Qinghai Ma, Guangsheng Liu, Xiaocong Yang, and Lijie Guo, Physical model investigation on effects of drainage condition and cement addition on consolidation behavior of tailings slurry within backfilled stopes, Int. J. Miner. Metall. Mater., 30(2023), No. 8, pp.1490-1501. https://dx.doi.org/10.1007/s12613-023-2642-5
Cite this article as: Qinghai Ma, Guangsheng Liu, Xiaocong Yang, and Lijie Guo, Physical model investigation on effects of drainage condition and cement addition on consolidation behavior of tailings slurry within backfilled stopes, Int. J. Miner. Metall. Mater., 30(2023), No. 8, pp.1490-1501. https://dx.doi.org/10.1007/s12613-023-2642-5
Research Article

Physical model investigation on effects of drainage condition and cement addition on consolidation behavior of tailings slurry within backfilled stopes

Author Affilications
  • Corresponding author:

    Guangsheng Liu      E-mail: liuguangsheng@bgrimm.com

    Lijie Guo      E-mail: guolijie@bgrimm.com

  • Estimation of stressses within the tailings slurry during self-weight consolidation is a critical issue for cost-effective barricade design and efficient backfill planning in underground mine stopes. This process requires a good understanding of self-weight consolidation behaviors of the tailings slurry within practical stopes, where many factors can have significant effects on the consolidation, including drainage condition and cement addition. In this paper, the prepared tailings slurry with different cement contents (0, 4.76wt%, and 6.25wt%) was poured into 1.2 m-high columns, which allowed three drainage scenarios (undrained, partial lateral drainage near the bottom part, and full lateral drainage boundaries) to investigate the effects of drainage condition and cement addition on the consolidation behavior of the tailings slurry. The consolidation behavior was analyzed in terms of pore water pressure (PWP), settlement, volume of drainage water, and residual water content. The results indicate that increasing the length of the drainage boundary or cement content aids in PWP dissipation. In addition, constructing an efficient drainage boundary was more favorable to PWP dissipation than increasing cement addition. The final stable PWP on the column floor was not sensitive to cement addition. The final settlement of uncemented tailings slurry was independent of drainage conditions, and that of cemented tailings slurry decreased with the increase in cement addition. Notably, more pore water can drain out from the cemented tailings slurry than the uncemented tailings slurry during consolidation.
  • Underground mining activities produce enormous voids (mine stopes and drifts) and generate a large volume of tailings after mineral processing. The mine-out voids may lead to instability of the surrounding rock mass, and the disposal of tailings on the ground surface can cause environmental problems [12]. To resolve the adverse impacts, backfill techniques have been extensively used in the mining industry to fill underground voids with tailings slurry, which is a mixture of mine tailings, water, and zero or low proportion of binders. Given the fluidity of fresh tailings slurry, barricades must be built in the access drifts to the stope to retain the backfill [35]. It is significant to estimate and manage the pressure to avoid barricade failure, which can result in serious safety and economic consequences [67]. This task requires to have a good understanding of the self-weight consolidation behaviors of tailings slurry.

    After placed in mine stopes, the tailings slurry will undergo several processes, including settling, sedimentation, and consolidation [8]. Initially, tailings particles are in a suspension of a fluid medium and tend to settle down under gravity. The particles are thus brought closer to each other, which leads to a denser backfill and the expulsion of pore water between particles [9]. However, given the poor hydraulic conductivity of tailings slurry with a large portion of fine-grain particles, pore water cannot drain from the slurry without delay [1011]. As a result, the excess pore water pressure is generated due to overburden. Along with the proceeding drainage, the pore water pressure (PWP) dissipates simultaneously with settlement, and the isolated particles can touch each other. Then, the particles build a skeleton whereby the tailings particles can transmit their weight to the floor of the backfilled stope [8], which results in the generation of effective stresses; then, the excess PWP caused by the overburden is transferred from the pore water to the contacted tailings particles. Self-weight consolidation is the process of load transfer to the tailings particles as the pore water escapes [12]. The consolidation of tailings slurry within stopes can be affected by many factors, including the drainage condition, cement content of the slurry, and tailings characteristics. Given the effects of these factors, the tailings slurry will show a very different consolidation behavior, which leads to various stress distributions and pressure on barricades. Therefore, the influences of factors on the consolidation behavior of tailings slurry should be understood to make a reasonable filling plan for mine stopes.

    Several analytical solutions have been proposed to evaluate the evolution of the settlement and PWP associated with the consolidation of tailings slurry [1011,1315]. These analytical solutions provide useful references to estimate the stress state of tailings slurry within the stope to make backfill strategy [2,16]. However, analytical solutions were almost developed for one-dimensional conditions with many assumptions. Practical backfill stopes have various drainage conditions that can affect the consolidation of tailings slurry but are difficult to consider in analytical models. For example, some mines usually build barricades with permeable materials or apply wick drains in stopes to accelerate drainage [17]. In addition, the side rock walls of stopes may have joints, fractures, and other geological structures through which water flow can occur to form permeable side walls [18]. The effects of diverse drainage conditions on PWP within tailings slurry during consolidation have been partly studied using numerical simulations [3,1719]. Nonetheless, the influences of drainage conditions on other consolidation characteristics, including slurry settlement, volume of drainage water, and residual water content in final drained and consolidated tailings slurry, were not considered in previous numerical models.

    Cement hydration is another factor that can evidently affect the self-weight consolidation [2022]; it can consume pore water and reduce the permeability of tailings slurry [2324], but these conditions are very difficult to simulate with the current commercial software. Some works concerning the in-situ measurement for the cemented tailings slurry in mine stopes had been conducted to learn the influence of cement hydration on consolidation, and valuable data were obtained [2529]. However, field monitoring of consolidation behaviors of the tailings slurry is not always feasible and is usually costly.

