Know what you're digging into

Aug. 31, 2001
Not long ago, a county road engineer from rural Kansas needed a partial subsurface evaluation of a bridge project

Not long ago, a county road engineer from rural Kansas needed a partial subsurface evaluation of a bridge project.

Not long ago, a county road engineer from rural Kansas needed a partial subsurface evaluation of a bridge project

Not long ago, a county road engineer from rural Kansas needed a partial subsurface evaluation of a bridge project. One of the county’s older bridges had deteriorated and needed to be replaced. To hold down costs, the county engineer decided to handle the engineering work internally rather than contracting the project out to a consulting firm.

In developing the bridge design, the county engineer settled on another cost saving strategy. Instead of planning and designing the foundation, she would simply tell the contractor to set the foundation into bedrock.

Bedrock would certainly provide a strong foundation for the bridge. Chances are, it would probably hold down the cost of the bridge as well by eliminating the need to plan and design foundations.

The engineer was probably right. Then again, she might be wrong.

Maybe it wouldn’t be necessary to go all the way to bedrock to accomplish her goal. Maybe excavating all the way to bedrock would cut into the savings of eliminating the design and analysis work.

A thorough geotechnical subsurface analysis is the only way to find out for sure. If such an evaluation proves the engineer right, is it a waste of money?

Don’t miss anything

No one quarrels with the need for some level of subsurface evaluation of a bridge site. The issue generally has to do with the question of how thorough the analysis should be.

A firm specializing in geotechnical exploration would, of course, recommend that every bridge project should base its design on a thorough evaluation. Whether or not a thorough evaluation might be a waste of money depends upon how a bridge engineer views the cost of such an analysis. As a rule, a complete geotechnical analysis will cost well less than 1% of the total cost for building a bridge. This small cost will cut into savings on a project if it simply proves out assumptions. But it will not eliminate the savings.

Over time, however, geotechnical analysis on many projects will likely save more than they cost. Now and then, the savings produced by an exploration and analysis will represent a sizable chunk of a budget.

Perhaps more important, consistent subsurface testing will ensure against disasters: a sink hole that no one would have known about, corrosive soil chemistry that might corrode a pier or foundation structure, a drainage surprise, a settlement issue related to the embankments or some other problem that only a more detailed subsurface survey can uncover.

In addition, state highway departments as well as county and city road departments have been building bridges for more than a century. Over time, these departments have developed a solid set of principles governing quality bridge design and subsurface exploration. Often, road department guidelines for subsurface testing involve only the foundation of the bridge and include elementary sampling procedures.

On certain bridge projects, however, a thorough geotechnical evaluation might uncover circumstances allowing the alteration of these standards and produce savings. Whether or not exceptions should be made for the sake of budgets on individual projects, of course, is up to the supervising engineers. The point is that some projects can make a cost advantage out of exceptions, given administrative approval.

Taking the steps

A thorough investigation of subsurface conditions at a bridge site follows a basic five-step plan.

Preliminary site work is the first step. This begins with a study of the ground surfaces across the site and includes mapping grades, slopes and modifications made perhaps to accommodate previous embankments. The study will set boring depths for the embankments, provide preliminary foundation analysis and any drainage structures that may appear necessary. The geotechnical engineer also will suggest boring locations.

The next step is the drilling itself. Engineers follow an outline created during the preliminary study of the site, drilling borings in areas that will support the bridge as well as along the embankment routes. The type and depth of the samples vary with the conditions prevailing across the site. Engineers will accommodate basic requirements set by government departments managing the project but will often suggest additional sampling or different kinds of sampling techniques than what existing criteria may require.

The third step in the process involves laboratory testing of the samples. Tests evaluate soil and rock samples in terms of strength, moisture, density and other properties. Once again, the nature of the bridge may call for specialized tests to determine if there is a potential for long-term settlement, corrosion, deterioration of foundation soils caused by changes in moisture content, or unstable slopes.

Fourth comes the geotechnical report, which covers the results of the tests and presents the geotechnical engineer’s recommendations. The report contains a full description of the site both above and below ground, at the embankments and beneath the bridge, along with recommendations related to fill areas and materials. In some cases, these reports may uncover conditions that suggest further drilling and testing.

Monitoring the site preparation work and foundation construction process rounds out the geotechnical engineering task. Practical experience suggests that the engineer who prepared the original geotechnical report makes a good choice for this work. When others handle monitoring duties, delays can arise when judgments become necessary and are made without benefit of the experience of developing the report.

All in all, a thorough approach to subsurface analysis will not replace the good judgment of experienced bridge engineers. But it can raise options that might not otherwise be available. For example, an analysis might provide options that would cut the cost of embankments through the acquisition of less land. It could uncover cut material that will make more appropriate fill for the embankments. It may discover alternatives that will reduce future maintenance requirements.

Any number of cost saving ideas worth considering may arise from a thorough geotechnical survey.

Given the relatively low cost of such a survey and the high cost of a bridge, wouldn’t it be better to search for those options?