Explosive compaction has been used in a variety of projects over the past few decades due to its cheap resource and easy installation process. Detonation charges are placed in boreholes in sandy-silty sands or gravel soils, and then light up the charge. Some charges are fired at the same time, with certain delays to allow the cyclic loading. Often several charges will be filled in one borehole with gravel stemming between each charge to prevent sympathetic detonation. Explosive compaction is attractive, as explosives are inexpensive source and it allows densification of the soil with substantial savings over other conventional methods. Explosive compaction only requires small-scale equipments such as geotechnical drill or washing boring rigs, in order to minimize mobilization costs. Explosive compaction can be conducted at deeper depths than conventional ground treatment equipment. Most explosive compaction has been driven by concerns over liquefaction, and has been conducted on loose soils below the water table (can achieve depths up to 50 m below the ground level). (W. B. GOHL, 2000) However, compaction also increases the stiffness and strength of soil profile, and explosive compaction has wide application for general ground improvement.
1.1 Backgrounds on explosive compaction
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In 1936, explosive compaction was first used for the densification of a railway embankment at the Svirsk hydroelectric power project in the former Soviet Union (Ivanov, 1967). Ivanov notes that up to 44cm of settlement occurred as a result of 3 blasting coverage, but the blasting caused extensive cracking of the overlying unsaturated soils and was not considered successful. The first successful application of explosive compaction was performed in the late 1930ââ‚¬â„¢s to dandify the foundation soils for the Franklin Falls dam in New Hampshire (Lyman, 1940). Soon following the work at Franklin Falls dam, the effectiveness of this technique was confirmed by its successful performance for compaction a hydraulic fill dike on the Cape Cod Canal and by several tests at the Dennison Dam in Texas and the Almond Dam in New York. These cases concluded that blast densification could be widely used for compaction loose cohesionless soils that are substantially saturated. In 1967, Ivanov presented a manual on explosive compaction which provides guidelines for the placement and sizing of the explosive charges used in compaction. However, in most explosive compaction projects several short columnar charges are placed in each blast hole, and neither set of available guidelines appears valid. More importantly, these guidelines present no method to estimate the impacts from the blasting or final soil properties achieved. (Mitchell, 1995)
2.0 Cohesionless soil
Explosive Compaction is conducted by installing the explosive charges in the soil profile, which is often applicable to cohesionless soil (W. B. GOHL, 2000). Cyclic straining of the soil is caused by the large explosive energy. This strain process, repeated over many cycles caused by the sequential detonation of explosives, induces a tendency for volumetric compaction of looser sub soils. It is thought that shearing strains are responsible for this volumetric compaction, particularly at distances more than a few meters from a blast hole. In saturated soils, the overburden pressures are thrown onto the pore fluid and excess pore pressures develop during blasting, which caused a shakedown settlement of the soil. If the number of cycles and amplitude of strains are large enough, this will caused liquefaction of the soil mass (i.e. pore water pressures temporarily elevated to the effective vertical overburden stress in the soil mass so that a heavy fluid created).
The reconsolidation of the soil mass caused by the dissipation of water pressures is time dependent, generally happens within hours to days. This depends on the permeability of the subsoils and drainage boundary conditions, and is reflected by release of large volumes of water at the ground surface. Immediate volume change can happen and is caused by passage of the blast-inducted shock front through the soil mass.
Concern about the explosive compaction is the large amount release of gas into soil, such as nitrogen oxide, carbon monoxide and carbon dioxide. Release of carbon dioxide may increase the ammonia level. Venting is necessary because some of the release gases are poisonous like nitrogen oxide and carbon monoxide, especially in confined areas. Hence, the by-product of explosive compaction should be checked before starting the project and assess its suitability for a particular area (W. B. GOHL, 2000).
2.2 Blast hole pattern
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A staggered rectangular grid of boreholes at spacing of 4 to 9 metres is generally used (W. B. GOHL, 2000). This blast-hole pattern is used to provide a pattern of two or more phases within the treatment area. The initial phase will destroyed any bonds existing between the cohesionless soil particles. Subsequent passes cause additional settlement after pore pressure dissipation. Repeated applications of blast sequences will cause additional settlement depending on soil density and stiffness. Bore holes are drilled over the full depth of soil deposit to be treated, and 75 to 100 millimetres diameter plastic casing is installed (W. B. GOHL, 2000). The casing will support the loaded explosive at one or more levels within the boreholes, together with charges separated by gravel. The number of holes detonated in any shot will depends on vibration control considerations, the liquefaction effect and settlement on adjacent slopes. The advantage of using multiple blast phases is the increase of settlement and more uniform densification. This is because local soil loosening can occur immediately after charge detonation, subsequent passes of blasting are designed to re-compact these initial loosened zones from surrounding boreholes. Therefore at least two phases are usually recommended for explosive compaction. (W. B. GOHL, 2000)
The instruments used for an explosive compaction projects generally includes the following (W.B.GOHL, Elliott, & Martin, 2000):
Surface geophones are used to measure the vibration response at critical location.
Pore pressure transducers are used to measure the residual pore water pressures generated by explosions.
Hydrophones installed in water filled casings close to blast zones are used to determine the charge detonation.
Sondex tubes are used to measure the settlements in soil after blasting.
Ground surface settlement measurements
Inclinometers are used to measure slope movement, if explosive compaction is carried out near slopes.
In some projects, additional confirmation of explosive detonations is required, electronic coaxial cables are installed down the blast holes. Some high speed data acquisition systems have been used to measure the firing times of explosive deck. Alternatively, high speed filming of the firing of non-electric delays can also be utilized to record the process of charge detonations.
