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Use of the explosive method in the soil improvement work of

a highway project

S. W. Yan and J. Chu

A highway was constructed in Jiangxi Province, China,

through a mountainous area. Some sections of the

highway went through valleys where a soft clay layer

6 8.5 m deep was encountered. A new explosive method

was developed and adopted for this project. The 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 tests.

1. INTRODUCTION

Explosive compaction has been a method used in past decades

for the compaction of loose granular soil. The method usesenergy 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.18

However, most of the studies are limited to

applications in granular soils. The use of explosive compaction

or similar methods in other types of soil, such as soft clay, is

rare. Recently a new explosive method has been developed in

China for removing and replacing seabed mud with rocks for

offshore dike construction.9

Encouraged by this development, a

new method to replace soft clay with crushed stones by

blasting has been tested in conjunction with a highway

construction in Jiangxi Province, China. After some

experiments, a workable method has been established and

adopted in the soil improvement work for this highway project.

This new method is called the explosive replacement method.

The detail of the method and its application in this highway

project are presented in this paper.

2. BACKGROUND TO THE PROJECT

To cater for the rapid economic development, a highway

connecting Ganzhou and Dingnan in Jiangxi Province (see Fig.

1) was constructed from January 2002 to January 2004. The

highway is 127 km long and 26.5 m wide. A few sections of

highway run through valley zones. The valleys were 2050 m

wide, with the water table typically at the ground surface. A

typical soil profile is shown in Fig. 2. The soils were alluvial in

nature. The first layer was silty clay 68.5 m thick with a

vegetation layer 0.5 0.6 m thick on top. The second layer was

silty gravel 1.32 m thick overlying weathered sandstone. The

top 12 m of the sandstone was highly decomposed. Typical

soil properties of the soft clay are shown in Fig. 3. The water

content of the soil was generally higher than the liquid limit

(Fig. 3(b)). The undrained shear strength of the silty clay was

only about 20 kPa (Fig. 3(c)). How to improve the soft clay

layer rapidly for highway construction becameoneof the

challenges to this project. Some of the commonly adopted soil

improvement methodscould 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 drains10

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.11

However, this

method was considered too expensive to be used over a longsection. Furthermore, the use of this method might not meet

the construction schedule either. The explosive replacement

Article number = 13713

Fig. 1. Location map

Geotechnical Engineering 152 Issue GE1 Yan Chu 1Use of explosive method in soil improvement

_________________________________________________________________________________________http://www.paper.edu.cn


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method was considered feasible for this project for the

following three reasons:

(a) The construction was in a remote mountainous area, so

blasting was permitted.

(b) Plenty of rocks were generated from the tunnelling work

for the same project.(c) The soft clay layer to be replaced was only about 6 8.5 m

thick.

After some experiments, a workable method was established

and adopted for this highway project.

3. ANALYSIS

The use of the explosive replacement method depends on the

selection of design parameters such as the positioning of the

charges, the charge messes, and the detonation sequence. Some

relationships between these parameters have been used for

explosive compaction:

Hopkinsons number:12

HNW1=3

R1

Normalised weight:8

NW

W=Lc 1=2

R2

Energy input attenuation:5

E1 X Wi

R2vi

3

whereWis the charge weight (kg), R is the effective radius in

plan (m),Lc is the charge length (m), Wi is the weight of

individual charges around a point in the soil mass (g), and Rviis the minimum vector distance from a charge to a point in the

soil mass (m). HN, NWand E1 are constants. Unfortunately, for

a given value ofHN,NWorE1, the above relationships may

provide infinite combinations of charge weight with radius.

Furthermore, it is difficult to select suitable valuesofHN,NW

orE1 in practice. Based on blasting mechanics,13

anew set of

equations is derived as follows.

