What Is Soil Compaction?
Soil compaction occurs when soil particles are pressed together, reducing the pore space between them. Strongly compacted soils contain few large pores, less total pore volume, and, consequently, higher density. The compacted soil has a reduced rate of infiltration and water drainage.
Different Types of Soil Compaction Equipment
Soil compaction equipment can be divided into two groups
- Light Soil Compaction Equipment
- Heavy Soil Compaction Equipment
Light Soil Compaction Equipment
These equipment are only used for compacting the soil in small areas and where the required compaction effort is less. Below is the light equipment for soil compaction:
- Vibratory Plate Compactors
- Vibro Tampers
Compactors are used to compact small areas, providing an impact load to the soil. This equipment is lightweight and can be operated manually or by machine.
The base size of the compactors can be 15cm x 15cm or 20cm x 20cm or more.
Compactors for Soil Compaction
For machine-operated compactors, the usual weight ranges from 30 kg to 10 tons (6 pounds to 22,000 pounds).
These hammers weighing 2 to 3 tonnes (4400 to 6600 pounds) can fall freely from a height of lm to 2 m (3 feet to 7 feet) on the ground for compacting rock fragments.
Compactors are suitable for compacting cohesive soils and other soils — this machine in areas with difficult access.
2. Vibratory Plate Compactors:
Vibratory plate compactors are used for compacting coarse soils with 4 to 8% fines. This equipment is used for small areas. The usual weights of these machines range from 100 kg to 2 tons, with plate areas between 0.16 Sq.m. and 1.6 Sq.m.
3. Vibro Tampers
Vibro compactors are used for compacting small areas in confined spaces. This machine is suitable for compacting all types of soil by vibrations installed on a base plate through a spring-activated by an alternative mechanism driven by a motor.
They are usually manually guided and weigh between 50 and 100 kg (100 to 220 lbs).
Heavy Soil Compaction Equipment
Heavy compaction equipment is selected based on soil moisture content and soil types. The following are different types of equipment:
- Smooth Wheels Rollers
- Sheeps-Foot Rollers
- Pneumatic Tyred Rollers
- Impact Roller
- Grid Rolls
- Pad Foot/ Presser Rollers
1. Smooth Wheels Rollers:
This roller imparts static compression on the soil through large drums as per the above fig. These types of rollers are generally used for compacting granular base courses and paving construction for highway and airfields.
The rollers are self-propelled by a diesel engine and have weight distribution, which, if necessary, can be altered by the addition of ballast to the rolls.
Weight can range from 10-200 kN, the range of the commonly used weight being 80-100 KN. The pressure exerted may be of the order of 300 N per linear centimeter of a width of the rear roll.
The number of passes varies with the desired compaction, while the usual number of passes is generally eight to achieve the equivalent of standard Proctor compaction. The smooth-wheeled rollers are of three types. They are
- Three-wheel type
- Tandem rollers
- Three-axle tandem rollers.
The main difference between them is in their weights. The first two weigh between 1 ton and 18 ton, while the third is heavier and weighs from 12 ton to 18 ton.
In some rollers, there is a facility to adjust the weights in the rollers by ballasting them with water or by means of a heavy sliding weight.
The performance of smooth-wheeled rollers depends mainly on the size of the wheels and the load transmitted through them to the soil per unit area.
The parameters governing the performance of roller are
- Width and diameter of the rolls, and
- Load per unit width under the compaction rolls.
The former determines the depth to which the compaction is effective, while the latter affects the net pressure applied on the surface of the soil.
Thus, specifications for any compaction work should include the load per unit width as well as the gross weight of the smooth-wheeled roller.
Generally, a weight of 10 ton capable of transmitting about 23 kg/cm pressure over the width of tread is specified.
The steel drums of the rollers may be ballasted with water or sand to increase the weights.
If a machine is designated as 7.3-12.8 ton, it means that the minimum weight of the machine only is 7.3 ton and that it can be ballasted to give a maximum weight of 12.8 ton.
Smooth drum rollers may have steel drums, front, and rear, or one drum may be replaced by smooth steel wheels.
Smooth drum rollers may be combined with accessories to produce vibrations for compacting sand, gravel, and rockfill.
Smooth drum rollers are not recommended for compacting cohesive soils in earth dams because of their low unit pressures and the smooth embankment surface that they leave.
2. Sheep Foot Rollers
The sheep’s foot or tamping roller consists of a hollow steel cylindrical drum with steel projections or “feet” extending in a radial direction, outward from the cylindrical surface as per above fig.
The diameter and width of the drum and length and shape of the feet of the rollers are varied to get different capacities. The most common type is one having two drums 122 cm in width and 107 cm in diameter.
The feet are 15 to 20 cm long and are spaced in staggered rows in a way so as to obtain a contact of about two tamping feet per square foot of the tamped area.
The sheep’s foot roller is especially good for cohesive soils because of the small width of the loaded area of its feet.
