In order to reveal water content influence on shear strength, swelling, and creep properties of red-layers in Guangzhou Metro, Southern China, the typical red-layers rock and soil specimens were experimentally studied by direct shear test, UU triaxial test, swelling test, and creep test, and the measured data were analyzed. The results showed that soil internal friction angle exponentially decreased with the water content increase and cohesion in accordance with the Gaussian function firstly increased and then decreased with the increase of water content. Expansion rate significantly decreased with the initial water content increase. The red sandstone had very strong isotropic expansion and disintegration properties. The mechanism of water content effect on red-layers properties was water induced microstructures and mineral compositions change which caused the macro physical and mechanical characteristics degradation. The results should provide the reference for further research for water induced damage mechanism or creep damage control of red-layers in engineering practice.
In the southeast of China, there are widely distributed red-layers, for example, in Guangzhou city, Guangdong province. The red-layers are rich in minerals such as expansive clay minerals, soluble minerals, and organic minerals, which lead the special engineering geological properties of red-layers in comparison with common clay; for example, the expansion and strong viscosity properties, caused by engineering construction disturbance and water intrusion induced microstructures damage or/and mineral compositions change of red-layers, are the special features of red-layers which are responsible for many engineering problems; for instance, when Guangzhou Metro Lines were excavated in the red-layer grounds by shield tunnel, the expansion and strong viscosity properties of red-layers could lead to the large squeezed muck wear formation ahead of shield cutters, which would further lead to the cutter torque decrease, the thrust sharply increase, and the excavation speed significantly decrease; once the shield screw conveyor was failure to dump the squeezed muck in time, the increase of the shield face pressure would lead to severe spewing collapse. Such accidents have occurred three times in the Guangzhou Metro No. 2 Line tunnel construction under the Pearl River [
The physical and mechanical properties of red-layers, including the mudstone, argillaceous silty sand rock, silty sand, and fully or strongly weathered granite, are largely affected by water content. In general, the red-layers have the favorable engineering properties in natural state, but the water content change and construction disturbance could lead to significant difference in the strength and deformability of red-layers, which would present serious security and stability problems for those regions of engineering construction especially for Guangzhou Metro Line construction [
The physical and mechanical properties of red-layers with different water contents have been widely studied. Zhang et al. [
However, limited studies have been conducted on the mechanism of water content influence on the red-layers properties. In most studies, only water content influence on the physical or mechanical features of red-layers specimens was considered, but the mineral compositions and microstructures change was ignored; some studies have showed that the water inside the samples did effect the mineral compositions and microstructures of red-layer soft rock, which yielded macroscopic degradation of mechanical parameters (Liu et al. 2009; [
In this paper, a series of experimental tests were conducted on red-layers specimens with different water contents. The specific details of experimental tests were listed as follows.
(1) The specimens of different typical red-layers, including red clay, moderately weathered red clay, moderately to strongly weathered red clay, sandstone, red sandstone, weathered granite, and shield tunnel excavated soil, with intact or reconstituted state, were described and prepared.
(2) Static tests were carried out on typical red-layers rock and soil specimens with different water contents by direct shear test, UU triaxial test, and unconfined swelling test to determine its mechanical properties, and the measured data were analyzed.
(3) The X-ray diffraction (XRD) was used to determine the mineral compositions and the scanning electron microscope (SEM) was adopted to explore the microstructures of typical red-layers rock and soil specimens, respectively. And the collected photos were analyzed to reveal the mechanism of water content influence on the red-layers properties.
The typical red-layers specimens were sampled at Tiyu West Road Station, Zhujiang New Town Station, Wushan Road Station, Kecun Road Station, and Tianhe Road Station of Guangzhou Metro Line, separately. The details of specimens were shown in Table
Main test items.
Test method | Specimen type | Sampling site | Sampling depth | Test groups | Number of specimens | Lithology description |
---|---|---|---|---|---|---|
Fast shear test | Remolded soil | Zhujiang New Town Station | 12 m |
8 | 32 | Red clay |
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Triaxial test | Remolded soil | Zhujiang New Town Station | 12 m |
10 | 30 | Red clay |
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Expansion test | Undisturbed and disturbed rock | Wushan Road Station | 12 m | 1 | 5 | Red sandstone |
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Creep test | Undisturbed rock | Zhujiang New Town Station | 17.8 m |
1 | 10 | Red sandstone |
Typical borehole rock or soil cores.
