«Leaders Guide Trenching and Shoring Safety - Competent Person The manual for Excavations Trenching and Shoring is produced in Adobe Acrobat. The ...»
Type B Soils are cohesive soils with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa). Examples of other Type B soils are: angular gravel; silt; silt 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 fissured or 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
Type C Soils are cohesive soils with an unconfined compressive strength of 0.5 tsf (48 kPa) 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 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.
Layered Geological Strata Where soils are configured in layers, i.e., where a layered geologic 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 top of stable rock.
More on Soil Classifications Soil is a mixture of sand, gravel, silts, clay, water, and air. The amount of these ingredients determines its "cohesiveness", or how well a soil will hold together.
Cohesive Soil Cohesive soil does not crumble. It can be molded easily when wet, and is hard to break up when dry. Clay is a very fine grained soil, and is very cohesive or ―sticky‖. Calcium carbonate also may be present and it may provide a cemented property.
Granular Soils Sand and gravel are course grained soils, having little cohesiveness and are often called. Generally speaking, the more clay that is in the soil being excavated, the better the trench walls will hold up.
Water Content of Soil Another factor in soil cohesiveness is water. Soil that is filled with water is termed. Saturated soil does not hold together well, and is particularly dangerous in excavation work. However, the opposite can also be true. Another dangerous soil is soil that has little or no water in it, or if, can crumble easily, and will not hold together when excavated.
Weight of Soil
Soil is heavy. A cubic foot can weigh as much as 125 pounds, and a cubic yard can weigh over 3,000 lb. — as much as a pick-up truck!
Most workers don‘t realize the force that will hit them when a cave in occurs. A person buried under only a few feet of soil can experience enough pressure in the chest area to prevent the lungs from expanding. Suffocation can take place in as little as three minutes. Heavier soils can crush and distort the body in a matter of seconds. It‘s no wonder trench accidents involve so many deaths and permanently disabling injuries.
Discussion of Four Categories of Soil
OSHA classifies soils into four categories:
Solid Rock is the most stable, and Type C soil is the least stable. Soils are typed not only by how cohesive they are, but also by the conditions in which they are found. Stable rock is practically unachievable in the excavation of a trench. This is because the excavation of rock typically requires drilling and blasting, which fractures the rock, making it less stable.
A soil be considered Type A if it is fissured (Has cracks) or other
conditions exist that can adversely affect it, such as:
Subject to vibration from heavy traffic, pile driving, or similar effects Having been previously disturbed/excavated Where it is part of a layered system, where less stable soil is near the bottom of the excavation, with the more stable soils on top.
Subject to other factors which would make it unstable-such as the presence of ground water, or freezing and thawing conditions.
Many OSHA compliance personnel believe that construction equipment at the site create enough vibrations to prevent any soil from being typed as "A". If vibrations can be felt while standing next to an excavation, the competent person should consider downgrading Type A soil to Type B or C.
They include silts, sandy loams, medium clays, and unstable rock. Soils that might be classified as A, but have fissures, or are subject to vibration, may also be classified as "B" soils They are easily recognized by the continual sloughing of the sides of the walls of excavation. If soil is submerged, or water is seeping from the sides of an excavation, it‘s very probably "C" soil. Soil may be classified as Type C if an excavation is dug in "layered" soils, where different soil types lay on top of each other. When an unstable soil type is underneath a stable soil type in an excavation, the "weakest link" will soon give way.
Previously Disturbed Soil In many construction projects, the soil that is being excavated has been This means the soil has been dug up or manipulated in the past. This is another factor a competent person must consider when typing soils. Previously disturbed soils are rarely as strong as undisturbed soils, and are usually typed as type "C" soil.
Previously disturbed soil is commonly found above existing utilities, such as water, sewer, electrical and gas lines. This could also apply to lots where the solid has been disturbed in order to building homes. This makes work around these utilities more dangerous due to the unstable nature of the soil. Much of the work utility worker perform in excavations are along right-of-ways, where the soil is almost always likely to be Type C. Because of where you may dig, it‘s important to understand that once soil has been excavated, it will never be returned to the way it was naturally formed.
To Type or Classify Soils Per standard, a competent person must type or classify soils by using at least one and one test. A visual test can include inspecting the soil as it is being removed, and examining the spoil pile and the color and make-up of the excavation walls.
A manual test means working with the soil with either your hands or with an instrument designed to measure soil strength. For example, if you can roll the soil in your hands into a long "worm" or ribbon, the soil is cohesive and may be classified as A or B, depending on other conditions. One useful instrument for measuring soil strength is a. When you press this instrument into a soil sample, it measures its unconfined compressive strength in tons per square foot (tsf).
Regardless of the methods used, the typing of soils must be done by the competent person prior to anyone entering the excavation. The weaker or less stable the soil, the greater the need for protective systems.
Remember: soil can be very heavy, weighing as much as 125 lbs per cubic foot.
A typical cave-in often covers a victim with 4-6 yards of soil, this equates to 18,000 pounds of material. By law, all soil must be classified prior to work to determine the proper trench protection methods. Generally the Competent Person will be responsible for classifying the soil typically found in excavations.
