Transportation Research Record 1849 47Paper No. 03-3116

Dry jet mixing has been widely used since the 1980s for stabilization ofsoft soil. The quality and strength of the dry-jet-mixed columns must beevaluated to confirm the success of the stabilization. The standard pen-etration test (SPT) is shown to be a simple and effective method for thistask. The strength characteristics along the length of the column weredetermined, and correlations between the SPT blow count and theunconfined compressive strength were developed.

Dry jet mixing (DJM) is a soil improvement technique that pneu-matically delivers powdered reagent into the ground and mixes itwith in situ soils to form a soil-cement column. The chemical reac-tions between soils and dry reagents such as cement powder or limeincrease the strength and reduce the compressibility of soft ground.Because of its many advantages, DJM has attracted increasing atten-tion compared with other in situ soil stabilization methods since itsdevelopment by Kjeld Paus in Sweden, in 1967. In the 1980s a largeincrease in DJM usage occurred in the Nordic countries and inJapan. Today the method is used worldwide, especially in Europe,North America, and Asia (1, pp. 1525).

The DJM method was introduced to China through Japan in theearly 1980s. The first DJM equipment in China was developed in1983 at the Wuhan Research Institute of Engineering Machinery ofChina. The equipment was successful in soft soil improvement forrailway engineering. Although there were some early failures ofDJM column foundations for buildings during the 1980s (2), DJMrapidly spread throughout China in the 1990s, especially for roadand railway embankment applications. Han et al. (3) presented adetailed review of the state of the practice of deep soil mixing inChina.

The binder material for DJM in China is typically cement, butoccasionally a lime-cement mixture is used. The binder materialcontent is usually 10% to 15% of the soil weight. Column diametersare normally 500 to 600 mm, and the maximum depth of improve-ment has been 15 m. The column pattern is generally triangular withplan area ratios (Acolumns/Atotal) between 0.10 and 0.25. Columns areconstructed by mixing the soil with cement one to three times (themixing equipment goes all the way down and back up). However,cement is only added during the initial mix. Small pilot studies thatinclude in situ column testing are typically conducted before pro-duction installation to determine the optimum cement contents for aparticular site and soil conditions. To ensure quality control, a load

transducer measures the weight of cement applied per length of col-umn. Mix uniformity is verified by in situ testing, as will subsequentlybe discussed.

Many theoretical and empirical methods for calculating settle-ments and analyzing the stability of DJM-improved soils weredeveloped during the last decade (4, pp. 125153). Although themodulus and strength of the soil-cement column material can bemeasured by laboratory tests, there can be large differences betweenthe performance of samples prepared in the laboratory and field per-formance of full columns. Thus, a dynamic column penetrometerwas developed in Sweden and Finland by Halkola (5) for investi-gating column strength and integrity in situ. The cone penetrationtest (CPT), vane penetrometer test, and other in situ tests are alsofinding their way into practice for in situ testing of soil-cementcolumns. However, these methods are not sufficient to evaluate soil-cement columns (6, pp. 285294), because they can only be used inthe upper portion of each column where penetration is not met withrefusal. Furthermore, none of the above-mentioned tests provides asample of the DJM column for visual inspection.

In this paper, use of the standard penetration test (SPT) for esti-mating the strength and split-spoon sampling for visual inspectionof the DJM soil-cement column material is presented. The distribu-tion of strength along the length of soil-cement columns is shown.The relationships between SPT blow count and unconfined com-pressive strength are developed, and the engineering applications ofsoil-cement columns are discussed.

SOIL-CEMENT STRENGTH ALONG LENGTH OF COLUMNS

The shear strength or unconfined compressive strength of a soil-cement column is a function of many factors, including soil type,binder content, construction method, time, and the ambient envi-ronment, specifically temperature. To comprehensively investigatesoil stabilization by DJM soil-cement columns, SPTs were per-formed on cement columns in the LianYunGong section of the Lian-Huo Freeway in China. The cement columns were 0.5 m in diameterand the lengths ranged from 8 to 13 m corresponding to the typicalthickness of an upper soft clay deposit. Table 1 summarizes the rangeof properties of the clay encountered on the project. The amount ofcement used ranged from 55 to 75 kg/m depending on the soil watercontents. As water content increased, the percentage of cement wasincreased.

