Identifying soil types and compaction of soil subgrades

by jason_cramp | September 3, 2015 4:25 pm

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By Brian Burton

Understanding soils as they relate to interlocking concrete paver (ICP) installation is vital to a successful project. Identifying soil types and knowing what effect each one can have on project length and labour requirements is also imperative. Further, knowing the soil type and achieving an adequate level of compaction is integral to the long-term performance of an installation and can often help reduce unnecessary callbacks.

Gradation fundamentals, sieve analysis

Generally, the suitability of a soil for use under a pavement decreases with its particle size. The measurement of various particle sizes is called soil gradation and is accomplished via a series of sieves. These circular sieves have various size openings to allow any material smaller than screen size to pass through. Soil samples are dried, weighed, and then placed in the coarsest sieve, while stacked over increasingly finer sieves. As they are shaken, each one blocks a specific size particle (or larger) from passing to the next smaller sieve, while smaller particles pass to the next smaller sieve, and so on. The material remaining on each sieve is weighed and the per cent passing each is calculated.

Classification of soils by this gradation technique indicates how it will perform under a pavement. (Aggregate base materials and bedding sands are also classified by gradation.) A sieve analysis typically produces particle sizes that fall into the following soil categories. Most soils are a mix of the following different sized particles:

• Coarse sand: 2 to 0.25 mm (0.0787 to 0.0098 in.)
• Fine sand: 0.25 to 0.076 mm (0.0098 to 0.003 in.)
• Silt: 0.076 to 0.005 mm (0.003 to 0.000196 in.)
• Clay: Smaller than 0.005 mm (0.000196 in.)

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Soil classification

Most contractors should be familiar with the general soil variations in their area; however, there can be significant soil type differences within a small geographic area. When a soil classification is known, its tendency to hold water and ease of compaction can be predicted. Identifying soil types also assists contractors when selecting compaction equipment and when estimating the time needed to complete a project. For example, a heavy clay soil may require more time to excavate and compact than a sandy soil.

Quick field identification

A simple way to classify soils in the field is by visual appearance and feel. If coarse grains can be seen and the soil feels gritty when rubbed between the fingers, then it is likely a sandy soil. If grains cannot be seen with the naked eye and it feels smooth, then the soil is likely a silt or clay.

A primary factor in the performance of soil under pavement is its ability to hold water. The more water the soil will hold, the poorer it performs as a foundation for pavement. There are some easy ways contractors can perform quick field identification and assess the soil’s water holding capacity.

Patty test

• Mix the soil with enough water to make a patty of putty-like consistency and let it dry completely.
• The greater the effort required to break the patty with fingers, the greater the plasticity, or ability to hold water. High-dry‑strength is characteristic of clays, while silts and silty sands will break easily.

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Shake test

• Mix one tablespoon (15 ml [0.5 oz]) of water with the soil sample by hand. (The sample should be soft, but not sticky.)
• Shake or jolt the sample in a closed palm a few times.
• If water comes to the surface, the soil is fine sand.
• If no water or very little moisture comes to the surface, it is silt or clay.
• If squeezing the soil between the fingers causes the moisture to disappear, the soil is sandy.
• If moisture does not readily disappear, then the soil is silty.
• If moisture does not disappear at all, the soil is clay.

Snake test

• Moisten a small sample of soil to the point where it is soft, but not muddy or sticky.
• Roll the sample into a thread or ‘snake’ between the palms.
• The longer the thread can be rolled without breaking, the higher the clay content.

These field tests can be performed quickly and easily to classify soils and obtain a relative measure of their water holding capacity. In cases where a contractor is unsure, or unable to classify the soil, a civil or geotechnical engineer should be hired to test and/or determine the exact soil classification.

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Moisture content

Controlling the moisture content of the soil or base during compaction is critical to achieving maximum density. The correct amount of water is necessary to allow soil or aggregate particles to slide by each other. The water, in effect, acts as a lubricant. If there is too much water in the soil, however, the water will take up space between the particles and prevent them from staying together.

A simple field test for estimating optimum moisture

To achieve adequate compaction, the soil must have the right amount of moisture. The following is a simple field test to evaluate the soil for the right amount of moisture for compaction:

• Squeeze a handful of the soil to be compacted into the size of a tennis ball.
• Drop the ball about 0.3 m (1 ft) from the ground.
• If the ball breaks into a small number of fairly uniform fragments, it is close to optimum soil moisture.
• If the soil does not form into a ball at all, the soil is too dry and water must be added. (Gravel and mostly sandy soils often will not form into a ball.)
• If the soil is too moist, the ball will not break apart, unless the soil is very sandy. The soil to be compacted should be allowed to dry.

