Infrastructure Projects - Construction Technology
A Practical Guide to Construction Techniques and Practice
Welcome to the class on safe support of excavation. We begin by reviewing some of the codes and requirements for safely supporting excavation (SOE). I am referring o 29 CFR, which is the federal code for this kind of work. If you did a search on 29 CFR, you see that there are volumes of material available and commentary on this code. I've tried to somehow boil this down to just two rules. And I think that this is at least an effective summary of what is included in the code.
The first rule says, excavations over 5 feet must be supported. And this is helpful and useful because there's no ambiguity here. If you're Excavating deeper than 5 feet, you must support the sides.
However, this is also misinterpreted to mean that if the excavation is less than five feet deep, no supports are required. That is a mistake and can lead to real difficulties. The depth of excavation is only one consideration. The nature of the soil is probably more important and more of a determining Factor. If the soil is cohesive. 5 feet might be just fine. If the soil is not cohesive and a kind of a running flowing soil, there may be no depth at which that can be safely excavated, without supporting the sides.
The other consideration, which is very important, is the presence or absence of live loads. Any excavation which is subjected to live loads needs to be supported. And on a construction site, it's hard to predict where the construction equipment will be. And you really need to be conservative and assume that at some time or another, the construction equipment will be right at the edge of the excavation reaching inside, raising or lowering material. And you need to design with that potential live load in mind.
Rule number two is supervision by a competent party. It would be wonderful if every activity on a construction site was supervised by a competent party. But nowhere is that more important than in creating a safe support for excavations. You really need some experience people, and if they are experienced with the local soil conditions, so much the better. This is not a place for someone who's just beginning the learning curve. You really need some knowledgeable, experienced help here.
I'm suggesting a new rule that all excavations subject, the live load must be supported. I think that is much safer and really describes and anticipates the conditions on a construction site.
Here we have one of the simplest, most direct devices for supportive excavation. This is a portable box called a trench box. You can excavate a rather crude opening and lower the box in place. You can continue to excavate inside the box to get it to the grade that you want. The ends of the box are open so you can advance length of pipe or whatever it is you're trying to install.
You've quickly created very safe environment. No special tools are required. The same backhoe that excavated the trench can handle this box as well. There are some shortcomings here. You can see it requires quite a bit of space. You are unlikely to ever see one of these in some downtown street. Also, it has to be an area which is free of utilities.
If you are new to this business, you may not have heard much discussion about utilities, but on any construction project, they are extremely important and need to be well understood and accounted for before construction gets underway. Very often they are neglected, and they come back to really haunt you during construction. This box requires an environment that is free of utilities to work.
In a downtown location, because of space limitations and utilities, the trench box would be infeasible. This location is quite typical. This happens to be in lower Manhattan. The excavated trench is filled with utilities. And by the way, these are only up near the surface. You can expect that there would be many more utilities, gas mains, water mains and sewers that would exist farther down in the excavation.
If you're designing a project, you need to carefully assess what utilities are in the area and how they are going to be relocated or otherwise accommodated. You need to do is careful job as possible. And when questions arise, the use of test pits are a very effective way of gathering more information so that you have anticipated to the extent possible the impact of utilities. You can be quite sure there will still be surprises during construction.
The goal is to minimize those surprises because each uncovered utility, which is not anticipated, will result in delays, additional costs and possibly disputes over who pays for these unidentified utilities. This is an area which is often I put on the back burner and really belongs in the forefront of any design effort.