Because of installation, the central 100-mm core of a soil-cementcolumn is not representative of the overall column. Therefore, SPTswere performed at a distance of two-fifths of the column radius

Evaluation and Quality Control of Dry-Jet-Mixed Clay Soil-Cement Columns byStandard Penetration Test

Songyu Liu and Roman D. Hryciw

S. Liu, Institute of Geotechnical Engineering, Southeast University, Nanjiang,210096, Peoples Republic of China. R. D. Hryciw, Department of Civil andEnvironmental Engineering, University of Michigan, 2340 GG Brown, Ann Arbor,MI 48109-2125.

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(0.10 m) from the center. Disturbed samples from the split-spoonsampler were visually inspected for cement mix uniformity. In addi-tion, undisturbed 88-mm samples were taken every 1.5 m along thelength of the columns by dry coring. The cores were protected by atube and sealed with wax for transport to the laboratory, whereunconfined compressive strength tests were performed. The coreswere taken from just above the SPT depths. Thus, each reportedunconfined compressive strength (qu) corresponds to an SPT blowcount (N ) at approximately the same location.

48 Paper No. 03-3116 Transportation Research Record 1849

Unit Weight 15.4 kN/m3 to 17.5 kN/m3 Void Ratio 1.0 to 2.1 Water Content 46.5% to 85.0% Plasticity Index 16 to 38 Liquid Limit 40 to 65 Clay Fraction (

Liu and Hryciw Paper No. 03-3116 49

Column 23-12 Column 19-16 Column 14-20 Depth (m) N qu

(kPa) N qu

(kPa) N qu

(kPa) 1.5 17 455 16 507 23 585 4.0 16 520 22 640 21 620 6.0 15 486 18 523 18 495 8.0 14 430 17 540 16 508

10.0 14 452 20 621 19 465

TABLE 2 N and qu for Three Typical DJM Columns

qu= 6.8N+ 20

0

0.1

0.2

0 5 10 15 20SPT-N

q u (M

Pa) qu= 8N + 150

0

0.2

0.4

0.6

0 5 10 15 20 25 30 35SPT-N

q u (M

Pa)

R2=0.62 R2=0.52

qu = 10N + 270

0

0.2

0.4

0.6

0.8

1

10 15 20 25 30 35SPT-N

q u (M

Pa)

R2=0.37

qu = 8.3N + 365

0

0.2

0.4

0.6

0.8

1

10 15 20 25 30 35SPT-N

q u (M

Pa) R2=0.14

qu = 6N+ 445

0

0.2

0.4

0.6

0.8

1

10 15 20 25 30 35SPT-N

q u (M

Pa)

R2=0.13

(e)

(c) (d)

(a) (b)

FIGURE 3 Relationships between SPT-N and qu at (a) 7 days, (b) 14 days, (c) 28 days, (d ) 60 days, and (e) 90 days.

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As expected, the unconfined compressive strengths increase withSPT blow count at all times. Furthermore, the slopes of the lines cor-responding to different ages are almost identical, as observed in thecomparison of regression lines in Figure 4.

Equations 1a through 1e have the following general form:

where A is the slope of the linear regression, and quo is the uncon-fined compressive strength extrapolated to N = 0 at each age, t (indays). Figure 5 illustrates the functional relationship, quo, which canbe fit very well by the following equation:

Since the lines in Figure 5 are virtually parallel, an average slopevalue of A = 8 can be assumed. Finally, combining Equations 3 and2 obtains

In the foregoing equations N is the SPT blow count modifiedfrom the raw blow count (Ni) according to Chinese practice (8).This practice requires multiplying Ni by the following amounts forthe rod lengths specified: 1.00 for 3 m, 0.92 for 6 m, 0.86 for 9 m,0.81 for 12 m, 0.77 for 15 m, and 0.73 for 18 m. Although in Chinathis procedure is referred to as a rod-length correction, this is notthe same rod-length correction used in the United States. In fact, theChinese correction actually appears to be a correction for over-burden pressure. The SPTs were performed using a 63.5-kg automatictrip hammer.

