A new type of cement-soil mixing pile reinforced by basalt fibre is proposed for increasing the bearing capacity of cement-soil mixing piles. This work primarily consists of three parts. First, the process of construction technology is proposed, which could allow uniform mixing of the basalt fibre in cement-soil. Second, the optimal proportions of the compound mixtures and the mechanical properties of the pile material are obtained from unconfined compression strength test, tensile splitting strength test, and triaxial shear test under different conditions. Third, the reliability of the construction technology, optimal proportions, and mechanical properties are verified by testing the mechanical properties of the drilling core sample on site.
Cement-soil mixing piles are widely used for reinforcement and waterproofing curtains in soft soil sites due to the flexible reinforcement form, decreased construction disturbance, and low cost. In foundation pit engineering, the lower horizontal bearing capacity of the pile is lower relative to the vertical bearing capacity, so cement-soil mixing piles are only used for waterproof curtains. The enhancing of horizontal bearing capacity and lateral rigidity would enhance the nondeformability of the soil behind the piles, decrease the deformation that causes upheaval and settlement, and reduce the project cost by decreasing the thickness of the gravity retaining wall and the amount of rigid retaining piles.
Several methods have been studied in regard to the defection of cement-soil mixing piles by adding one or more materials, primarily including soil condensates, mineral admixtures, and additives [
Basalt fibre is a filament drawn from cooled basalt ore that has been melted in a furnace at a temperature of 1450°C~1500°C. Basalt fibre has the same composition as basalt ore and can directly break down to be soils [
A new type of cement-soil mixing pile reinforced by basalt fibre is proposed to enhance the tensile and shear strength and further enhance its lateral rigidity and horizontal bearing capacity. The new pile could be used not only in waterproofing curtain but also in horizontal load-bearing to resist deformation. The following three areas were examined in detail: the optimal ratio of materials to form the cement-soil mixing pile, the method to uniformly mix the basalt fibre in the cement-soil mixing pile, and the engineering effect of cement-soil mixing piles reinforced by basalt fibre.
Cement-soil reinforced by basalt fibre can have an enhanced strength by increasing the contact frictional bond stress between the cement-soil and the basalt fibre. However, insufficient basalt fibre mixed in the cement-soil will have a limited impact on the strength and a cement-soil with excess basalt fibre will not cause significant contact friction for strength enhancing. Therefore, the mixing ratio is a crucial parameter. In this study, an unconfined compressive strength test, a tensile splitting strength test, and a triaxial shear strength test, which are closely related to the horizontal bearing capacity of the mixing pile, were conducted on prepared samples to determine the optimal mixing ratio and optimal fibre length.
Three types of materials, namely, soft soil, cement, and basalt fibre, were prepared to manufacture the samples. Soft soil is an underconsolidated, mucky, silty clay from a beach of Yangzi river in the Jiangyin region, Jiangsu province, China. It has a grey colour, liquid-plastic state, slight lustre, medium shake vibration reaction, low tenacity strength and dry strength, and a medium-high compressibility. The cement used was selected composite Portland (P.C32.5). The basalt fibre was a high-strength basalt fibre (see Figure
The physical and mechanical indices of soft soil.
Index | Natural moisture | Weight | Void ratio | Saturability | Liquid limit | Plastic limit | Plastic index | Statistical results of grain size | ||
---|---|---|---|---|---|---|---|---|---|---|
0.25~0.075 | 0.075~0.005 | <0.005 | ||||||||
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/kN·m−3 |
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% | |||
Values | 36.2 | 18.0 | 1.014 | 96 | 33.8 | 22.9 | 10.9 | 3.7% | 85.5% | 10.8% |
The basic parameters of cement.
