The reinforcement treatment for embankment using the fly ash deposits as the filling material for roadbed was clarified in this paper. Studies have shown that fly ash can be used as the filling material for embankment, but the subgrade bearing capacity from the original fly ash deposits cannot meet the requirements for operating. Fly ash has a good condition to run the dynamic consolidation for meeting the requirements of embankment compaction. The modulus of resilience and the California bearing ratio (CBR) of fly ash is close to that of general filling material for embankment. Fly ash also has the engineering properties of high void ratio and low cohesion. The maximum level of compaction of the fly ash deposits can be 93% and the bear capacity can be about twice over before after the treatment.
Fly ash is generated during the combustion of coal for energy production with the characteristics of large amount of emissions and wide distribution. Fly ash is generally grey in color and powdery in physical character. However, in China, only 20% of fly ash is utilized as a component of building materials or concrete. The large amount of fly ash that has not been used seriously pollutes the environment. How to make waste fly ash profitable is a subject worth studying.
Ahmaruzzaman [
In China, with the fast development of highways, a large number of high embankments have been constructed [
The research work in this paper was supported by the reinforcement project from the fly ash deposits by means of the laboratory tests and the outdoor tests. The engineering properties of fly ash were captured, and the reinforcement technology of the embankment construction based on fly ash deposits on expressway was analyzed.
The area, where the expressway passes is flat, lies in the alluvial plain between the Mount Tai and the lower Yellow River. The surface water in the area mainly comes from surface runoff, ditch, and irrigation. The surface of the area belongs to quaternary loose overburden with a shallow groundwater level; the buried depth in the south of the Yellow River is about 1.5 m to 15 m, about 0.6 m to 7 m in the north. The landform in this area is the typical alluvial plain of the Yellow River, soil layers mainly consist of alluvium, in which sand and clay also exist. The surface is largely composed of clayey silt, mild clay, clay, and fine sand.
K22 + 947∼K23 + 420 belongs to the discharge site of fly ash in the fossil-fuel power station, covering 29584 m2 and 1.85 × 105 m3 in the storing amount, as shown in Figure
The approximate location of K22 + 97∼K23 + 420.
The engineering properties of fly ash and bearing capacity of natural foundation need to be determined, to better decide the reinforcement technology aiming at the fly ash deposits. In this paper, laboratory geotechnical test, microcosmic test on fly ash, field loading test, and cone penetration test (CPT) were carried out for the fly ash distributed in the K22 + 947∼K23 + 420 section, and the field bearing capacity was comprehensively evaluated.
Samples of undisturbed and disturbed fly ashes were used in the laboratory test in which all the samples were acquired by the thin wall sampler in the fly ash deposits. The laboratory test is carried out in accordance with the
Laboratory test results of fly ash.
Original sample number | Sample depth (m) | Moisture content (%) | Wet density (g/cm3) | Dry density (g/cm3) | Proportion | Initial void ratio | Compression modulus |
Permeability coefficient |
Saturated quick shear test | |
---|---|---|---|---|---|---|---|---|---|---|
Cohesion (kPa) | Internal friction angle (°) | |||||||||
A1-1 | 1.0∼1.3 | 33.4 | 1.38 | 1.03 | 2.62 | 1.562 | 19.58 | 2.38 |
12.0 | 27.8 |
A1-2 | 1.7∼2.0 | 42.4 | 1.31 | 0.92 | 2.62 | 1.881 | 8.89 | — | 35.0 | 26.3 |
A2-1 | 1.0∼1.3 | 48.0 | 1.54 | 1.03 | 2.62 | 1.547 | 11.44 | — | 14.0 | 24.6 |
A2-2 | 1.7∼2.0 | 53.7 | 1.40 | 0.91 | 2.62 | 1.909 | 6.53 | — | 1.0 | 28.6 |
A2-3 | 3.0∼3.3 | 61.7 | 1.41 | 0.87 | 2.62 | 2.039 | 8.12 | 9.32 |
0.0 | 30.5 |
Laboratory test results of fly ash.
Maximum dry density (g/cm3) | Optimal moisture content (%) | Test of CBR | Liquid limit |
Plastic limit |
Plastic index |
Modulus of resilience (MPa) | |||
---|---|---|---|---|---|---|---|---|---|
Compactness (%) | Dry density (g/cm3) | CBR2.5 (%) |
|
|
|||||
1.19 | 29.3 | 90 | 1.071 | 6.9 | 49.8 | 30.1 | 19.7 |
|
51.56 |
93 | 1.107 | 8.3 |
| ||||||
95 | 1.131 | 8.7 |
| ||||||
97 | 1.154 | 9.7 |
|
Results of heavy compaction test on fly ash.
