How to Lay Soil-Cement Paving

Soil-cement paving is a durable, economical paving solution that involves mixing soil and cement to create a stabilized base or surface course for roads, parking lots, driveways, and other applications. Laying high-quality soil-cement paving requires attention to details such as proper soil selection, pulverization, proportioning of materials, mixing, compaction, finishing, curing, and quality control. This comprehensive guide provides step-by-step instructions on how to properly lay soil-cement paving for long-lasting results.

Selecting the Right Soil

Choosing the appropriate soil type is critical for successful soil-cement paving. The soil should have these ideal characteristics:

Well-graded with a good distribution of particle sizes from clay and silt to sand and gravel. Avoid gap-graded soils with a lack of particles in the middle size ranges.
Plasticity index (PI) between 10-30. Lean clays and sandy soils often fall within this range.
Liquid limit less than 50.
Minimum 25% passing the #4 sieve.
Clay content between 10-40%.
Free of roots, stumps, leaves, trash, and other organics.

Soil testing helps verify that the soil meets specification. Send samples to a qualified geotechnical engineering lab for testing. Critical tests include sieve analysis, PI, and clay content.

Adjust less-than-ideal soils by blending in materials to achieve proper gradation, plasticity, and clay content. For example, mix lean clay with granular particles like crushed stone. Or blend excessively plastic soils with concrete sand. Work with a materials engineer if needed to design an optimal soil blend.

Key Soil Selection Tips

Sample soil from multiple locations and depths to account for variability.
Test soil well in advance to allow time for blending adjustments if needed.
Soil selection and processing can make or break the quality and longevity of soil-cement paving. Invest the effort upfront to get it right.

Pulverizing the Soil

Once an appropriate soil is selected, the next step is to pulverize it to break up clods and achieve the desired particle size distribution.

Pulverization goals:

100% passing 1-inch sieve
At least 80% passing the No. 4 sieve

Equipment choices for pulverization include:

Rotary tillers
Disk harrows
Scrapers
Graders
Soil pulverizers

Make multiple passes over the soil with the equipment to thoroughly break down clods and homogenize the gradation.

Moisture content affects the efficiency of pulverization. Dry soils are difficult to break down. Wet soils tend to stick together and “ball up” in clods. Moisture in the range of 2% below to 2% above the optimum moisture content for compaction usually allows for effective pulverization.

After pulverization, inspect the soil to confirm the particle size distribution meets requirements before continuing to the next steps.

Proportioning the Cement

With the soil properly prepared, the next task is to add the right amount of cement. The cement proportion determines the ultimate strength and longevity of the soil-cement pavement.

Typical cement content ranges from 4% to 10% by weight when using Type I or Type II Portland cement. Higher cement contents produce stronger material. Cement requirements depend on factors like:

Traffic loading
Subgrade strength
Environmental exposure
Required design life

The preferred method is to have a geotechnical lab determine the optimum cement percentage through a mix design procedure. This will confirm the right cement rate for your specific project.

When a formal mix design is not performed, follow local standards or use 5% to 7% cement as a starting point for most paving applications. Materials can be proportioned by weight or volume, but weight provides the most consistent results.

Steps for Proportioning by Weight:

Determine the maximum dry density (MDD) of the prepared soil using the standard Proctor test method.
Select the target cement rate, such as 6% by dry weight of soil.
Calculate the weight of compacted soil needed for the section being constructed. For example, a 6 inch thick soil-cement layer over a 10 foot wide by 50 foot long area:

Volume = 10 ft x 50 ft x 0.5 ft = 250 cubic feet
Weight of soil = 250 cu ft x 110 lb/cu ft MDD = 27,500 lbs

Multiply the total soil weight by the cement rate to determine required cement weight.

Following the example above with 6% cement:
27,500 lbs soil x 0.06 = 1,650 lbs cement

Add the cement in the calculated amount and mix thoroughly with the soil.

Mixing the Materials

Proper mixing blends the pulverized soil and cement powder together to achieve a uniform mixture. Mixing can be performed using pugmill mixers, rotary tillers, disk harrows, and other equipment.

Aim to achieve a consistent color indicating uniform distribution of cement throughout the soil. Over-mixing can degrade the soil particles, so find the minimum time needed to achieve consistent blending.

Typical mixing guidelines:

Add cement in small increments during mixing to avoid clumping
Mix for 1 to 3 minutes after last cement is added
Avoid mixing when soil moisture exceeds optimum level

Tip: Adding a small amount of water during mixing helps with dispersion of the cement. About 2-3% by weight of the soil is usually sufficient.

Test mixing adequacy by taking samples from several locations and observing color consistency after spreading out thin.

Compacting the Mixture

Once soil and cement are thoroughly mixed, the next step is compaction. Adequate compaction is critical to achieve proper density, strength, and long-term durability of soil-cement paving.

Compact to 95% to 100% of the maximum dry density according to AASHTO T 134 (Standard Proctor) or to the density indicated by the mix design.

Compaction equipment exerts pressure on the soil-cement mixture to squeeze particles together and increase density. Steel-wheel rollers, padfoot rollers, and pneumatic rollers are commonly used.

Key Compaction Guidelines

Use a moisture content at or up to 2% above optimum moisture content. Material that is too dry or wet will not compact adequately.
Compact immediately after mixing and spreading. Soil-cement loses workability with time.
Roll longitudinally first, then make passes at an angle to achieve uniform density.
Compact layers no thicker than 6 inches for best results.
Check density frequently with nuclear gauge testing or other methods to verify compaction meets specifications.
Protect compacted material from traffic until finishing is complete.

