A detailed manufacturing procedure of a synthetic soft rock is presented, as well as its applications on the laboratory experiments of socketed piles. With the homogeneity and isotropy of the simulated soft rock, the influence of different variables on the bearing performance could be investigated independently. The constituents, cement, gypsum powder, river sand, concrete-hardening accelerator, and water, were mixed to form the specimens. Both uniaxial and triaxial compressive tests were conducted to investigate the stress-strain behavior of the simulated soft rock. Additionally, the simulated soft rock specimens were used in model pile tests and simple shear tests of the pile-rock interface. Results of the simulated soft rock in both the uniaxial and triaxial compressive tests are consistent with those of natural soft rocks. The concrete-hardening accelerator added to the mixtures improves the efficiency in laboratory investigations of soft rock specimens with a curing time of 7 days. The similarities between the laboratory tests and the field observations provide convincing evidence to support its suitability in modeling the behavior of soft rocks.
Physical models have served important functions in geotechnical engineering research and practice, which can clearly portray complex, nonlinear geotechnical mechanisms and phenomena with economic feasibility. In the laboratory physical modeling for geotechnical engineering, the variables and the testing conditions can be controlled easily, and thus a quantitative rule is obtained for the research objects. In the design of physical models, the most important thing is the manufacture of model materials. With regard to model materials to simulate soft rock, the majority of past work has mainly been concerned with mixtures of cement and fine aggregates such as sand or kaolin (e.g., [
Johnston and Choi [
Soft rock is part of the continuous spectrum of materials with strength properties that are intermediate between soil and rock. Soft rocks are harder, more brittle, more dilatant, and more discontinuous than soil. But soft rock is also softer, less brittle, more compressible, and more susceptible to changes induced by variations in effective stress than other types of rock. There are many different criteria to define soft rock: criteria for strengthen deformability, durability, weathering degradation strength-stress relationship, and so forth. Finally it seems that an agreement has been reached between major international associations (ISRM, IAEG, and ISSMGE) and researchers to use the simple compressive strength as a criterion to separate soft rocks from hard soils at the lower limit and from hard rocks at its upper limit. The simple compressive strength is a property commonly used by professionals involved in the design of engineering projects, and in practice, soft rocks will commonly display uniaxial compressive strengths in the range of 0.6~12.5 MPa and mass stiffness values of 100~1000 MPa [
Drawing on previous research in the literature, mixtures containing the following materials were used to form synthetic soft rock in this study: Portland Cement P.C32.5, Duo-bang high-strength gypsum powder, river sand with a maximum grain diameter of 1.00 mm, concrete-hardening accelerator, and water. These materials (shown in Figure
Constituents of simulated soft rocks: (a) cement; (b) plaster; (c) medium sand; (d) concrete-hardening accelerator.
Grading curve of the medium river sand.
A specified quantity (see Table
UCS testing programs and results.
Number | Cement | Plaster | Water-cement ratio | Sand | Hardening accelerator | Density | Curing time | Average UCS value | Average deformation modulus |
---|---|---|---|---|---|---|---|---|---|
(%) | (%) | (%) | (%) | (g/cm3) | (d) | (MPa) | (MPa) | ||
A1 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.70 | 7 | 1.12 | 158.62 |