Cement | SO3/% | MgO/% | Cl−/% | Initial setting time/min | Final setting time/min | Rupture strength/MPa | Rupture strength/MPa | ||
---|---|---|---|---|---|---|---|---|---|
3 d | 28 d | 3 d | 28 d | ||||||
P.C 32.5 | ≤3.5 | ≤6.0 | ≤0.06 | ≥45 | ≤600 | ≥2.5 | ≥5.5 | ≥10 | ≥32.5 |
The performance parameters of basalt fibre.
Type | Density/(kg/m3) | Tensile strength/MPa | Elastic modulus |
Acid-base resistance property | Melting point/°C | Elongation at break/% |
---|---|---|---|---|---|---|
Bunchy monofilament | 2650 | ≥2000 | 90~110 | ≥99% | 1250 | 3.5 |
Basalt fibre.
The preparation and curing processes were as follows: Dry and smash the mucky silty clay, filter the soil with a 2 mm-bore-diameter filter sieve, and test its moisture content. Mix the soil and basalt fibre, and stir evenly; recover the moisture content of the soil based on the natural moisture content, which was 36% in this study, and seal the mixed fibre soil in a hermetic bag to avoid evaporation for 24 h. Mix the fibre soil and cement paste in which the mass ratio of water and cement is 0.5 and stir evenly. Weigh the mixture sample and divide the weight into four equal parts by weight. Use the sample kit to manufacture the sample and divide the vibratory compaction into four layers using the four divided weights, respectively. The prepared sample can be seen in Figure Cool it for 24 h before forming the stripping, test its mass, and dispose of any sample whose weight is greater than 20% of the average. Number the samples, and place them in the curing room in which the relative humidity is greater than 95% and the temperature is One day before the expired age, remove the sample from the curing room to underwater maintenance for 24 h.
Mixture example with basalt fibre and cement-soil.
An orthogonal test scheme was used considering the factors of the basalt fibre content, basalt fibre length, and mixing amount of cement. The ratios of the test materials are shown in Table
The ratio of test materials.
Number | Cement mixing ratio/% | Water cement ratio | Fibre content/% | Fibre length/mm | Age | ||
---|---|---|---|---|---|---|---|
7 d | 28 d | 90 d | |||||
C18% | 18 | 0.5 | 0 | 0 | 6 | 6 | 6 |
C15% | 15 | 0.5 | 0 | 0 | 6 | 6 | 6 |
B0.2% | 15 | 0.5 | 0.2 | 12 | 6 | 6 | 6 |
B0.4% | 15 | 0.5 | 0.4 | 12 | 6 | 6 | 6 |
B0.6% | 15 | 0.5 | 0.6 | 12 | 6 | 6 | 6 |
B6 mm | 15 | 0.5 | 0.4 | 6 | 6 | 6 | 6 |
B24 mm | 15 | 0.5 | 0.4 | 24 | 6 | 6 | 6 |
C is the abbreviation of cement; B is the abbreviation of basalt fibre.
The mixing pile in a horizontal loading would suffer unidirectional compressive stress. An unconfined compressive strength test was conducted to evaluate the compression resistance of the cement-soil. The test method refers to the assay method of concrete. In the process of the test, the pressure should be uniform, the sample should be kept on the centre axis of the pressure plate, and the head face should be parallel to the pressure-bearing surface and the top loading board. The testing machine was acquired from Bert Industry & Trade Co., LTD, in Jinan city, China. The loading rate was 8 mm/min, and the test was stopped when the stress reached a stage of rapid decreases.
In the test, if the compressive strength of the 6 samples in one group is not larger than 15% of the average value, the average value is taken as the compressive strength. If the strength value is larger than 20% of the average, the average value of the middle four samples is taken as the compressive strength. If the compressive strength of the middle four samples is also larger than 20% of average, the test of this group is considered to be invalid.
Unconfined compressive strength can be obtained from
While bearing a horizontal loading, the pile would suffer from tensile stress. A tensile splitting strength test was conducted to calculate the tensile strength of the samples. Before the test, the position of the splitting line should be determined, and the split line should be consistent with the centre line at the bottom of the pressure plate. When the pressure attenuates quickly after the peak value, the test is ended, and the peak pressure as the failure load is recorded. In the test, the loading rate was 8 mm/min, and the precision of strength can reach 0.01 kN.