    Physical model tests can be carried out to investigate consolidation behaviors of cemented tailings slurry by setting reasonable conditions based on field backfill properties in mines. To date, some column tests on the tailings slurry have been reported [24,3037]. But these reported tests only consider one kind of drainage condition and a single cement dosage. The influences of drainage conditions and cement addition on the slurry settlement, PWP evolution, drained water volume, and residual water content of tailings slurry were seldom investigated in previous studies.

    This study aimed to investigate the effects of drainage condition and cement addition on self-weight consolidation behaviors of tailings slurry with physical model tests. The prepared slurry with different cement contents (0, 4.76wt%, and 6.25wt%) was poured into 1.2 m-high columns, which allowed three different drainage scenarios: undrained, partial lateral drainage near the bottom part, and full lateral drainage boundaries. The self-weight consolidation behaviors were then analyzed in terms of PWP, settlement, volume of drainage water, and residual water content in the final drained and consolidated backfill.

    The raw materials used for the tailings slurry preparation included unclassified tailings and cement, which were both sampled from an underground copper mine located in Anhui province, China. Tap water was used to prepare the tailings slurry in the laboratory. Before the physical model tests, necessary tests were conducted to obtain the physical properties, including particle size distribution (PSD), bulk density, solid density, porosity, hydraulic conductivity, chemical and mineral properties of the tailings, and the density and specific surface area of the used cement.

    Fig. 1 presents the PSD curve of the tailings tested with a Malvern Mastersizer S2000 laser particle analyzer. Fig. 1 also shows that the tailings particles finer than 20 μm (particle contents Pd < 20 μm) accounted for 44.60vol% of the total particles. The percentages of clay- (particle size d < 2 μm), silt- (2 μm < d < 75 μm), and sand-sized particles (75 μm < d < 5000 μm) were 9.43vol%, 67.60vol%, 32.40vol%, respectively. Most of the PSD belonged to medium to fine sand type. According to the Unified Soil Classification System, the tailings used in this paper can be classified as low plastic silts. More details of physical and chemical characteristics of the tailings and cement used in tests can be obtained from the Supporting Information (Tables S1 and S2).

    Fig. 1.  PSD curve of the unclassified tailings.

    A column constructed with acrylic glass and steel plates was used to simulate the placement of tailings slurry in underground mine stope and its self-weight consolidation. The column measures 120 cm in height and has a horizontal square section of 30 cm × 30 cm. Drainage conditions of the column were controlled by inserting elastic rubber mats and nonwoven geotextiles between the front plate and the main body of the column. Three seepage boundaries were considered in tests: undrainable (UD) along the whole height of the column, bottom drainable (BD) only along the lower part of the filled tailings slurry where drainage could occur on the lower 1/10 height of the column, and full drainable (FD) along the whole height of the filled tailings slurry where drainage is allowed along the entire height of the column. These three seepage boundaries could be achieved by adjusting the length of the rubber mat and nonwoven geotextiles. Other details can be obtained from the Supporting Information (Figs. S1–S3).

    The PWP sensors were installed along the vertical central line of a side plate at elevations of 4, 19, 39, 59, and 79 cm from the inside bottom surface, and a floor PWP sensor (0 cm) was installed at the bottom plate. The designed locations of PWP sensors enabled the use of measurements for the evaluation of pressure on barricades, which are usually located at the bottom of mine stopes. The type of PWP sensor was DMKY, which was made by NanJingDanMo Electronic Technology Co., Nanjing, China. The strained full-bridge circuit was used in DMKY, and a high-precision full-bridge strain gauge was pasted inside the sensing membrane of the sensors. The DMKY sensors measured PWP from 0 to 30 kPa with a resolution of 0.005 kPa. All the sensors were connected to a matched data logger, which was made by the same manufacturer of DMKY sensors, to collect the PWP of the consolidating slurry every 10 s once the prepared tailings slurry was poured into the column.

    A soft plastic ruler was pasted on the outside surface of the transparent acrylic front plate to measure the vertical settlement of the tailings slurry by aiming at the interface between the water pond and the settled tailings surface during consolidation. For details, the initial height of the tailings slurry was recorded after the column was fully filled. Then, the height of the settled tailings surface was read once an hour during the consolidation tests. The differences between the current recorded value and initial slurry height were calculated to achieve the corresponding settlement of the consolidating slurry at different times.

    The drainage water expelled from the tailings slurry was collected into two buckets by water collectors. The buckets, which were covered with cap covers to reduce evaporation, were put on an electronic balance to weigh the drainage water once an hour during the tests.

    Two groups of physical model tests with the constructed column were conducted to investigate the effects of drainage conditions and cement addition on the self-weight consolidation behaviors of tailings slurry. The solid content of the prepared slurry was fixed at 68.00wt%. No cement was added in the tailings slurry of the first group to focus on the effects of drainage conditions. However, in the second group, the cemented tailings slurries with 4.76wt% and 6.25wt% (i.e., mass ratios of cement to tailings are 1:20 and 1:15, respectively) cement addition were used to mainly explore the influence of cement dosages on the consolidating characteristics, with a fixed BD drainage condition.

    Table 1 presents the detailed programs in the physical model tests considering tailings slurry drainage conditions and cement addition. The selected cement content (4.76wt% and 6.25wt%) and solid content (68.00wt%) in Table 1 are typical parameters of the unclassified tailings backfill slurry used in the final pour of the underground stopes, especially in secondary stopes, in metal mines [2527].