Cone Penetration Testing (CPT), Standard Penetration Testing (SPT) and Becker Penetration Testing (BPT) are commonly used to measure the improvement in soil density after explosive compaction. For sand and silt areas, CPT could provide the most reliable and reproducible results than other two tests. (W. B. GOHL, 2000).
3.0 Cohesive Soil
Explosive Compaction has used in the past few decades to compact the loose granular soil. However, the use of explosive compaction for cohesion soil, such as clay, is rare. Yan and Chu developed a new explosive compaction design method, which replaced soft clay with crushed stones. (S.W.Yan & J.Chu, 2004), which is called explosive replacement method. Meanwhile, this method has been used in conjunction with a highway construction in China.
3.1 Outline of the method
There are three main steps described by Yan and Chu (S.W.Yan & J.Chu, 2004) to achieve the replacement method, which are:
The explosive replacement is set up as shown in fig1. Firstly, explosive charges are installed in the soil profile; secondly crushed stones (gravel) are piled up next to it.
The soft soil is exploded out and spherical cavities are formed after the area has been shot. Meanwhile, the crushed stones collapse into the cavities. Then the cohesive soil is replaced with crushed stones in a fast way. The soil which is exploded into the air will flow away. The crushed stones after collapsing could from a slope of 1V:3H or 1V:5H, as shown in fig1(b).
As a result of explosion, the shear strength of the soil is reduced instantaneously and crushed stones can sink into the soft clay layer. Soil at the bottom will consolidate and the clay will remain part of its original strength. The explosion also could densify the gravel layer below the clay layer. More crushed stones are filled to from a leveled ground and steeper slope, as shown in fig1(c).
Fig 1.(a)Before explosion; (b) After explosion; (c) After backfill
3.2 Ground-probing radar (GPR) tests
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The distribution of the crushed stones in the soft clay can be detected by conducting the Ground-probing radar tests.The radar system transmits repetitive, short pulse electromagnetic waves into the ground from a broad bandwidth antenna. Parts of the waves are reflected when hitting discontinuities in the sub-surface, and parts of the waves are absorbed by the materials that they contact with. Then receiver picked up the reflected wave and the elapsed time between wave transmission is recorded (Koerner, 1984).
4.0 Explosive Compaction Design
Explosive Compaction Design is based on empirically methods, which had been presented by Narin van Court and Mitchell (J.K.Mitchell & W.Narin, 1994). Wu (G.Wu, 1996) developed the explosive compaction design by using the finite element model. His model applies dynamic cavity expansion theory and assumes that the blast pressure generated from charge detonation applied normal to the surface of cavity. The charge weight per delay is direct proportional to the size of the spherical cavity, thus in order to gain larger cavity size and larger detonation effect, charge weight should be increased. Wuââ‚¬â„¢s model also considers the non-linear shear stress- strain response of the soil and rate dependent effect. Parameters are estimated based on the densities of the granular soils and by conducting some single and multiple-holes tests at site.
Cavity expansion theory indicates: a) it is more effective to use multiple cycles of blasting than single cycles (b) charge weight increases as the depth increases (c) the influence zone of charge detonation increases as the size of spherical cavity increases (W. B. GOHL, 2000).
The design of explosive compaction often begins with Hopkinsonââ‚¬â„¢s number (HN) and Normalised Weight (NW) as (S.W.Yan & J.Chu, 2004):
Where Q is the charge weight in kilogram and R is the effective Radius in plan (metre). However, due to the infinite combinations of charge weight with radius, a suitable HN can be difficult to select.
Meanwhile, explosive compaction typically uses columnar charge and a good correlation of energy attenuation by the square root method is demonstrated, so this attenuation function is used in the following analyses, and the energy input attenuation is derived as (W. B. GOHL, 2000):
where Wi is the weight of individual charges around a point in the soil mass(g), and Rvi is the minimum vector distance from a charge to a point in the soil mass(m).
The distance between charges can be estimated as:
Where, to allow some overlapping, should be taken to be less than 2.
In those equations, HN, NW and E are constants. Based on blasting mechanics, a new set of equation has been derived by Yan and Chu (S.W.Yan & J.Chu, 2004), and the finally radium could be govern as follow:
Where Pk is a pressure constant in Pascal, is the density of the explosive in kilogram per cubic metre, D is the velocity of the explosive in metre per second, Pa is the atmospheric pressure in Pascal, Qis the mass of the explosive,is the unit weight of soil in Newton per cubic metre and hc is the thickness of the soil above a cavity in meter.
In addition, Gohl (W.B.GOHL, Elliott, & Martin, 2000) has developed an equation to approximate the charge effectiveness in a given soil type and it is derived based on the Hopkinsonââ‚¬â„¢s Number and it is given as the following:
Where e is the fraction of maximum achievable vertical strain and k is a site factor related to the soil properties and damping. From past project, k was found to be 81 to 143.
Explosive compaction uses the energy released by completely contained detonations within the soil mass to rearrange the particles into a denser configuration. This technique offers several advantages over other soil improvement techniques. especially with regard to the cost, soil type, and depth effectively treated. Moreover, explosive compaction is an effective and predictable method for both cohesive and cohesionless soil. In which explosive replacement method for cohesive soil is newly developed. Although this compaction method has been used for decades, under a variety of site and environmental conditions, explosive compaction has not achieved general acceptance in civil engineering. Therefore, further development is encouraged and due to the physical testing restrains, possibly numerical simulation will develop in future.