Research of Characteristics of Deep Cement Mixing Columns in Treatment of Soft Soil

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Research of Characteristics of Deep Cement Mixing Columns in Treatment of Soft Soil

Tuan Anh Nguyen, Lecturer, PhD.

Ho Chi Minh City University of Transport 2 Vo Oanh Street, Binh Thanh District, Ho Chi Minh City, Viet Nam.

Dat Thanh Nguyen

Ho Chi Minh City University of Transport 2 Vo Oanh Street, Binh Thanh District, Ho Chi Minh City, Viet Nam.

Abstract – The article presents the content assessment of the application of deep cement mixing technology. From laboratory and field experiments with geological conditions in Duyen Hai – Tra Vinh areas, Vietnam, we will find out the factors that affect the quality and durability of the deep cement mixing column. At the same time, we can determine the optimal ratio about content of cement and water for soil samples after being reinforced to meet the economic – technical requirements. Finally, test data is analysed to serve calculations and simulation based on linear regression models of Microsoft Excel.

Keywords- Deep cement mixing columns, soft soil, optimal ratio of water and cement, linear regression models.

  1. INTRODUCTION

    Deep cement mixing columns (DCMCs) are the soil at the construction site and the cement is grouted to the ground by the injection grouting pump. The drill bit is drilled down for loosening the soil until it reaches the depth of the soil layer, that needs to be reinforced, then it comes back and moves up. In the process of moving up, cement is grouted into the ground. This is a new technology applied in flooded areas where other types of columns do not meet the requirements.

    DCMCs are widely applied in the treatment of foundation and soft ground for public construction works such as bridges, ports, embankments, repairing the leaking for the sewer sides and sewer bottoms, retaining walls, reinforcing the soil around the tunnel, preventing landslide of the slope, strengthening the roadbed, bridge abutments, etc. Especially in the Mekong Delta, DCMCs are frequently used because this area is often flooded due to high tides and climate change.

    With specific advantages in soft ground treatment, the technology of deep cement mixing column is widely used to reinforce the ground, control the subsidence when the work is put into use. The calculation for design of the ground reinforced by the method of DCMCs is based on many different points of view and assumptions.

    However, based on the specific conditions of soft soil, topography, geological conditions, construction methods, working conditions of DCMCs and experience, the most appropriate calculation method is chosen.

    In addition, the theory of calculating DCMCs is quite limited, when parameters of durability of the pile are used in calculation, they need to be experimented with each specific soil and construction.

  2. RESEARCH METHODS

    Empirical method: Performing experiments in the laboratory and on the site to determine the specific characteristics and factors affecting the durability and quality of DCMCs.

    Theoretical method: Summarizing and selecting experimental data based on linear regression model.

  3. TESTING METHODS FOR DETERMINING CHARACTERISTICS OF DCMCs

  1. Parameters of soil and materials used for the experiment

    Samples of soil ground used in the experiment were taken in the field of Duyen Hai Thermal Power Plant, Duyen Hai District, Tra Vinh Province, Vietnam which all have the same characteristics of soft grey brown clay silt. Samples are taken in the field and carefully stored to ensure the natural criteria for the experiment. Parameters of ground soil and materials used for the experiment are synthesized in Tables I-IV.

    TABLE I. PHYSICAL-MECHANICAL PARAMETERS OF SOIL

    Parameter

    Unit

    The average value

    Moisture W

    %

    41.6

    Unit weight

    kN/m3

    17.36

    Void ratio e

    1.123

    Cohesive force C

    kN/m2

    7.4

    Angle of internal friction

    Degree

    1.74

    Plastic limit WP

    %

    22.92

    Liquid limit WL

    %

    39.99

    Plasticity index IP

    %

    17.07

    Liquidity index IL

    1.1

    Parameter

    Unit

    Standard (TCVN)

    Limit

    Sample test results

    Specific weight

    kN/m3

    4030-2003

    29.6

    Volumetric mass

    kN/m3

    1772-87

    1.21

    Standard consistency

    %

    6017-1995

    26

    Time to start setting

    minute

    6017-1995

    > 45

    115

    Time to end setting

    minute

    6017-1995

    < 420

    320

    Volumetric stability

    mm

    6017-1995

    < 10

    2.33

    Fineness of grinding of the remainder on the 0.09mm sieve

    %

    4030-2003

    < 10

    1.5

    Bending strength at 28 days

    kN/m2

    6017-1995

    9.75

    Compressive strength at 28 days

    kN/m2

    6017-1995

    40

    42.9

    Parameter

    Unit

    Standard (TCVN)