The “geotechnical engineering test method standard,” recommended by Ministry of Water Resources of People’s Republic of China, and the “geotechnical engineering technical manual,” recommended by Geotechnical Department of Nanjing Hydraulic Research Institute, were adopted as reference for specimen preparation and experiments.
For direct shear test, the preparation of red-layers specimens is as follows: firstly, the field sampled specimens were completely air-dried and then sieved by 2 mm sieve, and water was added to make remolded soil samples with different water contents. A total of 32 samples, with four specimens as a group, were tested. The shield tunnel excavated soil had high initial water content, that is,
The automatic triaxial test specimens were made with different water contents by the reconstituted Zhujiang New Town Metro Station red-layers, and the Unconsolidated-Undrained (UU) triaxial test was adopted. The specimens were remolded as a cylinder with 39.1 mm in diameter and 80 mm in height with different water contents (Figure
Part reconstituted soil specimens for automatic triaxial shear test.
The typical red sandstone of Wushan Road Metro Station was used for unconfined swelling test. The red sandstone specimens were completely air-dried in natural condition and then were remolded into cylinder with 61.8 mm in diameter and 130 mm in height. Three transverse and three vertical direction strain gauges were glued on the cylinder surface with 120-degree intervals; the layout of strain gauges was shown in Figure
Strain gauges layout on red sandstone swelling specimen (T.No.: Transverse Number; V.No.: Vertical Number).
Front view of strain gauges layout
Top view of strain gauges layout
Typical red sandstone specimens with dry and saturated state of Zhujiang New Town Metro Station were chosen for creep test, and the specimens’ sampled depths were 17.8 m
Part specimens of creep test.
The STGD-3 type photoelectric liquid-plastic limit tester was used to measure the water content of reconstituted soil specimen (Figure
STGD-3 type photoelectric liquid-plastic limit tester.
ZJ type direct shear apparatus.
The KTG automatic triaxial test apparatus was used for shear strength test (Figure
KTG automatic triaxial shear test apparatus.
The unconfined swelling test was adopted for Wushan Road Metro Station typical red sandstone. And the creep testing program was as follows: after the red sandstone specimens were prepared and data acquisition instrument and its supporting software relevant parameters were set up and checked, a red sandstone specimen was placed on the bearing platform, the load sensor was putted on the specimen upper surface, and the oil pressure was slowly increased to ensure the lower surface of oil pump closely contact to the load sensor; then the classification incremental loading creep test could be started. The test results were automatically collected by data acquisition instrument. The three levels of loading were 8 MPa, 16 MPa, and 24 MPa, and each level loading duration was 144 h and the cumulative loading time was 18 days for one sample experiment.
For further discussion of water-weakening red-layers mechanism [
The shear strength envelopes of the remolded soil specimens were shown in Figure
Direct shear test results with different water contents.
Contents | Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6 | Group 7 | Group 8 |
Water content (%) | 9.99 | 13.16 | 16.92 | 17.27 | 18.348 | 20.308 | 28.226 | 37.551 |
Internal friction angle (°) | 40.987 | 39.328 | 25.561 | 24.6447 | 23.623 | 10.238 | 1.946 | 1.356 |
Cohesion (kPa) | 17.19 | 14.249 | 30.78 | 30.472 | 32.113 | 45.458 | 12.573 | 8.235 |
Shear strength and vertical load relationship curves.
(1) Internal friction angle
Fitting curve of water content and internal friction angle.
(2) Cohesion
Fitting curve of water content and cohesion.
After careful investigation of Figures
The main stress difference and the axial strain curves and other valuable results were obtained after the test, as shown in Figures
Triaxial test data with different water contents.
Contents | Group 1 | Group 2 | Group 3 | Group 4 | Group 5 |
Water content (%) | 13.61 | 14.04 | 15.0 | 15.81 | 18.5 |
Max. |
99.3 | 116.2 | 111.6 | 95.7 | 202.3 |
Max. |
63.1 | 67.5 | 99.2 | 184.3 | 199.4 |
Max. |
81.2 | 96.4 | 112.1 | 156.3 | 240.4 |
Cohesion (kPa) | 24 | 39 | 48 | 62 | 88 |
Principal stress difference and axial strain with 13.61% water content.
Principal stress difference and axial strain with 14.04% water content.
Principal stress difference and axial strain with 15.00% water content.
Principal stress difference and axial strain with 15.81% water content.
Principal stress difference and axial strain with 18.5% water content.
Relationship of water content and peak strength.
Relationship of water content and strain.
