If Ground Improvement is necessary, What methods are available ?

Ground Improvement by Explosives (G.I.E.)

1.1-If Ground Improvement is necessary, What methods are available ?

Many methods for ground densification and improvement are available, including dewatering, compaction preloading with and without vertical drains, admixture stabilization, grouting of several types, deep mixing, deep densification and soil reinforcement.

Many of these techniques, such as dewatering, compaction pre-compression and some types of grouting, have been used for many years. However, there have been rapid advances in the areas of deep densification (vibrocompaction, deep dynamic compaction, compaction piles and explosive densification), jet and compaction grouting, deep mixing, and stone column systems in recent years.

These methods have become practical and economical alternatives for many ground improvement applications.

1.1.1- Abstract:

Explosive compaction has been a method used in past decades for the compaction of loose granular soil. The method uses energy from confined detonations of explosive charges placed within the soil mass to densify loose, saturated sand or gravel. Various theoretical and case studies have been published in the past. However, most of the studies are limited to applications in granular soils.

1.1.2- Why Blast?

Compare to remove or replace dynamic compaction, grouting, etc	Inexpensive
1- Maximum depth implemented: 40m 2-EC readily gives volume changes 2-3 times larger than volume that might occur under large earthquake motions, 3-final average relative densities often greater than 70%	Improves soil
Controlled blasts leave nearby structures unaffected	Low impact to surroundings
Only small-scale equipment is needed (e.g. geotechnical drill or wash boring rigs),	Simple method

1.2- Over view of the paper:

Here we deal with two methods of Ground Improvement by Explosives (G.I.E).

A) Explosive replacement method.

B) Explosive Compaction for Tailings Volume Reduction.

1.2.1-ENVIRONMENTAL ISSUES (Disadvantages):

Below-ground explosive detonations result in large amounts of gas being released into the soil-water system, in the form of nitrogen oxide carbon monoxide and carbon dioxide. Nitrogen oxide is inert in terms of environmental effects on groundwater.

Release of carbon dioxide may lower the "PH" of the groundwater temporarily, while ammonia levels may also be temporarily elevated. But both nitrogen oxide and carbon monoxide are poisonous in air and venting, it is necessary if blasting is carried out within conned spaces.

1.2.2- Explosive replacement method:

This method uses the energy of the explosion to remove the soft clay and replace it with crushed stones. Explosive charges are placed in the soil to be improved according to a specific pattern. Crushed stones are piled up next to the area where the charges are installed. The explosion creates cavities in the soil and causes the pile of crushed stones to slide into the exploded area. The detail of the method and its application to the highway project are described. The effectiveness of the method is evaluated using borehole exploration, plate load tests and ground probing , radar(GPR) tests.

1.2.3- Explosive Compaction for Tailings Volume Reduction:

Applications of the sequential detonation of explosives to cause tailings volume reduction in tailings ponds are discussed. The method is expected to be effective where the dominant tailings components comprise granular, low to non-plastic silt/sand mixtures. The sequential detonation of below ground explosives placed in cased boreholes has been widely used in civil and mining engineering for over 70 years. Previous applications have included foundation compaction of predominantly granular soils for earth dams, bridges, buildings and offshore oil structures (Goal et al, 2000). This has been largely driven by the need to increase seismic or static liquefaction resistance in granular soils. A potential new applications of explosive compaction (EC) is discussed in the present paper with respect to volume reduction of previously impounded mine tailings in tailings ponds.

Explosive replacement method (E.R.M.)

2- Part (1) : Explosive replacement method (E.R.M.) :

2.1- Uses :

The Explosive replacement method (E.R.M.) is used in other types of soil than loose saturated sand or gravel, such as silty clay, soft clay soil.

For a case in china, a high way was constructed through a mountainous area. Some sections of the highway went through valleys where a soft clay layer (6–8.5) m deep was encountered. The valleys were 20–50 m wide, with the water table typically at the ground surface. A typical soil profile is shown in (Fig. 1). The soils were alluvial in nature. The first layer was silty clay 6–8.5 m thick with a vegetation layer 0.5–0.6 m thick on top. The second layer was silty gravel 1.3–2 m thick overlying weathered sandstone. The top 1–2 m of the sandstone was highly decomposed.