    Table  1.  Test programs considering tailings slurry drainage conditions and cement addition
    GroupMass ratio of cement to tailingsCement content
    / wt%
    Drainage conditionSolid content / wt%Tailings
    / kg
    Cement
    / kg
    Water
    / kg
    Uncemented slurry00UD68.00122.40057.60
    00BD68.00122.40057.60
    00FD68.00122.40057.60
    Cemented slurry1:204.76BD68.00116.505.9057.60
    1:156.25BD68.00114.707.7057.60
     | Show Table
    DownLoad: CSV

    The tailings, cement, and water were prepared and weighed once for a column in advance and then mixed in a concrete mixer for 5 min until a homogenized state. The prepared fresh slurry was lifted with a bucket to the top of the column and poured gradually into the column along the inside wall with the help of a funnel and tube to reduce disturbance. Each column was totally filled within 30 min to an initial height around 108 cm (the total height of the column was 120 cm).

    After the model was fully filled with the tailings slurry in a laboratory with controlled temperature and humidity (22–24°C and 35%–40%), the data of PWP, settlement, and drainage water of the tailings slurry were collected based on the monitoring plan. When the monitored PWP and drainage water had no evident changes with time (typically 7 days after pouring), the column was dismantled, and tailings backfill samples were collected from different elevations to measure the residual water content in the drained and consolidated backfill using the oven-drying method.

    Fig. 2 presents the evolution of PWP at different heights of the three columns during the first 2 h. The column was filled with ten buckets of slurries within 30 min (Fig. 2). The PWP increased sharply once a bucket of slurry was poured into the column and remained transitorily unchanged before the next bucket was poured. As a result, the trends of the PWP over time occurred stepwise during pouring. The illustrated PWP jumps were caused by the poured slurry into the column, which accorded with the reported results of Ghirian and Fall [24], Nujaim et al. [38], and Saleh-Mbemba and Aubertin [33]. In their studies, column backfilling was completed in 2–3 layers, and the PWP also increased sharply once the new layer was placed. As shown in Fig. 2, after pouring, the PWP did not decrease significantly in the short term (within 2 h) because of the poor permeability (2.37 × 10−6 m/s) of the unclassified tailings used and the drainage paths for bleeding water, which indicates the slow rate of self-weight consolidation.

    Fig. 2.  PWP evolution at different heights of three columns within the first 2 h after pouring: (a) UD, (b) BD, and (c) FD.

    The PWP evolution in the three columns showed a slight difference in Fig. 2. At the lower part of the column (0–19 cm), the PWP in the FD column was slightly smaller, followed by those in the BD and UD columns, and an approximately equal PWP was observed at the upper part of the three columns. This finding can be explained by the weak effect of drainage conditions on the PWP evolution in the short term (the first 2 h).

    Fig. 3 shows the distribution of PWP along the full height of the three columns with different drainage conditions at t = 2 h. The figure also displays the overburden stress of the tailings slurry and hydrostatic water pressure for comparison. The tailings slurry density (ρs) was 1.757 g/cm3, and the water density (ρf) was 1 g/cm3, which resulted in the 18.98 kPa maximum overburden stress at the bottom of the slurry and 10.8 kPa maximum hydrostatic pressure. As shown in Fig. 3, the PWP of the three cases was considerably higher than the corresponding hydrostatic pressure and close to the overburden stress. This finding indicates that the self-weight of the tailings slurry was mainly supported by the pore water during the initial period when the column was backfilled with tailings slurry, and the effective stress was close to zero (less than 1 kPa in Fig. 3). Some results from field monitoring [27], column tests [38], and numerical simulations [39] also revealed that the PWP was initially equal to the overburden pressure (total stress), and the tailings slurry behaved as a fluid. Therefore, barricade failures had the highest risk after the stope was fulfilled, especially when the continuous filling strategy was used for the narrow stope. The possible risks and optimal continuous filling height must be investigated to develop a safe filling strategy in mine stopes based on the estimation of PWP during slurry consolidation.

    Fig. 3.  Monitored PWP at different heights of three uncemented columns at 2 h after pouring.

    Fig. 4 shows the evolution of PWP over time (t) on the floor of the three columns for a maximum period of 187 h (more than one week). After pouring, the PWPs in the three columns reached approximately equal peak values (18.21, 18.08, and 17.94 kPa for the UD, BD, and FD columns, respectively) due to their similar initial slurry heights and then decreased progressively over time. The PWP in the FD column was always the quickest to reach the minimal state and reached the stable PWP value of 3.7 kPa at 96 h. The end of PWP dissipation clearly illustrated the termination of self-weight consolidation for the tailings slurry in the FD column. The PWP in the UD column did not reach a stable value during the test, which meant that self-weight consolidation continued, and the PWP further decreased over time. For the BD column, the PWP was expectedly between the values of the FD and UD columns. As shown in Fig. 4, increasing the length of the drainage boundary can promote the dissipation of PWP and thus accelerate the self-weight consolidation of the tailings slurry.

    Fig. 4.  PWP evolution at the floor of three uncemented columns with different drainage conditions.

    In practical stopes, the drainage of the tailings slurry usually occurs through barricades at the bottom of stopes, which corresponds to the BD condition in this paper. Jaouhar and Li [18] conducted numerical simulations and studied the influence of the number of draining holes installed through barricades on the PWP exerted on barricades. The results showed that increasing the number of draining holes can lead to a significant decrease in the PWP. The addition of draining holes on barricades is equivalent to increasing the length of the drainage boundary. As a result, the PWP dissipation is accelerated, which is in accordance with the results in Fig. 4.