(1) Cohesion
(2) With the same confining pressure, the specimens with different water contents had different peak strength and axial deformation. In general, the peak strength increased with the increase of water content. When the water content was no more than 18.5%, the peak strength firstly increased and then decreased with increase of the
(3) Generally, the strain slightly decreased and then sharply increased with increase of water content; the inflection point occurred at
(4) When the water content was 18.5%, the peak strength reached the maximum value under different confining pressure, and the stain was 1%
The intact and disturbed typical red sandstone specimens of Wushan Road Metro Station were used for unconfined swelling test. After being immersed in water, the air-drying red sandstone specimen rapidly expanded with numerous cracks occurring and debris falling, which indicated that red sandstone had strong swelling and disintegration potential. The swelling damage mode of red sandstone specimen was shown in Figure
Typical specimen upper face after 10 minutes in water.
Swelling rate versus time curves of red-layers specimens.
Vertical strain and time curves of No. 3 specimen.
Transverse strain and time curves of No. 3 specimen.
(1) Water contents had the strong influence on the expansion of the disturbed specimens; generally, the expansion decreased with increase of water content, which illustrated that the water may weaken the mineral compositions or microstructures to reduce the sample expansion.
(2) There was very different swelling between the different water content samples. The smallest expansion of the highest water content sample was about 0.65%, while the biggest swelling of the air-drying samples was about 14.3%; the expansion has very big difference between the two specimen.
(3) The microstructures had big influence on the expansion, too. With the similar water content, that is, 13.311% of intact sample and 13.401% of disturbed specimen, the expansion was about 1% and 9.8%, respectively, almost ten times difference in swelling with different microstructures.
(4) In general, there were similar vertical stains in different parts of samples, and comparatively, there were different transverse strains in different parts of samples. The lower part of sample had bigger transverse strain, that is, about 2.75 mm, while the upper and middle part of specimen had smaller strain, that is, about 2.15 mm and 1.75 mm, respectively.
The creep tests of typical dry and saturated Zhujiang New Town Metro Station red sandstone were conducted using equipment under uniaxial compression. Based on the tested results as shown in Figure
Time-strain curves of the typical red sandstone.
(1) All of the time-strain curves exhibited three typical stages, that is, accelerated, dumped, and steady-state stages. The deformation at accelerated stage was significant. Generally, the accelerated stage spent short period of time. In contrast, the dumped and steady-state states took long period of time.
(2) The strain of different deformation stages all increased with the increase of the loading stress; that is, the higher the loading stress, the higher the strain of different deformation stages.
(3) Water content has very significant effect on the creep properties of red sandstone; in the same loading level, the strain of air-drying samples was only 40.1 to 49.3 percent of that of the saturated samples. Therefore, the influence of water content on the creep characteristics of red sandstone could not be ignored in the engineering design and construction of major projects in this area.
Studies have shown that the properties of rock or clay were often close to its mineral compositions, water content, and microstructures (Aziz et al. 2010; [
X-ray diffraction results of the remaining samples.
Number | Sampling site | Sampling |
Lithology | Clay mineral relative content (%) | Mixed layer ratio (% S) | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|
S | I/S | I | Kao | C | C/S | I/S | C/S | ||||
KD01 | Tiyu West Road Station | 16 | Moderate weathered red clay | 54 | / | 46 | / | / | / | / | / |
KD02 | Zhujiang New Town Station | 17.2 |
Red clay | 64 | / | 36 | / | / | / | / | / |
KD03 | Wushan Road Station | 12 | Moderate-strong weathered red clay | 2 | 46 | 42 | 4 | 6 | / | 15 | / |
KD04 | Zhujiang New Town Station | 18.1 |
Sandstone | / | 47 | 44 | 3 | 6 | / | 15 | / |
KD05 | Kecun Station | 27.6 | Red sandstone | 3 | 75 | 20 | 1 | 1 | / | 40/25 | / |
KD06 | Tianhe Station | 16 | Shield excavated soil | / | / | 56 | 38 | 6 | / | / | / |
All mineral X-ray diffraction analysis test results.