Now we have a big challenge, How to improve the soft clay layer rapidly for highway construction ?

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2.2- Advantages of using (E.R.M.) :

A) Some of the commonly adopted soil improvement methods could not be applied, as these methods could not offer an expedient solution to meet the project schedule. For example, surcharge or vacuum preloading together with vertical drains 10 are normally used for similar projects. However, the time required for consolidation was too long for this project. Deep cement mixing is also used sometimes for highway construction in China. However, this method was considered too expensive to be used over a long .

B) The construction was in a remote mountainous area, so blasting was permitted.

C) Plenty of rocks were generated from the tunneling work for the same project.

D) The soft clay layer to be replaced was only about 6–8.5 m thick.

2.3- Parameters that (E.R.M.) depends on :

The use of the explosive replacement method depends on the selection of design parameters such as the positioning of the charges, the charge messes ,the detonation sequence, the charge weight, the effective radius in plan, the charge length, the weight of individual charges around a point in the soil mass , and the minimum vector distance from a charge to a point in the soil mass .

2.4 – Illustration of the method :

The explosive replacement method is illustrated in Fig. 2. As shown in Fig. 2(a), explosive charges are first installed in the soil layer, and then crushed stones are piled up next to it on the side of the road 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 soft soil is replaced with crushed stones in a 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 form a slope of 1V : 3H or 1V : 5H, as shown in Fig. 2(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 regain 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 form a leveled ground and a steeper slope, as shown in Fig. 2(c). The above process is then repeated to remove and to replace the soil in another section.

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2.5- Application

(Details of the method):

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A typical cross-section of the highway is shown in Fig. 3. The road surface is elevated to up to 6 m above the ground to counter flooding. The width of the soft soil layer to be improved is required to be 5 m wider than the toe of the embankment on each side, as shown in Fig. 3.

2.5.1- Placement of charges:

A TNT type of explosive was used for the project. As the purpose of blasting is to remove the soft clay, the thickness of the soft clay above a cavity should be controlled to be small. The weight of each charge is tabulated in Table 1 for different depths of soft clay. As the charges used were cylindrical rather than spherical, the cavity would be a teardrop shape, for this reason, the actual depth of the clay above the cavity was small. A factor of 1.3–1.5 was also applied to the weight of charge to ensure a complete collapse of the cavities formed. The real weight of each charge with respect to different depths of soft clay is given in Table 1. The charges were installed at a horizontal spacing of 2 m in one row. The embedded depth of the charge was determined based on the depth of the clay layer as calculated in Table 1. The charges were installed using a 16 t excavator, as shown in Fig. 6. A pipe 21.3 cm in diameter and 12 m long was driven into the soft clay using a 11 kW vibrator.

Once the pipe reached the required depth, a cylindrical charge 19 cm in diameter was placed. For a charge weight of 16–24 kg, the length of the charge would be 50–80 cm. Water was filled into the pipe as an overburden pressure to the charge before the pipe was pulled out. Sequential detonation was used to reduce the impact of the explosion. A picture taken at the moment of explosion is shown in Fig.4.

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2.5.2- Placement of crushed stones :

The source of the crushed stones was the sandstone rock excavated from tunneling construction for the same highway project. The particle size of the stones ranged from 10 to 70 cm. The pile of crushed stones was typically 1–2 m high and 5–6 m wide (see Fig. 2(a)). After the explosion, the stones fell into the cavities and formed a slope of 1V : 3H to 1V : 5H. More stones were backfilled to form a leveled ground and a boundary slope of

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1V: 0.8H. With every round of explosion, the improvement section could be advanced for 6–8 m.

2.5.3- Construction of embankment:

An embankment typically 6 m high (see Fig. 3) was constructed after the soil improvement work. The fill used for the embankment was mainly clayey sand with basic properties as given in Table 2. The maximum dry density and the optimum water content as obtained from standard Proctor compaction tests were 1.8 Mg/m3 and 14.5% respectively. Roller compaction with 0.3–0.5 m for each lift was used. The relative compaction values specified were 95% for the

top 0.8 m, 93% for the fill between 0.8 and 2.3 m, and 90% for the fill below 2.3m. The pavement cover consisted of a 600 mm thick concrete layer and a 160 mm thick bitumen concrete layer on top.