    Wick drains or WRI are also used to facilitate drainage of the tailings slurry. The numerical simulations conducted by Li [19] showed that the PWP within the backfilled stope with five wick drains (corresponding to the FD column) was considerably lower than the PWP within the stope with only five draining holes through barricades (corresponding to the BD column), followed by the PWP within the stope without wick drains and draining holes (corresponding to the UD column). The results of the physical model test performed by Saleh-Mbemba and Aubertin [33] showed that the PWP decreased more rapidly when the WRI was at the center of the column (corresponding to the FD column) compared with the PWP in the column without WRI (corresponding to the UD column). Saleh-Mbemba and Aubertin explained that the decrease in PWP was caused by horizontal water drainage toward WRI due to a horizontal seepage path allowed on the entire height of the slurry. During the sequential backfilling in practical stopes, if the drainage is only allowed at barricades, the length of seepage path of the newly filled slurry at the surface of the previous deposited slurry will increase with the rising filling height, which can cause difficulty for the newly filled slurry to drain and cause a higher PWP. After the wick drains or WRI was applied in stopes, a horizontal seepage path was provided for the upper slurry, which accelerated the PWP dissipation. Therefore, the results in Fig. 4, together with the findings of numerical simulations conducted by Jaouhar and Li [18] and Li [19] and the physical model test performed by Saleh-Mbemba and Aubertin [33], indicate that increasing the length of drainage boundary can accelerate the dissipation of PWP and lead to a lower PWP in the backfilled stope.

    During the tests, bleeding water accumulated on the upper part of the tailings slurries to form a water pond. This part of the water was removed (by siphoning) from the UD and BD columns at 127 and 173 h, respectively, which resulted in a 2.2 and 0.97 kPa drop in PWP. The PWP reduction after the removal of bleeding water at the surface of the tailings slurry was also observed in the column tests conducted by Saleh-Mbemba and Aubertin [33], but no further explanation was given for this phenomenon in their study.

    The water pond removal was equivalent to the removal of hydraulic load at the surface of the tailings slurry, and the PWP on the column floor decreased correspondingly. The hydraulic load at the surface of the UD and BD column backfills can be calculated as 2.19 and 1.83 kPa, respectively, based on the depth of the two water ponds. The discharged hydraulic load was approximately equal, and the decrease in PWP (2.2 kPa) in the UD column was twice as much as that (0.97 kPa) in the BD column. This result was due to differences in the degrees of slurry consolidation when the water pond was removed.

    The degree of consolidation at one point can be defined as the ratio of the amount of PWP dissipation and the initial PWP at this point in the tailings slurry. When the UD and BD columns were fulfilled, the initial PWPs at the bottom were 18.21 and 18.08 kPa, respectively. When the water pond was removed, the PWP in the UD column was 11.49 kPa at t = 127 h, and the value was 7.82 kPa in the BD column at t = 173 h. The degrees of consolidation of the tailings slurry at the bottom of UD and BD columns when the water pond was removed were 36.90% and 56.75%, respectively. A higher degree of consolidation in the BD column means that less amount of self-weight was supported by pore water, which resulted in the relative insensitivity of the PWP to the removal of hydraulic load at the surface. Therefore, a longer drainage boundary can lead to a higher degree of consolidation of the tailings slurry, and a steadier backfill structure can be expected to resist changes in external loads.

    Fig. 5 shows the variations in collected drainage water of the uncemented tailings slurry in the FD and BD columns. The volume of drainage water increased gradually with time, and at the end of the tests, 17.7 and 4.7 kg of water drained from the FD and BD columns through the seal of the nonwoven geotextile, accounting for 30.7% and 8.2% of the initial mixing water of 57.6 kg, respectively. These values were more than three times their differences. This result can be attributed to the fact that for the FD column, bleeding water was not accumulated at the surface to form a water pond but flowed through the drainage boundary near the top and was collected into the bucket. Meanwhile, the bleeding water on the top of the BD column could not drain through the drainage boundary directly and must flow through the entire height of the tailings slurry to the bottom drainage boundary before being collected into the bucket, which means that the pore water stayed in the BD column for a longer time. Therefore, a larger amount of pore water can be expelled from the FD column compared with the BD column at the end of the tests.

    Fig. 5.  Drainage water evolution of the uncemented tailings slurry in FD and BD columns.

    Fig. 6 presents the mass proportion of pore water that expelled from the column, stored in the backfill and accumulated at the surface to form water pond at the end of the tests. The largest amount of expelled water (17.7 kg) was from the FD column, and the biggest water pond (containing 20.07 kg pore water) was found on the top of the UD column. However, the amount of pore water that remained in the backfill was approximately equal to an average value of 37.02 kg in the three columns (36.75, 36.80, and 37.53 kg) with different drainage conditions (Fig. 6). Thus, regardless of the drainage condition, most of the pore water (averagely 64.27wt% of the initial amount of water 57.60 kg) cannot be expelled out from the tailings slurry under the overburden condition because of the poor permeability of the tailings used (2.37 × 10−6 m/s). This part of pore water supported the overburden together with the skeleton formed by contacted tailings particles, and the PWP was sustained for a long time until the pore water evaporated or was consumed partly by the chemical process of cement hydration. As shown in Fig. 4, the PWP in the FD column backfill remained unchanged at 3.7 kPa after 96 h, which indicates the end of consolidation, and evidently, the non-zero PWP for the FD column (Fig. 6) was attributed to the pore water stored in the tailings slurry.

    Fig. 6.  Mass proportion of pore water that was expelled, stored in the backfill, and accumulated at the surface to form water pond at the end of tests.

    Fig. 7 shows the evolution of the settlement of tailings slurries in the three columns. The slurries settled more rapidly when the column had a longer drainage boundary before 72 h. For example, at t = 24 h, the settlements were 8.4, 10.6, and 13.2 cm for the UD, BD, and FD column backfills, respectively. The settlement during the self-weight consolidation of the tailings slurry was relevant to the drainage of pore water. For the UD column, pore water can only be expelled through bleeding at the top surface of the tailings slurry, and for the FD column backfill, apart from bleeding, drainage was allowed mainly through the nonwoven geotextile along the entire height of the column, and it provided a horizontal drainage path for pore water. Thus, settlement can benefit from this condition. After the same period, more water can be expelled from the FD column, which resulted in a larger void space for the rearrangement of the tailings particles’ position. Therefore, the tailings slurry in the FD column with the longest drainage boundary settled more rapidly.