As the explosion takes place very rapidly, it can be assumed to

be aninsulated process. Based on the LondauStanyukowich

theory,13

we have

ForPvp > Pk:

Gravel 1

8

05

Soft clay

Vegetation

8m

Weathered sandstone bedrock

Fig. 2. Typical soil profile in the valley

0

2

4

6

8

10

12

Void ratio

Depth:m

(a)

0

2

4

6

8

10

12

WC, LL and PL: %

Depth:m

WC

LL

PL

(b)

0

2

4

6

8

10

12

cu: kPa

Depth:m

(c)

0 1 2 30 25 50 75 100 0 20 40 60

Fig. 3. Typical soil properties: (a) void ratio; (b) water content (WC), liquid limit (LL) and plastic limit (PL); (c) undrained shearstrength,cu

Geotechnical Engineering 152 Issue GE1 Yan Chu Use of explosive method in soil improvement

________________________________________________________________________________________http://www.paper.edu.cn


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Pvp

Pe

Ve

Vvp

k4

ForPvp , Pk:

Pvp

Pk

Vk

Vvp r

5

wherePkand Vkare the pressure and volume at the conjugate

point, which is specified as the point where k 3 and r 4/3;

Pvp is the pressure in the cavity (Pa); Vvp is the volume of the

cavity (m3);Ve is the volume of explosive; and Pe is the

explosive pressure (Pa), which can be calculated from:13

Pe re D

2

2 k 1 6

where re is the density of the explosive (kg/m3), and D is thevelocity of the explosive (m/s).

For a spherical explosive, Ve 4r30=3 and Vvp 4R

3v p=3,

wherer0 is the radius of the explosive and Rvp is the radius of

the cavity. Therefore:

ForPvp > Pk:

Pvp

Pe

Ve

Vvp

k

r0

Rvp

97

Thus, forPvp > Pk:

Pvp re D

2

8

r0

Rvp

98

At the conjugate point Pvp PkandRvp Rk. Then, from

equation(8),we have:

Rk re D

2

8Pk

1=9

r09

When the cavity expands furtherthat is, whenPvp , Pkwe

have

Pvp PkVk

Vvp

4=3 Pk

Rk

Rvp

410

When the cavity due to explosion ceases expansion,Pvp P0,

whereP0 is the atmospheric pressure, Pa, plus the verticaloverburden pressure,

Pni1i hc i, wherehc is the thickness of

the soft clay above a cavity. Therefore the final radius of cavity

due to explosion can be calculated as

Rvd Pk

P0

1=4re D

2

8Pk

1=9r0

0:794P5=36k r

1=9e D

2=9 Pa Xni1

i(hc)i

" #1=4r0

11

As the mass of the explosive,Q, can be calculated as

Q 3rer30=4Q 3rer0

3=4, equation(11)can also be written

as

Rvd 0:4926P5=36k

D

re

2=9Pa

Xni1

i hc i

" #1=4Q1=312

4. OUTLINE OF THE METHOD

The explosivereplacement method is illustrated in Fig. 4. As

shown in Fig.4(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 chargesare 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: 3Hor

1V: 5H, as shown in Fig. 4(b). The impact of the explosion also

causes an instantaneous reduction in the shear strength of the

soil below the level of explosion14

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 itselfwill also regain part

of its original strength after explosion.14

The explosion also

has a densification effect on the gravel layer below the clay

layer. More crushed stones are backfilled to form a levelled

ground and a steeper slope, as shown in Fig. 4(c). The above

process is then repeated to remove and to replace the soil in

another section.

5. APPLICATION

A typical cross-section of the highway is shown in Fig.5. 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 thetoe of the

embankment on each side, as shown in Fig.5.

5.1. Placement of charges

A TNT type of explosive was used for the project. The design

parameters for the explosive were: density, re 1100 kg/m3;

velocity,D 6200 m/s;Pk 3.0 3 108 Pa. The unit weight of

the soil was 18 000 N/m3. Substituting all the parameters into

equation(12),we have:

Rvd 10:4Q1=3

98000 18000hc 1=413

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. Usinghc 0 in equation(13),we have:




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Rvd 0:59Q1=314

equation(13)is equivalent to equation(1) when NH 0.59.

ForQ 1020 kg, we getRvd 1.271.6 m from equation

(14).The distance between charges, L, can be estimated as

L Rvd15

where 1.5 2.0. When 2,L the diameter of the

cavity. To allow some overlapping, , 2 should be used.

Based on the above analysis, a constant value ofL 2 m was

used for this project. The weight of each charge can then be

calculated using equation(13),and the values are 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, with its vertical dimension much larger than

the calculatedRvd.13

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.

