Why Subgrade Machine Control Models May Not Be the Best Choice for Your Project

In the world of construction technology, precision is everything. GPS machine control models have revolutionized earthmoving, making projects faster, more efficient, and more accurate. However, one common point of confusion among contractors and grading professionals is whether to use subgrade machine control models or finish grade models. While subgrade models may seem like a logical choice for certain applications, in most cases, they introduce more challenges than benefits.

At Quantum Land Design, we’ve built well over 10,000 GPS models and have found that subgrade models often cause more headaches than solutions. In this article, we’ll break down why subgrade models tend to create issues, explain when they might be necessary, and demonstrate why finish grade models are the superior option for most projects.

The Challenges of Subgrade Models

A subgrade machine control model represents the base layer before rock, pavement or topsoil have been placed. While this approach might seem beneficial for early-stage grading work, it introduces several significant issues:

1. Mismatch Between Model and Plan Sheets

One of the biggest problems with subgrade models is that the grades in the model are not what is stated on the official plan sheets. Since grading plans are typically designed for finish grade, trying to adjust them for subgrade can lead to grade offsets and transitions that can be difficult to compare to plan.  This can cause confusion among project managers, surveyors, and operators. This plan to model misalignment increases the risk of costly mistakes and rework.

2. Increased Potential for Errors and Higher Cost

Because subgrade models are derived from finish grade plans, there is an added layer of complexity in building the model and checking it's accuracy.  Due to every spot grade and contour requiring a vertical offset, a subgrade model requires more time to build and check against the plan.  When it comes revision time, which it almost always does, contractors always need the new model back as quickly as possible.  Patching in a revision to a subgrade model that does not match plan grades takes more time, slowing our turnaround and you getting back to earthmoving.  

3. Difficult Transitions Between Varying Subgrades

Construction sites all have varying subgrade depths, such as different depths for pavement, sidewalks, or topsoil. Modeling these transitions accurately and smoothly adds a step to the modeling process, increasing modeling time.  In addition to this, some projects offer alternate subbase materials that each require a different subgrade depth.  The subbase material may not be known when the model is built.

 The extra effort spent adjusting subgrade models does not provide enough value to justify the additional expense and complexity.

When Subgrade Models Might Be Necessary

While subgrade models generally introduce more challenges than benefits, there are a few specific situations where they may be required:

1. Projects with Special Subgrade Design Requirements

If a project calls for a specific and clearly designed subgrade design a separate surface model may be necessary.  These types of designs are usually for unique subgrade drainage systems. They are explicitly designed and called out in the plans just as finish grade is. 

The permeable paving project in the plan sheets below is a great example of a project that had unique subgrade drainage. Note the elevations and typical sections are clearly called out in the plans.

2. Liner Projects with Subgrade Profiles 

Some projects, such as railroad lines and some roads, require distinct subgrade profiles that differ from the final grading plan. In these cases, subgrade models may be necessary or even preferred over finish grade.   

The road cross section below shows a subgrade profile that cannot be graded by directly offsetting the finish grade model down. The contractor requested a subgrade model to the bottom of the base layers then a finish grade model to use for final grade checks and staking.

3. Over-Excavation and Demo

Road and Demolition jobs may require unusually deep or wide excavations to remove unsuitable material.  When the over-ex does not align with the finish grade model a subgrade model might be advantageous.  City street removal and replacement projects can be a good example of this.  

If your project falls into one of these categories, Quantum Land Design can build a subgrade model tailored to your needs. However, for the vast majority of grading projects, a finish grade model is the more effective choice.

Why Finish Grade Models Are the Best Option

1. Directly Aligns with Engineer’s Plans

Finish grade models are designed to match plan grades and design intent, eliminating the need for additional calculations and reducing the risk of misinterpretation. This plan-to-model alignment ensures that what operators see in their GPS machine control system directly corresponds to the engineer’s specifications.

2. Simplifies Communication and Reduces the Opportunity for Mistakes

Because the finish grade model matches the official design, it eliminates confusion between contractors, surveyors, and project managers. Everyone works from the same reference point, leading to fewer misunderstandings and errors.  Model accuracy checks can be performed quickly against the plan sheets

3. Faster Turnaround Time

In earthmoving, time is money.  By eliminating the need for additional adjustments, finish grade models can be completed more efficiently, helping keep your project on schedule. At Quantum Land Design, we  provide a three-business-day turnaround for most machine control models.

4. More Cost-Effective

Since finish grade models require fewer adjustments and reduce the likelihood of costly errors, they are a more cost-effective option in the long run. Contractors can keep model costs down, avoid unexpected rework and keep their grading operations running smoothly.

Making the Right Choice for Your Project

If you’re considering a subgrade model for your next project, we encourage you to think twice. In most cases, a finish grade model will provide a more accurate, efficient, and cost-effective solution. However, if your project has unique subgrade requirements, Quantum Land Design is happy to work with you to develop a custom solution.

Our goal is to help contractors and grading professionals maximize efficiency and minimize risk with the best possible GPS machine control models. If you have questions or need a model for an upcoming project, reach out to us today. Our team is ready to provide expert guidance and fast, high-quality modeling services to keep your project moving forward.

Get in Touch

Need a GPS machine control model for your next project? Contact Quantum Land Design today to discuss your needs and get a quote. Our team is committed to providing fast, accurate, and cost-effective solutions tailored to your project requirements.