Insufficient compaction leaves soil-cement vulnerable to early failure. Putting in the effort during this step pays off through improved longevity.

Finishing the Surface

Once compaction is complete, the compacted material must be shaped and finished to final grade tolerances. This provides a smooth, durable pavement surface.

Finishing steps include:

Trim to Grade – Use a motor grader, trimmer machine, or edge shapers to trim off any over-compacted material above finished grade. Remove loose material prior to final rolling.
Final Rolling – Roll the trimmed surface to flatten ridges and provide initial compaction. Avoid driving on untreated material alongside the soil-cement area to prevent contamination.
Final Shaping – Use equipment like a steel-wheel roller or pneumatic roller in static mode to impart pressure and create a smooth, close-knit surface. Make light passes to avoid deep roller marks.
Edge Compaction – Compact edges next to curbs or gutters using suitable equipment that can access tight areas. Avoid under-compaction at edges.
Touch Up – Hand tamp any soft or low spots and fill with fresh material to achieve grade. Smooth rake marks before final finish rolling.
Finish Rolling – Make light final passes over the surface immediately before curing to lock in a smooth, dense finish. Roll longitudinally and laterally.
Surface Straightedge Check – Check for flatness with a 12-foot straightedge. Mark high spots and cut down until within tolerance.

Consistent attention to proper finishing lays the foundation for a quality pavement surface that will stand the test of time. Do not rush this critical step.

Curing the Soil-Cement

Curing is the process of maintaining proper moisture in the freshly laid soil-cement to allow the chemical reaction between the soil and cement to fully develop strength-enhancing hydration products.

Proper curing improves durability and prevents early raveling or cracking. Soil-cement should be cured using one or more of these methods:

Moisture Curing – Apply water frequently to keep surface continuously moist for 5-7 days. Avoid erosion of surface fines.
Ponding – Contain water on the surface using temporary berms for 5-7 days. Avoid undermining uncompacted edges.
Wet Burlap – Place burlap strips on surface and keep saturated by sprinkling for 5-7 days. Use weights to hold burlap in place.
Plastic Sheeting – Cover surface with plastic film, overlapping seams, and leave in place for 5-7 days. Hold down firmly with fabric weights.
Liquid Membrane Compounds – Apply curing compound to surface at the specified application rate and allow to air cure. Avoid ponding or traffic on liquid membrane.

Monitor weather forecasts and have curing equipment/materials on hand before paving begins. Curing compounds and materials should meet ASTM specifications. Maintain cure protection until opening pavement to traffic.

Quality Control Testing

It is important to perform quality control (QC) testing throughout soil-cement construction to verify materials, proportioning, placement, compaction, and curing meet specifications.

Typical QC tests include:

Soil Analysis – Gradation, PI, clay content, etc. on soil samples.
Moisture Content – Check during pulverization and mixing steps.
Density – Use nuclear gauge to check compaction at regular intervals.
Smoothness – Straightedge testing on finished surface.
Strength – Molded cylinder samples tested at 7 days.
Thickness – Measure cores taken from hardened pavement.

Correct any deficiencies found through QC testing as paving progresses. Thorough QC gives confidence that the soil-cement meets quality standards.

Troubleshooting Guide

Some potential issues and solutions in soil-cement paving:

Raveling

Insufficient curing leading to weak surface
Over-pulverization degrading particles
Too much clay in soil mix

Cracking

Subgrade movement due to soft soils or frost
Too much clay causing shrinkage
Improper curing and hydration

Poor Density

High moisture content
Low soil plasticity
Layer thickness too thick
Insufficient compactive effort

Surface Failing

Contamination from organics or poor base material
Lack of protection from construction traffic

FAQ

Here are answers to some frequently asked questions about soil-cement paving:

What thickness should the soil-cement layer be?

4 to 8 inches is typical. The required thickness depends on factors like traffic loading, soil strength, climate conditions, and design life. Performing a mix design procedure can help optimize the thickness.

What types of soil are not suitable?

Soils to avoid include gap-graded gravels, organic soils, expansive clays, and any soils not meeting plasticity, gradation, or clay content requirements. Conduct testing well ahead of time to change course if needed.

How long does soil-cement paving last?

Properly proportioned, mixed, placed, compacted, and cured soil-cement can last from 10 years up to 25 years or longer. The quality and attention to details during construction have a big impact on longevity.

Can soil-cement be used in cold weather?

Yes, but extra precautions are needed to prevent the material from freezing during curing. Use heated water for mixing, compressive strength accelerators, insulated curing blankets, or heated enclosures if paving in cold conditions.

How long does soil-cement take to gain strength?

50% of ultimate strength is reached in 5 to 7 days after proper curing. 90% strength is achieved within 28 days. Strength continues to increase slowly over several months.

When can traffic be allowed on the pavement after construction?

Keep all traffic off during curing, then restrict heavy loads for 7-14 days after final curing. New soil-cement can withstand light vehicles after 3 days if necessary.

Conclusion

From soil preparation to final curing, executed properly at each step, soil-cement pavements can provide excellent, cost-effective service life under traffic. Paying attention to important details like proper proportioning, thorough mixing, adequate compaction, and moisture curing is the key to success. Using the techniques outlined here will enable you to effectively lay high-quality, durable soil-cement paving. With care taken during construction, soil-cement offers an economical paving option that can stand the test of time.