose the greatest risk with these activities and are much more likely than other excavation-related accidents to result in worker fatalities. Other potential hazards include falls, falling loads, hazardous atmospheres, and incidents involving mobile equipment. One cubic foot of soil can weigh 100 pounds. One cubic yard of soil can weigh as much as a car, and the kinetic energy of soil falling 3, 4, 10 feet, and you can see the danger here. It has been said that an unprotected trench is an early grave. So let me start by stating the obvious: do not enter an unprotected trench.
So at what point do we need to protect a trench? According to OSHA, trenches 5 feet (1.5 meters) deep or greater require a protective system unless the excavation is made entirely in stable rock. Oh, and forget about stable rock, I will explain that later. If less than 5 feet deep, a competent person may determine that a protective system is not required.
OSHA standards require that employers inspect trenches daily and as conditions change by a competent person before worker entry to ensure the elimination of excavation hazards. A competent person is an individual who is capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary, or dangerous to workers, soil types and protective systems required, and who is authorized to take prompt corrective measures to eliminate these hazards and conditions.
OSHA standards require safe access and egress to all excavations, including ladders, steps, ramps, or other safe means of exit for employees working in trench excavations 4 feet (1.22 meters) or deeper. These devices must be located within 25 feet (7.6 meters) of all workers. Here are some other requirements to follow:
Keep heavy equipment away from trench edges. In the fire department, we tried to keep all heavy rigs at least 25 feet away. But this was to prevent vibrations to already unstable trenches during rescue operations. But heavy equipment that could pose a hazard of falling into the trench or even knocking materials into the trench has to be located back at a safe distance.
The standard requires that you keep excavated soil (spoils) and other materials (like sand and gravel used for backfill or pipes being installed) at least 2 feet (0.6 meters) from trench edges. Ongoing inspections are needed to ensure this requirement is being met.
Know where underground utilities are located before digging. This is a requirement in the OSHA standards, but it requires you to follow State laws that actually govern locating and marking PUBLIC utilities. Private utilities still need to be located and marked as well. So, familiarize yourself with the 811 "call before you dig" system.
Test for atmospheric hazards such as low oxygen, hazardous fumes, and toxic gases when > 4 feet deep. This is when you suspect there could be a hazardous atmosphere or one could reasonably be expected to exist, such as in excavations in landfill areas or excavations in areas where hazardous substances are stored nearby.
The COMPETENT PERSON must inspect trenches AND the adjacent areas at the start of each shift and as needed throughout the shift, as well as following a rainstorm or other water intrusion event.
Do not work under suspended or raised loads and materials. This means workers down in the trench and the track hoe bucket swinging over their head to drop gravel, sand or pulling the material out.
Also, Walkways have to be provided where workers or equipment are required or permitted to cross over excavations. Guardrails which comply with §1926.502(b) shall be provided where walkways are 6 feet (1.8 m) or more above lower levels. Again, only when workers are to CROSS OVER the open trench of 6 feet deep or more.
Ensure that all personnel has high visibility or other suitable clothing when exposed to vehicular traffic. This is a requirement and you have to reference the Manual on Uniform Traffic Control Devices for minimum requirements for highways as well as flaggers being used. This tells you the ANSI rating needed for certain reflective apparel needed for day and night work.
Also, prepare for an emergency, especially if in an unfamiliar area, a rural area and/or working with a hazardous atmosphere by contacting the local emergency response service and determining whether or not they are equipped and prepared for a potential rescue. This may also drive the need to have someone trained in 1st aid/CPR at the site as well. Also, know where the nearest emergency medical center is located. For job sites, this should ALWAYS be a part of a site-specific safety plan anyway.
Now, let’s get into soil classification. You need to understand what OSHA deems to be stable or unstable soil and how the class of soil drives the protective system you choose. So let’s get into some definitions you need to know and the different types of soil classifications as well as HOW to test soils. Remember, these terms are important to understand as we move forward:
"Cemented soil" means a soil in which the particles are held together by a chemical agent, such as calcium carbonate, such that a hand-size sample cannot be crushed into powder or individual soil particles by finger pressure.