A2 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.75 | 7 | 1.56 | 204.60 |
A3 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.80 | 7 | 1.97 | 283.91 |
A4 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.85 | 7 | 2.56 | 378.97 |
A5 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.90 | 7 | 3.00 | 428.85 |
A6 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.95 | 7 | 3.43 | 471.38 |
B1 | 4.5 | 5.0 | 0.50 | 84.71 | 1.04 | 1.80 | 3 | 1.08 | 107.50 |
B2 | 4.5 | 5.0 | 0.50 | 84.71 | 1.04 | 1.80 | 5 | 1.22 | 138.80 |
B3 | 4.5 | 5.0 | 0.50 | 84.71 | 1.04 | 1.80 | 14 | 1.62 | 204.64 |
B4 | 4.5 | 5.0 | 0.50 | 84.71 | 1.04 | 1.80 | 21 | 1.70 | 219.56 |
B5 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.95 | 3 | 2.33 | 336.85 |
B6 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.95 | 5 | 2.95 | 407.23 |
B7 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.95 | 14 | 3.63 | 496.99 |
B8 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.95 | 21 | 3.80 | 522.17 |
C1 | 4.5 | 5.0 | 0.50 | 84.71 | 1.04 | 1.80 | 7 | 1.44 | 157.88 |
C2 | 6.0 | 5.0 | 0.50 | 82.12 | 1.38 | 1.80 | 7 | 1.74 | 237.90 |
C3 | 8.0 | 5.0 | 0.50 | 78.66 | 1.84 | 1.80 | 7 | 2.62 | 375.71 |
C4 | 11.0 | 5.0 | 0.50 | 73.47 | 2.53 | 1.80 | 7 | 3.37 | 442.15 |
C5 | 14.0 | 5.0 | 0.50 | 68.28 | 3.22 | 1.80 | 7 | 3.72 | 560.08 |
D1 | 4.5 | 3.0 | 0.50 | 87.71 | 1.04 | 1.80 | 7 | 1.22 | 211.32 |
D2 | 4.5 | 8.0 | 0.50 | 80.21 | 1.04 | 1.80 | 7 | 1.52 | 256.99 |
D3 | 4.5 | 11.0 | 0.50 | 75.71 | 1.04 | 1.80 | 7 | 1.61 | 273.64 |
D4 | 4.5 | 14.0 | 0.50 | 71.21 | 1.04 | 1.80 | 7 | 1.84 | 312.89 |
E1 | 4.5 | 5.0 | 0.35 | 86.135 | 1.04 | 1.80 | 7 | 0.85 | 148.56 |
E2 | 4.5 | 5.0 | 0.43 | 85.375 | 1.04 | 1.80 | 7 | 1.02 | 186.18 |
E3 | 4.5 | 5.0 | 0.58 | 83.95 | 1.04 | 1.80 | 7 | 1.57 | 254.22 |
E4 | 4.5 | 5.0 | 0.65 | 83.285 | 1.04 | 1.80 | 7 | 1.48 | 219.09 |
F1 | 4.5 | 5.0 | 0.50 | 85.75 | 0.00 | 1.80 | 7 | 1.41 | 188.39 |
F2 | 4.5 | 5.0 | 0.50 | 85.3 | 0.45 | 1.80 | 7 | 1.39 | 201.22 |
F3 | 4.5 | 5.0 | 0.50 | 84.85 | 0.90 | 1.80 | 7 | 1.53 | 224.98 |
F4 | 4.5 | 5.0 | 0.50 | 84.53 | 1.22 | 1.80 | 7 | 1.62 | 199.23 |
F5 | 4.5 | 5.0 | 0.50 | 84.4 | 1.35 | 1.80 | 7 | 1.53 | 208.34 |
Taking several key factors that affect the uniaxial compressive strength of analogue soft rock samples into consideration—such as density, curing time, cement content, plaster content, water-cement ratio, and concrete-hardening accelerator content—uniaxial compressive tests were conducted on 124 specimens with 32 different sets of properties, as shown in Table
Typical stress-strain curve of simulated soft rock in uniaxial compression.
The relationship between the deformation modulus at 50% of the ultimate strength (
Relationship between the deformation modulus and uniaxial compressive strength of simulated soft rock in this study and previous studies (data from [
Obviously, the behavior of the simulated soft rock is highly dependent on density, cement content, plaster content, water-cement ratio, concrete-hardening content, and curing time, as indicated in Table
Figures
The variation of parameters against curing time: (a) uniaxial compressive strength; (b) deformation modulus.
To achieve a comprehensive understanding of the properties of this simulated soft rock, cylindrical specimens (101 mm × 200 mm) with proportions of cement, plaster, medium sand, water, and concrete-hardening accelerator of 4.5%, 5.0%, 84.71%, 4.75%, and 1.04%, respectively, were used in triaxial compressive tests at confining stresses varying from 10 kPa to 500 kPa. Specimens with a density of 1.8 g/cm3 and a curing time of 7 days were prepared. After the uniaxial compression tests, the average uniaxial compressive strength was 1.44 MPa. Figures
Behavior of the simulated soft rock specimen in triaxial compressive tests: (a) stress-strain curves; (b) volumetric strain-strain curves.
Mohr’s circles and strength envelope of the simulated soft rock.
