Compaction is known to be one of the most important methods used to improve the shear strength of soils. In the construction of roads, dams, buildings, airfields, etc., different methods of compaction come into play. Common methods of compaction involve the use of tampers, rollers, or vibrators. These methods are very suitable but they are usually effective at compacting layers of soil from 0.1 m – 1 m.
In situations where deep compaction is required, especially at greater depths, other methods are more suitable for this type of compaction. These methods are called deep compaction methods and they include:
Vibroflotation
Vibroflotation is one of the popular methods of deep compaction. This method is particularly suited for compacting thick deposits of loose, sandy soils up to 30 m depth. The method is not effective in cohesive soils but it can alternatively be used to form sand piles to reinforce the soil deposits and accelerate consolidation. When used for sandy soils, a relative density of 70% or more can be achieved.
A vibroflot is made of a cylindrical tube, about 2 m in diameter, fitted with water jets at the top (upper jet) and the bottom (lower jet). It also contains a rotating eccentric mass which develops a horizontal vibratory motion.
Using the lower water jet, the vibroflot is lowered into the loose soil to the required depth. The water jet’s discharge reduces the soil’s shear strength by briefly inducing a quick condition ahead of the vibroflot. Due to the vibroflot’s own mass, it settles. After reaching the necessary depth, the vibrator is activated laterally, which compacts the soil in a horizontal manner in a 1.5 m radius.
The water from the lower jet is transferred to the top jet and the pressure is reduced so that it is just enough to carry the sand poured from the top to the bottom of the hole (Note: the spacing of the holes is usually kept around 2 – 3 m on a grid pattern – see Figure 1). Vibration continues as the vibroflot is slowly raised to the surface. Additional sand is continually dropped into the space (crator) around the vibroflot. By raising the vibroflot in stages and simultaneously backfilling, the entire depth of the soil is compacted.
The grain-size distribution of the backfill material is an important factor that controls the rate of densification. Brown (1977) has defined a quantity called the suitability number for rating backfill as
Where,
D50, D20, and D10 are the diameters (in mm) through which, respectively, 50, 20, and 10% of the material passes.
The smaller the value of SN, the more desirable the backfill material. Following is a backfill rating system proposed by Brown:
| Range of SN | Rating as backfill |
| 0 – 10 | Excellent |
| 10 – 20 | Good |
| 20 – 30 | Fair |
| 30 – 50 | Poor |
| >50 | Unsuitable |
Terra Probe Method
This method is similar to the vibroflotation method in many aspects; however, it is considerably faster than the vibroflotation method but less effective than vibroflotation. This is because the method has a smaller zone of influence when compared to vibroflotation and thus, lower relative density is achieved.
The terra probe consists of an open-ended pipe, about 75 cm in diameter. This pipe is provided with a vibratory pile drive. The vibratory pile drive when activated gives vertical vibrations to the terra probe and it goes down. After reaching the desired depth, the terra probe is gradually raised upward while the vibrodriver continues to operate. Thus, the soil within and around the terra probe is densified.
The terra probe method has been successfully used up to a depth of 20 m. The spacing of the holes is usually kept at about 1.5 m. Saturated soil conditions are ideal for the success of the method. For the sites where the water table is deep, water jets are fitted to the terra probe to assist in the penetration and densification of the soil. The method can even be used at offshore locations because it does not require backfilling of sand.
Compaction by Pounding (Dynamic Compaction)
The method is also known as heavy pumping, dynamic compaction or high-energy compaction has been used recently to densify large deposits of loose, sandy soils and compact fine-grained soils usually to a great depth.
Pounding is done by dropping heavy mass (2 to 50 Mg) from a large height (7 to 35 m) on the ground surface. The actual mass and the height selected depend upon the crane available and the depth of the soil deposit. A closely spaced grid pattern is selected for the pounding locations. At each location, 5 to 10 pounds are given. The depth (D) in metres to which the method is effective can be determined from the following relation:
D = C √MH
Where,
C = Coefficient (0.5 to 0.75)
M = mass (Mg)
H = height of drop (m)
When using this method, it is necessary to ensure that harmful vibrations are not transferred to adjacent buildings. This can be done by determining the radius of influence (R) in metres beyond which no harmful vibrations are transmitted using the expression below:
R = 130 √MH
Where,
M = mass (Mg)
H = height of drop (m)
Compaction by Explosives
In this method, buried explosives can sometimes be used to densify cohesionless soils up to a depth of 25 m. The method is most suitable in fully saturated cohesionless soils. The shock waves developed in these soils cause liquefaction of sand, which is followed by densification. In partially saturated cohesionless soils, compressive stresses develop due to capillary action and prevent the soil particles from taking closer positions. Thus the method is not effective for partially saturated soils.
The explosives charges used in this method usually consist of about 60% dynamite and 30% special gelatin dynamite and ammonite. The charges are placed at two-thirds the thickness of the stratum to be densified. The spacing of explosive points is kept between 3 to 8 m. Three to five blasts are generally required at each location. The radius of influence (R) of compaction can be determined using the relation
R = (M/C)1/3
Where,
R = Radius of influence (m)
M = mass of charge (kg)
C = constant (= 0.04 for 60% dynamite)
Densification is achieved by the shock waves and vibrations produced by the explosives which are somewhat similar to that produced by vibratory, compaction equipment. The uppermost zone of the soil up to a depth of about 1 m gets displaced in a random manner and is, therefore, not properly densified. This zone should be compacted using conventional methods by rollers.
Precompression
This method is more suitable to improve the properties of cohesive soils such as silts, clays, organic soils, and sanitary landfills. In this method, the soils are preloaded before the application of the design loads. Preloading causes settlement before actual construction begins.
The preload is generally in the form of an earth fill which is left in place for a long time so as to induce settlement. The preload must be carefully selected so as not to cause shear failures in the soil. During preloading, it is important to check the progress of settlement through the use of a monitoring system consisting of a settlement plate and piezometers. To reduce the time of settlement, vertical sand drains may be used.
After the required compression has been achieved, the preload is removed prior to compaction, and the stability of the soil deposit is checked as well. Sufficient soil data should be collected to predict the rate and magnitude of settlement.
Use of Compaction Piles
Compaction soils can be densified by constructing compaction piles. A capped, pipe pile is driven into the soil. The soil surrounding the pile is compacted due to vibrations caused during driving. The pile is then extracted and the hole formed is backfilled with sand. Thus the compaction pile is formed.
Further Reading
Arora, K.R. (2014). Soil Mechanics and Foundation Engineering (7th edition). Standard Publishers Distributors, New Delhi, India.