    Fig. 7.  Settlement of uncemented tailings slurries in the three columns.

    As displayed in Fig. 7, the settlement of the three tailings slurries mainly occurred within the first 72–96 h and remained virtually unchanged at approximately 22 cm regardless of drainage conditions. This finding was due to the identical tailings used for column tests. Given the identical tailings and the same mix recipe used, the uncemented tailings slurry in the three columns had identical initial properties, including PSD and void ratio. During consolidation, pore water in the tailings slurry was expelled under the force of gravity. Then, settlement occurred, and the void ratio decreased, which resulted in a stiff skeleton formed by the contacted tailings particles and made the skeleton less deformable. When the stiffness of the skeleton was large enough, the settlement ceased and was no longer significantly affected by gravity [32]. As observed in Fig. 6, a nearly equal volume of pore water was stored in the void space of the tailings slurry at the end of the tests, and this finding suggests a nearly equal void ratio and stiffness of the tailings slurry in the three columns. Therefore, the equality of the initial and final void ratios and stiffness led to a similar settlement of the tailings slurry.

    The results on the influences of drainage conditions on the settlement of the tailings slurry differed from those reported by Belem [36]. In his study, columns with three drainage conditions were developed: a full lateral drainage column where drainage was allowed along the entire height of the column, a partial drainage column where drainage was allowed only on the lower half of the column, and an undrained column where drainage was not allowed. The results showed that the settlement of the full lateral drainage column was twice that of the similar settlements of the partial drainage and undrained columns. However, the settlements of columns with three drainage conditions were almost the same as those observed at the end of the tests in Fig. 7. The difference can be caused by the filling sequence, column scale, type of the tailings slurry used in the column tests, and cement addition in all columns slurry by Belem et al. [36]. In Belem’s study, column backfilling was completed by two layers with a rest time of 12 h, and the settlement of layer 1 was not measured during the tests. In addition, the column height in Belem’s study was 3 m, which allowed the settlement to occur more considerably under gravity. Moreover, the CPB containing 4.5wt% binder and with a solid mass concentration of 75.8% was used.

    Fig. 8 presents the residual water content distribution along the height of the tailings slurry in the three columns with different drainage conditions at the end of the tests. The residual water content was high (42.00wt% to 46.00wt%) near the surface of the tailings slurry in the three columns and close to the initial water content (47.10wt%) of the slurry after preparation. This finding can be attributed to the downward seepage of the bleeding water that accumulated at the surface of the slurry. In addition, the residual water content had a significant reduction toward the height of 70 cm and reached 30.00wt% to 33.00wt%. This means that the area affected by downward seepage on the upper part (70~86 cm) was approximately 18.60% of the slurry height (86 cm), and the residual water content of the slurry below 70 cm was not affected.

    Fig. 8.  Distribution of residual water content along the height of the tailings slurry in the three columns at the end of tests.

    The higher water content at the surface in Fig. 8 is inconsistent with the results of previous studies. The column tests conducted by Ghirian and Fall [24] revealed that wet density and saturation at the surface were lower than those in other parts. This difference can be caused by the used CPB and evaporation in their study [24]. The solid content of the CPB was 75wt%, which resulted in less bleeding water accumulation at the surface. In addition, the top of the column remained open during tests, and evaporation can occur. Thus, the wet density and saturation of the CPB at the surface can be lower, and the microcracks caused by evaporation and unsaturation were observed at the surface. In this study, the solid content of prepared slurry was 68wt%, and evaporation was not allowed during tests. Thus, water content at the surface was high due to the downward seepage from the formed water pond.

    The residual water content of the tailings slurry below 70 cm decreased noticeably toward the bottom of the column, and that in the UD column was always the maximum, followed by those in the BD and FD columns. For example, the residual water contents at the height of around 60 cm reached 31.85wt%, 30.66wt%, and 29.45wt%, and at the height of 40 cm, the values decreased to 30.90wt%, 29.10wt%, and 28.51wt% for the three columns, respectively. This result can be attributed to the high overburden near the bottom and the slurry was compressed and became denser. As a result, the water content was lower than the upper part. The FD column had the longest drainage boundary. Thus, more pore water can be expelled, and the tailings slurry had a low residual water content.

    Fig. 9 presents the PWP evolution at different heights of the tailings slurry with 6.25wt% cement within 2 h after pouring. The PWP showed a stepwise increase over time during pouring, which was similar to the results of the uncemented tailings slurry shown in Fig. 2. After pouring, PWP decreased slightly with time. For example, the PWP on the column floor (height h = 0 cm) decreased from 18 kPa at 0.4 h to 17.5 kPa at 2 h. This result showed the effect of water absorption for cement hydration on PWP dissipation.

    Fig. 9.  PWP evolution at different heights of the cemented tailings slurry with 6.25wt% cement addition within 2 h after pouring.

    Fig. 10 shows the evolution of PWP with time monitored by the floor sensor in the cemented tailings slurry with different cement dosages. The monitored PWP of the uncemented backfill in the FD column is also presented in Fig. 10 for comparison. At the end of pouring, the PWP of the three BD columns reached approximately equal peak values (18.43, 18.33, and 18.08 kPa for the tailings slurry with 0, 4.76wt%, and 6.25wt% cement addition, respectively), which were close to the initial overburden stress of 18.98 kPa. This result demonstrates that the cemented slurry exhibited a fluid-like behavior at an early age [26,38]. After pouring, the PWP of the two cemented slurries decreased rapidly and remained unchanged at 7.5 kPa after 96 h, and that of the uncemented slurry in the BD column was the highest and decreased slowly but could not reach a stable value during the testing period. The difference in the PWP evolution between cemented and uncemented slurry indicates that cement hydration can accelerate PWP dissipation during consolidation.