    Limit

    Sample test results

    Specific weight

    kN/m3

    4030-2003

    29.6

    Volumetric mass

    kN/m3

    1772-87

    1.21

    Standard consistency

    %

    6017-1995

    26

    Time to start setting

    minute

    6017-1995

    > 45

    115

    Time to end setting

    minute

    6017-1995

    < 420

    320

    Volumetric stability

    mm

    6017-1995

    < 10

    2.33

    Fineness of grinding of the remainder on the 0.09mm sieve

    %

    4030-2003

    < 10

    1.5

    Bending strength at 28 days

    kN/m2

    6017-1995

    9.75

    Compressive strength at 28 days

    kN/m2

    6017-1995

    40

    42.9

    TABLE II. PHYSICAL-MECHANICAL PARAMETERS OF CEMENT

    TABLE III. PHYSICAL-MECHANICAL PARAMETERS OF DOMESTIC WATER USED FOR SAMPLE PREPARATION

    Determination parameter

    Unit

    Water used to prepare the samples

    Result

    TCVN 302-

    2004

    Color

    Level

    Colorless

    Colorless

    Greasy scum

    Level

    No scum

    No scum

    pH

    Degree

    7.4

    4÷12.5

    Total amount of dissolved salt

    mg/L

    9.2

    200

    Content of (SO4)2-

    mg/L

    23.5

    600

    Content of Cl-

    mg/L

    52.6

    350

    Total amount of suspended solid (SS)

    mg/L

    25

    200

    TABLE IV. PHYSICAL-MECHANICAL PARAMETERS OF WATER AT THE SAMPLING LOCATION USED FOR SAMPLE PREPARATION

    Parameter

    Unit

    Water at the sampling location

    Amount

    According to TCXD 3994-

    1985

    Temperature

    0C

    28.125

    pH

    7.885

    Clearness

    Sensing

    Turbid

    Odor

    Sensing

    Non

    Free CO2

    mg/L

    12.194

    No corrosion

    Corrosive CO2

    mg/L

    0

    Na+ + K+

    mg/L

    9109.644

    Ca2+

    mg/L

    285.82

    Mg2+

    mg/L

    863.208

    No corrosion

    Cl-

    mg/L

    15575.86

    (SO4)2-

    mg/L

    1772.608

    Strong corrosion

    (HCO3)-

    mg/L

    318.066

    (CO3)2-

    mg/L

    0.75

    Total hardness

    mg/L

    85.25

    Total mineralization

    mg/L

    27927.95

    Medium corrosion

  2. Uniaxial unconfined compression test

    1. Experimental results for mixing samples using domestic water with a ratio of water / cement respectively as 1.4; 1.0; 0.8

      1. Experiment 1: Water/Cement =1.4

        – Based on Fig. 1, we can see that the value of qu (kPa) from

        7 to 14 days of age increases rapidly by about 70 times compared to the original soil without reinforcement, then it tends to increase slowly until 28 days of age.

        • The intensity of deep cement mixing sample is proportional to the amount of cement in preparation. But when the amount of cement quickly grows from 14% to 20%, the intensity of cement tends to increase slowly. (Table V)

          TABLE V. UNIAXIAL COMPRESSIVE STRENGTH QU FOR MIXING SAMPLE USING DOMESTIC WATER WITH THE RATIO OF W/C =1.4

          Day

          Uniaxial compressive strength qu (kPa)

          M100 (6%)

          M150 (9%)

          M200 (12%)

          M250 (14%)

          M300 (17%)

          M350 (20%)

          0

          15

          15

          15

          15

          15

          15

          7

          218.3

          533.3

          687.3

          790.2

          914.3

          1206.7

          14

          293.4

          702.6

          1068.1

          1226.8

          1376.3

          1609.1

          28

          322.2

          943.9

          1339.9

          1687.7

          1779.5

          1960.4

          Fig. 1. Uniaxial compressive strength of experiment 1

      2. Experiment 2: Water/Cement =1.0

        Uniaxial compressive strength qu for mixing sample using domestic water with the ratio of W/C=1 is synthesized in Table VI.

        TABLE VI. UNIAXIAL COMPRESSIVE STRENGTH QU FOR MIXING SAMPLE USING DOMESTIC WATER WITH THE RATIO OF W/C=1

        Day

        Uniaxial compressive strength qu (kPa)

        M100 (6%)

        M150 (9%)

        M200 (12%)

        M250 (14%)

        M300 (17%)

        M350 (20%)

        0

        15

        15

        15

        15

        15

        15

        7

        284

        569.7

        982.4

        1097.9

        1447.3

        1818.5

        14

        421.5

        957

        1394.5

        1618.2

        1959.2

        2360.4

        28

        554.1

        1365

        2076.7

        2247

        2736.5

        3389.8

        Fig. 2. Uniaxial compressive strength of experiment 2

        The result in Fig.2 indicates that the value of qu (kPa) in this experiment increases more strongly compared to that of experiment 1, specifically at 28 days of age the value of qu (kPa) increases by about 137 times in comparison with that at 28 days of age after being reinforced. The main reason is due to the change of the water amount when mixing sample. Compared with experiment 1, in this experiment, when the water amount decreases by 40% when mixing, the value of qu (kPa) of the sample will increase by 55.24% at 28 days of age.

