Shrink Swell Clay Soils in Collin County: Engineering Buried Utilities for Blackland Prairie Conditions

Why is Blackland Prairie clay hard on buried utilities?

The ground keeps moving after the trench is closed. Much of Collin County lies in the Blackland Prairie, where dark, high plasticity clays dominate. The NRCS description of Houston Black clay, a major Blackland Prairie soil, rates it high shrink swell, with dry season cracks up to 4 inches wide.

A pipe in that soil gets pushed and dropped unevenly every season. Joints open, gravity sewers lose grade, and the transition from a rigid manhole to a moving pipe takes the strain. Pavement cracks along the trench line are often the first sign anyone sees.

We covered the state sewer rules for this, embedment under 30 TAC §217.54, resilient manhole connectors and the 30-day mandrel test, in our post on reactive clay soils and buried utility design. This post is about what McKinney adds on top.

What soil data does McKinney require before streets are built?

A sealed geotechnical report for every roadway and alley. Section 3 of McKinney's Engineering Design Manual (Revision 7, adopted May 5, 2026) requires a Geotechnical Report for Roadways, signed and sealed by a Texas licensed engineer, with the city's checklist and a summary form.

The field and laboratory minimums are specific:

• Borings on the roadway centerline at 500-foot spacing or less, or on a 400-foot grid through a subdivision, to at least 20 feet below finished subgrade or to competent rock, whichever is shallower.
• Samples every 3 feet or less to 10 feet below finished subgrade, then every 5 feet or less.
• Atterberg limits, percent passing the No. 200 sieve, and moisture and density for each soil type.
• Swell tests (ASTM D 4546), at least two samples per boring at depths from 0 to 10 feet, plus a potential vertical rise by TxDOT's Tex-124-E method for 20 feet of moisture penetration, with the more conservative result governing.
• Sulfate testing in the upper 3 feet of each boring.

The season matters. Samples taken June through September use the mean swell. Samples taken October through May use the mean plus one standard deviation. Samples wetter than normal for the season are air dried to represent the drier part of the year.

How deep does the moisture-treated zone go?

At least 5 to 8 feet under the street on swelling clay, and deeper if the PVR calls for it. Table 3-1 of the manual sets the minimum moisture conditioning depth by average swell to 10 feet: 60 inches at 2 to 6 percent, 72 inches at 6 to 8 percent, 84 inches at 8 to 10 percent and 96 inches at 10 percent or more. The treatment must also hold potential vertical rise to 4.5 inches.

Compare that with the manual's minimum cover for water mains: 4 feet for 6 to 8 inch mains, 5 feet for 12 to 18 inch, and 6 feet for 20 inch and larger. On a high swell site, the pipes in the street section sit inside the zone the geotechnical engineer is working to stabilize.

That is the coordination point. In McKinney's typical layout, residential and collector sewers run along the roadway centerline, so the trench cuts through the moisture-treated subgrade. Ask the geotechnical engineer how trench backfill should be placed and compacted so it does not become a wet, soft stripe through the treated zone.

What embedment does McKinney require for water and sewer pipe?

Set classes for small water mains, calculations for large ones. Under Section 5.2.J, water mains smaller than 16 inches get NCTCOG Class "H" embedment extended at least 12 inches over the top of the pipe. For 16 inches and larger, the engineer picks the embedment class for the pipe material and loads, and submits calculations.

Sewer embedment and backfill follow the city's Construction Standard Details under Section 5.3.J. For both, the Director of Engineering may require trench dams based on groundwater potential, pipe slope and the length of sloped segments.

Trench dams matter on clay sites. A granular pipe zone in a clay trench can act as a drain, carrying water along the pipe and wetting the clay downhill. A dam breaks that path.

Who decides the pipe material on a clay site?

The engineer of record, based on the soil. Section 5.2.H says pipe material specification is the engineer's responsibility, based on site and soil conditions, loading and pressure, and that the city's pipe size table does not relieve the engineer of that call.

For sewers, the manual requires a minimum structural life of 50 years, a corrosion resistant liner or a demonstration where the material could deteriorate, and no change of pipe material between manholes. All buried metal is wrapped in polyethylene.

On a moving clay site, that means joint type and pipe stiffness are design decisions with a reason written down, not a copy of the last job's sheet.

Does sulfate in the clay change anything?

Yes, for the subgrade, and it is worth reading for the pipes. Formations with more than 6,000 ppm (0.6 percent) sulfates must be lime stabilized with a double application method under Section 3.4.B.

The manual ties sulfate testing to the pavement. The same results belong in front of whoever chooses pipe, manhole and coating materials, since the buried structures sit in the same soil.

Is your McKinney utility design ready for expansive clay?

Answer yes or no. Any no is a gap to close before final plans.

1. Does the geotechnical report meet the Section 3 boring spacing, depth and testing minimums?
2. Was the design swell set for the season the samples were taken?
3. Do you know the moisture treatment depth along each street, and where it changes?
4. Has the geotechnical engineer given trench backfill recommendations for lines that cross the treated zone?
5. Do water mains under 16 inches show Class "H" embedment 12 inches over the pipe?
6. Are calculations ready for any 16 inch or larger main?
7. Is the need for trench dams addressed on sloped runs?
8. Is the pipe material choice tied to the soil and sulfate data?

A note on limits: this is general education from McKinney's Engineering Design Manual, Revision 7, and public soil data as of October 2026. Other Collin County cities set their own standards, and your site's report governs, so confirm the design with your geotechnical engineer and the reviewing city.

Frequently asked questions

Does McKinney's geotechnical report cover the utility trenches?

Not by name. Section 3 is written for pavement and subgrade, with borings on the roadway centerline. Because most public mains sit in the street section, those borings usually describe the utility corridors too. Lines outside the streets, in easements or along creeks, may need borings of their own.

Can we use flexible pipe in Collin County clay?

Yes, if the design supports it. Flexible pipe tolerates some movement through deflection, but it depends on embedment that delivers the soil support the design assumed. Pick it with the soil data in hand.

I'm a developer with a McKinney subdivision in design, and my geotechnical report was done in March. Is the swell number right?

Check it. Samples taken October through May must use the mean swell plus one standard deviation. If the report used the plain mean for March samples, the moisture treatment depth may be too shallow.

I'm the civil engineer on a Collin County site outside McKinney's limits. Does this manual apply?

Only in McKinney and its ETJ, to the extent state law allows. Elsewhere, use that city's or county's standards, and treat McKinney's swell testing approach as a sound reference for your geotechnical scope.

Related resources

• McKinney's 2026 Engineering Design Manual: Utility Changes Developers Need to Catch Before Plan Submittal
• Stormwater and Utility Coordination in McKinney's High Growth Subdivisions: Avoiding the Common Redesign Trap
• Water Storage and Pressure Zone Design for McKinney's Rolling Terrain: Engineering for Collin County Topography
• Gravity Sewer vs. Pressure Sewer: Choosing the Right System for Your Community

Designing water and sewer lines on Collin County clay?

We build the utility design around the geotechnical findings for your streets and corridors, so the embedment, pipe and backfill fit the ground they sit in.

We specialize in:

• Water and sewer utility design
• Gravity sewer and lift station design
• Water distribution design
• Utility corridor and easement layout
• Site civil design for land development
• Construction administration

Modern Engineering Solutions, McKinney, Texas. Contact: (214) 833-6748 or mod-eng.com