Experimental Study on Mechanical Behavior of a New Backfilling Material : Cement-Treated Marine Clay

Backfilling mining method is an overlying strata control way, which is widely used in underground coal mine. *is method is effective in preventing and controlling geological disasters such as surface subsidence, mine water inrush, rock burst, and other disasters. Cement-treated marine clay (CMC) is a typical porous media, which has abundant reserves and can be used as a new backfilling material.*erefore, the mechanical characteristics of CMC are very important for overlying strata control in coal mine. To investigate stress-strain behavior of CMC, isotropic consolidated drained (CID) triaxial test and isotropic compression test (ICT) were conducted with different confining pressures in the range of 50–800 kPa. Stress-strain behavior was found similar to those of the overconsolidated stress-strain behavior as well as the pore water pressure versus strain. Stress versus strain curves under lower confining pressure 50–250 kPa presented shear dilatancy. *e result shows that the peak strength increased linearly with increasing confining pressure.*e internal friction angle and cohesion are 48° and 590 kPa, respectively. Before the confining pressure reaches 727 kPa, which is the primary yielding point, the secant modulus E1 (the secant modulus at 1% axial strain) and the secant modulus E50 (corresponding to the 50% of the peak point) increase initially and decrease afterwards with the increasing of confining pressure. Afterwards, the two parameters increased with increasing confining pressure. *e yielding stress occurred in the stage, generating a dramatic decrease in tangent modulus.*is study can be a theoretical basis for engineering application of this new backfilling material.


Introduction
In underground mining, the stress balance of stope is destroyed as a result of overlying strata moving and fracturing.e overlying strata rupture at the gob, and "three zones" were formed from coal seam to upper strata, which are named caving zone, fractured zone, and bending zone, respectively.e rupture of overlying strata usually leads to disasters such as mine water inrush, rock burst, gas outburst, surface subsidence, and other disasters (Figure 1).Backfilling mining method is widely used in underground coal mine for overlying strata control, and the overlying strata are supported by solid backfilling materials (Figure 2). is mining method effectively prevents the rupture of overlying strata and then decreases the geological disasters in mine [1,2].e support ability of backfilling body depends on the mechanical characteristics of materials [3], which are usually composed of gangue, fly ash, tailings, or limes.Wu et al. [4] studied thermal, hydraulic, and mechanical performances of cemented coal gangue-fly ash backfill.Deng et al. [5,6] developed a new type of CPB with waste rock as coarse aggregate, fly ash as fine powder, slag as activator, and ordinary Portland cement as binder.Niroshan et al. [7] studied relationship of the microstructure and long-term mechanical properties of the cemented filling material.De Araújo et al. [8] studied the shear strength of a cemented paste backfill under high confining pressure condition.Yilmaz and Fall proposed that the mechanical characteristics of tailing paste backfilling materials are influenced by chemical and physical factors [9].
Marine clays are composite materials that include fine sand and clay and rich in calcareous and siliceous sediments.In general, marine clays are soft cemented and have the characteristics of soft sensitive and highly compressive [10].Marine clays have abundant reserve in the offshore area, and they can be used as backfilling materials in the mine located on the coastal region.As a backfilling material, the mechanical characteristics of marine clays are changing with increasing of mine depth, which caused by different confined stresses.us, it is important to study the mechanical characteristics of marine clay under different confined stresses because it determines the control effect of overlying strata.Compared to other chemical stabilization methods, using cement to improve the strength of soft clay is a popular solution because cementation can cause formation of weak to strong bonds between soil particles, as well as it is more economic and effective for subgrade engineering, embankments, deep excavation, and underground construction.Selection criteria for the choice of a particular cementing agent for soil improvement include factors such as increase in density, strength (primarily the unconfined compressive strength), and durability.Shear strength of cemented soils is one of the most important parameters in geotechnical engineering.Many researchers have investigated unconfined compressive strength q u of cement-stabilized clay [11,12].Several studies have been conducted on the tensile strength of cement-improved clay [13,14].e physical-chemical and microstructural aspects of cement-treated Singapore marine clay have been studied [15].Some researchers have examined the isotropic compression and the undrained shear behavior of cement-treated soils [16][17][18][19].Very little attention has been given to the influence of confining pressure on the overall mechanical behavior [20,21].
is paper presents a study on effect of the primary yielding confining pressure on shear characteristics of CMC by using the drained triaxial test and isotropic compression test, including stress-strain behavior, volume-strain relationship, pore water-strain relationship, shear strength behavior, failure modes, and deformation behavior.It is important to the engineering design of marine clay materials backfilling method.