The tensile strength can be obtained from
While bearing a horizontal loading, the pile would suffer from shear stress. A triaxial shear strength test was conducted to determine the strength of the pile. The test using the UU shear test refers to the shear strength test of the concrete. In the test, the American GCTS static dynamic triaxial test apparatus was used with an HCA-300 static triaxial test module. The axial strain rate was controlled at 1%/min and the axial strain value was recorded every 0.5% of axial strain. The test concluded when the axial strain reached 15%. Four types of confining pressures were taken at 100 kPa, 200 kPa, 300 kPa, and 400 kPa.
Figure
Failure modes of the unconfined compression test and tensile splitting test.
Unconfined compression
Tensile splitting
The unconfined compressive strengths of plain cement-soil and cement-soil reinforced by basalt fibre of three different ages and fibre content are shown in Table
The unconfined compressive strength.
Sample | 7 d compressive strength/MPa | Variable coefficient | 28 d compressive strength/MPa | Variable coefficient | 90 d compressive strength/MPa | Variable coefficient |
---|---|---|---|---|---|---|
C18% | 1.08 | 0.01 | 1.52 | 0.04 | 2.28 | 0.07 |
C15% | 1.03 | 0.03 | 1.29 | 0.05 | 1.90 | 0.02 |
B0.2% | 1.08 | 0.04 | 1.58 | 0.09 | 2.31 | 0.01 |
B0.4% | 1.16 | 0.03 | 1.67 | 0.03 | 2.43 | 0.07 |
B0.6% | 1.14 | 0.01 | 1.62 | 0.01 | 2.38 | 0.01 |
B6 mm | 1.12 | 0.08 | 1.54 | 0.03 | 2.40 | 0.04 |
B24 mm | 1.09 | 0.01 | 1.51 | 0.07 | 2.18 | 0.07 |
The relations of fibre content and strength.
The relationship between the basalt fibre length and the unconfined compressive strength can be determined from Figure
The relations of fibre length and strength.
Figure
The 3D envelope surface of the compressive strength and tensile strength.
Unconfined compression
Tensile splitting
Figure
The tensile strengths of the plain cement-soil and cement-soil reinforced by basalt fibre are shown in Table
The tensile strength of basalt fibre samples.
Sample | 7 d tensile strength/MPa | Variable coefficient | 28 d tensile strength/MPa | Variable coefficient | 90 d tensile strength/MPa | Variable coefficient |
---|---|---|---|---|---|---|
C18% | 0.24 | 0.03 | 0.36 | 0.03 | 0.60 | 0.08 |
C15% | 0.20 | 0.01 | 0.28 | 0.03 | 0.45 | 0.05 |
B0.2% | 0.23 | 0.01 | 0.34 | 0.09 | 0.58 | 0.01 |
B0.4% | 0.29 | 0.04 | 0.42 | 0.08 | 0.66 | 0.05 |
B0.6% | 0.25 | 0.07 | 0.39 | 0.02 | 0.59 | 0.03 |
B6 mm | 0.27 | 0.02 | 0.37 | 0.01 | 0.56 | 0.05 |
B24 mm | 0.25 | 0.08 | 0.38 | 0.05 | 0.54 | 0.07 |
The relations of fibre content and strength.
The relationship between the basalt fibre length and the tensile strength can be determined from Figure
The relations of fibre length and strength.
Figure
The stress-strain curves of the plain cement-soil samples and cement-soil samples reinforced by basalt fibre under different confining pressures are shown for the specific case of the 28 d samples and are shown in Figure
Related parameters of shear stress and shear strain after 28 days.