2.6- VERIFICATION:

2.6.1- Borehole exploration:

Boreholes were drilled to examine the depth of the crushed stones after soil improvement. One of the borehole logs is shown in Fig. 8. The stones were found to be present up to 9 m, in which the top 5–6 m was densely packed whereas the remaining 4–5 m was embedded in clay. Below the crushed stone layer were the silty gravel layer and the weathered sandstone layer.

2.6.2. Plate load tests

Plate load tests were conducted using a square plate 1.0 m *1.0 m. The load was applied via a hydraulic jack reacted against a steel beam, which was counterbalanced by dead weight. The plate was placed on the ground surface before the 6 m of embankment was built. The results of a typical plate load test are shown in Fig. 9. The results indicate that the improved ground had adequate bearing capacity. Using the load–settlement curve shown in Fig. 9, the modulus of

subgrade reaction, ks, which is used for pavement design, 15can be determined as the secant modulus for a specified point on the curve. 16 The modulus of subgrade reaction determined from the initial linear portion of the curve was 120 MPa (see Fig.9). It should be pointed out that the plate load test results reflected only the condition of the upper layer of 1.5–2 m depth in the compacted stone layer. The critical area for settlement would be the deeper zone where the stone was mixed with soft clay, which was not significantly stressed by the plate load tests. Therefore the plate load test results gave an optimistic picture of the load settlement behaviour. The settlement of the improved foundation soil measured 3 months after the opening of the highway was more than the maximum settlement shown in Fig. 9, but less than 30 mm. The total

settlement of the highway measured at the same time was less than 100 mm. The total allowable settlement as specified by the Ministry of Transport for the design of expressways in China was 300 mm. able 2 Basic properties of the fill used for embankment

2.6.3. Ground-probing radar(GPR) tests

GPR was 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 broadband width 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. More explanation of the method can be found in reference. The GPR system used in this project adopted a frequency of 100 MHz. This frequency was chosen to suit the depth of the crushed stone layer. GPR tests were conducted along six lines of a total length of 417 m. Of these, two lines were along the longitudinal direction and four were along the transverse direction of the highway. One scanned profile is shown in Fig.10. The crushed stones in the top 5 m of the soil profile were detected. Soft clay pockets within this layer could also be identified from the image, as indicated by arrows in Fig. 10. However, the stones in the deeper layer could not be identified Clearly from the image. This could be because the radar wave became much less effective when it penetrated the layer of stones embedded in clay.

Applications of Explosive Compaction for Tailings Volume Reduction

3- Part (2): Applications of Explosive Compaction for Tailings Volume Reduction :

3.1-introduction :

The method is expected to be effective where the dominant tailings components comprise granular, low to non-plastic silt/sand mixtures. The paper discusses the mechanisms associated with explosive compaction, factors influencing design, and typical ground response (vibrations, settlements, pore water pressures) observed.

The sequential detonation of below ground explosives placed in cased boreholes has been widely used in civil and mining engineering for over 70 years. Previous applications have included foundation compaction of predominantly granular soils for earth dams, bridges, buildings and offshore oil structures (Gohl et al, 2000). This has been largely driven by the need to increase seismic or static liquefaction resistance in granular soils. A potential new application of explosive compaction (EC) is discussed in the present paper with respect to volume reduction of previously impounded mine tailings in tailings ponds.

3.2-Who does it work :

EC involves placing single or multiple (decked) charges in a borehole drilled over the depth of soil to be densified . Several charges are fired sequentially, with delays selected to minimize offsite vibrations and also to promote cyclic loading of the subsoil. In general, the process is repeated a number of times to cause progressive soil compaction. Pore pressures generated in saturated soils following each blast sequence are allowed to dissipate before further blasts are carried out.

