- LATEST HEADLINES
- THE MAGAZINE
Avoiding errors in construction is something we all strive to accomplish.
In this column, I’ve put together a checklist that contractors should follow to minimize potential issues, mistakes and liabilities on a project. These items range from due diligence that should be done in the field to some that affect your business in the office as well. Overlooking any one of these items could be costly to your business.
Let’s start in the field.
Are You Surveying Existing Conditions?
For many contractors, this is a must; though for some, it doesn’t seem to be an issue. In all likelihood, the site was surveyed by the design engineering company prior to commencing design. But how much time has passed since the design firm’s survey and the actual construction? If one or more years have transpired, it would be good policy to resurvey it as the contractor to ensure that something hasn’t changed over time. There is also a possibility the design firm took shortcuts in determining existing conditions because nowadays low/no cost topographic data can be obtained from Google, ESRI, U.S. Geological Survey, SiteOps and USAPhotoMaps, among others. In any case, the contractor may not want to use the design firm’s existing data, unless appropriate legal language is provided for protection.
I’ve noticed that some contractors perform a spot check on the engineer’s existing ground survey. If the spot checking matches, they move ahead with the survey data. While this is practical, it could be fraught with potential trouble if things changed in the areas that were not checked.
I recommend that contractors have an adequate survey performed of the existing conditions to ensure they know exactly what they are facing on a project. This could include a rapid laser scan of the site instead of a manual survey. It is worth noting that a contractor working on the Houston Light Rail system a few years ago was liable for any damage to structures within the construction corridor. Because of this, he laser scanned the entire corridor to develop an accurate 3D model. This served two purposes: 1) he could use it as his existing conditions survey prior to construction and 2) use it as a risk mitigation task to ensure protection against false claims.
Once on the site, you can have a surveyor set your control. The surveyor may be a licensed or contract employee. If you use a subcontractor, did your agreement force him to retain responsibility of the control he set? Once control is set, how often do you or the surveyor actually check it?
A daily spot check of all control points and staking is a good idea. I have seen land owners move construction stakes to try to get more land for themselves. This has happened on a golf course where the homeowner had a golf easement, and moved the stakes to enlarge the backyard. In another case, the GPS equipment fell off of the tripod. A well-meaning-but untrained-technician simply stood it back up again and proceeded with work! Protect yourself with a daily spot check of control.
If you are working with 3D models of the site, its surfaces or subgrades, are you watching the units closely? There have been numerous situations where the design is computed using U.S. survey foot (3.28083333...feet/meter). When the project is awarded, the model building often begins. If done in-house, it is simple to verify the working units. If the model is built by a subcontractor, it may be harder to confirm. The international foot is 0.3048m/foot. The National Geodetic Survey indicates this difference equals 2 feet in 1 million feet. To add perspective, a 4-mile road would be off 0.04 feet everywhere.
Several major software programs in our industry set their data export tools to default to international foot, so check these parameters carefully and hold your subcontractors to a high standard on this. When using LandXML, check that the import/export settings reflect U.S. survey units.
Did the Engineer Design to the Topsoil Stripped Surface?
The model used for 3D machine control should be derived from an existing ground model that has been stripped of topsoil where appropriate. The design surface should be created using the stripped surface (which is really how we build projects). Some engineering firms completely skip this step and can generate inaccurate and poor finished surfaces as a result.
Many designers may not know how to strip a surface of topsoil, and so they just design to the surveyed existing ground. In other words, tie-outs hit daylight on the unstripped existing surface. To further stress how this can affect construction, the stripped topsoil may not be constant throughout the project. In fact, it usually is not constant. Most projects have a deviating topsoil thickness, and when you extract this from the existing ground model, I refer to it as “differential topsoil stripping.”
The soil reports may indicate that some areas can achieve a 1-foot strip while others may be only 6 inches. Indeed, other areas may simply be unsuitable, and there is no useable topsoil. In the figure showing section view, note that the top linework represents the original ground while the parallel linework illustrates the stripped surface data. The existing ground surface should reflect the void left by the strip process.
