Equipment For Soil Compaction

Note: This highly informative and elaborate paper was written by the Author many years back and holds valuable information. Readers should update themselves with the latest developments that might have occurred after the presentation of this paper.

A fundamental requirement of all civil engineering projects is proper compaction of the soil over which a structure is being raised. This is necessary to receive good serviceability for the entire designed life of the structure built over the soil. Good compaction of soil brings a bountiful of advantages to the engineers. It results in:

Minimizing soil settlement on application of load.
Increase in density of soil resulting in increasing its shear strength.
Decrease in permeability of soil leading to a fall in its water absorption.
Reduction in swelling or shrinkage of soil.

Whether the soil is being laid beneath a building or under a floor, in an embankment or a canal bank, its compaction has a direct effect on the future behaviour of the structure built over it. Take example of an earthen dam. Good compaction of soil laid in it in layers to raise it to its desired height will make the soil dense, thereby decreasing the seepage and loss of water stored behind the dam. The dam structure itself will have a longer life as water will not be able to make way into it. In case of an embankment, the chances of settlement of soil due to movement of heavy traffic over it will be minimized thus reducing the maintenance cost of the road built over it and making the embankment stand against the ravages of time. Compaction of soil should therefore be given as much attention as we pay to jobs involving concreting work.

The methods: Compaction of soil can be done either by expulsion of air entrapped in the soil or by expulsion of water present in its pores or by elastic compression of the soil particles. Among these, the elastic compression of soil particles contributes negligibly to overall compaction, whatever amount of compaction we may be doing. Proper compaction therefore depends upon the expulsion of air and water from the soil. It has been noted that usually no water gets expelled from the voids during compaction. Only a gradual expulsion of water may take place from the pores of saturated cohesive soils under the action of continuously static loads. Thus during the compaction of soils, mainly, it is the expulsion of air only that brings desired results and increases the density of soil to the maximum.

Soil considerations: Before selecting the equipment for compaction of soil, it is important to study the soil characteristics. Among these, the most prominent are the moisture content of soil, the type of soil and the method of compaction. The compaction of soil is always measured in terms of dry density of soil. More is the dry density, more compact the soil is. The moisture content at which maximum dry density is achieved is called Optimum Moisture Content (O.M.C). Here it must be noted that this O.M.C is for a particular ‘compaction effort’ applied to the soil. If we increase the Compaction effort, the O.M.C decreases for this new and increased effort and the density of soil further increases. Thus O.M.C and Compaction Effort are two variable factors. More and more density can be achieved by increasing the Compaction effort and decreasing the O.M.C for that effort. It should be borne in mind that the density of soil will increase only if the shear stress produced by the ‘compaction effort’ is more than the shear strength of soil at its existing density and moisture content. The compaction effort should therefore be quite adequate. The compaction equipment should therefore be able to deliver the required ‘compaction effort’.

Coarse Vs Fine soils: Well graded coarse soils can be compacted to a high density as compared to fine grained soils. Optimum Moisture Content for fine grained soils is more than that for coarse grained soils because finer particles have larger surface area and need more water to wet themselves. Thus for the same Compaction Effort applied, maximum dry density of fine grained soils shall be lesser than that for coarse grained soils. To achieve good results for a particular soil, a graph should first be prepared between the Compaction Effort and the O.M.C for that effort by applying different efforts and noting the O.M.C for each effort. The desirable as well as possible result should then be chosen as the target to be achieved. This is important because sometimes, the best Compaction Effort and the O.M.C may not be possible to achieve at a particular site.

Compaction Rollers: Rollers are used to carry out the compaction of soil in the field. A number of types of rollers are available for this purpose. However the type of rollers to be used depends upon the nature of soil to be compacted. For clayey soils, kneading type rollers are most suitable. Heavy vibratory compactors or sheep foot rollers shall therefore prove better for such soils as these rollers penetrate into the soil. Sheep foot rollers are chosen when the clayey soil has high water content. 95% of the modified proctor density can be achieved in these soils by using rubber tyred rollers with tyre load of 11 MT. For sandy or cohesionless soils, double or tandem vibratory rollers are most suitable. Even 100% of modified proctor density can be achieved in these soils by using proper vibratory equipment. If rubber tyred vibratory rollers with 13.5 MT load are applied, even 103 to 104% of modified proctor density is achievable in these soils after making 30 passes of these rollers. Once the thickness of layers of soil to be laid and compacted is decided, the type and weight of rollers can also be decided as the thickness of layer also affects the choice of rollers. Lesser is the thickness of a layer, more is the compaction achieved for a given type of roller. After making right choice of rollers, the number of passes required to achieve the desired density should be observed in the field and followed for each layer to be laid.

