1. Introduction
Grouting has been widely applied in tunnels, subways, foundation pit of buildings and many other underground constructions [
1]. The commonly used grouting materials in transportation tunnels are cement-based silicate engineering materials. As reported in [
2], the flowing slurry can transfer to the inner cracks in surrounding rocks or artificially drill holes. After solidification and hardening of the grouting body, the integrity of the broken surrounding rock or the bonding properties between anchor rod and rock can be improved significantly, and thus increase the security of supporting structure. As one of the primary means of disaster prevention and control, the reinforcement effect of grouting has achieved good results in most tunnel constructions. However, with the development of transportation engineering, more and more mountain tunnels worldwide are being built under high-geothermal conditions. Especially in the Sichuan-Tibet railway project of China, dozens of transportation tunnels are facing the challenge of high-geothermal problem during the construction [
3]. The rock temperature tested in the holes on site is almost always above 40 °C, which has a great influence on the reinforcement effect of grouting material, and thus poses a potential threat to the long-term safety of bolt-supported structure [
4]. Therefore, deeper investigations on the mechanical characteristics of cement-based grouting material in high-geothermal tunnels are necessary.
In previous studies, scholars have conducted many experiments on the mechanical properties of grouting material under normal temperature conditions. Some studies have discussed grouting methods, various grouting parameters, and the properties of grouting material appropriate for tunnel construction [
5]. Generally, there are many factors that have a significant impact on the mechanical behavior of cement-based grouting material, such as w/c ratio, curing condition, substance components, specific surface area, viscosity and so on [
6]. Li et al. [
7] conducted many experiments on modified additives for grouting material and proved that higher water to binder ratio led to good fluidity, but the compressive strength decreased significantly. Li et al. [
8] studied a new cementitious anti-washout grouting material (CIS). The results indicated that the CIS grout had a high early compressive strength due to the admixture of water glass. The hydration products of C-S-H cause CIS grout to be denser, lending it a higher strength, but the increase of xanthan gum has an opposite effect. Some investigations have indicated that the early strength of most cement-based grouting materials can be improved by mixing lithium carbonate hardening accelerators, but the test results by Won et al. [
9] suggested that this additive may also lead to a decrease in long-term strength. However, the focus of those investigations was mainly on the characterization of cement grouts at standard room temperature.
As reported in [
10], the real construction environment, particularly as presented recently for high-geothermal tunnels, has a temperature of about 40 °C to 90 °C. The impact of high temperature on the mechanical properties of grouting material has to be fully taken into consideration. On the one hand, several studies on the temperature effect (<40 °C) have indicated that the hydration degree of the grouting material, which determines the hardening, is known to depend largely on temperature [
11]. Consequently, in [
12], Mirza et al. found that the setting time of grouting material significantly depended upon the temperature variation, and the increase of temperature can accelerate the condensation of most kinds of grouting material. In addition, curing temperature has a remarkable influence on the strength properties of cement grout specimens. Elkhadiri et al. [
13] studied cement pastes specimens cured under higher temperature (e.g., at 22 °C and 40 °C), finding that they had a higher strength than those cured under lower temperature (e.g., 4 °C). Holt et al. [
14] proved that early strength is vital to supporting tunneling, as the cement-based grouting material used in tunneling engineering and the lower curing temperature may obstruct the development of compressive strength in the early stages, resulting in the invalidation of rock bolts or the failure of the supporting system. Bohloli et al. [
15] also found that the rapid increase in temperature in grouting material had a negative effect on the strength of grout specimen. The experimental results indicated that a high temperature of 20 °C led to a lower uniaxial compressive strength of the grouting body than that found at 8 °C. Although those findings made great contributions to the study on the impact of temperature on grouting material properties, more and more high-geothermal tunnel project cases are presenting rock temperatures far beyond the previously investigated temperature range. On the other hand, many scholars have also carried out research on grouting materials that have experienced fires [
16]. Wang et al. [
17] found that under temperature conditions of 550 °C, the ultimate bearing capacities of cement-based material samples cured at 14 and 28 days decreased to 41% and 60%, respectively. Li et al. [
18] studied the residual compressive strength of cement-based grouting material by exposing the samples to the temperatures of 150 °C, 350 °C, 550 °C, and the testing results indicated that being exposed to higher temperature results in lower mechanical behaviors. Similar results were obtained in the investigation on the elastic modulus of grouting material with early age after fire [
19]. However, the cement-based grouting materials in those researches had been hardened sufficiently before being exposed to the fire condition, and there exist essential differences between the two kinds of high-temperature environments when compared to the grouting material used in high-geothermal tunnels.
