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.