Sequence of activities: In view of above explanation, the sequence of activities for compaction of soil shall be as under:

Check the type of soil.
For clayey soils, choose heavy vibratory compactors or sheep foot rollers.
For sandy or cohesion less soils, choose tandem or double vibratory type rollers.
Decide thickness of soil layers to be laid and compacted.
Decide number of passes of the chosen rollers to achieve desired density.
Decide weight of rollers with respect to thickness of soil layer.
Pass the chosen rollers of decided weight for the decided number of passes over the soil layer.
Check the density of soil after compaction.

How to check the density of compacted soil: It is very important to check the actual density of soil rather than just passing the rollers over each layer a few times and believing that the soil has been compacted. To determine the maximum dry density of soil in the laboratory, Standard Proctor Test Apparatus and Modified Proctor Test Apparatus are used. The particulars of these apparatuses are given in Table 1.

Table 1

Sr.	Item	Standard Proctor Test	Modified Proctor Test
1.	Mould diameter	10 cm	10 cm
2.	Mould Height	11.7 cm	11.7 cm
3.	Shape of Mould	Cylindrical	Cylindrical
4.	Weight of Hammer	2.5 kg	4.5 kg
5.	Fall of Hammer	30 cm	45 cm
6.	Type of fall	Free ( in a sleeve )	Free ( in a sleeve )
7.	Hammer diameter	5 cm	5 cm
8.	Number of layers	3	5
9.	Number of blows per layer	25	25

In case a mould of 15 cm diameter is used instead of 10 cm diameter, the number of blows should be increased from 25 to 56. Rest of the test procedure remains the same. From the above table, it can be seen that as the number of layers is lesser and the energy input is also lesser in case of Standard Proctor Test, it gives lesser values of maximum dry density as compared to modified proctor test. After checking the maximum dry density achieved by Standard Proctor Test or Modified Proctor Test, the values are set for the field densities. During actual compaction of soil in field, field densities are taken from time to time and compared with these values. To check the field densities, core cutters are used. Samples of compacted soil are drawn by use of core cutters, weighed, dried to find the moisture content and the dry density achieved is arrived at and compared with the results obtained from laboratory.

Choosing vibratory rollers: While choosing vibratory rollers, following guidelines should be kept in mind:

More is the thickness of soil layer to be compacted, lesser should be the speed of the roller. Speed should be around 4 kilometers per hour.
More is the density required of the soil to be compacted, lesser should be the speed of the roller.
Lesser are the number of vibratory drums in the roller, more is the number of passes of the roller over the stretch to be compacted.
More are the number of vibrations per minute of the roller, more is the compaction of soil and to a greater depth too.
More is the amplitude of the roller, more is the compaction of the soil. However, thickness of soil layer shouldn’t be too thin in this case.
More is the weight of the drum, obviously, more is the pressure on soil and better is the compaction.

Equipment available: Many top bracket manufacturers are producing soil compaction equipment to suit any soil conditions. Caterpillar, the king of equipment manufacturers, Greaves Cotton, Larsen & Toubro, Escorts and Volvo are a few more big names. Marsman, Dynapac and Changlin can’t be left behind. Greaves Cotton is producing vibratory soil compactors, heavy as well as light tandem rollers and pneumatic tyred rollers. Caterpillar’s vibratory rollers hold a class in themselves and provide highly superior compaction effort.

These days, vibratory rollers are ruling the compaction scene. These produce vibrations by rotating eccentric weights in the rollers at high speeds. Most of these have a smooth cylindrical steel drum with one rotating eccentric weight placed at the center of the drum. The rotating eccentric drum produces vibrations causing the roller to bounce against the soil. Vibrations do not allow the top layer of an embankment to achieve density. So the top layer is compacted by switching off the vibratory mode of the roller. As the compaction performance of a vibratory roller depends upon its frequency, vibrating mass and amplitude, these parameters are examined while selecting a vibratory roller for the density to be achieved.