The Hoek-Brown failure criterion of simulated soft rock and natural rocks are presented in Figure
Hoek-Brown failure representation of simulated soft rock and various rocks (data from [
Based on the factors discussed above, it was found that the strength and deformation properties of the simulated soft rock in both uniaxial and triaxial compression tests were consistent with those of natural rocks. In addition, the constituents are universally and economically obtainable. Moreover, the specimens can be shaped and cured easily. As a consequence, the analogue material and its manufacturing method can be used to simulate soft rock in model experiments. However, the simulated materials may have a little difficulty in simulating the fissures, joints, structural surfaces, discontinuities, and stress history of the actual rocks since they are found to be homogeneous and isotropic. And the small laboratory specimens are also not representative of the actual field behavior, which is influenced by a much larger scale effect.
On the basis of the reasonability of simulated soft rock, these materials and manufacturing techniques have be used in the investigations of socketed piles in soft rock. Indeed, it can also be used in other research of rock mechanics, such as rock slope and rock tunneling. In this section, the experimental applications of the simulated soft rock in socketed piles are discussed briefly and other details of the applications are presented by Huang [
The applications of the simulated soft rock included an experimental model study of bearing behavior of piles in soft rock using the apparatus developed by our team, as detailed in Figure
Schematic illustration of the model test apparatus.
As a result of these studies, the measured total bearing capacity of piles in soft rock showed a similar relationship and varying tendency compared to the calculated result using the spherical cavity expansion theory. And the measured shaft resistance increased to a peak and then decreased near the tip of the pile, which was consistent with the results of in situ loading tests [
Pile tip failure model (settlement/diameter = 1.7).
The shear behavior of pile-rock interface is a critical factor in the performance of socketed piles in soft rock. Direct and simple shear tests are the two common laboratory testing methods used to investigate the behavior of the pile (concrete)-rock interface. In fact, the measured shear strength parameters using the direct shear tests are overestimated for the limitation of test conditions. Simple shear tests are created as an attempt at improvement over the performance of the direct shear box. Based on the two main types of simple shear devices designed by the Norwegian Geotechnical Institute and by the Cambridge University, an improved simple shear device with rotatable plates was created for minimizing the influence of shear boxes on the sample deformation along the shear direction, as shown in Figure
Schematic illustration of the simple shear devices with rotatable plates.
The simulated soft rock samples were shaped easily with specific surface geometries, and then the samples were tested in the simple shear apparatus against a concrete section simulating the pile under different vertical pressures. It was found that the shear strength of the simulated soft rock samples is higher than the pile-rock interface, suggesting that the failure zone of socketed piles may occur in the pile-rock interface under shear progress of loading. The pile-rock interface performance was highly dependent on the level of roughness and normal stress on the contact. Figure
Roughness profiles used in simple shear testing.
Failure status of simulated soft rocks after testing.
Based on the comparison on mechanical properties of the simulated soft rocks and natural soft rocks in both uniaxial and triaxial compressive tests, these constituents of cement, gypsum powder, river sand, concrete-hardening accelerator, and water are recommended to simulate the soft rocks with good performances in laboratory investigation. It substantially improves the efficiency of the preparation of specimens with the aid of concrete-hardening accelerator. The uniaxial compressive strength and deformation modulus of the simulated soft rock are in the range of 0.85~3.80 MPa and 107.50~560.08 MPa. All these percentages of each constituent and its mechanical parameters could be the reference for the similar rock mechanic problems. In addition, it is reasonable to obtain the simulated soft rocks that those mechanical parameters are beyond the range mentioned above, with the extension of the quantitative relationship of the mechanical parameters and its percentages of constituents. The applications of the simulated soft rocks to socketed piles are presented, which focus on the load-transfer mechanisms and bearing performance of piles in soft rock. The homogeneous and isotropic simulated soft rock is manufactured into defined sizes and shapes readily with constant mechanical properties under laboratory conditions.
The authors declare that they have no conflicts of interest.
The work described in this paper was supported by the National Natural Science Foundation of China (Grant nos. 51378403 and 51309028) and Ph.D. Short-Time Mobility Program of Wuhan University. The authors thank Benjiao Zhang, Bin Huang, Zifeng Qiu, Lei Xiao, and Gang Luo for their valuable contributions to the model tests. The project has been also supported by POWERCHINA Hubei Electric Engineering Corporation and State Grid Hubei Electric Power Company. Mr. Qing Fang is the senior engineer in charge of technical work of this project. And Mr. Jiangang Yin is the project manager to organize and coordinate this project completely.