Field Compaction of Soil: 4 Procedures | Soil Engineering

The following are the various elements of field compaction: 1. Selection of Fill Material and Borrow Area 2. Selection of Field Compaction Equipment 3. Compaction of Soil in the Field 4. Field Compaction Control.

Procedure # 1. Selection of Fill Material and Borrow Area:

The local soil sources are surveyed and samples are collected and tested. The relative suitability of different soils is determined and suitable borrow area is selected based on the suitability of soil, availability in required quantity, the cost of compensation for using the borrow area, and the cost of transportation. Once the basic properties such as grain size analysis and index properties are determined, the soil can be classified as per IS – 1498-1970 that also gives the relative suitability for compaction.

Soils with high plasticity index or large shrinkage ratio should be avoided as they tend to expand or compress or become too sticky or shrink. Organic matter that can decay should generally be avoided as fill material unless special precautions are taken.

IS light or heavy compaction tests are conducted on the selected soil to determine the OMC and MDD. The OMC and MDD serve as a guide for developing compaction specifications to be achieved in the field.

Procedure # 2. Selection of Field Compaction Equipment:

The following are the different types of equipment used for compaction:

i. Smooth wheel rollers.

ii. Pneumatic tired rollers.

iii. Sheep’s foot rollers.

iv. Vibratory rollers.

v. Vibrating plate compaction equipment.

vi. Tamper.

vii. Grid rollers.

These equipment are described in the following subsections:

i. Smooth Wheel Rollers:

Smooth wheel rollers are suitable for proof rolling subgrades and for finishing operation of fills. They provide a contact pressure of 30-40 t/m² with 100% coverage under the wheels. They are effective in compacting soils in thickness of 20-30 cm and are not efficient for thicker layers. They are suitable only for cohesionless soils.

Smooth wheel rollers are self-propelled or towed steel rollers ranging from 2 to 20 t capacity. They are suitable for well-graded sands and gravels, silts, and clays of low plasticity. They are unsuitable for uniform sands, silty sands, and clays of high plasticity.

ii. Pneumatic Tired Rollers:

Pneumatic or rubber tired rollers consist of heavily loaded wagons with several rows of closely spaced tires, four to six in a row. They provide a contact pressure of 60-70 t/m² with 70%-85% coverage under the wheels.

Pneumatic tired rollers are suitable for compacting most granular soils. Although they are also used for compacting cohesive soils, they are not as effective as sheep’s foot rollers for compacting fine-grained clays. Compaction is achieved by kneading action and also due to static weight of high tire pressure.

iii. Sheep’s Foot Rollers:

Sheep’s foot rollers consist of hollow drums with a large number of projections called shanks or sheep’s foot. These shanks have areas of 25-75 cm² for each projector. During rolling, these shanks pierce into the soil giving a high contact pressure of 150-750 t/m² resulting in most effective compaction of cohesive soils. Compaction is achieved by a combination of kneading action under concentrated high pressure.

The initial passes of the sheep’s foot roller compacts the lower portion of the lift. Subsequent passes compact the middle and upper portions. Sheep’s foot rollers are not suitable for compacting granular soils. The lift thickness should be generally limited to about 20 cm. They do not adequately compact the upper portion of the lift. Hence, compaction of last lift is generally followed by a few passes of a pneumatic or smooth wheel roller.

The sheep’s foot rollers are also known as tamping rollers. They are either self-propelled or towed and available in the range of 5-8 t. They are very effective for fine-grained soils and sands and gravels with more than 20% fines. But they are unsuitable for very coarse soils and uniform gravels.

iv. Vibratory Rollers:

Vibratory rollers are very efficient in compacting granular (cohesionless) soils. The vibration is produced by rotating off center or eccentric weights. Vibrators can be attached to smooth wheel rollers, pneumatic tired rollers, or sheep’s foot rollers. Vibratory roller compacts the soil from top to bottom. Hence, the lift used should be small, generally limited to 10 cm. Vibratory rollers apply high frequency of about 2000-6000 blows/min of low magni­tude. The vibratory action sets the particles in motion and rearranges them to a denser packing.

v. Vibrating Plate Compaction Equipment:

The vibrating plates are similar in principle to vibratory rollers but are small in size. They are used to compact granular soils over small areas, which are inaccessible to the large size compaction equipment. They are also more economical than other compaction equipment.

Vibrating plate rollers are suitable for most soils with low-to-medium fines content. They are unsuitable for large volume work and for wet clayey soils.

vi. Tampers:

Small hand-operated or machine-driven tampers can be used for compacting small areas of cohesive soils. They are similar in principle to the rammers used in laboratory compaction test and can be effectively and economically used compared to other compaction equipment.

Power rammers are also called trench tampers and are pneumatic tampers. They are suitable for trench back fill and work in confined and inaccessible areas. They are unsuitable for large volume work, mainly because of very low day’s output.

vii. Grid Rollers:

Grid rollers consist of 30-50 mm bars at a spacing of about 10 cm. Their mass is in the range of 5-12 t. Grid rollers are suitable for well-graded sands, soft rocks, and soils with stones containing fine faction. They are unsuitable for uniform sands, silty sands, and highly plastic clays.

Table 12.6 summarizes the compaction equipment suitable for different types of soil.

Procedure # 3. Compaction of Soil in the Field:

Compaction equipment can efficiently compact the soil up to a limited depth. Hence, the fills or embankments are compacted in several layers, called lifts. If lift thickness is too large, soil at the top of the lift will be well compacted whereas that at the bottom will not be compacted completely. For certain compaction equipment, the reverse also happens. For most compaction equipment, lift thickness should be of the order of 15 cm.

The soil from the borrow area is transported and spread in several layers. The compacted thickness is about two- third of the loose thickness. Each layer is compacted by passing the roller over the soil required number of times. About 10-15 passes of the roller are the most efficient and economical ways to achieve MDD during compaction. After this, the next layer of loose soil is spread and compacted. The procedure is repeated until the required height of the fill, embankment, or earth dam is reached.

Water content should be closely monitored during compaction. If the soil gets too wet, it will be impossible to compact. If the soil is too dry, compaction will not be effective. Since it is easier to add than remove water, water is added in increments during the rolling operation.

Procedure # 4. Field Compaction Control:

Improper compaction of soil in the field will lead to the dry density of compacted soil less than the required maximum achievable dry density, resulting in the following problems:

i. Low shear strength of compacted soil leading to shear failure of the soil and foundation supported on the compacted soil.

ii. Excess total and differential settlements leading to the failure of foundation and structures supported on the compacted soil.

iii. Slope failure of slopes of compacted fills, embankments, and earth dams.

iv. Excessive volume changes in the compacted fill in terms of excessive swelling and shrinkage.

v. Increase of permeability of the compacted fill leading to the loss of water, failure of hydraulic structures due to piping, etc.

It is, therefore, necessary to adopt proper field compaction control to ensure that the dry density and the water content of the compacted soil is within specified limits of MDD and OMC, respectively.