Materials.
e studied materials include Singapore upper marine clay and ordinary Portland cement (OPC), the type of which is 42.5R.e main physical characteristics of the marine clay are presented in Table 1.e liquid limit and plastic limit of marine clay are, respectively, 74.07 and 31.00%,which indicates that the plasticity index of 43.07%.e constituents of marine clay are approximately 24.1% colloid, 21.8% clay, 47.7% silt, 6.2% fine sand, and 0.2% medium sand.Table 2 shows the chemical compositions of OPC by weight.In addition, the organic content of this soil is less than 1%, in which effect on the mechanical test can be neglected.

Sample Preparation and Testing.
e mix ratio of CMC studied is 213, and the curing time is 28 days.e mix ratio 213 means that the mass ratio value of dry soil solid (S): dry cement solid (C): water (W) is 2 : 1 : 3 [23].e mix ratio 213 was used in several cement clay stabilization researches, such as deep mixing and jet-grouting studies and projects including soft, fine-grained soils [24,25].
e main procedure of sample preparation is the same with that used by Chin et al. [26].It has been noted by Kamon that clay particles are aggregated with average diameter of up to 60 mm (0.05-0.6 mm) [27].e first step was to use a sieve size of 0.3 mm filter to ensure the homogeneity of the clay after soaking in water for days and clearing its small rocks, pebbles, sea shells, etc. en, the marine clay was first mixed in a Hobart mixer for 5 minutes and then added the prescribed amount of water to achieve 100% moisture content for another 5 minutes; after which, both cement and water left according to the desired ratio 213 were added to the Hobart mixer and mixed for 10 minutes.And then, the mixed soils were removed and stored in a plastic bag to preserve the water content and to ensure the fixed environment atmosphere.Later, the prepared mixture was placed into the prepared molds, which diameter is 38 mm and height is 76 mm.Finally, the specimens were labeled and stored in a PVC box which contains distilled water.It should be noted that all samples should be completed within half an hour.Figure 3 shows the whole specimen preparation procedure.Table 3 presents the basic parameters of samples tested in this study.e confining pressure was maintained in the range 50-800 kPa.
e treated marine clay specimens were tested using isotropic consolidated and drained triaxial test, consolidation test, and constant stress ratio test.
e triaxial GDS equipment is a fully computer controlled triaxial stress path equipment, as shown in Figure 4.   Advances in Materials Science and Engineering e mechanical test used a back pressure of 500 kPa, and the B-value reached at least 0.95.e vertical consolidation force and the shear force were designed as follows: 50, 200, 250, 350, 500, 600, and 800 kPa.It should be noted that the primary yielding pressure is 727 kPa, and therefore, the maximum loading force is 800 kPa.Since shearing and friction effect are negligible to the pore pressure changes, a shearing rate of 0.005 mm/min was selected to ensure the pore pressure changes, which was demonstrated to be true by the tests results [26,28].
Isotropic compression procedures followed the prescribed procedures in ISO/TS 17892-9 [29,30].It was done by using a high-pressure test system supplied by GDS, as shown in Figure 4. e sample was saturated under back pressure until the B-value approached at least 0.9 [31], which requires a back pressure of 400 kPa approximately. is test used back pressure of 500 kPa, and the B-value reached at least 0.95.Consolidation and swelling were conducted at a stress rate of 1 kPa/min with side drainage.

Stress-Strain
Behavior.Figure 5 presents the curves of the axial strain ε 1 -q-ε v -pore pressure with different confining pressures in the range of 50-800 kPa of the CMC with the mix ratio 213 as well as the curing time 28 days.All stressstrain curves show an apparent peak point, which increases obviously with the increasing confining pressure.e stress deceases quickly after reaching the peak point.e curves are similar to those of the overconsolidated stressstrain curves, which are all smooth hump shapes, showing a plastic failure (Figure 5(a)).
e stress-strain behavior is approximately the same with that tested by Haeri et al. [32] and Hao et al. [33,34].Advances in Materials Science and Engineering When the confining pressure is lower, e.g., 50-250 kPa, ε 1ε v relationship can be expressed as dilatancy; however, other under higher confining pressure conditions show an opposite trend.As the confining pressure increases, the dilation increases because of the reduction in the brittleness in the cemented soils (Figure 5(b)).
As indicated in Figure 5(c), the pore water pressure curves present a nonlinear increasing trend with increasing confining pressure.e pore water pressure reaches to the peak when the responding deviator stress reaches to the peak.It can be concluded that the main trend of deviator stress-axial strain and pore water pressure-axial strain is similar.Pore water pressure changes little under 500 kPa or lower than it, but it increases obviously when the confining pressure is more than 500 kPa, which can reach as high as 430 kPa during the shearing procedure.Pore water pressure is equal to the primary point at the beginning of the shear test, which is constant after axial strain reached approximately 10%, no matter what confining pressures.