Thecharges 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 1624 kg, the

length of the charge would be 5080 cm. Water was filled intothe 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. 7.

5.2. Placement of crushed stones

The source of the crushed stones was the sandstone rock

excavated from tunnelling construction for the same highway

Section that has been

replaced by crushed

stones

Pile of crushed stones

56

1

1V

08H

46

Charge685m

Soft clay

(a)

(b)

(c)

Profile before explosion

Profile after explosion

1V

(35)H

Soft clay

Soft clay

Backfill

1

Fig. 4. Explosive displacement procedure: (a) before explosion;(b) after explosion; (c) after backfill

Gravel

Replaced crushed stones

Embankment

265 m

5 m 445 m 5 m

85m

1

8m

6m

15

1

Fig. 5. Typical cross-section of the road




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PROOFS

project. The particle size of the stones ranged from 10 to

70 cm. The pile of crushed stones was typically 12 m high

and 56 m wide (see Fig. 4(a)). After the explosion, the stonesfell into the cavities and formed a slope of 1V: 3Hto 1V: 5H.

More stones were backfilled to form a levelled ground and a

boundary slope of 1V: 0.8H. With every round of explosion,

the improvement section could be advanced for 6 8 m.

5.3. Construction of

embankment

An embankment typically

6 m high (see Fig.5) was

constructed after the soil

improvement work. The fill

used for the embankment wasmainly a 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 2.3 m. The pavement cover consisted of

a 600 mm thick concrete layer and a 160 mm thick bitumen

concrete layer on top.

6. VERIFICATION

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 56 m was densely packed whereas the

remaining 45 m was embedded in clay. Below the crushed

stone layer were the silty gravel layer and the weathered

sandstone layer.

6.2. Plate load tests

Plate load tests were conducted using a square plate 1.0 m 3

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

loadsettlement curve shown in Fig.9, the modulus of

subgrade reaction, ks, which is used for pavement design,15

can 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 .52 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 anoptimistic picture of the loadsettlement behaviour. The

settlement of the improved foundation soil measured 3 months

after the opening of thehighway 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

Depth of soil, h: m 5 6 7 8

Distance betweencharges, L: m 2 2 2 2 in equation(15) 1.7 1.6 1.5 1.5Radius of cavity, R vd: m 1.18 1.25 1.33 1.33Thickness of soft clay above cavity, hc: m 2.65 3.50 4.33 5.34Embedded depth for charges, hc + R vd: m) 3.83 4.75 5.66 6.67Weight of each charge, Q: kg 10.9 14.0 18.0 19.3Weight of each charge used, Qu: kg 16 20 24 24

Table 1 Calculation of the weight of each charge

Fig. 6. Installation of charges

Fig. 7. During explosion

Soil type Liquidlimit

Plasticlimit

Plasticityindex

Fines content(less than 75 m)

Organiccontent

Clayey sand (SC)* 25.3% 12.3% 13% 18% 2.7%

*According to the Unified Soil Classification System.

Table 2 Basic properties of the fill used for embankment




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PROOFS

Ministry of Transport for the design of expressways in China

was 300 mm.

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 broad-

bandwidth antenna. Some of

the waves are reflected when

they hit discontinuities in the

subsurface, and some are

absorbed or refracted by thematerials 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 reference17.

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.

7. SUMMARY

A new explosive replacement method was developed and

adopted for the soil improvement work of a highway project in

Jiangxi, China. The method uses the energy of explosion toremove and replace soft clay with crushed stones. Explosive

charges were placed in the soil to be improved according to a

specific pattern. Crushed stones were piled up next to the area

where the charges were installed. The explosion created

cavities in the soil and caused the pile of crushed stones t o

slide into the exploded area. Based on blasting mechanics,13

a

set of equations regarding the weight of the charge and the

radius of the explosive cavity were derived to calculate the

weight of the charges and the spacing between the charges. The

design of the charges was made using the equations and the

results were found to be satisfactory. A method to install

charges was also developed and used in this project. Borehole

exploration after the soil improvement revealed that the

crushed stones were densely packed in the top 56 m, but were

embedded in clay in the bottom 34 m. The results of plate load

tests indicated that the top layer of the improved ground had

sufficient bearing capacity and a high modulus of subgrade

reaction. The ground-probing radar adopted was effective in

detecting the layer of the densely packed crushed stones.

