An Examination Of Explosive Compaction Environmental Sciences Essay

1.0 Introduction

Explosive compaction has been used in various projects throughout the world over the last 80 years. Explosive compaction involves placing a charge at depth in a borehole in loose soil (generally sands to silty sands or sands and gravels), and then detonating the charge. Several charges are fired at one time, with delays between each charge to enhance cyclic loading while minimizing peak acceleration. Often several charges will be stacked in one borehole with gravel stemming between each charge to prevent sympathetic detonation. Explosive compaction is attractive, as explosives are an inexpensive source of readily transported energy and allow densification with substantial savings over alternative methods. Only small-scale equipment is needed (e.g. geotechnical drill or wash boring rigs), minimizing mobilization costs and allowing work in confined conditions. Compaction can be carried out at depths beyond the reach of conventional ground treatment equipment. Most explosive compaction has been driven by concerns over liquefaction, and has been on loose soils below the water table (and to depths of nearly 50 m). (W. B. GOHL, 2000) However, compaction also increases ground stiffness and strength, and explosive compaction has wide application for general ground improvement .

1.1 Backgrounds on explosive compaction

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 1930’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 an 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 setting off explosive charges in the ground often applicable to cohesionless soil. The explosive energy will caused cyclic straining of the soil. 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 strain amplitudes and number of cycles of straining are sufficient, 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).

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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.

2.1 Disadvantage

Issue associated with explosive compaction is it results in large amount of gas being released into the soil – water system, in the form of nitrogen oxide, carbon monoxide and carbon dioxide. Release of carbon dioxide may lower the PH of the ground water and this may increase the ammonia level. Both nitrogen oxide and carbon monoxide are both poisonous substance in the air and venting is necessary if blasting is carried within confined areas. Hence, the chemical make-up of a particular explosive and its by-product should be reviewed for every project in order to assess its suitability for use at a particular area.

2.2 Blast hole pattern

The blast hole pattern generally use a staggered rectangular grid of boreholes at spacing of 4 to 9 metres. This 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. Once the area has been shot and pore pressures have largely dissipated, 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. The casing will support the loaded explosive at one or more levels within the boreholes, with each charge separated by gravel stemming. The stemming will reduce the ‘back blast’ and encourage the crater effect. The number of holes detonated in any shot will depends on vibration control considerations and the effect of liquefaction and settlement on adjacent slopes and structures.

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 around a charge, subsequent passes of blasting from surrounding boreholes are designed to re-compact these initial loosened zones. Therefore at least two phases are usually recommended for explosive compaction.

2.3 Instrumentation

The instruments used for an explosive compaction projects generally includes the following:

Surface geophones to measure vibration response at critical location.

Pore pressure transducers to measure residual pore pressures generated by blasting.

Hydrophones installed in water-filled casings near blast zones used to identify charge detonations.

Sondex tubes to measure settlements with depth in a soil profile after blasting.

Ground surface settlement measurements

Inclinometers where blasting is carried out near slopes to measure slope movement.

In some projects, additional confirmation of explosive detonations is required, electronic coaxial cables are installed down the blast holes and used to measure firing times of explosive deck using high speed data acquisition systems. Alternatively, high speed filming of the firing of non-electric delays can also be employed to monitor charge detonations.

Standard Penetration Testing (SPT), Becker Penetration Testing (BPT) or electronic Cone Penetration Testing (CPT) is commonly used to assess the improvement in soil density after explosive compaction. For sand and silt areas, CPT is considered to provide the most reliable and reproducible results.

3.0 Cohesive Soil

Explosive Compaction has been a method used in past decades for the compaction of loose granular soil. However, the use of explosive compaction for cohesionless soil, such as clay, is rare. A new explosive method for replacing soft clay with crushed stones by blasting has been development by Yan and Chu[8], 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 [6] to achieve the replacement method, which are:

The explosive replacement is set up as shown in fig1. The explosive charges are first installed in the soil layer, and then crushed stones are piled up next to it on the side of the site that has been improved.

When the charges are detonated, the soft soil is blown out and cavities are formed. At the same time, the crushed stones collapse into the cavities. In this way, the cohesive soil is replaced with crushed stones in rapid manner. The soil that is blown into the air will form a liquid and flow away after it falls to the surface. The crushed stones after collapsing from a slope of 1V:3H or 1V:5H, as shown in fig1(b).

The impact of the explosion also causes an instantaneous reduction in the shear strength of the soil below the level of explosion so that the crushed stones can sink into the soft clay layer. The stones help the soil at the bottom to consolidate, and the clay itself will also remain part of its original strength after explosion. The explosion also has a densification effect on the gravel layer below the clay layer. More crushed stones are backfilled 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

GPR test is used to detect the distribution of the crushed stones in the soft clay. The radar system transmits repetitive, short pulse electromagnetic waves into the ground from a broad bandwidth antenna. Some of the waves are reflected when they hit discontinuities in the subsurface, and some are absorbed or refracted by the materials that they come into contact with. The reflected waves are picked up by a receiver, and the elapsed time between wave transmission and reception is automatically recorded.[Koerner R.M. Construction and geotechnical methods in foundation engineering. McGraw-Hilll, New York,1984]

4.0 Explosive Compaction Design

Explosive Compaction Design is based on empirically methods, which had been presented by Narin van Court and Mitchell (refer 1). Wu (refer 2) developed the explosive compaction design by using the finite element model. His model applies dynamic cavity expansion theory and assumes that a charge detonation may be idealized by assuming a blast pressure-time input applied normal to the surface of a spherical cavity. The charge weight per delay is proportional to the size of the spherical cavity, thus larger charge weight could result in larger cavity size and larger detonation effect. Wu’s model also considers the non-linear shear stress- strain response of the soil and rate dependent viscous damping. Parameters used in the Wu model are calibrated based on initial estimates of the relative densities of the granular soils and analysis of single and multiple-hole test blasts at a site.

Cavity expansion theory indicates: a) multiple cycles of blasting will be more effective than single cycles; (b) the zone of influence of a given charge detonation increases as the size of the cavity increases (c) charge weight should be increased as the depth increases. (Refer 3 Gohl et al, 2000).

The design of explosive compaction often begins with Hopkinson’s number (HN) and Normalised Weight(NW) as:

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:

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. However, for a given value of HN, NW or E, the above relationships may provide infinite combinations of charge weight with radius. Furthermore, it is difficult to select suitable values of HN, NW or E1 in practice. Based on blasting mechanics, a new set of equation has been derived by Yan and Chu (2004) [6], 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.

The distance between charges can be estimated as:

Where, to allow some overlapping, should be taken to be less than 2.

In addition, Gohl has developed an equation to approximate the charge effectiveness in a given soil type and it is derived based on the Hopkinson’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.

5.0 Conclusion

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.