3.3-Effectiveness on the soil improvement

In saturated ground, the energy released by an explosive detonation causes liquefaction of the soil close to the blast point and causes cyclic shear straining of the soil. This process increases pore water pressures and provided strain amplitudes and number of strain cycles are sufficient, the soil mass liquefies. Liquefaction of the soil followed by time-dependent dissipation of the water pressures causes reconsolidation within the soil mass. This re-consolidation happens within hours to days following blasting, depending on the permeability of the subsoil and drainage boundary conditions, and is reflected by release of large volumes of water at the ground surface or up blast casings. “Short term” volume change is also caused by passage of the blast induced shock front through the soil mass. At close distances from a charge detonation, the hydro-dynamic pressures are large enough to cause compression of the soil water system even though the bulk compressibility of the system is relatively small.

The EC method is expected to be most effective where the tailings comprise granular, low to non-plastic, silt/sand mixtures. Experience has indicated that the degree of volume change obtained by blasting depends on the initial density of the subsoil, the total amount of explosive used per unit volume of soil (charge density), and the geometry of the blast pattern. The density of initially loose deposits typically increases considerably to relative densities of up to 70-80% for higher charge densities. Soils with initial relative densities of 60 to 70% can only be densified by a small amount. Thus, the initial density of the tailings controls how much settlement can be expected from the EC process.

3.4-Case study:

2 EC test programs (Sites 1 and 2) in loose mine tailings (sand/silt mixtures) at a tailings dam site in Northern Ontario. The principal author was also involved in initial design of blasting trials for the Canadian Liquefaction Experiment, referred to as Site 3. The latter was carried out at Syncrude’s J-pit outside of Fort McMurray, Alberta (Wride et al, 2000). Other EC data reported in the engineering literature concerning compaction of silt/sand mine tailings has also been reviewed (Klohn et al, 1981; Handford, 1988; Gohl et al, 2000), referred to as Sites 4, 5 and 6, respectively.

The basic geotechnical characteristics of the above mine tailings materials and the charge density used for each EC project are summarized in Table 1. Charge density is defined as the sum of charge weights used in blast holes located within the interior of the test area to be densified plus ½ the sum of charge weights for blast holes located around the perimeter of the area, all divided by the total volume of soil. This definition of charge density accounts approximately for blast energy radiated away from the zone to be densified for perimeter blast holes.

3.5-Post-EC Settlements :

Table 1 indicates that a range of loose silt/sand tailings mixtures have been successfully blast densified, achieving average settlements over the loose layer thickness of up to about 10%. Maximum settlements are achieved using higher charge densities, which implies the use of relatively close blast hole spacing, multiple decks of explosives, and repetitive blast sequences. Post-EC settlements averaging 2 to 4% of the loose layer thickness over the test area have been achieved using relatively low charge densities of around 5 gm/m. The latter have been based on larger blast hole spacing, typically in the range of 10 to 20 m

Settlements decay with distance from a blast hole, forming a bowl-shaped depression around each blast hole as pore pressures generated by blasting dissipate. Since shear strains and hydrodynamic blast pressures attenuate with distance from a charge detonation, this settlement reduction with distance should be expected. Depending on the spacing and pattern of blast holes fired, these depressions gradually level out with successive blast sequences, corresponding to the effects of adjacent blasts causing additional cyclic straining within a partially settled area.

The effect of several blast sequences on local settlement within the center of a test EC area (Sites 1,2 and 4) is shown in Figure 1, showing the progressive tailings shakedown with each blast series. For Sites 1 and 2, each blast sequence involved the sequential detonation of several charges. For Site 4, only 1 charge detonation was used for each blast series. The data indicate that initial soil liquefaction following the first blast series typically caused vertical settlements equal to 5 to 6% of the loose layer thick-ness within a few meters of a blast hole. This corresponds to zones of largest shear strain in close proximity to the blast hole. Several blast series result in large vertical settlements around the blast hole, in the range of 12 to 14% of the loose layer thickness. These localized settlements typically exceed the average settlement over the test area.

Ishihara and Yoshimine (1992) suggest that post-liquefaction settlements in loose sands (initial relative densities of 40 to 50%) equal to 3.5 to 4.5% of the loose layer thickness should be expected following earthquake shaking. Larger earthquake settlements should be expected for looser sands. For blast loading, the effects of large shear strain amplitude, number of strain cycles, and hydrodynamic blast pressures apparently leads to larger post-liquefaction

settlements locally around a blast hole compared to that caused by earthquake loading.