    Fig. 10.  Evolution of PWP with time monitored by the floor sensor in the tailings slurry with different cement dosages.

    As shown in Fig. 10, within the first 96 h, the contribution of cement hydration to PWP dissipation was significant when 4.76wt% cement was added to the uncemented tailings slurry. At 72 h, the PWPs of the slurry containing 0 and 4.76wt% cement were 12.2 and 8.4 kPa, respectively, and they showed a decline of approximately 31%. However, as more cement was added, the effect of cement hydration became less considerable. For example, at 72 h, the PWP of the slurry with 6.25wt% cement was near 8.2 kPa, which is close to the PWP of the slurry containing 4.76wt% cement. Thus, increasing the cement dosage by 31.25% (from 4.76wt% to 6.25wt%) can only lead to a 2.38% (from 8.4 kPa to 8.2 kPa) decline in PWP at the bottom of the BD column.

    After 96 h, the PWP of the uncemented slurry in the BD column decreased gradually and was close to the stable value of the two cemented slurry. The removal of the water pond did not affect the similarity of PWPs at the bottom of the three BD columns.

    For the cemented slurry, the similarity of PWPs can be explained by the high stiffness of the tailings slurry at the bottom and the water supply from the upper part slurry. The PWP reduction caused by cement hydration is due to the net volume reduction (or self-desiccation), which means that the volume of the hydrated cement is less than the combined volume of the cement and water prior to hydration [1,24,32]. Helinski et al. [23] proposed an analytical model to quantify the relationship between the PWP change and the amount of volume change associated with cement hydration and the incremental stiffness change in the slurry. Given that no inflow of water is allowed in Helinski’s model, the lost volume associated with cement hydration can be accommodated by pore water expansion and skeleton compression, leading to a lower PWP. If no inflow of water comes from the upper part to the bottom of the BD column, the PWP on the floor should decrease with the increase in cement content, according to Helinski’s model. However, at the bottom of the BD column, the inflow of water was allowed in the column. Indeed, the pore water flowed from the upper part to the bottom drainage boundaries. The water supply from the upper part dominated the accommodation of the lost volume associated with cement hydration instead of skeleton compression because the skeleton of the tailings slurry was less deformable after 96 h (Fig. 11). In addition, pore water expansion was less likely to occur due to the high water bulk modulus (2.1 GPa). The water supply from the upper part eliminated the effects of cement hydration on the PWP at the bottom, and this gravity-driven seepage had no effect on the PWP. Therefore, increasing the cement dosage cannot result in further PWP reduction at the bottom of the BD column.

    Fig. 11.  Evolution of the settlement of cemented tailings slurry with different cement contents in the BD column.

    For the uncemented slurry, PWP dissipation mainly relied on pore water drainage. After a long period, the skeleton of uncemented slurry became less deformable under the overburden condition, and the settlement almost ceased (Fig. 11). Then, the pore water drainage associated with consolidation and PWP dissipation stopped. Given that the overburden pressures (total stress) of the slurry at the bottom of three columns were similar, and no more stress was transferred to the skeleton due to high stiffness, the PWP of the uncemented slurry after 168 h can be approximately equal to those of the cemented slurry after sufficient time has passed.

    As shown in Fig. 10, the final stable PWP of the two cemented slurry was nearly 7.5 kPa, which was relatively higher than the PWP (approximately 3.7 kPa) of the uncemented slurry in the FD column. The considerable difference between them indicates that a good drainage condition may be more favorable to PWP dissipation than cement addition. This scenario will be discussed in more detail later in Discussion Section.

    Fig. 11 presents the evolution of the settlement of the tailings slurry with different cement contents in the BD column. The cemented tailings slurry consumed less time to reach the final stable settlement than the uncemented tailings slurry. Meanwhile, the final settlement of cemented slurry was considerably smaller. The settlement of the cemented slurry with 6.25wt% and 4.76wt% cement addition mainly occurred within the first 96 h and reached stable values of 13.4 and 14.6 cm, respectively. However, the settlement of uncemented slurry mainly occurred within the first 120 h and did not reach a distinctly stable value at around 21 cm before the end of the tests. The tailings slurry’s settlement was associated with the stiffness of the skeleton formed by the contacted tailings particles. The hydration products can fill the pores between the tailings particles, which resulted in increased stiffness [20,22]. The stiffer skeleton caused additional difficulty for the backfill to compress under gravity. As a result, the cemented backfill settled less as more cement was added.

    Fig. 12 shows the effect of cement dosage on the volume of water drained from the column. The volume of drained water increased with consolidation time. At the end of tests, 4.70 kg water drained from the uncemented slurry, and 5.6 and 5.7 kg water drained from the cemented tailings slurry. This finding indicates that more water can be drained from the cemented slurry compared with the uncemented slurry, which is counterintuitive. Many studies [20,2324] have demonstrated that the permeability of cemented tailings backfill decreased as the cement content increased; thus, the cemented slurry would have lower hydraulic conductivity and drain less volume of water than the uncemented slurry. However, the existing research on the effect of cement content on the permeability of cemented slurry is unsuitable for explaining the phenomenon in Fig. 12 because the previous results were mainly tested on cylindrical backfill samples measuring 50 mm in diameter and 100 mm in height. In addition, the influences of settlement and consolidation on the permeability of the tailings slurry were not considered. In the large-scale physical model test using the columns in this study, the uncemented slurry can undergo considerable compression and settlement due to overburden. Then, the tailings particles contacted tightly, which resulted in less connected capillary pores for water flow paths leading to a significant reduction of permeability. For the cemented slurry, although cement hydrates grew and filled the void space between tailings particles to decrease permeability, the stiffness of the cemented tailings skeleton increased to improve the resistance of compression and settlement, which left more void space for water flow to suppress the reduction of permeability. Therefore, the hydraulic conductivity of the cemented slurry depends on the competition between the gravity-driven settlement to compress the drainage path and cement hydration to increase the stiffness of cemented tailings skeleton to support the drainage pores. As shown in Fig. 12, the effect of cement hydration on increasing stiffness accounted for a larger advantage, and the cemented slurry showed a relatively good permeability for more water drainage. However, on the contrary, the uncemented slurry settled more and had a poorer permeability, which led to a lower volume of pores for water drainage. However, if more cement is added, the water drainage of cemented slurry will decrease because the surplus hydrate will seal the flow paths and consume more water.