Equation 4 has two practical uses. First, it can predict the uncon-fined compressive strength of soil-cement column material at anytime t based on an SPT performed on the column at t. Second, itcan also be used to conservatively estimate the strength at a futuretime t based on an SPT performed at an earlier time. The conser-

q N tu kPa days( ) = + ( ) 8 162 286 4ln ( )

q tuo kPa days( ) = ( ) 162 286 3ln ( )

q AN qu uokPa( ) = + ( )2

q N d

q N e

u

u

kPa 0 days

kPa 0 days

( ) = + ( )

( ) = + ( )

8 3 365 6 1

6 0 445 9 1

. ( )

. ( )

50 Paper No. 03-3116 Transportation Research Record 1849

vatism stems from the fact that N actually increases with time, andtherefore the first term in Equation 4 should gradually increase.Nevertheless, the second term in Equation 4 appears to account formost of the observed time effects, especially if the SPT is per-formed after 14 days. If the SPT is performed at only 7 days, theauthors recommend doubling the value of N for estimation of qubeyond 28 days.

Equation 4 is very practical for finding the right design parameters,such as optimum cement contents for a given soil based on 7- or14-day tests on pilot soil-cement columns.

ENGINEERING APPLICATIONS

Foundations

For foundations, the bearing capacity of the soil-cement column sys-tem must be evaluated. Broms (9, pp. 177184) evaluated short-term capacity based on cavity expansion theory and indicated thatwhen u (column) = 30 and h < 100 to 150 kPa,

and when u (column) = 0 and h > 100 to 150 kPa,

where

p = bearing capacity of the cement column,h = horizontal stress on the column, andu = undrained angle of internal friction.However, for shallow failures of foundations, qu is much larger

than h and therefore it can conservatively be assumed that

Liu et al. (10, pp. 153158) found that boring and sampling of thesoil-cement column causes a considerable decrease in the uncon-

p uq ( )6

p u hq b= + ( )5

p u hq a= + 3 5( )

7 d

14 d

28 d60 d

90 d

0

100

200

300

400

500

600

700

0 10 20 30 40SPT-N

q u (k

Pa)

FIGURE 4 Comparison of best-fit relationships between SPT-Nand qu at 7, 14, 28, 60, and 90 days (d days).

00 20 40 60 80 100 120

Time (days)

100

200

300

400

500

600

700

q u (kP

a)

FIGURE 5 Variation of unconfined compressive strengthwith time.

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fined compressive strength of recovered specimens compared withthose cured in the laboratory. The strengths of the laboratory-curedspecimens reflect the in situ strengths. Thus, the strength predictedby Equation 4 can be increased by a factor, k, to account for thestrength loss due to sampling:

where k is an index of disturbance equal to the ratio of undis-turbed (laboratory or DJM column) strength to disturbed strength.For the soils in this study, k was found to be 2.5 (10, pp. 153158). This value can conceivably be extended to other CH and CLsoils. For different soil types, other values of k would have to beestablished.

Finally, since loading is distributed over both the column and thesurrounding soil, the bearing capacity depends on contributions ofboth the column and the unstabilized soil. Thus, for engineeringapplications, it may be estimated by a weighted volume average ofthe column capacity and the soil bearing capacity:

where

sp = bearing capacity of composite ground,s = bearing capacity of unstabilized soils, anda = ratio of column area to total area (0.10 to 0.25).

Considering the relatively uniform observed N-values along thelengths of cement columns, the average SPT blow count can be usedin Equation 7. However, should N-values be significantly lower inthe upper portion of the column, more deference should be given tothese values.

Table 3 shows a comparison of bearing capacities of single columnsdetermined by SPT and the static load test (SLT), where the SPT wasperformed following the SLT for the same column. There appears tobe excellent agreement between the load test results and predictions byEquation 7.

The settlement of DJM soil-cement columns can be predicted inseveral ways. The Youngs modulus of the column, E, could be esti-mated using empirical relations between modulus and unconfinedcompressive strength such as E/qu = 250, where qu is computed byEquation 4. Alternatively, since SPT N-values are available, directcorrelations between E and SPT-N could be employed to computeE for the soil-cement column.

sp p sa a= + ( )1 8( )

p k N t= + ( )8 162 286 7ln ( )

Liu and Hryciw Paper No. 03-3116 51

Slopes and Embankments

For slopes or embankments stabilized by DJM soil-cement columns,the global shear resistance sp corresponds to the weighted averageshear strength of columns and unstabilized soils:

where p is the shear strength of the cement column and s is theshear strength of the unstabilized soils.