Additionally, on the basis of our previous study results [
20,
21], various cooling measures have been taken in the engineering of high-geothermal tunnels in order to create an appropriate working environment. Therefore, the grouting material used in actual high-geothermal tunnels is hydrated and solidified under variable temperature curing (VTC) conditions instead of under constant high-temperature curing conditions [
10]. In addition to the problem of high temperature, the influence of relative humidity (RH) in high-geothermal tunnels on grouting materials should not be underestimated. Some studies have suggested that it is easy to cause decreases in compressive strength and bond strength in grouting material in a hot-dry environment [
22,
23]. There exists a significant combined effect of temperature and relative humidity. In summary, the real environmental conditions in high-geothermal tunnels is far complex, and current studies considering the environmental effect (temperature and humidity) on the mechanical properties of grouting material are limited. Thus, the major aims of this paper are:
a. Perform a series of experiments on the compressive strength of grouting material cured under VTC conditions (all combinations of temperature and RH).
b. Study the failure characteristics and mechanism of grouting materials.
c. Discuss the impact of temperature and RH, including the coupling effect on the mechanical properties of grouting material, and establish a compressive strength prediction formula.
d. Obtain a constitutive model of common cement-based grouting material subject to high-geothermal environments.
The results of this investigation may provide valuable information and theoretical support for grouting technology, numerical models of grouting materials, and the supporting design in high-geothermal tunnel.
4. Conclusions
In this paper, mechanical strength experiments on cement-based grouting material cured under VTC conditions were carried out to study the compressive characteristics under high-geothermal environments. The coupling effect of temperature and relative humidity on the mechanical properties of grouting material at different ages were further studied. Based on the failure characteristics and comparison to the stress-strain curves of specimens cured under SC conditions, the changing rules of typical points (peak stress and strain) of curves under different curing conditions were investigated, along with the compressive constitutive model of cement-based grouting material under high-geothermal environments. The conclusions are as follows:
(1) The compressive strength of grouting material significantly decreases with the increase of temperature. When the temperature of VTC condition is over 40 °C, it may cause a decline of 10%–40% in the mechanical properties of the grouting material, and the strength degradation is more serious under a hot-dry environment.
(2) High RH level makes a great contribution to the increase in compressive strength of grouting material, regardless of curing time. When the temperature exceeds 56.3 °C and 75 °C, the relative humidity begins to play an increasingly important role in the strength of grouting material. Moreover, the coupling effect of temperature and humidity markedly improve the grouting materials strength at early ages and decrease the degradation of long-term strength.
(3) There are similarities and differences between the compressive stress-strain curve of cement-based grouting material cured under VTC condition and SC condition. Higher temperature and lower relative humidity cause the lower relative peak stress of grouting material, and the RH effect is slightly greater than the temperature effect. As for relative peak strain, the influence of the environmental effect is to the contrary, and the temperature has a more significant influence than relative humidity on the peak strain of grouting material.
(4) According to the relationship between mechanical properties and high-geothermal environmental conditions, the calculation formula of relative stress and strain of grouting material under different environmental conditions is established. Moreover, from numerous experimental data and the complete stress-strain curves of the grouting material, a new segmental constitutive model of compressive strength in high-geothermal environment is obtained, considering both temperature and relative humidity.