"Cohesive soil" means clay (fine-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 moist. Cohesive soil is hard to break up when dry and exhibits significant cohesion when submerged. Cohesive soils include clayey silt, sandy clay, silty clay, clay, and organic clay.
"Dry soil" means soil that does not exhibit visible signs of moisture content.
"Fissured" means a soil material that has a tendency to break along definite planes of fracture with little resistance, or a material that exhibits open cracks, such as tension cracks, in an exposed surface. So anywhere in the Standard where you see reference to whether or not the slope or bench of a trench is “fissured”, that is what it means.
"Granular soil" means gravel, sand, or silt (coarse-grained soil) with little or no clay content. Granular soil has no cohesive strength. Some moist granular soils exhibit apparent cohesion. Granular soil cannot be molded when moist and crumbles easily when dry.
"Layered system" means two or more distinctly different soil or rock types arranged in layers. Micaceous seams or weakened planes in rock or shale are considered layered.
"Moist soil" means a condition in which a soil looks and feels damp. Moist cohesive soil can easily be shaped into a ball and rolled into small diameter threads before crumbling. Moist granular soil that contains some cohesive material will exhibit signs of cohesion between particles.
"Saturated soil" means a soil in which the voids are filled with water. Saturation does not require flow. Saturation, or near saturation, is necessary for the proper use of instruments such as a pocket penetrometer or sheer vane. Which I will get into shortly.
"Stable rock" means natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.
"Submerged soil" means soil which is underwater or is free seeping.
"Unconfined compressive strength" means the load per unit area at which a soil will fail in compression. It can be determined by laboratory testing or estimated in the field using a pocket penetrometer, by thumb penetration tests, and other methods.
So let’s dig into classifying soil. “Soil classification system" means, for the purpose of this subpart, a method of categorizing soil and rock deposits in a hierarchy of Stable Rock, Type A, Type B, and Type C, in decreasing order of stability. The categories are determined based on an analysis of the properties and performance characteristics of the deposits and the characteristics of the deposits and the environmental conditions of exposure.
"Type A" means:
Cohesive soils with an unconfined, compressive strength of 1.5 ton per square foot (tsf) (144 kPa) or greater. Examples of cohesive soils are clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. Cemented soils such as caliche and hardpan are also considered Type-A.
However, no soil is Type-A if:
(i) The soil is fissured; or
(ii) The soil is subject to vibration from heavy traffic, pile driving, or similar effects; or
(iii) The soil has been previously disturbed; or
(iv) The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H:1V) or greater; or
(v) The material is subject to other factors that would require it to be classified as a less stable material.
"Type B" means:
(i) Cohesive soil with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa); or
(ii) Granular cohesion-less soils including: angular gravel (similar to crushed rock), silt, silt loam, sandy loam and, in some cases, silty clay loam and sandy clay loam.
(iii) Previously disturbed soils except those which would otherwise be classed as Type C soil.
(iv) Soil that meets the unconfined compressive strength or cementation requirements for Type A, but is fissured or subject to vibration, or
(v) Dry rock that is not stable; or
(vi) Material that is part of a sloped, layered system where the layers dip into the excavation on a slope LESS steep than four horizontal to one vertical (4H:1V), but only if the material would otherwise be classified as Type B.
"Type C" means:
(i) Cohesive soil with an unconfined compressive strength of 0.5 tsf (48 kPa) or less; or
(ii) Granular soils including gravel, sand, and loamy sand; or
(iii) Submerged soil or soil from which water is freely seeping; or
(iv) Submerged rock that is not stable, or
(v) The material in a sloped, layered system where the layers dip into the excavation or a slope of four horizontal to one vertical (4H:1V) or steeper.
In order to properly classify the soil, OSHA requires at least on visual analysis and one manual test be performed on the soil.
Visual analysis is conducted to determine qualitative information about the excavation site. In general, you need to consider the soil adjacent to the excavation, the soil forming the sides of the open excavation, and soil samples taken from excavated material. Here are the steps:
1. Observe samples of soil that are pulled out of the ground as well as the soil in the sides of the excavation. You need to estimate the range of particle sizes and the relative amounts of the particle sizes. Soil that is primarily composed of fine-grained material (like a putty) is cohesive material. Soil composed primarily of coarse-grained sand or gravel is to be considered granular material.
