Beware of the excavation rats—Part 1

Concrete Construction, May, 2002 by Michael W. Hayslip

How terrible it must be to slowly suffocate in a collapsed excavation! To feel the earth close in against your body with every outgoing breath. And to know, for so brief a moment, that you may never again hear a child's laughter or see joy in a loved one's face because today is the day you might die, all alone. If only you had heeded the warnings of those "running rats"!

"Running of the rats" is a common term in sloped excavation work, especially where there is some stickiness to the soil. The rats are the rocks, boulders, or clods of dirt that roll down the face of an open cut in the earth. Every cut in the earth will try to "heal" itself because the force holding it in place has been removed. This healing may be so slow that one may not notice it happening--until it's too late.

In parts 1 and 2 of this article we focus on the causes and prevention of excavation failures--things everyone, not just the Competent Person, on a jobsite should understand. OSHA regulations (29CFR1926.32F) define a Competent Person as one who is capable of identifying existing and predictable hazards on the jobsite or unsafe working conditions and who has the authority to take corrective measures. This month we begin with a fundamental overview of soil mechanics to explain why soil fails, then offer a few preventative measures for avoiding cave-ins. In the following months Part 2 then goes on to highlight a few relevant portions of OSHA's Federal excavation standard, and Part 3 concludes by providing suggested actions for anyone involved in the rescue of a trapped craftsperson resulting from a collapsed excavation.

Basic soil mechanics theory

Simply put, a soil's stability depends on not exceeding its shear strength capacity.

To fully understand this, we must know several things. First, what is shear strength? Second, what gives soil its shear strength? And third, what factors, if any, reduce a soil's shear strength?

First, shear is an engineering term that describes forces that seem to "slide" past each other. Shear strength is the capacity of a material to resist these internal and external forces. The greater a soil's shear strength, the greater its resistance to failure. Perhaps shear in soil is best understood in contrast with compressive forces that act head-to-head (push)and tensile forces that act in opposite directions (pull).

Second, there are two principal properties that give soil its shear strength: cohesion and internal friction.

Cohesion is the stickiness of a soil. Clay is an example of a cohesive soil, while sands are not cohesive soils. Clay particles act differently when placed together than sand particles do. The magic of cohesion in clay comes from the size and shape of its particles.

Imagine if a clay particle were the size and shape of a shelled sunflower seed. Relative to that particle of clay, a medium-sized sand particle would be well over 100 feet in diameter! Due to a clay particle's small size and relatively flat shape, it develops particle-to-particle attractions that are the result of electrical charges on the faces of these microscopic minerals. Clay particles are attracted to each other, which is not so true for sand.

Sands are generally rounded and do not exhibit cohesion in the truest sense of the word. Unlike clay, the particles are not attracted to each other. Sands instead have what is termed apparent cohesion (sometimes known as the sand castle effect). With sands, an intermediate substance, such as water, temporarily binds the particles together. Too little or too much water and the apparent cohesion (false stickiness) fails.

Internal friction is a geotechnical engineer's way of saying that the particles do not want to slide past each other. Internal friction is mostly a result of particle shape. For instance, rounded bank run material will not stand as steeply (or resist shear as well) as rough, angular gravels and sands. Rounded particles generally have less internal friction than angular ones and, therefore, are less stable. Imagine rolling marbles in your hand; then imagine rolling crushed aggregate. The marbles are rounded and have less internal friction to resist shear.

So if cohesion and internal friction give a soil its strength, the ultimate question remains: What factors reduce a soil's capacity to resist shear?

Factors that by themselves or in combination may reduce the shear strength of soil are:

* Water from any source such as precipitation, surface runoff, aquifers, and the like

* Vibrations of all sorts, from construction equipment, vehicular traffic, and machines like compactors, skidsteers, and generators

* The degree of soil compaction or density

* Freeze/thaw cycles

* Surcharge loads from spoils, stockpiled materials, or neighboring structures

* External point loads from equipment and materials

* The existence of previously disturbed soil

* Depth of excavation

* Unit weight of the soil

* Chemical properties of the soil

* Erosion

* Earthquakes

* Utility locations

* Moisture content--too much or too little