Article

In the first part of this three-piece article on trench collapses, the civil engineers at Robson Forensic discuss soil identification, which is an important step in determining which protective method is appropriate for protecting workers in, and around, trenches.

Trench Collapses - Soil Identification

Standards addressing excavation safety are derived from a number of sources including the OSHA Construction Industry Regulations (29 CFR 1926, Subpart P – Excavations), as well as ANSI/ASSE A10.12 Safety Requirements for Excavation. One of the most important safety aspects in dealing with this matter is that workers must be trained on how/why excavation accidents occur, and then what can be done to minimize and/or eliminate the risk of collapse. There are a number of factors that may contribute to a trench collapse, including:

  • Type of soil;
  • Weight of the soil;
  • Moisture content of soil and height of the surrounding water table;
  • Changes in weather, and freezing and thawing of soil;
  • Vibrations and/or weight from vehicles and adjoining structures.

Classifying Different Types of Soil

Soil is categorized in decreasing order of stability. The most stable type of soil is stable rock. The next most stable form of soil is Type A, then Type B, and finally, Type C soil. On earthwork construction operations, OSHA requires that the soil and/or rock being excavated be classified by a competent person. In order to do this, the competent person must perform a visual test, a manual test, or both. In basic terms, the competent person examines the soil being excavated to determine what type of soil it is and how long it will “stand-up” under its own weight.

To determine the stand-up time of the soil, during their analysis, the competent person must consider the following factors:

  • Depth of trench;
  • Slope of trench wall;
  • Type of soil;
  • Unit weight of soil;
  • Depth of the water table in area of excavation;
  • Surrounding surface vibrations.

The inspection conducted by the competent person should not be done just once at the beginning of each day and/or shift, because disturbances during bulk or trench excavation procedures may change a soil’s classification during the course of a work day/shift.

Soil Classifications

  • Stable Rock - A stable rock formation is normally a solid mineral or material that can be excavated with side walls that are vertical. Additionally, soil that is rock will remain intact and not fall apart when exposed to the weather.
  • Type A Soil - The next most stable type of soil, Type A, is normally defined as a clay or sandy clay soil. It is fairly stable and holds up well when exposed to the elements. Type A soil is cohesive in nature, but does not contain any fissures. Lastly, Type A soil cannot be subjected to vibration from heavy traffic, or have been previously disturbed or dug. If such is the case, the Type A soil is automatically downgraded to the next lower soil classification (Type B).
  • Type B Soil - Commonly including angular gravel, silt, and sandy loam, type B soil is somewhat cohesive but has a lower compressive strength than a Type A. As the case with Type A soil, any previously disturbed Type B soil drops a category to Type C because of the changes in soil characteristics.
  • Type C Soil - Often referred to as “plain” or “farmers” dirt, Type C usually has some combination of loose gravel, sand, or loam; sometimes with water leaching from it. Type C soil can be cohesive, but breaks down easily when exposed to the weather. This is the most dangerous type of soil for trenching operations because it is the weakest, and requires that the most stringent protective measures.


Analysis of Soils

During earthwork and/or trenching operations, both a visual test and a manual test must be performed by a competent person, unless the visual test determines that soil is Type C.

Visual Soil Test - In a visual test, the competent person should inspect the soil and the surrounding area for the following:

  1. Cracks that have developed in the soil or on the vertical wall(s).
  2. Evidence that the soil has been previously excavated.
  3. To determine if there are source(s) of vibration nearby.
  4. To determine if there are source(s) of water nearby.

Manual Soil Tests - There are five (5) generally accepted manual tests used in excavations procedures. The first three (3) tests listed do not use any instruments, just one’s hands. The final two (2) tests generate actual measured readings.

  1. Plasticity Test – A moist soil sample is taken from the area being excavated, and molded/formed into a ball. The ball is then rolled between your hands until it becomes the shape of a dirt “snake”, about 1/8” wide. A two (2) inch sample of the “snake” is broken-off and held by one end. If the soil holds together, the soil is considered to be cohesive.
  2. Dry Strength Test - A soil sample is taken and attempted to be crushed in one’s hands. If the soil crumbles on its own, the soil is granular. If the soil sticks together in smaller, stiff clumps, it is most likely comprised of clay. If the dry soil breaks into clumps, and there is no evidence of fissures, the soil may be considered to be un-fissured soil.
  3. Thumb Penetration Test – In the industry, this test is not viewed as the most accurate method of testing the soil, but it is accepted nonetheless. An area of undisturbed soil is located, and the competent person attempts to press his extended thumb into the soil. If it is very difficult to penetrate the soil, the soil would be considered Type A. If the thumb penetrates no further than the length of a fingernail, it is probably Type B. If the thumb penetrates easily into the soil, then the soil is Type C.
  4. Pocket Penetrometer Test – This instrument test is a simple method used to measure the compression strength of exposed trench soil. The penetrometer has a rod that when pressed into the wall of a trench generates a reading. The measurements generated indicate the compressive strength and type of the soil:
  • Type A: 1.50 tsf or greater
  • Type B: 1.50 - 0.50 tsf
  • Type C: 0.50 tsf or less tsf - tons per square foot
  1. Hand-Operated Shearvane - A shearvane is another direct reading instrument to determine the unconfined compression strength of soils. This instrument is pressed into the vane of a soil and rotated. The soil strength can then be read directly on the wheel of the device.

Once the competent person has properly analyzed the soil and made a determination as to what the soil type is, the best method to protect the workers from an excavation collapse can be determined.

INVESTIGATING TRENCH INJURIES

There are a number of variables on any site that determine the selection of protective trench measures; these variables are dynamic and in many cases, will be influenced by weather and surrounding conditions and activities.

Among the civil engineers at Robson Forensic, there are many construction professionals with firsthand experience managing site safety and engineering protective systems for trenches and other hazards.

Submit an inquiry or contact the author of this article to discuss your case.

 

Featured Expert

J. David Gardner, P.E.

Civil Engineer & Construction Safety Expert

​Robson Forensic offers construction safety experts from many of our regional offices. This article was assembled with the assistance of several members from our construction safety group, but spearheaded by David Gardner.

Dave Gardner is a civil engineer with more than twenty-five years of professional experience. He provides investigations, reports, and testimony in matters involving construction claims and injuries. Dave’s project experience crosses various disciplines within civil engineering, including heavy highway and bridge, municipal engineering, and compressor stations/site work for the natural gas industry.

Contact David or your local Robson Forensic office to discuss your case. Based on geographic location and technical specifics of your case, we can help determine which of our experts is most appropriate to aid in your investigation.