Stress-rebound deformation curve of fly ash at maximum dry density.
Figure
Conclusions can be obtained from Tables The moisture content of fly ash in this testing site is larger than the optimal value, e.g., 29.3%, and increases with increasing depth. The moisture content is above the liquid limit (49.8%) when the depth is more than 3 m. The dry density in the site is about 0.87 g/cm3 to 1.03 g/cm3, which means the compactness of the fly ash is about 73% to 86%. The foundation of the undisturbed fly ash needs to be compacted and reinforced according to the standards on the degree of compaction. The void ratio of fly ash in the site has a relatively large value and increases with increasing depth. The compression modulus of fly ash in the site is relatively low with the compressibility of 0.13 MPa−1 to 0.44 MPa−1, which is medium compressibility, and the middle to lower parts are above the medium compressibility. The cohesive value is relatively low, e.g., 0 kPa to 14 kPa, and the internal friction angle is 24.6° to 30.5°, which means fly ash cannot be directly used in roadbed. Under the compactness value of 90%, 93%, 95% and 97%, CBR2.5 is 6.9%, 8.3%, 8.7% and 9.7%, respectively. The fly ash can meet the requirements of the bearing capacity of the embankment after compacted treatment since the CBR2.5 value is between the general value ranges of the filling material, e.g., 5% to 15%.
The requirements in the technical specifications for design and construction for fly ash embankment were met of the grain composition through the gradation test. The specifications require the ratio of particles with particle size less than 0.075 mm is more than 45% in which the test result was 57.7%.
It can be concluded that the fly ash in the site can be used as the filling material for embankment after processing the reinforcement treatment. Settlement after construction will occur without reinforcement due to high moisture content, low compression modulus, and high discreteness of the fly ash. Fly ash can meet the requirements of the bearing capacity of the embankment after compacted treatment since the modulus of resilience and CBR are close to that in general filling material. Drying time can be shortened because the optimal moisture content is high. Dynamic consolidation is suitable because fly ash has a large void ratio and low cohesion. Partly liquefaction will be caused on the surface of fly ash under dynamic consolidation; then, internal water will be removed through gap inside fly ash.
To clarify the characteristics on the grain composition of fly ash, microstructural analyses of fly ash using scanning electron microscope (SEM) were carried out, as shown in Figure
Microstructure of fly ash. (a) Precision of 500
Figure
The bearing capacity of the field site needs to be clarified before reinforcement to assess the bearing capacity of the field site after reinforcement. For foundation in expressway, plate loading tests are usually used for determining the subgrade bearing capacity. Therefore, four points were settled in the field test, in which, test point 1 was set at K23 + 010, test point 2 was set at K23 + 130, test point 3 was set at K23 + 250, and test point 4 was set at K23 + 370.
The plate loading test was carried out in a test pit, and a specially made 0.6 m × 0.6 m thick steel plate was used as the bearing plate. The slow maintained load test method was adopted in the loading test to observe the displacement of the bearing plate by using the dial indicator. The oil jack was used for pressuring, heavy weight such as sand pocket, and steel ingot was used for loading, as shown in Figure
Arrangement of plate loading test.
The stress-deformation curve of four points using plate loading test is shown in Figure
Stress-deformation curve of plate loading test.
It can be concluded from Figure
The cone penetration test (CPT) on 12 test points was carried out in this paper, and cone resistance
According to the
It can be seen from Figure
Results of CPT.
Treatment methods such as dynamic consolidation and cement injection pile are usually used to improve the bearing capacity of fly ash. K22 + 947∼K23 + 420 is a typical fly ash deposit that cannot be directly used for embankment without reinforcement treatment. The dynamic consolidation method has been widely used in the projects related to foundation treatment due to the following reasons: it can improve the strength and reduce the compressibility of the foundation soil through vibro compaction, dynamic consolidation, and thixotropy effect with low cost and short time [
As stated previously, groundwater level is an important factor affecting the dynamic consolidation method. Generally, precipitation should be carried out once the groundwater level is too high in case to ensure the groundwater level is below the proposed reinforcement depth. The thickness of fly ash and groundwater depth in the field site were determined through drilling before the dynamic consolidation, as shown in Table
Thickness of fly ash and groundwater depth.
