Caliche Road Base: The Texas Highway Standard (TxDOT Item 247)

Across Central, South, and West Texas, as well as parts of New Mexico and Oklahoma, caliche is the undisputed backbone of regional road construction. From rural ranch roads and oilfield drilling pads to Texas Department of Transportation (TxDOT) secondary highways, caliche offers an economical, naturally cementitious flexible base.

In this technical guide, we examine the geological formation of caliche, TxDOT Item 247 engineering specifications, moisture requirements, and best installation practices for hot, semi-arid environments.

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1. What is Caliche? Geological Origin & Chemistry

Caliche (pronounced kuh-lee-chee) is a sedimentary rock formed by the precipitation of calcium carbonate (CaCO3) within soil horizons in arid and semi-arid climates.

Unlike virgin crushed granite or basalt—which rely solely on geometric particle friction—caliche possesses a unique self-cementing chemical mechanism:

1. When caliche is crushed, spread, and conditioned with water, fine calcium carbonate particles dissolve slightly into a mild slurry.
2. Under high-energy compaction (pneumatic-tire or heavy vibratory rollers), the slurry fills microscopic voids between limestone fragments.
3. As the base cures and dries under the Texas sun, the calcium carbonate recrystallizes, chemically cementing the aggregate into a dense, semi-rigid pavement crust.

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2. TxDOT Item 247 Specification Grades

The Texas Department of Transportation governs flexible base materials under TxDOT Standard Specifications Item 247 (“Flexible Base”). It divides materials into specific Grades based on strength, plasticity, and durability:

TxDOT Specification Property	Grade 1-2 (Highway & Commercial)	Grade 3 (County & Ranch Road)	Grade 4 (Non-Spec / Private Driveway)
Master Sieve Top Size	1-3/4 in (45 mm) Max	1-3/4 in (45 mm) Max	Varies by pit
Retained on 7/8 in Sieve	10% – 35%	10% – 35%	Unregulated
Retained on #4 Sieve	45% – 75%	45% – 75%	Variable
Retained on #40 Sieve	70% – 85%	60% – 85%	Variable
Max Liquid Limit (LL)	35 – 40	40 – 45	None
Max Plasticity Index (PI)	Max 10 – 12	Max 15	None
Min Compressive Strength	35 – 45 psi (at 0 psi lateral)	Not Specified	Pit-run
Wet Ball Mill Max Degradation	Max 40% – 45%	Max 50%	None

Contractor Note: When ordering for a residential or commercial driveway, always request TxDOT Item 247 Grade 1 or Grade 2 Flexible Base. Grade 3 or uncertified “pit-run” caliche contains excessive unbonded plastic clay, which turns into slick, wheel-spinning mud during heavy Texas downpours.

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3. Caliche Density, Moisture & Compaction Rules

Due to its chalky, porous nature, caliche has a lower unit weight than heavy crushed granite or basalt:

Measurement Type	Caliche Base Density	Crushed Limestone Density	Crushed Granite Density
Loose Delivered Weight	1.35 – 1.45 tons/yd³	1.50 – 1.58 tons/yd³	1.55 – 1.65 tons/yd³
Compacted In-Place Density	1.70 – 1.85 tons/yd³	1.82 – 1.95 tons/yd³	1.90 – 2.05 tons/yd³
Bulk Dry Unit Weight	125 – 135 lbs/cu ft	135 – 145 lbs/cu ft	140 – 150 lbs/cu ft
Optimum Moisture Content (OMC)	8.0% – 12.0%	5.5% – 7.5%	4.5% – 6.5%

Why Caliche Requires More Water

Because porous caliche aggregates absorb water internally into their calcium carbonate structure, they require a higher moisture content (8% to 12%) during rolling than standard limestone. If rolled dry, the surface will “dust out” and ravel quickly under tire traffic.

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4. Caliche vs. Crushed Limestone: Pros and Cons

Criteria	TxDOT Caliche Base	Crushed Limestone (Type D)
Quarry Material Cost	$12 – $22 per ton (Very affordable)	$26 – $42 per ton
Haul Distance in Central/West TX	Local pits within 15–25 miles	Rail-head or 50+ mile haul
Dry Weather Strength	Extremely high (bonds like concrete)	High (frictional interlock)
Wet Climate Performance	Moderate (vulnerable if unsealed)	Superior (low moisture sensitivity)
Dust Generation	Moderate to high (produces white dust)	Low to moderate

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5. Best Practices for Caliche Driveway Construction

1. Subgrade Stabilization: In expansive black clay regions of Texas (such as the Houston/Austin I-35 corridor), install a heavy woven geotextile fabric beneath the caliche to prevent plastic clay intrusion.
2. Build a 2% Drainage Crown: Water is caliche’s primary enemy. Ensure water sheds immediately to road ditches so it cannot pond on the surface.
3. Pneumatic (Rubber-Tire) Proof Rolling: After initial steel drum vibratory passes, finish caliche roads with a multi-wheel pneumatic rubber-tire roller. The kneading action of rubber tires draws the fine calcium carbonate slurry to the surface, creating a smooth, sealed protective crust.
4. Surface Sealing: For maximum longevity, apply a Two-Course Surface Treatment (Chip-Seal) using asphalt emulsion and Grade 4 crushed limestone cover stone.

Calculate precise material requirements for Texas projects using our Road Base Calculator Texas and Caliche Calculator.