Experience with blasting in loose sands and silts suggests that the zone of significant settlement is approximately ½ of the radius of liquefaction. The looser the soil, the broader the radius of liquefaction and radius of significant settlement. Field trials are typically carried out to confirm the amount of actual settlement achieved since this will depend on the initial densities and other geotechnical properties of the tailings, the size of charge detonation per delay, the number of charges detonated sequentially, and the depths of charge burial.

3.6-Pore Pressure Response :

Charge detonations lead to stress wave propagation away from a blast hole, producing dynamic changes in mean stress and shearing stress within the soil medium. The changes in mean stress are typically very high (MPa range) within a few meters of a blast hole (for the typical charge sizes used by the authors in EC projects), leading to transient hydrodynamic pore pressures. The shearing stresses developed are limited by the undrained strength of the tailings materials, leading to permanent shear strain offsets in a soil element following passage of the shock front. The shear strain pulses are considered primarily responsible for build-up in residual pore water pres-sure. In close proximity to a blast hole, high amplitudes of dynamic mean stress may also lead to residual pore pressure build-up if the soil-water compressibility is such that soil skeleton volume change occurs. This would also lead to residual pore pressure buildup. High pressure, load-unload hydro-static compression tests on saturated soil elements would be required to confirm this effect.

He typical pattern of transient pore pressure pulses and the gradual buildup of residual pore pressures are seen in Figure 2 for a multiple hole detonation sequence at Site 2. The rate of data acquisition used was not rapid enough to accurately capture all the hydrodynamic pressure pulses, but the general trend of gradual buildup in residual pore pressure is evident. The distance of the nearest blast hole to the point of pore pressure measurement is considered far enough that the effects of dynamic mean stress on residual pore pressure change is minor. Pore pres-sure build-up resulting from shear straining is considered to be the dominant factor.

The amount of residual pore pressure buildup at a reasonable distance from a blast hole (neglecting hydrodynamic pressure effects) depends primarily on shear strain amplitude and number of strain cycles (Dobry et al, 1982). These in turn will depend on charge weight per delay, distance from a charge detonation, and number of charges detonated sequentially. In the extreme, if the residual pore pressures over and above the pre-blast static water pressures in the ground equal the initial vertical effective stresses, then a condition of soil liquefaction results. The residual excess pore pressures divided by the initial vertical effective stress is defined as a pore pressure ratio, PPR. Figure 2. Pore pressures versus time during sequential blasting at Site 2, showing peak hydrodynamic pressures and residual pore pressure buildup. The rate of data acquisition was not high enough to accurately capture all the hydrodynamic pres-sure –pulses. train pulses.

3.7-Parameters :

A plot of PPR versus scaled hypo central distance (R) and average charge weight (kg) per delay (W) for Sites 1 to 4 is shown in Figure 3. Reliable pore pressure data were not available for Sites 5 & 6.

Here the hypo central distance R refers to the distance (meters) between the pore pressure measurement point and the nearest charge detonation. The data include both single and multiple charge detonations. The data indicate that PPR increases with in-creasing charge weight per delay, decreasing distance between a blast point, and increasing number of charge detonations. The data indicate that a radius of liquefaction (where PPR ≥ 0.9) can be estimated as equal to αW^0.33 in loose tailings material with α values in the range of 3 to 9. For design, the available data suggest that an average radius of liquefaction equal to 6W^0.33 may be presumed, assuming multiple charge detonations are employed.

Data from cyclic, strain controlled triaxial tests on loose sands (relative densities of about 45%) indicate that it is necessary to achieve peak shear strains during one strain cycle of about 0.3% to produce soil liquefaction after 4 to 10 strain pulses (Dobry et al (1982). Thus, design estimates of maximum shear strain versus distance from a charge detonation may be used in advance of blasting field tests to estimate maximum radii of liquefaction around blast holes. The procedures used are described in Section 3 on Blast Design, involving the use of nonlinear blast analysis.

