4 Inch Minus Rock

What Is 4 Inch Minus Rock?

If you're building a new home or adding a structure to a rural Colorado property, you're almost certainly going to need 4 Inch Minus Rock at some point — whether for your septic system, your construction entrance, or both. It's one of the most-ordered products at Mule Creek Gravel, and for good reason: it does jobs that no other aggregate can do quite as well.

The name describes it simply: all rock pieces that pass through a 4-inch screen, ranging from about 2½ inches up to 4 inches in size. It's unwashed and uncoated — what comes out of the pit after screening is what gets delivered to your project. The large, angular pieces create substantial void space — empty air gaps that water moves through quickly and freely.

Unlike Class 6 Road Base, which is engineered to compact into a solid mass, 4 Inch Minus is engineered to stay loose. It resists compaction, holds its void structure under load, and keeps water moving even when buried under several feet of soil — which is exactly what a septic leach field or a stormwater tracking pad needs it to do.

Where Is 4 Inch Minus Rock Used?

This product has a narrower — but critically important — set of applications. When one of these jobs comes up, there's usually no substitute.

• Septic System Leach Fields — The primary use in Teller, Park, Fremont, Jefferson, Douglas, El Paso, and Pueblo County. 4 Inch Minus surrounds perforated distribution pipe in leach trenches, providing the drainage void space required by Colorado Regulation 43 for on-site wastewater treatment systems.
• Construction Entrance Tracking Pads — Also called a Vehicle Tracking Control (VTC), a thick pad of 4 Inch Minus at a job site entrance is required under CDOT's stormwater BMP guidelines for construction sites that disturb more than one acre.
• Retaining Wall Core Fill — Behind large boulder or block retaining walls, a free-draining core of coarse rock prevents hydrostatic water pressure from building up behind the wall.
• Erosion Control Along Channels — The mass and size of 4 Inch Minus makes it effective for lining drainage ditches, culvert outlets, and swale channels where fast-moving water would displace smaller rock.
• Sub-base Drainage Layer — In situations where groundwater is close to the surface, a thick layer of 4 Inch Minus below a gravel driveway base creates a capillary break.
• Culvert Outlet Protection — Placed at the discharge point of a culvert, coarse rock dissipates the energy of flowing water and prevents scour of the surrounding soil.

4 Inch Minus Rock — Construction Entrance Tracking Pad, Park County CO

Using 4 Inch Minus Rock — Practical Guidance

Coverage varies significantly depending on application. For a septic leach field trench, Colorado guidelines require at least 6 inches of drainage rock below the distribution pipe and 2 inches above it — so a trench 24 inches wide and 100 feet long needs approximately 8–10 tons. For a construction tracking pad at CDOT's minimum dimensions (12 feet wide × 70 feet long × 6 inches deep), plan for 25–30 tons.

• Always use geotextile fabric — Whether you're installing a septic trench or a construction tracking pad, always line the excavation with geotextile separation fabric before placing 4 Inch Minus. In a septic trench, the fabric prevents fine soil particles from migrating into the rock over time and clogging the drainage void space.

Technical Specifications

CDOT Vehicle Tracking Pad Specifications

Under CDOT's Erosion Control & Stormwater Quality Guide (Chapter 5, Section 208), aggregate-based Vehicle Tracking Control systems must meet the following minimum design criteria:

• Pad Width — Minimum 12 feet (full width of access point)
• Pad Length — Minimum 70 feet (provides approximately four full tire rotations)
• Aggregate Depth — Minimum 6 inches, placed over geotextile class 2 fabric
• Permit Trigger — Required under CDPHE COR400000 for sites disturbing ≥1 acre
• Maintenance — Must be inspected and redressed with fresh rock when sediment accumulation compromises effectiveness