Because the permeability of the cement column is very low, theundrained shear strength can be used for the stability analysis (9,pp. 177184). Therefore,

where qu can be obtained by Equation 4 times the index of disturbance,k = 2.5.

CONCLUSIONS

A large number of SPTs and laboratory unconfined compressivestrength tests have been collected on cement columns formed byDJM for soft ground improvement. The strength characteristicsalong the lengths of the cement columns were studied. The followingconclusions can be drawn:

1. The SPT is an economic and effective method to evaluatethe strength and quality of stabilized soil-cement columns. Thetest is highly recommended as the quality control method forDJM.

2. The strength parameters along the cement columns are gener-ally homogeneous, with a logarithmic increase in strength with time.

3. Unique empirical relationships between SPT blow count Nand unconfined compressive strength qu of the cement column weredetermined for various curing times.

4. The SPT results can be used for determining the bearingcapacity both at the time of testing and by procedures establishedherein, in the future. This conclusion makes the SPT particularlyuseful for analysis of pilot test results.

5. The shear resistance and modulus for stability analysis andsettlement of DJM can likewise be estimated.

p uq= 0 5 10. ( )

sp p sa a= + ( )1 9( )

Bearing Capacity (kN)Column

No.

Time After Installation

(days)

Column Length

(m)

Column Diameter

(mm)

Average N

By SPT

By SLT

Error

16-13 63 13 500 13 240 240 0

10-11 45 12 500 16.4 227 270 15%

11-19 67 13 500 15 252 270 7%

13-9 46 13 500 14 219 240 8%

TABLE 3 Comparison of Bearing Capacities Determined with SPT and SLT

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ACKNOWLEDGMENTS

Sponsorship from the Jiangsu Provincial Highway Administrationfor this study is gratefully acknowledged. The field and laboratoryefforts of L. C. Miao, R. M. Li, S. Li, and D. W. Zhang and others atSoutheast University were essential to the successful completion ofthe research.

REFERENCES

1. Bruce, D. A., M. E. C. Bruce, and A. F. Dimillio. Dry Mix Methods:A Brief Overview of International Practice. In Proc., InternationalConference on Dry Mix Methods for Deep Soil Stabilization, Balkema,Rotterdam, 1999.

2. Gong, X. N. Composite Foundation (in Chinese). Zhejiang UniversityPress, Hangzhou, China, 1992.

3. Han, J., H.-T. Zhou, and F. Ye. State-of-Practice Review of Deep SoilMixing Techniques in China. In Transportation Research Record: Jour-nal of the Transportation Research Board, No. 1808, TRB, NationalResearch Council, Washington, D.C., 2002, pp. 4957.

52 Paper No. 03-3116 Transportation Research Record 1849

4. Broms, B. B. Design of Lime, Lime/Cement, and Cement Columns. InProc., International Conference on Dry Mix Methods for Deep SoilStabilization, Balkema, Rotterdam, 1999.

5. Halkola, H. In Situ Investigations of Deep Stabilized Soil. In Proc., 8thEuropean Conference on Soil Mechanics and Foundation Engineering,Balkema, Rotterdam, 1983.

6. Halkola, H. Quality Control for Dry Mix Methods for Deep Soil Stabi-lization. In Proc., International Conference on Dry Mix Methods forDeep Soil Stabilization, Balkema, Rotterdam, 1999.

7. Braaten, A., R. Aaboe, and F. Oset. Development of In Situ ControlMethods for Lime/Cement Columns. In Proc., International Conferenceon Dry Mix Methods for Deep Soil Stabilization, Balkema, Rotterdam,1999.

8. Ministry of Construction, China. National Code of Geotechnical Investi-gation (in Chinese). Chinese Construction Industrial Press, Beijing, 1995.

9. Broms, B. B. Progressive Failure of Lime, Lime/Cement, and CementColumns. In Proc., International Conference on Dry Mix Methods forDeep Soil Stabilization, Balkema, Rotterdam, 1999.

10. Liu, S. Y., M. L. Shi, Y. Xu, Y. J. Du, and H. Zhu. New Approaches toEvaluate Deep Mixing Piles in Soft Ground Improvement. In Proc.,International Symposium on Lowland Technology, Saga, Japan, 1998.

Publication of this paper sponsored by Committee on Transportation Earthworks.

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