    Fig. 12.  Drainage water of the cemented tailings slurry with different cement dosages.

    The increase in stiffness, outweighing the reduction in permeability during cement hydration, has also been reported by Helinski et al. [40]. In his study, numerical simulations were performed to explore the influence of cementation on the PWP evolution in backfilled stopes. The results showed that the PWP of the cemented case (self-desiccation was disabled) dissipated more rapidly than that of the uncemented case. Helinski et al. also explained that the increase in stiffness, outweighing the reduction in permeability, caused the more rapid PWP dissipation in the cemented case. The experimental results in Fig. 12, together with Helinski’s numerical simulation, illustrate that the competition between the increase in stiffness and the reduction in permeability can produce counterintuitive outcomes.

    Fig. 13 presents the residual water content distribution along the height of the tailings slurry with different cement contents at the end of the tests. The water pond at the surface significantly affected the residual water content of the tailings slurry above 80 cm, where the water content was over 35.00wt%. This result can be attributed to the re-entry of the downward seepage to the tailings slurry, which is similar to that in Fig. 8. As shown in Fig. 13, variations in the residual water content of the tailings slurry with 4.76wt% cement were not uniform along the height. At the height below 80 cm, the residual water content of the backfill with 6.25wt% cement was around 35.00wt% and the highest among the three tailings slurries. This result can be caused by the low settlement of the slurry containing 6.25wt% cement and the method that was used to measure water content. The lower settlement of the slurry with 6.25wt% cement compared with that of the uncemented slurry (Fig. 11) led to the formation of more pores between tailings particles to accommodate water. In addition, the oven-drying method was used to measure the residual water content of the slurry, and a relatively high temperature of 100°C was set. The bound water converted by cement hydration from the pore water evaporated into the air due to the high temperature. Then, the bound water was included in the mass loss during calculations. Therefore, at the height below 80 cm, the slurry with 6.25wt% cement had the highest residual water content at the end of the tests.

    Fig. 13.  Distribution of residual water content along the height of the tailings slurry with different cement contents at the end of the tests.

    The physical model tests considered three drainage conditions, including UD, BD, and FD columns, to investigate the influence of drainage conditions on the self-weight consolidation behaviors of the tailings slurry. The drainage conditions of the column were achieved by inserting the elastic rubber mat and nonwoven geotextiles between the front plate and the main body of the column. This scenario means that the drainage of the slurry was only allowed to occur through the interfaces located on two vertical sides of the front plate. However, in practical stopes, drainage can transpire through the joints or fractures distributed in the surrounding rock walls. Therefore, the drainage boundaries in practical stopes can be the whole surface of the surrounding rock walls rather than uniformly arranged together in two interfaces. The difference in the spatial distribution of the drainage boundaries between the physical model and practical stope was not considered in the column design. However, the results of the column tests with the three drainage conditions, including UD, BD, and FD columns, can still provide references for stope backfilling. More work should be conducted to study the influence of the distribution of drainage boundaries on the self-weight consolidation behaviors of the tailings slurry.

    The experimental results presented in Fig. 10 indicate that the PWP of the uncemented tailings slurry in the FD column was consistently lower than the PWP of the cemented tailings slurry in the BD column after 24 h. This shows that a good drainage condition can be more favorable to PWP dissipation than increasing the cement dosage. A common practice of underground mining is dividing mine stopes into primary and secondary stopes. The former is first mined and filled with cemented backfill and must remain self-supporting during the recovery of secondary stopes, and the latter is excavated and usually filled with uncemented slurry because no side walls are exposed, and the self-supporting performance is not required like in primary stopes. The main security issue in secondary stopes is keeping the barricades stable during and after filling. Therefore, constructing an effective drainage structure in secondary stopes, such as the application of wick drains and draining holes on barricades, rather than increasing the cement dosage to promote self-weight consolidation, can be a better way to maintain barricade stability and reduce the backfill costs of underground mining. However, the result in Fig. 10 was obtained from the low-density (solid content is 68wt%) tailings slurry. More work is required in the future for the high-density paste slurry without water bleeding.

    To avoid the pressure exerted on the barricades exceeding their capacity, scholars generally divide the stope filling into plugging and final pouring, which are separated by an interval of several days. During this interval, the plug pour must be consolidated and gain sufficient strength to prevent barricade failure before the final pour begins. After consolidation and curing, the permeability of the plug can be very small because the slurry used for the plug pour usually has high cement content (15wt%–25wt% in China). As a result, the drainage condition of the final pour will be close to the UD condition if no wick drains are applied in the stope and the surrounding rock mass is intact. In addition, the PWP in the final pour can be very high and is difficult to dissipate (Fig. 4). A large amount of the tailings slurry (the height of the final pour can be more than 30 m) with a high PWP in the stope can lead to serious consequences, such as liquefaction, when mining the adjacent stopes. Furthermore, a longer waiting time is allocated to dissipate the PWP in the tailings slurry, which reduces mining productivity. The situation will be aggravated if more cement is added to the plug pour to improve its strength because the permeability of the plug becomes poorer accordingly. As a result, the drainage condition of the final pour is closer to the UD condition. Therefore, measures should be implemented to increase the plug strength, but the corresponding changes in the drainage condition of the final pour should also be considered because they concern the security and mining productivity in backfilled stopes.