Mileage number | Thickness of fly ash (m) | Groundwater depth (m) |
---|---|---|
K23 + 010 | 4.6 | 4.7 |
K23 + 130 | 7.6 | 6.7 |
K23 + 250 | 7.1 | 6.2 |
K23 + 370 | 6.7 | 4.4 |
It can be calculated from Table
The tamping pattern and maximum energy requirements using the dynamic consolidation method can be acquired through the empirical formula of reinforcement depth
Based on equation (
The main tamping energy was adopted as 1000 kN·m in the testing site, and there was 12 hits for one tamping time. The distance between points of the main tamping was 3.5 m × 3.5 m, and the settlement subjected to the tamping was limited to 5 cm for the last two tampings for controlling the quantities of tamping in each point without uplift or destruction. The full tamping energy was adopted as 600 kN·m, and there was 2 tampings for one time. Each tamping printing overlapped the 1/4 diameter of the other printings for the full tamping. Both the tamping patterns were arranged in a plumb blossom shape. After the excess pore water pressure was dissipated, the total settlement subjected to tamping will be clarified. The work site of dynamic consolidation is shown in Figure
Site work of dynamic consolidation.
Certain amount of pressure is generated by pore water in the fly ash layer under the vibration caused by tamping. It is necessary to monitoring the pore water pressure in the tamping area according to the
Vibrating wire piezometers were selected for the monitoring of water pore pressure, in which the primary precision was 0.1% to 0.2%F·S, and the range was 0.2 MPa to 2.0 MPa. Three monitor stations were set at the area without tamping, three monitor points in each station and one piezometer in each point. 12 piezometers were used in the test; the detailed information of the layout of piezometers is shown in Figure
Layout of piezometers.
Embedment of piezometers.
The average value of the measured value after the embedment and 24 hours before the construction was taken as the reference value of the piezometers. After the embedment of piezometers was finished, the frequency of reading the piezometers should be 3 times per day in order to make sure the status and the reference value is under control. Meanwhile, the frequency of reading times can be reduced to one or two per day when the surrounding environment is stable. The water pore pressure before and after the dynamic consolidation is shown in Figure
Pore pressure of fly ash area before and after dynamic consolidation.
Figure
The round plate with diameter of 60 cm was adopted in the plate loading test after the tamping, for the purpose to test the bearing capacity of the foundation. The results are shown in Figure
Stress-deformation curve of plate loading test on fly ash after dynamic consolidation.
Based on the plate loading test, Figure
Meanwhile, the cone penetration test (CPT) was carried out between K23 + 020 to K23 + 040, and 9 test holes were set. The layout of the test holes is shown in Figure
Layout of CPT holes after dynamic consolidation.
The results of CPT are shown in Figure
Results of CPT on fly ash after dynamic consolidation.
Results of CPT on fly ash after dynamic consolidation.
Status | Hole |
|
|
Thickness (m) | Bearing capacity (kPa) |
---|---|---|---|---|---|
After tamping | 0 | 3.875 | 135.9 | 6.2 | 184.1 |
1 | 4.282 | 144.8 | 6.4 | 198.8 | |
2 | 4.291 | 147.5 | 6.3 | 199.0 | |
3 | 4.017 | 136.2 | 6.2 | 189.2 | |
4 | 3.978 | 136.1 | 6.4 | 187.8 | |
5 | 4.215 | 135.5 | 6.3 | 196.3 | |
6 | 4.054 | 147.6 | 6.3 | 190.5 | |
|
|||||
Native | 7 | 2.528 | 81.9 | 7.4 | 135.6 |
8 | 2.839 | 91.7 | 7.2 | 146.8 |
Table
This paper presents a reinforcement treatment for embankment using the fly ash deposits as filling material for roadbed. Based on the technical specifications for design and construction for fly ash embankment, fly ash is qualified to be used as the filling material for embankment but lack of certain reinforcement treatment. According to the physical properties of fly ash, dynamic consolidation is put forward as the reinforcement treatment in the field test. The bearing capacity after the dynamic consolidation of the testing area is obviously increased in values, which means dynamic consolidation is suitable for the fly ash deposits in the testing area.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was financially supported by the Shandong Luqiao Group Co., Ltd, Science and Technology Project of Transportation in Shandong Province (2017027b).