Confining pressure/kPa | Parameters | C15% | B0.2% | B0.4% | B0.6% | B6 mm | B24 mm |
---|---|---|---|---|---|---|---|
100 | Failure strength/MPa | 2.55 | 2.95 | 3.20 | 3.01 | 2.71 | 3.07 |
Failure strain/% | 1.55 | 2.32 | 3.22 | 5.72 | 1.46 | 1.87 | |
|
0.28 | 1.69 | 1.67 | 1.82 | 0.82 | 0.93 | |
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200 | Failure strength/MPa | 2.78 | 3.15 | 3.42 | 3.27 | 2.97 | 3.23 |
Failure strain/% | 2.40 | 2.91 | 3.14 | 3.19 | 5.40 | 1.52 | |
|
0.61 | 1.19 | 1.20 | 1.16 | 0.82 | 0.51 | |
|
|||||||
300 | Failure strength/MPa | 2.82 | 3.30 | 3.63 | 3.39 | 3.13 | 3.44 |
Failure strain/% | 1.86 | 4.57 | 4.63 | 3.43 | 2.88 | 3.92 | |
|
0.74 | 1.2 | 1.62 | 1.15 | 0.54 | 0.30 | |
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400 | Failure strength/MPa | 3.02 | 3.51 | 4.07 | 3.65 | 3.47 | 3.74 |
Failure strain/% | 1.77 | 4.84 | 3.90 | 2.50 | 5.13 | 5.55 | |
|
0.55 | 0.90 | 1.39 | 1.04 | 1.97 | 2.40 |
The curves of shear stress and shear strain after curing for 28 days.
Curing age 28 d, confining pressure 100 kPa
Curing age 28 d, confining pressure 200 kPa
Curing age 28 d, confining pressure 300 kPa
Curing age 28 d, confining pressure 400 kPa
The shear strengths of the samples for different curing ages and different confining stresses are shown in Table
The data of the shear strength test.
Age/d | Confining pressure/kPa | Shear strength/MPa | ||||||
---|---|---|---|---|---|---|---|---|
C18% | C15% | B0.2% | B0.4% | B0.6% | B6 mm | B24 mm | ||
7 | 100 | 2.23 | 1.89 | 2.19 | 2.45 | 2.26 | 2.35 | 2.05 |
200 | 2.41 | 2.01 | 2.32 | 2.82 | 2.47 | 2.36 | 2.17 | |
300 | 2.72 | 2.16 | 2.76 | 2.98 | 2.70 | 2.54 | 2.64 | |
400 | 3.03 | 2.44 | 3.00 | 3.26 | 2.99 | 2.90 | 2.97 | |
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28 | 100 | 2.98 | 2.55 | 2.95 | 3.20 | 3.01 | 2.71 | 3.07 |
200 | 3.20 | 2.78 | 3.15 | 3.42 | 3.27 | 2.97 | 3.23 | |
300 | 3.34 | 2.82 | 3.30 | 3.63 | 3.39 | 3.13 | 3.44 | |
400 | 3.57 | 3.02 | 3.51 | 4.07 | 3.65 | 3.47 | 3.74 | |
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90 | 100 | 4.31 | 3.48 | 4.39 | 4.82 | 4.61 | 4.44 | 4.38 |
200 | 4.76 | 4.56 | 4.70 | 5.59 | 5.33 | 4.84 | 4.79 | |
300 | 5.01 | 4.92 | 5.04 | 5.85 | 5.74 | 5.37 | 5.32 | |
400 | 5.82 | 5.40 | 5.96 | 6.42 | 6.09 | 5.97 | 5.87 |
Shear strength index of the cement-soil with basalt fibre reinforced for different curing times.