2. Watch the soil as it is excavated. If it remains in clumps when excavated and dropped out of the bucket or shovel it is considered cohesive. If it breaks up easily, falls apart and does NOT stay in clumps you would consider it to be granular.
3. Look at the sides of the opened excavation and the surface area next to it. Crack-like openings, like tension cracks, could mean that you are dealing with fissured material. If chunks of soil spalls off a vertical face of the excavation, this is another sign the soil could be fissured. Small spalls are evidence of moving ground and are indications of potentially hazardous situations. So be sure to look for this.
4. Watch the area next to the excavation and the excavation itself for evidence of existing utilities and other underground structures, and to identify previously disturbed soil. An obvious sing would be indicated via utility markings made prior to digging or conduit, pipe, etc. being exposed as you dig. A less obvious sign of this would be a small patch of gravel or sand that you may have cut through or running alongside the trench. This may indicate backfill material and thus previously excavated soil.
5. Also, watch the opened side of the excavation for a possible layered system. Examine layered systems to identify if the layers slope toward the excavation. Estimate the degree of slope of the layers. This will also drive the slope angle allowed or if you can even bench the sidewalls.
6. Look for evidence of surface water, water seeping from the sides of the excavation, or the location of the level of the water table. Both in the excavation as well as at the surface.
7. Check the area for sources of vibration that may affect the stability of the excavation face. If you are running a front end loader up and down the length of an open trench carrying loads or straddling a part of the end of the trench with the track hoe then you might be subjecting the excavation to vibrations that could lead to unstable sections.
OSHA also requires at least 1 manual test be performed in order to determine quantitative as well as qualitative properties of soil. This provides more information in order to classify soil properly so as to get us to the next step; selecting appropriate protective measures. Let’s run down the options for manual testing:
1. Plasticity test. For this test, you simply mold a moist or wet sample of soil into a ball and then try to roll it into threads as thin as 1/8-inch in diameter. If the soil is cohesive (sticks to itself) it can be rolled into threads without crumbling and falling apart. For example, if at least a two-inch (50 mm) length of 1/8-inch thread can be held on one end, so dangling it, and it does NOT tear, the soil is cohesive.
2. Dry strength test. If the soil is dry and crumbles on its own or with moderate pressure breaks into individual grains or even a fine powder, it is granular - so, any combination of gravel, sand, or silt. If the soil is dry and falls into clumps which break up into smaller clumps, but the smaller clumps can only be broken up with difficulty, it may be clay in any combination with gravel, sand or silt. If the dry soil breaks into clumps which do not break up into small clumps and which can only be broken with difficulty, and there is no visual indication the soil is fissured, the soil may be considered un-fissured.
3. Thumb penetration test. The thumb penetration test can be used to estimate the unconfined compressive strength of cohesive soils. (This test is based on the thumb penetration test described in American Society for Testing and Materials (ASTM) Standard designation D2488 - "Standard Recommended Practice for Description of Soils (Visual - Manual Procedure).") Type A soils with an unconfined compressive strength of 1.5 tsf can be readily indented by the thumb; however, they can be penetrated by the thumb only with very great effort. Type C soils with an unconfined compressive strength of 0.5 tsf can be easily penetrated several inches by the thumb and can be molded by light finger pressure. This test should be conducted on an undisturbed soil sample, such as a large clump of spoil, as soon as possible after excavation to keep to a minimum the effects of exposure to drying. If the excavation is later exposed to rain, flooding, etc. the classification of the soil must be changed as well.
4. Other strength tests. Estimates of unconfined compressive strength of soils can also be obtained by use of a pocket penetrometer or by using a hand-operated shear vane. These are instruments used to penetrate a sample with a rod containing a resistance spring and a measuring cylinder to read unconfined compressive strength. The shear vane is a tool with a numbered dial and needle and requires a soil sample with a flat surface where you impress a disc with vanes and twist it until it shears off a section of the soil sample thus giving you a reading. These instruments come with instructions and also have some environmental limitations (springs acting differently in cold vs hot weather) and could be subject to wear over prolonged use.