Caterpillar’s latest series of soil compactors is named as E-series. Its CS533E vibratory soil compactor is an example worth consideration. This compactor ensures achievement of required density of soil through application of highly dynamic compaction force generated through eccentric weights and through minimum number of passes. It is equipped with a 130 Hp diesel engine and requires minimum maintenance despite presence of eccentric weights. It has an operating weight of 12.36 tonnes. The vibratory amplitude is 0.85 mm. CAT’s CS573E soil compactor has an operating weight of 13.92 tonnes. The engine is of 150 Hp and vibratory amplitude is as high as 1.7 mm.

L&T’s vibratory soil compactors are produced by the company in India in collaboration with CASE corporation, USA. Its 1107 Vibratory compactor is a single drum compactor in three models with an operating weight of 11.9 tonnes and 112 Hp diesel engine. Having a working speed up to 6 kmph, the amplitude of these compactors is 0.8 mm and 1.8 mm. Frequency of vibration system is 30 to 33 Hz. While the 1107 D model is suitable for sandy or cohesionless soils and has a plane surface drum, 1107 PD model is suitable for cohesive soils as its drum is pad footed. L&T vibratory equipment has a heavy duty articulation joint. In India, service and spares availability is additional advantage. L&T also produces a double drum tandem vibratory soil compactor for smaller jobs. Its weight is just 3.0 tonnes, amplitude is only 0.6 mm, frequency is 55 to 66 Hz and engine power is 46.5 Hp. Naturally, its speed is much higher, up to 12 kmph.

Escorts’s EC 5250 vibratory soil compactor comes in three variants like L&T’s 1107. 5250 D variant is for cohesionless soils while 5250 PD variant has roller with pad foots and is suitable for clayey soils. These compactors have an operating weight of 9.35 tonnes. HAMM 2420, another machine from Escorts has a plain surfaced drum. Its operating weight is 11.43 tonnes and engine power is 130 Hp. The frequency range is 0 to 22 Hz with amplitude as 0.8 mm and 1.9 mm. It can have a working speed up to 10.3 kmph. Tandem vibratory rollers are also produced by Escorts, the models named as ED 30 and HD 85. these two machines are suitable for smaller compaction jobs.

Dynapac produces a range of single drum vibratory rollers for soil compaction. This company has also produced a pad foot drum roller for cohesive soil compaction. Dynapac’s vibratory rollers are suitable for all types of soils except rockfill. Produced under CA series, the roller models have their operating weight ranging from 4.5 tonnes to 26.9 tonnes. Its LP8500 trench roller compacts granular and cohesive soils in backfill jobs.

Greaves Cotton has tied up with Bomag Gmbh, Germany to produce vibratory soil compactors for best results and least maintenance requirements. Greaves has been trying to lay more emphasis on minimizing service and maintenance requirements. An example in this direction is their claim of oil change requirement for rollers after 2000 hours of working which is described as ‘unmatched’. Greaves terms its soil compactors as ‘Work horses’ and believes that the soil should get compacted through minimum number of passes. This philosophy is making the company products rise on popularity charts. As the users are always inclined towards heavier vibratory compactors, Greaves’s latest product is 19 tonne compactor produced at their Tamil Nadu plant.

A word of caution: No compromise can ever be made on soil compaction as it leads to failure of structures, disintegration of roads, collapse of retaining walls and all sorts of troubles. An extra vigil in this area pays dividends. In case of embankments and earthen dams, many builders make such an arrangement that the fleet of dumpers has to pass over the laid layers to reach their location. Though it is a practical method of imparting more density to the laid layers of the soil, deployment of right equipment and its planned and controlled operation only leads to full soil compaction. We can’t work on presumptions and suppositions; we have to be sure of our work. An 80% of the Standard Proctor Test density is almost equivalent to soft soil. Therefore, higher percentages shouldn’t mislead the site staff but full compaction figures should be achieved. With such a vast range of machinery available within the country, there shouldn’t be any hitch in it.