    A water pond formed at the surface of the tailings slurry during the tests, and this phenomenon has also been observed in field monitoring [26]. The existence of the water pond means that the free drainage and zero PWP at the top boundary are not real situations in many analytical solutions and numerical simulations. Shahsavari and Grabinsky [41] studied the effect of this top boundary condition on the barricade pressure during consolidation, and the results showed that the presence of the overload associated with the water pond at the surface of consolidated backfill led to a higher maximum total pore pressure, which is consistent with the results of the current paper. As mentioned above, the removal of the water pond can cause a considerable drop in PWP (Fig. 4), especially in poor drainage conditions (UD). Moreover, the residual water content of the slurry near the surface increases considerably as a result of the re-entrant bleeding water. In practice, the re-entrant water can weaken the physical and mechanical properties of backfill skeletons, including elastic modulus and uniaxial compressive strength [24,32]. Furthermore, when mines use a strategy to backfill stopes in two stages that include a plug pour and a final pour, the bleeding water at the top of the plug will be locked in the stope after the final pour is placed [36]; this condition results in a zone of weakness between the plug pour and the final pour and may have an adverse effect on the stability of the side-exposed backfilled stopes. Therefore, certain measures must be applied to remove the water pond at the surface of the consolidating tailings slurry, which can lead to not only a lower PWP in the slurry but also a more complete and homogeneous backfill structure.

    For the uncemented tailings slurry in the FD column, at around 96 h after pouring, the settlement (Fig. 7) and PWP (Fig. 4) both reached stable values. Similarly, for the cemented tailings slurry in the BD column, the settlement evolution in Fig. 11 and the PWP evolution in Fig. 10 showed the same relationship. This finding reflects the correlation between the settlement and PWP of the tailings slurry during self-weight consolidation. The consolidation of the tailings slurry in stopes was driven by gravity, under which the skeleton deformed and imposed pressure on pore water and caused the generation of excess PWP. For the uncemented tailings slurry, the stiffness can increase during the settlement due to the closer contact of tailings particles. For the cemented slurry, apart from settlement, cement hydration can contribute to the increase in stiffness. When the stiffness increased to a value that was sufficient to sustain gravity, the tailings slurry stopped settling. The skeleton did not deform and extrude the pore water further, and no excess PWP was generated. Therefore, the settlement and the PWP reaching stable values can be regarded as a symbol of the end of the self-weight consolidation of tailings slurry.

    A series of column tests were conducted to investigate the effect of drainage condition and cement addition on the self-weight consolidation behavior in terms of PWP, settlement, the volume of drainage water, and residual water content in the final drained and consolidated backfill. The main conclusions are summarized as follows.

    (1) Increasing the length of the drainage boundary can reduce the time required to dissipate the PWP of the uncemented tailings slurry and result in a lower final stable PWP. Cement hydration can accelerate PWP dissipation, but the influence of cement addition on the PWP was not as significant as that of the drainage condition in the tests. In addition, the final stable PWP on the column floor was not sensitive to cement addition.

    (2) The settlement of the tailings slurry mainly occurred within the first 72–96 h after pouring. Increasing the length of the drainage boundary can accelerate the settlement, although the final settlement of the uncemented slurry was independent of the drainage condition. A larger amount of cement added to the slurry can accelerate the settlement and reduce the final settlement.

    (3) For the unclassified tailings used, most of the pore water (approximately 64.2% of the initial volume of water) remained in the slurry and cannot drain out at the end of tests regardless of the drainage condition. More pore water can drain out from the cemented slurry than the uncemented slurry in the BD column due to its higher permeability, which was caused by the reduced settlement of the cemented slurry.

    (4) Affected by the water pond, the residual water content of the slurry near the surface was substantially higher than that in the lower part of the column at the end of the tests. Beyond the affected area, the residual water content decreased with the depth of the slurry. Higher residual water content can be expected when the drainage boundary is shorter or when more cement is used.

    (5) A considerable reduction in the PWP occurred by removing the water pond at the surface of the slurry, and the reduction was dependent on the average degree of consolidation. The PWP of the slurry with a high average degree of consolidation showed a low sensitivity to water pond removal.

    This work was financially supported by the Young Scientist Project of the National Key Research and Development Program of China (No. 2021YFC2900600) and the Beijing Nova Program (No. 20220484057). The first author acknowledges the financial support from China Scholarship Council under Grant CSC No. 202110300001.

    Lijie Guo is a youth editorial board member for this journal and was not involved in the editorial review or the decision to publish this article. All authors declare that there are no competing interests.

    The online version contains supplementary material available at https://doi.org/10.1007/s12613-023-2642-5.

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    1. Wei Sun, Di Wu, Erol Yilmaz, et al. Delving into the role of microwave curing on the strength and cement hydration features of cemented tailings backfill. Green and Smart Mining Engineering, 2024, 1(3): 289. DOI:10.1016/j.gsme.2024.09.001
    2. Dapeng Chen, Shenghua Yin, Weiguo Long, et al. Heterogeneous information phase space reconstruction and stability prediction of filling body-surrounding rock combination. International Journal of Minerals, Metallurgy and Materials, 2024, 31(7): 1500. DOI:10.1007/s12613-024-2916-6
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    4. Shulong Liu, Yiming Wang, Aixiang Wu, et al. Early mechanical strength, hydration mechanism and leaching behavior of alkali-activated slag/fly ash paste filling materials. Journal of Building Engineering, 2024, 84: 108481. DOI:10.1016/j.jobe.2024.108481
    5. Qian Yin, Fan Wen, Zhigang Tao, et al. Effects of aggregate size distribution and carbon nanotubes on the mechanical properties of cemented gangue backfill samples under true triaxial compression. International Journal of Minerals, Metallurgy and Materials, 2024. DOI:10.1007/s12613-024-3014-5
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