5. Drying test. The basic purpose of the drying test is to
differentiate between cohesive material with fissures, un-fissured
cohesive material, and granular material. The procedure for the
drying test involves drying a sample of soil that is approximately
one inch thick (2.54 cm) and six inches (15.24 cm) in diameter
until it is thoroughly dry:
1. If the sample develops cracks as it dries, significant fissures are indicated.
2. Samples that dry without cracking are to be broken by hand. If considerable force is necessary to break a sample, the soil has significant cohesive material content. The soil can be classified as an un-fissured cohesive material and the unconfined compressive strength should be determined.
3. If a sample breaks easily by hand, it is either a fissured cohesive material or a granular material. To distinguish between the two, pulverize the dried clumps of the sample by hand or by stepping on them. If the clumps do not pulverize easily, the material is cohesive with fissures. If they pulverize easily into very small fragments, the material is granular.
Some of these tests are more for soils engineering and designing complex protective systems. Your average worker involved in excavation activities is probably going to use the visual analysis along with the plasticity test or thumb penetration test. Just getting your hands on a sample and trying to mold it, feel it, see how it behaves is really a good way to determine what you are dealing with.
ProTip: Forget solid rock; it can have any fractures/fissures, if you are scraping, blasting, hydraulic fracturing, pile-driving, drilling, then you are creating this condition. Which means pieces of varying size could potentially come loose and fall into the trench. Therefore you move to the next level: Type A soil. Well, if it is fissured or subject to vibration, etc. then it cannot be considered type A. And let’s face it, most excavations will be subject to varying degrees of activities that meet this criterion.
So, we are left with Type B soil and C. Cohesive or non-cohesive? (ok, there is a gray area known as non-cohesive type B, but honestly, just call it non-cohesive and get to work!). So is it Clay or Sand? It really is that simple. Once you know the soil type, you can determine what protective system to use.
Here your options are sloping, benching or shoring of some kind. The maximum allowable slope is as follows:
Stable rock - vertical (90)
Type A - 3/4:1 (53)
Type B - 1:1 (45)
Type C - 1 1/2:1 (34)
Notice I said MAXIMUM allowable slope! This means you may need to make it “flatter” if it is needed. Sloped means the angle at which it will lie and NO LONGER MOVE! So keep that in mind. If you have Type B soil based on the visual and manual test and slope it to a 45-degree angle and you get sloughing then you need to dress that slope back more.
Benching is the same; the angle of the bench as measured from the TOE of the trench (nearest bottom side). So you can tell how many steps will be needed to achieve the 45-degree angle. You are only allowed a 4-foot maximum face for the first bench step you make, then a 5-foot maximum face thereafter.
You can also use a single bench; make a 4’ maximum face, cut it back then go to the slope needed for the soil type. Or an unsupported vertically sided lower portion; you can come straight up no more than 3 1/2 feet then hit your slope. But keep in mind, Type C soil according to OSHA CANNOT be benched! Only sloped or shored.
And, these are ONLY allowed for trenches up to 20 feet deep. Trenches 20 feet (6.1 meters) deep or greater require that the protective system be designed by a registered professional engineer or be based on tabulated data prepared and/or approved by a registered professional engineer.
As for shoring; like trench boxes and speed shoring - you have to make sure you have what is called the tabulated datasheet for that system ONSITE. This tells you the depth limits based on soil type as well as width limitations, whether you can stack 2 or more trench boxes on top of one another, things like that. This gets tricky as there are all sorts of systems so I won’t get into specifics here. Just keep in mind, if your box or shield is below grade at any point, you must slope or bench as allowed per the soil type for any soil above the top of the device and you need at least an 18” lip to catch any rolling materials from going into the trench.
Same for the bottom; you can only raise the box or shield off the bottom of the trench up to 18” if conditions allow. So there are some little rules that go along with these systems; but again, you need to know the system requirements BEFORE beginning work and have that data onsite while work is taking place.
I hope you got some good info from this episode. Please follow up and seek more formal training for yourself and your co-workers on this topic. If you are overseeing work at your facility that involves this type of activity then I hope I gave you some good tips so that you can begin to go out and look at this work and assess whether or not things are compliant. Please let me know what you think, share your thoughts by emailing me at firstname.lastname@example.org.
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