3.8-Vibration control:

Induced vibrations on nearby structures need to be controlled where blast densication is carried out in developed areas. Blasting within 30±40 m of existing structures requires a reduction in the charge weights per deck (involving a reduction in blast hole spacing), and in the number of holes detonated at any one time. Also, when blasting is carried out on or adjacent to slopes, blast patterns are adjusted to restrict the zone of residual pore water pressure build-up and minimize the risk of slope instability. For the above reasons, the number of charges detonated .

Sequentially is often restricted to minimize the duration of shaking .

Longer-duration shaking causes more damage to structures and increases residual pore water pressure build-up. Individual charge delays are also selected so that destructive interference in the frequency range of interest occurs between ground waves from the sequence of blasts. Design of the appropriate charge delays between adjacent decks in each borehole and between adjacent boreholes is carried out using the following process:

(a) Ground vibration patterns (peak particle velocities and frequency content are determined at a particular location of concern remote from the blast point due to a single charge.

This is best done using old measurements, but can also be carried out theoretically.

(b) The frequency range of potentially damaging vibrations is selected based on structural vibration theory or other considerations.

(c) The effects of sequential charge detonation from a decked array of boreholes are assessed by a simple linear combination of the single charge wave trains in which time delays between decks and between adjacent boreholes are varied. Optimum blast delays are then determined to minimize the peak particle velocity or, alternatively, the vibrational energy content in the frequency range of interest.

Based on vibration measurements recorded during detonation of a single charge at various distances from a blast source at an alluvial site in Japan, the linear wave superposition model described above was applied to compute the likely peak particle velocities (PPV) at the ground surface for a particular direction resulting from multiple detonations. The computed and measured peak velocities are plotted against each other in Fig. 3 and indicate that the use of linear combination of wave motions (incorporating appropriate time-shifts in the waveforms based

on the prescribed detonation sequence and waveform scaling depending on charge weight ± distance relationships determined for the site) generally leads to a conservative over-prediction of PPV. This is particularly true for small source ± site distances, where non-linear effects caused by soil liquefaction around a blast point would be expected to reduce near old motions.

Blast hole layout and detonation sequencing Blast patterns generally use a staggered rectangular grid of boreholes at spacing of 4±9 m. Staggering is used to provide a pattern of two (or more) passes within a uniform grid. Boreholes are drilled over the full depth of soil deposit to be denied, and 75±100 mm diameter plastic casing installed (this casing size is convenient with the drills normally used). The casing is then loaded with explosive at one or more levels in the borehole (decks). A series of boreholes, each containing one or more decks, is then sequentially detonated. The number of blast holes detonated in any shot depends on vibration control considerations and on concerns about the effect of liquefaction and settlement on adjacent slopes and structures.

3.9-CONCLUSIONS

Explosive compaction has been used effectively for a wide variety of civil and mining engineering projects, primarily with respect to improving foundation soil resistance to static and seismic liquefaction. A new application of the EC process is proposed to re-duce the volume of previously impounded mine tailings, thereby increasing storage capacity within the tailings pond.

Previous experience with EC in non-plastic silt/sand tailings materials indicates that significant volume change can be induced by blasting in saturated materials. The amount of volume change largely depends on charge density, which is governed by blast hole spacing and the charge weights used in each hole. The geometry of the blast pattern further influences the uniformity of the compaction process. Data collected from previous EC field trials in tailings msign of the EC process. Estimates of post-EC settlement as a function of tailings depth and soil properties, blast hole layouts and charge densities are facilitated by application of nonlinear blast analysis.

If Ground Improvement is necessary, What methods are available ?

Many methods for ground densification and improvement are available, including dewatering, compaction preloading with and without vertical drains, admixture stabilization, grouting of several types, deep mixing, deep densification and soil reinforcement.

Many of these techniques, such as dewatering, compaction pre-compression and some types of grouting, have been used for many years. However, there have been rapid advances in the areas of deep densification (vibrocompaction, deep dynamic compaction, compaction piles and explosive densification), jet and compaction grouting, deep mixing, and stone column systems in recent years.

These methods have become practical and economical alternatives for many ground improvement applications।