Do Your 3D Models Have the Same Source?
Your 3D models for digital stakeout (3D machine control) or traditional stakeouts and cutsheets should emanate from a single 3D model source. The finished ground surface is an easy one since it is built using the features to be constructed for the finished ground. Problems occur when the subgrades or subsurfaces are developed by companies that have multiple people creating these surfaces, and they do so independently.
The objective when building 3D models should be to start with a 3D surface of the finished ground. Then you methodically subtract each intervening surface until you reach the “contractor’s box.” This is the cut to the existing ground strip surface where the contractor lays material. The “box” itself should be a surface to be used in the excavation for the site. There should be a surface developed for, say the stone sub-base. On top of that, there should be a surface containing the Sm-SA, and so on. This would allow a contractor to use the finished ground all by itself if the operators can read, interpret and dial-down the equipment accurately. If not, load in the subgrade surfaces, and let the machine control do all of the work directly.
What Liability Do You Have?
How qualified is your model-builder? Is he licensed and heavily experienced? Are the models being prepared for specific types of equipment that will actually be used on the project, or is it a generic model? For example, if you are building a parabolic road, special equipment is usually reserved for this. Was your 3D model built for that equipment, or was it a planar 2-percent cross slope?
What is the inspection of the project based on? How many lifts and of what thickness are required for laying material? Sometimes you may have 6 inches of material that must be spread and tamped, but it is inspected at intervals of 2 inches. If this is the case, then perhaps multiple 3D models of each lift need to be created.
Based on some of my personal experiences, I have often questioned the equipment the inspector uses to check constructed grades, slopes and elevations. Some use top-grade survey equipment while others use peanut prisms and hand levels. If substandard inspection is negatively rating your work, consider demanding more precise inspections.
Is the model building company that you contracted insured? You may want to use those who have Errors and Omissions (E&O) insurance or at least contractor’s insurance, sometimes known as “Plumber’s Insurance.” Are the people licensed professionals in either engineering or surveying? Some states require models for construction fall under the responsible charge of a licensed individual.
What Are Your Routine Maintenance Procedures?
Is your equipment’s firmware up to date? This can be checked according to your manufacturer’s recommendations. Do you have daily and weekly procedures for checking and maintaining equipment wear edges, blades, connections, etc.? Again, maintenance checks are standard practice for most contractors, but these procedures will keep your accuracies high, reduce your chances for surprises and keep liabilities minimized.
In order to keep your project on target deadline-wise, perform a daily as-built of your site. Use your equipment and its GPS to collect and store your work each day. At the end of the work day, drive the site where you worked and collect the top surface where you excavated or spread material. Then download the data in the office as a record of the work performed. Not only will you begin to identify areas in noncompliance, but you may also locate errors in the making, whether through blade wear, a base station that went awry or simply a human error.
Do You Keep Detailed Project Records?
Along with vehicle maintenance records, daily as-builts, change-orders, financials and other collected information, you might use a spreadsheet or project management software that has vehicle tracking, timelines and other project management tools.
You can also use asset management software so planned maintenance and replacement activities can happen at the most opportune times. This software can identify the equipment's productivity, problem areas and repair schedules. Proper management of equipment requires a detailed inventory of all major items and should include the manufacturer, warranties, model, year and number, attachments, and a list of the major components and parts required for normal service. Your records should also indicate major repairs.
Are you allocating enough time for all maintenance procedures? This should be evaluated to ensure that equipment in for service will not cut productivity. The servicing period can also be seen as an opportunity to practice preventative maintenance in mechanical adjustments that are required to keep the equipment operating at peak efficiency. Identify and repair minor defects before they become major problems. This involves periodic-sometimes daily-visual inspections to check for signs of possible trouble
As you can see, many things can affect your production, liability and success on the project. Although unforeseen circumstances may be unpreventable, we can plan, schedule and put certain procedures into place that could mitigate, minimize or eliminate many of them.