The medium-weight rollers can produce unit weights greater than the standard Proctor maximum in 15 cm to 30 cm thick layers at moisture contents slightly lower than standard Proctor optimum.
They can produce 95% of the maximum at optimum moisture content with 10 cm to 15 cm layers.
A sheep’s foot roller termed as the elephant’s foot roller, in which some of the feet are removed, and 20 cm to 25 cm flat plates are welded on the remaining is useful for silty soils of low cohesion because of the increased width of the loaded area produces greater confinement.
Sheep’s foot roller is employed even in the pre-compaction stage to break up slabby fragments of soft rock, to pulverize hard dry soil, or to aid in mixing the soil.
Sheep’s foot rollers are also towed by tractors. The roller is drawn back and forth over the area to be compacted until the feet penetrate hardly one inch into the soil.
The roller is said to have ‘walked out’ of the soil. Additional passes of the roller now will not be effective.
The performance of sheep’s foot roller is affected by the foot pressure and the coverage of the ground obtained per pass.
These, in turn, depend on the gross weight of the roller, the area of each foot, the number of feet in contact with the ground at any time, and the total number of feet per drum.
The drums can be mounted singly or in pairs and ballasted with water or wet sand to obtain different gross weights. The tamping feet are removable, and their sizes range from 32 cm2 to 78 cm2 of the cross-sectional area.
Pressures as low as 7 kg/cm2 and as high as 85 kg/cm2 can be achieved. It is necessary to carry out trials by varying the above parameters to obtain the combination that gives the best results for a given soil.
In sandy soils, mixing large quantities of water and using a towing tractor is advisable. The vibrations of the tractor, coupled with the high moisture content, give greater compaction.
3. Pneumatic Tyred Rollers
A common form of pneumatic-tyre rollers is shown in the above fig. It consists of a platform mounted on two axles. There is one more wheel on the rear axle compared to the front.
The front axle wheels lie between the rear ones in a plan. There is also a pneumatic-tired roller known as the wobble-wheel roller, in which the wheels are mounted at a slight angle with respect to the axle so as to get a slight kneading action.
The total width of contact of all the inflated tyres approximately equals 80% of the width of the roller. The rollers are normally towed by a tractor-like vehicle. They are rather unwieldy to use and function best where the surface to be rolled is not rough or steep.
The pneumatic-tired roller is good for compaction of cohesionless and slightly cohesive soils such as gravels, sands, clayey sands, silty sands, and even sandy clays.
It applies pressure over a relatively wide area so that failure does not take place. The light rubber-tired roller uses 7 to 13 wheels mounted in pairs on oscillating axles so that they can glide over the undulations in the ground.
The tyre contact pressure is typically 2.5 kg/m2, and the load is distributed over an area of a radius of about 10 cm.
The heavy rubber-tired roller consists of four large tyres mounted side by side on a suspension system, which permits them to follow the ground irregularities.
With a total weight of 35 tons to 200 tons, the tyre pressures range from 5 kg/cm2 to 10 kg/cm2.
The performance of a pneumatic-tired roller depends on the tyre inflation pressure and the area of contact between the tyre and the ground.
The total weight depends on the tyre size, number, and inflation pressure. A tyre inflation pressure of 2.5 kg/cm2 is commonly used.
The pneumatic-tyred rollers produce a smooth surface and may be used for finishing the top of an embankment after compaction by sheep’s foot rollers.
4. Impact Roller
The impact roller is simple, but in some cases, it is very effective in the surface compaction method as per the above fig.
A conventional tractor pulls a heavy prism-shaped mass, consisting of steel or concrete.
The impact generated by the rotation of the heavy mass (up to 50 tons) transfers sufficient energy to achieve medium compaction to a depth of several meters.
The compaction process is usually based on site-specific correlation, and a little documented evidence about actually achieved compaction effects is available.
The impact roller can be used on granular soils, including coarse-grained material such as gravel and rock fills.
5. Grid Rolls:
Grid rollers are used for compacting weathered rocks and well-classified thick soils. These rollers are not suitable for clay soils, silty clays, and uniform soils.
The main use of these rollers is in the subgrade and sub-base in road construction.
Grid Roller – Soil Compaction Equipment
As the name suggests, these rollers have a cylindrical surface of heavy steel that consists of a network of steel bars forming a grid with squire holes.
The weight of this roller can be increased by a ballast with concrete blocks.
Typical weights range from 5.5 net tonnes to 15 ballasted tonnes. The grid rollers provide high contact pressure but little kneading action and are suitable for compacting most coarse-grained soils.
6. Pad Foot/ Presser Rollers
These rollers are similar tosheep foot rollers with terminals with a larger area than sheep foot rollers.
Pad Foot/ Presser Foot
The static rollers of the support feet, also called compacting rollers, have static weights in the range of 15 to 40 tons, and their linear static loads on the drum are between 30 and 80 kg/cm.
These rolls are more preferable than sheep’s foot rolls due to their high production capacity and are replacing sheep’s foot rolls.