Shear Strength Behavior.
According to the drained test results, the peak strength under different confining pressures was sorted out, as shown in Figure 6. e peak strength p p increases linearly with increasing confining pressure σ 3 and can be fitted well by the following straight line: According to the test results, Mohr circles at failure and strength envelopes are compiled in Figure 7.As shown in Figure 7, the internal friction angle and cohesion are 48 °and 590 kPa, respectively, though under different confining pressures.e internal friction angle of CMC is approximately 2.2 times than that of without cement [25].Actual shear damage angle and destruction modes in the test are listed in Table 4, which can be drawn from Figure 8. e average actual shear damage angle is approximately 55 °. e failure modes of CMC are basically the same, with a shear zone; the stress-strain curves are strain-softening type (Figure 8).

Deformation Behavior.
In order to study the effect of confining pressure on the deformation properties of cementtreated soils, the secant modulus E 1 (the secant modulus at 1% axial strain) and the secant modulus E 50 (corresponding to the 50% of the peak point) are shown in Figure 9. Before the confining pressure reaches 727 kPa, which is the primary yielding point, the secant modulus E 1 and the secant modulus E 50 increase initially and decrease afterwards with the increasing of confining pressure.Afterwards, the two parameters increased with increasing confining pressure.
To research more in-depth, the isotropic compression test has been conducted to find the yielding point.As shown in Figure 10, the isotropic compression curve of OPC showed significant linear pre-and postyield segments, which allow its recompression and compression indices to be determined.
is is consistent with the linear v-lnp' relationship assumed by Ghee [35] and Xiao and Lee [36].e isotropic yielding stress is defined as the point where the compression curve starts to deviate from the initial linear behavior under a natural coordinate system [12,37].e yield stress was determined by best fitting a straight line with the initial part of the compression curve on the compression space by trial and error [23].e yielding stress occurs in the stage, generating a dramatic decrease in tangent modulus.Also, the yielding stress point of cement-treated soils in the test is approximately 727 kPa, and the changing trend of secant moduli E 1 and E 50 is nearly the same with conclusions by Wang et al. [38].

Conclusions
In order to investigate the influence of primary yielding stress on the mechanical characteristics of CMC, the isotropic consolidated drained triaxial test and isotropic compression test were conducted with different confining pressures in the range of 50-800 kPa.e stress-strain behavior, shear strength, and deformation behavior are studied in this paper.
e following conclusions can be drawn according to the test results:  (2) e shear strength of OPC stabilized marine clay increases when confining pressure increases.e peak shear strength can be fitted well by the straight line p p � 1.981σ 3 + 1003.6 kPa, as well as the fitting parameter R 2 � 0.933, which applies to the range of confining pressure applied in this study.e internal friction angle and cohesion are 48 °and 590 kPa, respectively.e failure modes of CMC, with a shear zone, are strain-softening types.
(3) Significant linear pre-and postyield segments can be drawn by the isotropic compression, which allow its recompression and compression indices to be determined.e yield stress 727 kPa approximately is determined by best fitting a straight line with the initial part of the compression curve on the compression space by trial and error.(4) Both the secant moduli E 1 and E 50 have an increasing, then decreasing trend till to the primary yielding stress point 727 kPa and at last increasing trend with increasing confining pressure.To CMC, the primary yielding occurs in the stage, generating a dramatic decrease in tangent modulus.
is study can be a theoretical basis for the engineering application of the marine clay backfilling material.And the further study should focus on the influences of primary yielding confining stress on micromechanical behavior of the CMC.

Figure 4 :
Figure 4: GDS controlled by computer in the CID test.

Figure 9 :
Figure 9: Relationship between confining pressure and E (E 1 and E 50 ).

Figure 10 :
Figure 10: Mean effective stress versus specific volume of CMC.

Table 3 :
Basic parameters of samples tested.
Note: tests 1-7 are isotropic consolidated drained (CID) tests; * test 8 is the isotropic compression test (ICT) for primary yield stress.Computer-control RAM Back pressure and back volume controllerCell pressure controller
(1) e brittleness in the CMC reduces as the confining pressure increases.Nonlinear stress-strain behavior shows similarity to the overconsolidated stress-strain trend, both of which have an apparent peak point, increase obviously with the increasing confining pressure, and are smooth hump shapes, showing a plastic failure.Pore water pressure-axial strain is similar to that of deviator stress-axial strain, which is constant after axial strain reached approximately 10%, no matter what confining pressures.