Most soils can be compacted to at least 95 per cent standard Proctor compaction test density. This is a minimum recommended guideline for pedestrian sidewalks and driveways. The depth of compaction to standard Proctor density in the soil should be at least 0.15 m (0.5 ft) for pedestrian traffic areas and 0.3 m (0.98 ft) for areas subject to vehicles. Soils that are continuously wet, very fine, or contain organic matter (e.g. decaying leaves, wood, etc.) will not compact to these recommended minimums. They may need other treatments such as replacing weak soils, stabilizing the soil with lime or cement, or replacement with aggregate.

Soil compaction

Compaction describes the process of mechanically increasing the density (weight per unit volume) of soil or aggregate base materials. Density is usually expressed in kilograms per cubic metre or pounds per cubic foot. Compaction moves soil or aggregate particles, rearranging them closer to each other, and forcing out the air or water trapped between them. Compaction removes air from bedding sand, aggregate base, dry soils, and water from moist clay and cohesive soils. This process also increases the load bearing capacity of the soil, preventing settlement and reducing swelling and contraction due to seasonal changes in moisture and temperature. With increased density, the soil, or base, is better able to support a load without settling or rutting.

Three factors govern the extent to which soil can be compacted:

• Nature, gradation, or physical properties of the soil or base materials.
• The moisture content of the soil.
• The type and amount of compacting effort required.

Soils are usually a mix of clay, silt, or sand and their particle size gradation determines their usefulness as a subgrade material. Silts and clays may require more time to compact than sandy soils because they have many small particles. In contrast, sand or granular soils are usually easier to compact than clay soils because the particles are larger.

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Compaction equipment and selection

Frequency and amplitude describe the motion of compaction equipment. Frequency is expressed in vibrations per second or hertz (Hz) and amplitude is expressed as half the distance travelled by the vibrating drum or base plate.

Machine types

Compaction equipment is generally classified into three machine types, each producing a different type of compaction effort: rammers, vibratory machines, and static rollers.

Rammers are distinguished by their low frequency (800 to 2500 blows per minute) and high stroke (40 to 90 mm [1.5 to 3.5 in.]). The stroke of a rammer is the height the ramming shoe or plate reaches from ground level while operating. The difference between ramming and vibratory action is rammers jump higher, but at a lower rate. The force of the impact is higher for a rammer. As a rule of thumb, rammers work well for clay soils.

Self-propelled rammer-plate machines use eccentric counter rotating weights to provide both the ramming force and travel speed. This machine can be used to compact clay soils and aggregate base materials. A handheld upright rammer (i.e. jumping jack) uses a spring mechanism.

Rammers can also be used for granular (sandy) soils if the soil being compacted is in a confined area (e.g. a trench). If the area is not confined, the rammer will push the granular soil to the sides rather than compact it. Rammer plates can also be used on granular materials in confined areas, or on open areas with extension plates.

Vibratory machines exert low amplitude and high frequency (2000 to 6000 Hz). Each rotation of the eccentric shaft in the drums forces energy into the ground. This vibration energy sets the soil particles in motion and rearranges them more tightly. They are suitable for compacting mixed dry soils and aggregate base materials.

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Soil compaction methods

A common error in compacting soil or base materials is completing this process all at once rather than in layers or lifts. A 50- to 75-mm (2- to 3-in.) thick lift of loose soil or aggregate should be placed and compacted, and then another loose lift is spread and compacted until the soil or base is brought up to the specified elevation. The material should be moist so it can achieve optimum density.

If the lift is too deep, the machine will need more time to compact the soil or base. The soil or base may never reach full compaction, especially if a hard layer is compacted at the top of the lift and the density is not consistent to the bottom. (Measuring soil compaction will be discussed in a future article on installation and compaction of base materials.)

Pay dirt

Accurate determination of soil types and achieving adequate compaction can save contractors time and money. In fact, failure to achieve adequate compaction is one of the most common deficiencies with the installation of interlocking concrete pavers. Although this activity may require more time and effort at the time of installation, it is generally much less costly than returning to the site to make repairs. The quality of workmanship and the finished product, according to experienced professionals, is the contractor’s best sales tool.

[7]Brian Burton operates a multidisciplinary firm that specializes in website design and development, and technical business writing. The firm also assists companies interested in selling goods and services to governments and institutions. He can be reached via e-mail at brianburton1995@gmail.com[8].

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