Sample | Strength parameters (cohesive force |
|||||
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7 d | 28 d | 90 d | ||||
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C18% | 633.7 | 24.3 | 823.5 | 26.4 | 711.5 | 36.3 |
C15% | 478.3 | 23.0 | 725.9 | 25.3 | 686.0 | 38.7 |
B0.2% | 644.3 | 24.5 | 822.3 | 26.6 | 954.3 | 40.3 |
B0.4% | 665.8 | 28.2 | 916.3 | 29.8 | 1087.8 | 44.9 |
B0.6% | 628.4 | 25.7 | 773.0 | 28.2 | 993.1 | 42.3 |
B6 mm | 597.6 | 25.2 | 824.9 | 26.7 | 837.9 | 43.1 |
B24 mm | 480.1 | 28.0 | 694.4 | 29.5 | 864.7 | 41.8 |
The relationship between fibre content and sample strength.
The relationship between the basalt fibre length and the shear strength can be observed from Figure
The relationship between fibre length and sample strength.
From the performed tests, the mass component of the materials to fabricate a mixing pile can be formed by basalt fibre 0.4, cement 15, water 7.5, and mucky soil 0.02, with an optimal ratio of basalt fibre content of 0.4% and an optimal length of 12 mm. By mixing the basalt fibre in cement-soil, the unconfined compressive strength increased by 26.3%, the splitting tensile strength increased by 50%, and the shear strength increased by 33%.
In the process of construction technology, only basalt fibre that is uniformly dispersed in the cement-soil mixing pile body can effectively improve the performance regarding the horizontal stiffness and horizontal load, which is also a crucial point in applying the design practice. Direct mixing cannot achieve sufficient uniformity; therefore, yellow sand, gypsum powder, plain fill, and mucky soil as additives were considered to be used in the mixing process to help the basalt fibre distribute evenly in the cement-soil.
The mass of the soil was taken as the reference quantity, and the mixing proportions of the cement and basalt fibre are 15% and 0.4% of the soil, respectively. The mass ratio of water and cement is 0.5, and the fibre length is 12 mm. The following processes were used in the test. As an example, the yellow sand as an additive process is shown in Figure Add 50 kg of a tiled additive to the mixing barrel and add 1 kg of dry basalt fibre using a blower. Mix the additive and basalt fibre under anhydrous conditions. Add 50 kg of cement into the mixing barrel and mix them together. Add 25 kg of water and mix.
Process of mixing with a slurry blender.
Step
Step
Step
Step
A picture showing the direct mixing process is shown in Figure
The mixing effect of the slurry blender with different additives.
Direct mixing
Slurry mixing gypsum powder
Dry mixing adding plain fill
Slurry mixing plain fill
Dry mixing adding mucky soil
Slurry mixing mucky soil
A picture showing yellow sand as the additive is shown in Figure
A picture showing gypsum powder as the additive is shown in Figure
Pictures showing plain fill as the additive are shown in Figures
Pictures showing mucky soil as the additive are shown in Figures
The mass component of the construction technological materials to create a mixing pile can be formed using basalt fibre 0.4, cement 15, water 7.5, and mucky soil 0.02, and the optimum fibre length is 12 mm.
The procedure of the construction technology is shown as follows (Figure Mix the basalt fibre and mucky soil in dry conditions using a rotary mixer. The mass ratio of basalt fibre to mucky soil is 20 : 1. Stir evenly, add the cement paste into the rotary mixer, and stir evenly again. The mass ratio of the water to the cement composing cement paste is 0.5. Locate, centre, and level the pile machine and move it to the designated spot. Centre and level it again. Adjust the perpendicularity of the guide frame using the odolite or plumbing bobs to be less than 1.0% of pile length according to the standard requirement. Premix and sink the blender into the deeper soil. Meanwhile, place the basalt fibre and cement into the aggregate bin. Start the rotary table of the mixing pile machine. When the rotation speed reaches the normal level, sink the drill pipe and stir at the same time. The sinking speed is controlled by gears. When the drill pile has been sunk to the designed depth, open the mortar pump, and when the mortar reaches the pulp mouth, start the mixing pile machine and tighten the chain device. The next procedure is guniting, stirring, and lifting the drill pile; the designed lifting speed is 0.50~0.8 m/min, yielding a fully mixed slurry and soil. When the drill bit up has been lifted to a location that is higher than the top of the pile by 500 mm, stir and sink the drill bit into the designed depth again. Repeat the step (h) until the blender is lifted to the ground.