In large projects 20‘ or more or where feasible, soil testing might be performed by a soils engineer.
Each excavation must be classified by a Competent Person as either:
Solid Rock Type A Type B Type C The classification must be based upon at least one visual and one manual soils test.
Note: Previously disturbed soil can never be Type "A" soil.
Visual tests are conducted to determine general information regarding:
The general site Soil adjacent to the excavation Soil forming sides of trench Samples from excavated soil Water content
1. Visually look to determine soil condition:
If the soil remains in clumps and is usually cohesive Soil that breaks up easily and does not stay in clumps may be granular
2. Observe the sides of the excavation, looking for:
Cracks Fissures Other evidence of moving ground Layered soil types, determine if layers slope toward the excavation and estimate the degree of the slope into the layers Water in the trench Sources of vibration
3. Carefully check the surrounding area for sources of vibration and other hazards which may affect stability.
MANUAL TESTSThe standard allows for a number of tests to be performed to help determine soil classification. Manual tests help to better identify the soil type and cohesiveness.
It is recommended that your organization retain the services of a soils engineer to provide further training on how best to determine soil type.
The following are common tests that are used for soil classification:
The plasticity test helps to determine if soil contains cohesive material. To perform this test, find a palm sized sample of moist or wet soil and mold it into a ball, and then attempt to roll it into threads about 1/8" in diameter.
If the soil contains cohesive properties, it can usually be rolled into threads without crumbing. If at least a two inch length of 1/8" thread can be held on one end without tearing, the soil appears to be cohesive.
The dry strength test help determine the soil type.
Obtain an undisturbed soil sample from the spoil pile and use your thumb or fingers to apply light to moderate pressure to the sample. If the sample crumbles into individual grains or powder with moderate pressure, the soil is usually considered granular. (combinations of gravel, silt or sand) If the soil is dry and falls into clumps which in turn break into smaller clumps, which can only be broken with difficulty, it may be clay in combination with gravel, sand or silt.
If the dry soil breaks into clumps which do not break up into small clumps and again can only be broken up with difficulty and there is no visual fissures, the soil may be considered un-fissured.
Thumb Penetration Test:
The Thumb Penetration Test can be used to estimate the Unconfined Compressive Strength of cohesive soil material. When performing the thumb penetration test it must be done on an undisturbed excavated soil sample such as a large lump of material. This test should be performed as soon as possible after excavation as to reduce the effects of drying or additional water.
Type "A" soils have an "Unconfined Compressive Strength" of 1.5 tsf or greater, and can not readily be indented by thumb pressure.
Type "B" soils are those with.5 to 1.5 tsf of "Unconfined Compressive Strength" and can generally be penetrated to the first joint of the thumb.
Type "C" soil has an "Unconfined Compressive Strength" of 0.5 tsf or less and can easily be fully penetrated by the thumb, and can be molded by light finger pressure.
NOTE: Type "A" soil is not common, as many factors can reduce the soils stability qualities.
1. Perform the test on a large undisturbed excavated sample of soil such as a large clump. Shave a clean spot that is free of voids and rocks, etc.
2. Following the manufacturers instructions, move the ring toward the handle to the lowest reading on the handle.
3. Grip the handle and with steady pressure, push slowly until the soil reaches the marking on the piston, about 1/4" from the end.
4. Read the "Unconfined Compressive Strength" in tons/square foot or tsf on the low side of the ring. (be sure to follow the manufacturers‘ instructions for your instrument)
5. Take 8-10 readings, throwing out high and low and calculating an average.
If using a "Shear Vane" again carefully read and follow the instructions.
Depending upon the instrument, the reading will need to be doubled to get the correct result.
The basic purpose of the drying test is to differentiate between cohesive material with fissures, un-fissured cohesive material and granular material. The drying test should be performed by a qualified individual, but the following information is for general awareness only.
The drying test is performed by using a sample about one inch thick and six inches in diameter. A small portable stove or engine manifold is often used for this purpose to heat the sample until thoroughly dry.
• If the sample develops cracks as it dries, significant fissures are indicated.
• Samples that dry without cracking are to be broken by hand. If considerable force is needed to break the sample, the soil generally has cohesive material content.
• If the sample is easily broken by hand, it is either fissured cohesive material or granular material. In order to tell which is cohesive and granular, attempt to pulverize the sample with hand or foot pressure. If they do not pulverize easily, the material is generally cohesive with fissures. If they easily break into small fragments, the material is usually granular.
Sedimentation or Olive Jar Test:
The "Olive" jar test can be used to estimate the amount of clay, silt, sand etc in a soil sample. 4-6 shovel full‘s of soil are mixed and all rock and debris discarded, cut sample into quarters and throw out two, mix and repeat at least two more times, mixing the final sample again. Generally a tall olive jar is used with a mark about 1 1/2" from the bottom. The soil sample is placed in the jar up to the line and water added to the top (about 5-6" of water). The jar is shaken vigorously, given a twist and the sample timed. The types of soil within the jar will dictate how quickly they settle out.
For example: If 80% of the soil settles out within the first 30 seconds; the soil is generally Type "C" material. If after 30 seconds 70-80% of the material does not settle out; it could be Type "B" soil.