0

56 m

89 m

9105 m

Crushed stones, densely

packed

Crushed stone embedded inclay

Siltygravel

Sandstone with top 12 m

heavily weathered

Siltygravel

Fig. 8. Borehole log of the replaced soil layer

0

2

4

6

8

10

12

14

16

18

Load: kPa

S

ettlement:mm

ks120 MPa

0 200 400 600 800

Fig. 9. Results of plate load test

Fig. 10. An image from ground-probing radar




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PROOFS

REFERENCES

1. HALLC. E. Compacting a dam foundation by blasting.

Journal of Soil Mechanics and Foundation Division, ASCE,

1962, 88, No. 3, 3351.

2. CHADWICKP., COXA. D. and HOPKINSH. G. Mechanics of

deep underground explosions. Proceedings of the Royal

Society, London, 1964, A256, No. 1070, 235300.

3. CHARLIE W. A., JACOBSP. J. and DOEHRING D. O.

Blasting-induced liquefaction of an alluvial sand deposit.

Geotechnical Testing Journal, ASTM, 1992, 15, No. 1,

1423.

4. NARIN VANCOURTW. A. and MITCHELLJ. K. New insights

into compaction of loose, saturated, cohesionless soils. In

Soil Improvement for Earthquake Hazard Mitigation, ASCE

Geotechnical Special Publication No. 49, 1995, pp. 5165.

5. NARIN VANCOURTW. A. and MITCHELLJ. K. Investigation of

predictive methodologies for explosive compactions.

Proceedings of the ASCE Specialty Conference on

Geotechnical Earthquake Engineering and Soil Dynamics,

Seattle, WA, ASCE Geotechnical Special Publication No.

75, 1998, pp. 639653.

6. GANDHIS. R., DEYA. K. and SELVAM S. Densification ofpond ash by blasting. Journal of Geotechnical and

Geoenvironmental Engineering, ASCE, 1998, 125, No. 10,

889899.

7. GOHLW. B., JEFFERIES M. G., HOWIEJ. A. and DIGGLE D.

Explosive compaction: design, implementation and

effectiveness.Geotechnique, 2000, 50, No. 6, 657665.

8. DEMBICKIE., IMIOLEKR. and KISIELOWAN. Soil compaction

with blasting method. In Geomechanics and Water

Engineering in Environmental Management(CHOWDHURYR.

N. (ed.)). Balkema, Rotterdam, 1992, pp. 599622.

9. JINL. and SHIF. Blasting techniques for underwater soft

clay improvement.China Harbour Engineering, 1999, No.

2, 10 17; No. 3, 1627.

10. YANS. W. and CHUJ. Soil improvement for a road using

the vacuum preloading method. Ground Improvement,

2003,7, No. 4, 165172.

11. LINK. Q. and WONGI. H. Use of deep cement mixing to

reduce settlements at bridge approaches. Journal of

Geotechnical and Geoenvironmental Engineering, ASCE,

1999,125, No. 4, 309320.

12. IVANOVP. L. Compaction of Noncohesive Soil by Explosions.

US Department of Commerce, Springfield, VA, 1967,

National Technical Information Service Report No.

TT70-57221.

13. HENRYCHJ. The Dynamics of Explosion and Its Use.

Elsevier, Amsterdam, 1979.

14. OSIPOVV. I., NIKOLAEVAS. K. and SOKOLOVV. N.

Microstructural changes associated with thixotropic

phenomena in clay soils. Geotechnique, 1984, 34, No. 2,

293303.15. AASHTO Guide for Design of Pavement Structures.

American Association of State Highway and Transportation

Officials, Washington, DC, 1993.

16. BOWLES J. E.Foundation Analysis and Design.

McGraw-Hill, New York, 1996.

17. KOERNER R. M. Construction and Geotechnical Methods

in Foundation Engineering. McGraw-Hill, New York,

1984.

Please email, fax or post your discussion contributions to the secretary: email: [email protected]; fax: 44 (0)20 665 2294;

or post to Mary Henderson, Journals Department, Institution of Civil Engineers, 17 Great George Street, London SW1P 3AA.



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