      3. Experiment 3: Water/Cement = 0.8

      Uniaxial compressive strength qu for mixing sample using domestic water with the ratio of W/C= 0.8 is synthesized in Table VII.

      TABLE VII. UNIAXIAL COMPRESSIVE STRENGTH QU FOR MIXING SAMPLE USING DOMESTIC WATER WITH THE RATIO OF W/C=0.8

      Day

      Uniaxial compressive strength qu (kPa)

      M100 (6%)

      M150 (9%)

      M200 (12%)

      M250 (14%)

      M300 (17%)

      M350 (20%)

      0

      15

      15

      15

      15

      15

      15

      7

      394.592

      777.7

      1518.9

      1725.4

      2107.4

      285.1

      14

      605.706

      1313.3

      2381

      2582.07

      2974.3

      4099.6

      28

      838.157

      1720.1

      2644.3

      3493.06

      4448.9

      25866.3

      Based on Fig. 3, it is said that the value of qu (kPa) of deep cement mixing sample in this experiment increases more significantly than those of experiments 1 and 2. The uniaxial compressive strength of natural soil increases about 211 times at 28 days of age after being reinforced. Similar to the first two experiments, when we perform the experiment of changing the ratio of water/ cement on preparing the sample, it will significantly change the value of qu (kPa) of DCMCs.

      Fig. 3. Uniaxial compressive strength of experiment 3

    2. Experimental results for mixing samples using water at the sampling location with a ratio of water/cement = 1.0

      Experiment 4: W/C=1 (water at the sampling location)

      TABLE VIII. UNIAXIAL COMPRESSIVE STRENGTH QU FOR MIXING SAMPLE USING WATER AT THE SAMPLING LOCATION WITH THE RATIO OF W/C=1

      Day

      Uniaxial compressive strength qu (kPa)

      M100 (6%)

      M150 (9%)

      M200 (12%)

      M250 (14%)

      M300 (17%)

      M350 (20%)

      0

      15

      15

      15

      15

      15

      15

      7

      159.992

      259.296

      505.251

      562.143

      903.598

      1091.86

      14

      216.238

      400.343

      871.621

      1509.04

      1645.84

      1896.05

      28

      304.228

      574.236

      1271.21

      1613.32

      2121.56

      2432.66

      • Compared to experiments 1, 2, 3 (W/C = 1.4; 1; 0.8), in this experiment 4, based on Fig. 4, we find that the value of qu (kPa) at 28 days of age is quite low, it declines about 36.79% compared to experiment 1 (W/C = 1), 57.82% compared to experiment 2 (W/C = 0.8) and 1.83% lower than experiment 3 with a W/C ratio of 1.4.

      The main reason is that the water for mixing sample has too high acid content which is shown in [Table IV] then, it facilitates the process of chloride corrosion and sulfate attacking, resulting in the formation of ettringite (sulfoaluminatehydrate: 3CaO.Al2O3.3CaSO4.32H2O) and gypsum (CaSO4.2H2O) softening of cement paste, changing the microstructure to increase porosity and reduce the strength of the deep cement mixing pile.

      Fig. 4. Uniaxial compressive strength of experiment 4

  3. Comparison of laboratory experimental results

    Based on Fig. 5, we see that experiments with the same type of water [Table III] and cement but with different mixing rates, the results are quite different. Water quality has a great influence on the strength and quality of DCMCs, so it is impossible to reinforce the soft soil in this area by dry mixing technology.

    Fig. 5. Uniaxial compressive strength qu of 4 experiments in laboratory

  4. Comparison with field experimental results

    Based on the results of 4 experiments in laboratory, the ratio W/C = 0.8 and the cement content of 14% will be the optimal content for mass construction. To verify the feasibility of this option, the comparison with the field results is implemented as Fig. 6.

    Fig. 6. Comparision of the value of qu (kPa) between lab and field experiments

    When the cement content is 14%, we see that the result of compressing samples in the field gives higher values than those of experiments in laboratory when the ratio of W/C = 1.4 and W/C = 1. With this content, the value of qu (kPa) of the field results is quite high but about 35.5% lower than that in the laboratory because the condition of sample preparation in the laboratory is almost ideal. Results of compression in the field also depend on: Modernity and accuracy of the mechanical system, geological conditions around the reinforced area, construction techniques, etc.