Site construction of cement-soil pile with basalt fibre.
Step
Step
Step
Step
Site core tests of the cement-soil are conducted on the cement-soil mixing pile reinforced by basalt fibre for 90 d.
The core samples used for the strength test are collected from three random points of the whole pile length (11 m) using a geological drilling rig. In the process, three (double) pipes with a single-acting rotary sampler were used, and the integrity and uniformity of the core samples were checked before the mechanical test as shown in Figure
Core sample of cement-soil on site.
The unconfined compressive strength tests, tensile splitting tests, and triaxial shear tests are conducted on core samples that have been soaked for 2 d. The results are given in Table
Strength of core sample (90 d).
Pile number | Pile depth |
Compressive strength/MPa | Tensile strength |
Shear strength | |
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1 | 0.5~4.0 | 1.73 | 0.47 | 773.9 | 42.2 |
1 | 4.0~7.5 | 1.83 | 0.50 | 819.2 | 43.1 |
1 | 7.5~11.0 | 1.80 | 0.49 | 805.8 | 42.5 |
2 | 0.5~4.0 | 1.75 | 0.48 | 783.4 | 42.3 |
2 | 4.0~7.5 | 1.78 | 0.48 | 796.8 | 42.5 |
2 | 7.5~11.0 | 1.89 | 0.51 | 846.1 | 43.0 |
3 | 0.5~4.0 | 1.75 | 0.48 | 783.4 | 42.3 |
3 | 4.0~7.5 | 1.78 | 0.48 | 795.2 | 42.2 |
3 | 7.5~11.0 | 1.86 | 0.51 | 832.6 | 43.3 |
This study aims to determine the effect of basalt fibre content, fibre length, curing age, and cement content on the cement-soil reinforcement based on the unconfined compressive strength, tensile strength, shear strength, and stress-strain characteristics and triaxial shear strength characteristics.
An appropriate basalt fibre content can effectively improve the shearing properties of cement-soil. The shear strengths of sample B0.4% under four different confining stresses are all larger than those of samples B0.2% and B0.6%, and the failure stress of sample B0.4% remained at a higher level after the peak stress. Obviously, there is an optimal fibre content that produces the highest shear strength, and the residual stress remains at a relatively higher level.
As the fibre length increases, increasing rates of shear strength and cohesive force cause the strength to increase first and then decrease, with a peak value appearing at 12 mm for both. Meanwhile, the fibre length signified no effect on the internal friction angle. Therefore, the length of the basalt fibre has a very important effect on the shearing performance of the cement-soil, and the optimal length is 12 mm.
Based on the triaxial shear tests, improvements in the cement-soil reinforced by basalt fibre are studied, and the following conclusions were drawn. Basalt fibre is an effective reinforcement material for cement-soil, which can effectively enhance its shear strength. The optimal ratios of basalt fibre mixed in the cement-soil obtained through different triaxial tests are consistent; the optimal fibre length is 12 mm, and the optimal mass ratio is 0.4%. Mixing basalt fibre in the cement-soil can effectively increase its cohesive force and internal friction angle. The cohesive force increased by 6.0~33.1%, and the internal friction angle increased by 8.8~16.2%. Using the proposed mix technology, the strength of the pile material collected at the site reached more than 70% of the artificial mixing cement-soil reinforced strength by the basalt fibre in the laboratory test.
In conclusion, the improvements in the tensile properties of the cement-soil, which is reinforced by basalt fibre, can effectively improve the overall mechanical performance. In practice, basalt fibre used in cement-soil mixing piles can enhance its horizontal bearing capacity to satisfy the needs of specific projects.
The authors declare that there are no conflicts of interest regarding the publication of this paper.