Soil Stabilization: Compaction, Chemical Additives and Geosynthetics

NCEES FE Civil spec: Geotechnical Engineering, K. Soil stabilization (e.g., chemical additives, geosynthetics). FE Reference Handbook: Civil Engineering chapter, Geotechnical section (phase relationships); Environmental chapter (landfill compaction).

What the FE Civil Spec Covers

Section K covers ways to improve soil that is too weak, too compressible, too plastic or too wet for the job. The spec names chemical additives and geosynthetics. Mechanical compaction is the baseline method and the most likely source of numeric questions, so it is included here too. Expect a mix of calculation (relative compaction, additive quantity) and concept (which method fits which soil).

Key Concepts

Compaction densifies soil by removing air. The Proctor test gives the maximum dry unit weight and optimum moisture content; field density (sand cone or nuclear gauge) is compared against it.
Lime (quicklime or hydrated lime) suits clays: it lowers the plasticity index, dries wet subgrades and builds strength through slow pozzolanic reactions. Typical dosages are a few percent by dry weight.
Portland cement suits sands, gravels and low-plasticity soils (soil-cement and cement-treated base).
Fly ash, asphalt emulsion and bitumen are other stabilizers, often for pavement layers.
Geosynthetics: geotextiles (separation, filtration, drainage), geogrids (reinforcement of walls, slopes and base courses), geomembranes (barriers such as landfill liners) and geocomposites.
Other methods: preloading with wick drains to speed consolidation, dynamic compaction and vibro methods for loose sands, and grouting.

Compaction Curve Basics

Plotting dry unit weight against water content gives a hump-shaped curve. The peak is the maximum dry unit weight at optimum moisture content. More compaction energy shifts the peak up and to the left. Standard Proctor uses a 5.5 lb hammer dropped 12 in on 3 layers (about 12,400 ft-lbf/ft³); modified Proctor uses a 10 lb hammer dropped 18 in on 5 layers (about 56,000 ft-lbf/ft³). Clays compacted dry of optimum are stiffer and more permeable but prone to swelling; wet of optimum they are more flexible, less permeable and preferred for liners and dam cores.

Core Formulas

Dry unit weight from a field test:

γ_{d} = \frac{γ}{1 + w}

Relative compaction:

R C = \frac{γ _{d, f i e l d}}{γ _{d, ma x}} \times 100%

Zero-air-voids (100% saturation) curve, the upper limit on any compaction curve:

γ_{z a v} = \frac{G _{s} γ _{w}}{1 + w G _{s}}

Additive quantity per unit area for a treated layer of thickness t at dosage p (by dry weight):

W_{a dd i t i v e} = p γ_{d} t

Where It Is in the FE Reference Handbook

The Civil Engineering chapter, Geotechnical section, has the phase relationships you need for dry unit weight, water content and saturation. The Environmental Engineering chapter has landfill compaction and liner items. Stabilizer selection and geosynthetic functions are not tabulated in detail, so treat those as concept questions.

Worked Example: Compaction Control and Lime Quantity

A clay subgrade is to be compacted to at least 95% of standard Proctor maximum dry unit weight, 18.0 kN/m³. A sand cone test gives a moist unit weight of 19.6 kN/m³ at a water content of 12%. (a) Does the lift pass? (b) The top 150 mm will then be treated with 4% lime by dry weight at a dry unit weight of 17.0 kN/m³. How much lime is needed per square meter?

Step 1: field dry unit weight.

γ_{d} = \frac{19.6}{1 + 0.12} = 17.50 kN/m^{3}

Step 2: relative compaction.

R C = \frac{17.50}{18.0} \times 100 = 97.2% \geq 95% \Rightarrow passes

Step 3: dry soil weight per square meter.

γ_{d} t = 17.0 (0.15) = 2.55 kN/m^{2}

Step 4: lime required.

W_{l im e} = 0.04 (2.55) = 0.102 kN/m^{2} = \frac{102 N}{9.81 m/s ^{2}} \approx 10.4 kg/m^{2}

Common Mistakes

Comparing the moist (total) field unit weight with the Proctor maximum dry unit weight.
Dividing by w instead of (1 + w) when converting to dry unit weight, or using w as a whole number (12 instead of 0.12).
Computing additive dosage on total weight instead of dry weight.
Recommending cement for a highly plastic clay or lime for clean sand.
Confusing geotextile functions: a geogrid reinforces, it does not filter.

Practice Problems

Try these from the free PolyCalc FE Civil problem bank. Each one has a full worked solution.

A field compaction test gives a dry unit weight of 17.2 kN/m³. The laboratory maximum dry unit weight from a standard Proctor test is 18.5 kN/m³. The relative compaction is most nearly: See the worked solution or practice the Soil Mechanics deck
A soil has moist unit weight γ = 19 kN/m³ and gravimetric water content w = 10%. What is its approximate dry unit weight? See the worked solution or practice the Soil Mechanics deck
A soil has water content w = 18%, specific gravity of solids G_s = 2.70, and void ratio e = 0.65. Its degree of saturation is most nearly: See the worked solution or practice the Soil Mechanics deck
A fine-grained soil has LL = 55 and PL = 25. Its plasticity index is: See the worked solution or practice the Soil Mechanics deck

Related Calculators and Examples

Frequently Asked Questions

Which additive is best for stabilizing a high-plasticity clay?

Lime is the usual choice for plastic clays. It reduces plasticity, makes the soil more workable and, over time, reacts with clay minerals to gain strength. Cement suits granular and low-plasticity soils, and fly ash or asphalt emulsion are other options.

What is relative compaction?

Relative compaction is the field dry unit weight divided by the maximum dry unit weight from a laboratory Proctor test, expressed as a percent. Specifications commonly require 90 to 95 percent of modified or standard Proctor.

What are the main functions of a geosynthetic?

Separation, reinforcement, filtration, drainage and containment (barrier). Geotextiles mainly separate, filter and drain; geogrids reinforce; geomembranes act as barriers.

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