  5. Direct shear test

    Using the optimal content to determine 02 basic characteristics of shear resistance as Table IX.

    TABLE IX. ANGLE OF INTERNAL FRICTION VALUE FROM DIRECT SHEAR TEST

    Day

    Angle of internal friction (degree)

    6%

    9%

    12%

    14%

    17%

    20%

    0

    1.74

    1.74

    1.74

    1.74

    1.74

    1.74

    7

    21.8

    20.22

    11.73

    14.01

    14.44

    15.63

    14

    20.53

    17.34

    14.76

    12.98

    12.05

    10.86

    28

    17.13

    14.63

    12.34

    8.61

    7.21

    11.09

    Day

    Angle of internal friction (degree)

    6%

    9%

    12%

    14%

    17%

    20%

    0

    7.4

    7.4

    7.4

    7.4

    7.4

    7.4

    7

    49.06

    75.145

    228.31

    239.41

    241.51

    267.05

    14

    93.218

    154.435

    267.506

    297.678

    325.509

    339.524

    28

    116.89

    208.979

    316.207

    448.162

    450.336

    457.167

    Day

    Angle of internal friction (degree)

    6%

    9%

    12%

    14%

    17%

    20%

    0

    7.4

    7.4

    7.4

    7.4

    7.4

    7.4

    7

    49.06

    75.145

    228.31

    239.41

    241.51

    267.05

    14

    93.218

    154.435

    267.506

    297.678

    325.509

    339.524

    28

    116.89

    208.979

    316.207

    448.162

    450.336

    457.167

    Fig. 7. Experimental results and relation between (degree) and time TABLE X. COHESIVE FORCE C FROM DIRECT SHEAR TEST

    Fig. 8. Experimental results and relation between C (kPa) and time

    Fig. 8 indicates that: At the time of 28 days of age, the cohesive force value of the sample at the rate of 6-12% is relatively small, but for the ratio of 14-20%, the value is nearly converged and it is not much different, with the average of about 451.89 (kPa). This also confirms that the optimal content selected for mass construction is reasonable, meeting economic

    • technical requirements when investment of the work. The correlation of (degree) and C (kPa) at 28 days of age is quite close with coefficient R2 = 0.8753 and they are inversely proportional when the content of cement increases.

      Fig. 9. Relation between (degree) and C (kPa) at 28 days

  6. Analyzing experimental data, select results according to linear regression model

Determination of value of qu (kPa) and summary of results of experiment 3 at 28 days of age are shown in table XI.

TABEL XI. SUMMARY OF RESULTS OF UNIAXIAL COMPRESSION TEST

The regression equation is as equation (1).

-(34.346 X1) – (3550.21 X2) + (116.92 X3) – (526.02 X4) +

(5531.98 X5) – (188.01 X6) + (325201.76 X7) + (237.94 X8) –

(181.18 X9) – (32.86 X10) + (1.14 X11) + (0.6 X12) + 58687.13 =

qui (kPa) (1)

Value of qu = 3491.90 (kPa) is found through the regression equation with the cement content of 14% (W/C=0.8).

With R2= 0.99996.

As table XII, the regression equation is as follows:

-(68.32 X1) + (0 X2) + (0 X3) + (0 X4) + (4132.37 X5) +

(81.75 X6) – (449.7 X7) + (12.39 X8) + (6.94 X9) – 57.3 = i (kPa)

(2)

The value of internal friction angle (degree) and cohesive force C (kPa) found through the regression equation with the

cement content of 14% (W/C=0.8) are = 8.80 (degree) and C= 437.10 (kPa), respectively. With R2= 0.995995.

TABLE XII. SUMMARY OF RESULTS OF DIRECT SHEAR TEST

Comment: Selecting data using a linear regression moel find that the typical values of DCMCs are 0.03% lower than the average value, at the same time, correlation coefficient R value is very high, approximately equal to 1, indicating that the relationship of the quantities and results is very close, the error during the experiment is low.

4. CONCLUSIONS

When reinforcing soft soil ground in coastal area of Vietnam, in the Southwest region in general and in Duyen Hai

  • Tra Vinh area in particular by deep cement mixing technology, it should be chosen with the ratio of W/C = 0.6 – 0.8, cement content of 14 – 16% (equivalent to 250 – 270 kg cement/m3 of natural soil), this will significantly improve the bearing capacity of soil up to hundreds of times. With the optimal ratio above, the compression result in the field gives quite high results, controlling the product quality and furthermore, ensuring economic – technical requirements.

The content and quality of water when mixing samples is one of the main factors that determine the quality and durability of deep cement mixing pillar.

Using a linear regression model based on Data Analysis application of Microsoft Excel software is very useful. This tool helps us re-test the experiment process, limit errors and at the same time increase the reliability of scientific products.

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