Number | Sampling site | Sampling depth (m) | Lithology | Mineral type and content (%) | Total clay minerals (%) | |||||
---|---|---|---|---|---|---|---|---|---|---|
Quartz | Feldspar | Plagioclase | Calcite | Hematite | Barite | |||||
PKD01 | Tiyu West Road Station | 16 | Moderate weathered red clay | 47.2 | / | 14.5 | / | 2.8 | / | 35.5 |
PKD02 | Zhujiang New Town Station | 17.2 |
Red clay | 45.7 | / | 10.9 | / | 3.5 | / | 39.9 |
PKD03 | Wushan Road Station | 12 | Moderate-strong weathered red clay | 33.6 | / | 14.8 | 12.2 | 2.8 | / | 36.6 |
PKD04 | Zhujiang New Town Station | 18.1 |
Sandstone | 27.7 | / | 5.4 | 21.1 | 5.7 | / | 40.1 |
PKD05 | Kecun Station | 27.6 | Red sandstone | 44.1 | 8 | 6.1 | 7.4 | 1.4 | / | 33.0 |
PKD06 | Tianhe Station | 16 | Shield excavated soil | 32.9 | 15.6 | 6.5 | / | / | 45.0 |
SEM picture of Tiyu West Road Station red clay sample, ×4140 Honeycomb I/S with a small amount of needle-like C.
SEM picture of Zhujiang New Town Station sample, ×20000 leaf shaped and laminated C.
SEM picture of Wushan Road Station sample, a×10000 filament I/S.
Energy spectrum picture of red clay sample in Tiyu West Road Station, a×2500 1-1 I/S; 1-2 I/S; 1-3 C/S.
X-ray diffraction pattern of red clay in Tiyu West Road Station.
Total minerals from X-ray diffraction pattern of red clay samples in Zhujiang New Town Station.
The following two points were made based on typical red-layers samples, SEM micrographs, and XRD photos.
(1) The microstructures of samples were relatively loose; the larger pores of the different samples were about 30–200
(2) Tiyu West Road Station sample mainly contained 54% smectite and 46% illite. Zhujiang New Town Metro Station sample relative contents were 39.9% clay minerals, 45.7% quartz, 10.9% plagioclase, and 3.5% hematite.
Influence of water content on the red-layers specimens was a complex phenomenon; the water induced change in microstructures and mineral compositions yielded degradation of physical and mechanical parameters. Therefore, it was important to analyze the effects of water content on the degradation mechanism of the specimens, as those conditions considered during above-mentioned experiments.
(1) The typical red-layers samples had layered or honeycomb microstructures and easily water swelling or weakening minerals such as chlorite and smectite, and the pores had good connectivity between the particles which provided the water invasion tunnels.
(2) With the lower water content, the particles and layers made good contact with each other and the contact surfaces were flat and smooth and formed the good microstructures which induced the relative good physical and mechanical properties.
(3) During the process of water content increasing, the water infiltrated inside the specimens. The chemical reactions such as dissolution and corrosion occurred in the mineral particles, and the chemical migration and dissolution occurred at the cemented or contact surfaces between mineral particles, which could make the mineral compositions and microstructures change and the macro physical and mechanical degradation of the specimens.
(4) Since the red-layers specimens are comprised of many minerals and then could easily expand inside the water, during water invasion, the cracks would likely propagate by the stress concentrated at the crack endpoints [
The widely spread red-layer in the Southeast China is hard and integrity, which has favorable physical and mechanical properties in natural condition, but it will rapidly expand, disintegrate, and soften after water invasion, which brings a great challenge for those region engineering construction. To reveal the effects of water content on the physical and mechanical properties of the typical Southeast China red-layers, the different tests such as the direct shear test, UU triaxial test, swelling test, and creep test were executed; the following conclusions could be made based on the findings of this paper.
(1) The internal friction angle
(2) The UU triaxial test results of the typical Zhujiang New Town Metro Station reconstituted red-layers further validated the trend of water content effect on cohesion.
(3) The red-layers specimens had strongly swelling potential; the expansion rate obviously decreased with the increase of water content but was not significant after water content beyond 23.5%.
(4) The air-drying red sandstone samples had strongly an isotropic expansibility and disintegration; the average vertical swelling was larger than average transverse expansion. The water contents and time curves of the unconfined swelling test could provide reference for solving the technical problems related to the civil engineering construction in typical red-layers.
(5) The mechanism of water content effect on red-layers properties was water induced microstructures and mineral compositions change which caused the macro physical and mechanical characteristics degradation.
The authors declare that they have no conflicts of interest.
The support of the Fujian Province Natural Science Fund, Grant no. 2016J01743, and the support of the Sanming University Science Research Development Fund and Fujian Province Young Teacher Education Research Project, Grants nos. JAT160454 and B201603, are gratefully acknowledged.