]]>

Pipeline Trenching

Now that we have discussed the different types of soil that OSHA recognizes, let's talk about how to test to determine these different soil types. When preparing trenching activities the three most common methods to test a construction site's soils are: a visual test, a plasticity test, the pocket penetrometer test. Testing soil is an important element in preventing trenches from collapsing.

First, a Visual Test is a purely qualitative evaluation of the condition in and around the entire pipeline construction jobsite. Here one looks at the soil being excavated as well as the soil next to the construction site. If the soil is in clumps, it is cohesive. Otherwise, it may be considered granular it if appears to be course-grained, gravel or sand-like. Signs of vibrations are checked, for i.e. crack-line openings along certain zones that would indicate tension cracks, or any existing utilities that indicate that the soil has been previously disturbed. Also, the exposed sides of the excavations are looked at for any layered geologic structuring. Any stresses, deformations and water intrusion should also be quickly taken into account and remedied accordingly, as these can cause trench failure.

Second, the Plasticity Test is used to determine whether the soil is cohesive, by rolling a moist soil sample into a 1/8" thick (half of a pencil width) that is no longer than two inches long. If the soil can hold itself together at one end without breaking, it is cohesive.

Third, the Soil Penetrometer Test consists of using a device that typically fits in one's pocket, and represents a more numeric measurement. Much like a tire pressure gauge, it is thin, where the metal piston is pushed against a soil sample. Generally, more than one pentrometer test is taken due to variability in results (+/- 20-40%). Therefore, it is advised that soil sample be taken from the same part of the excavation, to help ensure consistent results. It's important to realize that a penetrometer may give false readings if the soil contains rocks or pebbles since these substances don't compress. Because of this, other tests are also encouraged, such as the thumb penetration test, dry strength test and Shearvane (Torvane) test, mentioned below:

Soil-Penetrometer.jpg

Thumb Penetration Test- this test is used as a quick measure to estimate the compressive strength of cohesive soils. However, it is subjective, and the least accurate of the proposed methods. A person's thumb is pressed into a fresh clump of soil. If your thumb only makes an indentation with great effort, the soil is classified as Type A. In contrast, in Type C soil, the thumb sinks all the way into the clump of soil. Typically, in pipeline, the results vary somewhere in between these two types of soil.

Dry Strength Test checks whether soil fissures, splits and/or cracks and to what degree. When dry soil crumbles freely with some pressure into individual granules it is considered granular. If dry soil falls into clumps, which subsequently break into smaller clumps, and the smaller clumps can only be broken with great difficulty, the soil is mostly likely a combination of clay and gravel, sand, or silt. If soil breaks into clumps (which do not break up into smaller clumps, except with great difficulty), the soil is considered unfissured, unless there are other visual indications of fissuring.

Shearvane or Torvane Test determines the unconfined compressive strength of soil by digging in the blades of the vane into a level section of undisturbed soil. The knob, which is torsioned, is slowly turned until soil failure occurs. Then, a reading is taken, as per the manufacturer's instructions, to determine the tons (tsf) per square foot or kilograms (kPa) per square centimeter of the soil sample, which ultimately, guides one to better learn the type of soil being test.

In summary, we have included basic methods and tools used to determine the different types of soil , A, B or C that we typically work in. Determining the soil types will then give us a good indication of how to shield our trenches from collapse. The methods above are consistent with OSHA's present Technical Manual (OTM) on Trenching and Shoring.

The next step of a safety-conscious pipeline installation is shoring up trenches to prevent from trench collapse. In our next Blog, we will focus on the various ways that competent pipeline project superintendents use to shore up their soil trenches.

Safety is something we should live with. At Kana Pipeline, service first, safety always. Contact us today and let us be your safety-conscious pipeline contractor of choice.

Tags: , , , , , , , ,

Published on April 23, 2014 | Comments: 0

| Print Print

Leave a comment