Answering an Excavation Spoil Pile Question

Excavation Spoil Pile Set Backs.

As a safety consultant, I love answering the phone. You never know who’s on the other end or what question you’ll have to field.

Excavation Spoil pile set backs

Shoring and spoil pile set backs on Excavation site.

A few weeks ago a compliance officer from Oregon OSHA called. She had been having a discussion with her co-workers and they were at a bit of a impasse. They were considering the following scenario: A four-foot trench with two feet of spoil pile, but in this case the pile was right up against the trench lip and had not been set back two feet from the lip as required by the code. (She didn’t mention whether or not the spoil pile had been sloped as it should be.)

She and her colleagues were having a disagreement over their enforcement policy. Some thought that if the spoil pile wasn’t set back as required, OROSHA had a policy which said that the compliance officer should measure the depth of the trench from the top of the spoil pile down to the toe (or bottom) of the trench. Normally trench depth is measured from the lip of the cut (or original grade) down to the toe.

The result in this case was that a four-foot trench was now considered - from a compliance standpoint - a six-foot trench because the spoil pile was not set back the required two feet.

This can be important because when your excavation gets deeper than five-feet (four feet in some states) you are required to have a cave-in protective system which means you must slope, shore or shield. (Note: In shallower trenches the protective system is optional once the competent person has inspected the site and found no possibility of cave-in.)

In other words, having the spoil pile right next to the edge of the excavation meant the workers who originally dug a four-foot deep hole would now be responsible for the safety requirements of a six-foot hole.

It was an interesting question, but after thinking and talking it through, I convinced myself (and her I hope) that OROSHA couldn’t have a policy like that, and if they did it would be a regulatory mess.

Start with the rationale for a two-foot spoil pile setback. It doesn’t have anything to do with reducing cave-in forces (lateral earth pressure) on the trench walls. Most sets of tabulated data for protective systems assumes a two-foot spoil pile in the excavation area.

Instead, the two-foot setback appears in 1926.651 (j): Protecting Employees From Loose Rock and Soil. The regulations list the setback as one of the ways of protecting workers and goes on to say that if you cannot maintain this two-foot distance, you have to install retaining devices or use other means to protect workers. But there is no mention that doing this will require you to re-measure the depth of the trench and adjust your protective systems accordingly.

And consider a situation where you have a five-foot pile of dirt (spoil pile) on a construction site and someone digs a one-foot deep trench next to it without the setback. Would that result in a six-foot deep trench? How would you shore or shield this? It didn’t make sense to me.

Does the spoil pile right on the lip result in other hazards? Absolutely. It increases the forces on the lip and the likelihood that the lip can shear off. It creates an uneven walking surface on the trench edge, and can hide cracks in the soil which would indicate possible soil failure. So, maintaining this setback or taking other corrective measures would be an important consideration for the excavation’s competent person.

So it was an interesting question, and like I said, I love answering the phone.

Jim Johnson July 8, 2014

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