Barricade pressure monitoring in paste backfill

Transcription

1 GOSPODARKA SUROWCAMI MINERALNYMI Tom Zeszyt 4/3 M. YUMLU* Barricade pressure monitoring in paste backfill Introduction Paste backfill was first used in underground mining about 30 years ago in Germany. Since then mine operators have embraced this technology due its technical, operational and environmental advantages over other backfill methods. Paste backfill is an engineered high density non-newtonian slurry made from single or a combination of several suitable solid materials mixed with a preset amount of binder and produced to a toothpaste consistency. Paste is delivered underground through surface boreholes by gravity or pumping and reticulated underground using a network of internal boreholes and pipelines. Upon completion of ore, barricades are constructed at the draw points of open stopes to contain paste fill while the initial pour cures and forms a plug. Mobilization of uncured paste fill as a result of a barricade failure is a potential safety hazard and can lead to significant consequences, including endangering the safety of personnel, property damage and production losses and delays. The stability of barricades forms an integral part of a successful paste backfill application. Safe and efficient placement of paste fill requires a detailed understanding of paste fill characteristics from the production stage to the final fill exposures. Despite being a mature and well proven technology the early age behaviour of paste fill and the mechanism of pressure development within paste fill and the resultant loads acting on the barricades are not well understood. Mine operators usually borrow barricade designs and fill placement procedures from other mines without due consideration to their particular circumstances. Consequently, barricade failures continue to be reported in paste fill operations around the world. * AMC Consultants Pty Ltd, Melbourne.

2 234 This paper presents details of an in-situ instrumentation and monitoring program to ascertain the loads acting on the paste fill barricades during and after filling and to understand the pressure development mechanism within paste fill mass. Three stopes were instrumented using total pressure cells and piezometers. The pressures induced in the paste fill (horizontal and vertical) and on the barricade (horizontal) were recorded during and after placement of paste fill. 1. Paste fill application 1.1. Paste fill recipe and properties Paste fill is made using total mill tailings. The tailings are very fine with a P90 of 63 microns. Depending on the mill feed rate and the ore blend the material passing 20 microns can be 50 60%. The mine uses cement (CEM IV 32.5) as a binder and fills primary stopes at average cement content of 7% and secondary and tertiary stopes at 5% cement content. The slump varies between 175 mm and 190 mm and the target design strength is 1.0 MPa after 28 days curing age Construction of barricades and fill management system Shotcrete barricades are used to contain freshly placed paste fill in open stopes. The barricades are large measuring 7 m wide and 5 m high. The barricades are made up of 300 mm thick shotcrete reinforced with 28 mm diameter rebar, which are tied to the 20 mm diameter rebar bolts. There are limitations to the application of an arched shotcrete barricade at this particular mine due to the constraints imposed by the stope access geometry. The location of the sublevel haulage within the orebody along the footwall or hanging wall contact provides a restricted area for the construction of arched shotcrete barricade. As a consequence, a planar, flat shotcrete barricade is required. 2. Paste fill pressure monitoring tests 2.1. Pressure instruments Instruments used for the tests consisted of total pressure cells, contact pressure cells, piezometers and a data logger. Total pressure cells were used to monitor stress change in the paste fill while the contact pressure cell was used to monitor load on the barricades. Piezometers were used to monitor pore water pressure in the paste fill.

3 The piezometers used for these tests were Geokon Model 4500S units. These piezometers had a filter head of 50 micron size and the pressure capacity was 350 kpa with an accuracy of ± 0.1% of full scale. The instrument had a built-in thermistor for measuring the temperature in the fill. The total pressure cells used were Geokon Model 4800 units. The capacity of the total pressure cells used was 350 kpa with ± 0.1% of the full scale. They had built in thermistors for measuring the temperature during pressure recording. These cells are designed to measure soil and backfill pressures on structures. The back plate of the cell, which bears against the external surface of the structure, is thick enough to prevent the cell from warping. The other plate is thin and is welded to the back plate in a manner, which creates a flexible hinge to provide maximum sensitivity to changing soil or backfill pressures. The contact pressure cells used were Geokon Model 4810 units. The capacity of the contact pressure cells used was 350 kpa ± 0.1% of full scale. They had built in thermistors for measuring the temperature during pressure recording. All of the instruments were connected to an 8 channels Canary Data Logging System for real time recording. The sampling rate was set at 5 minutes and the readings were periodically downloaded to a lap top computer Instrumentation setup and barricade construction In all tests a 300 mm thick rebar reinforced planar shotcrete barricade was constructed at the base of the open stopes. Figure 1 shows the instrumentation setup Figure 2 shows the sequence for placement of instruments and construction of shotcrete barricades. The pressure cells were placed at two locations within the stope; in the centre of the stope and behind the shotcrete barricade. Only horizontal pressure was monitored at the barricade. In the stope vertical pressure and horizontal pressure along the length of stope were monitored. Fig. 1. Paste fill pressure instrumentation setup Rys. 1. Konfiguracja przyrz¹dów do monitorowania ciœnienia

4 236 Fig. 2. Placement of instruments and construction of shotcrete barricades Rys. 2. Umieszczenie przyrz¹dów i konstrukcji barykad torkretowych

5 Stope instruments were mounted on a rigid steel frame attached to a heavy metal box to prevent it from floating. The instrument frame was 1.2 m long, 0.60 m wide and 0.60 m high. A contact pressure cell was mounted at the back of a large steel plate (1m wide 1m high and 5 mm thick) in order to protect it from damage during shotcreting. The contact cell was installed in the centre of the shotcrete barricade at 0.5 m from the floor of the stope. No piezometer was installed at the barricade. The stope floor was leveled and the frame was pushed into the stope in a bucket of a remote controlled LHD. All instrument lead wires were protected using 25 mm plastic hoses. Table 1 lists summary information for the three tests described in this paper. 237 Description of pressure monitoring tests Opis prób monitorowania ciœnienia TABLE 1 TABELA 1 Description of the tests Test 1 Test 2 Test 3 Stope type Primary, Open Secondary, Blind Primary, Blind Average stope size L W H m m m Stope volume 2,185 m 3 1,862 m 3 1,546 m 3 Instruments locations Inside stope-middle Vertical [email protected] m m m Barricade Contact m Inside stope-middle Vertical [email protected] m m m Barricade Contact m Inside stope-3m away Vertical [email protected] m m m Barricade Contact m Barricadesize 7mW 5mH 7mW 5mH 7mW 5mH Barricade offset None 1 m None Barricade type Shotcrete Shotcrete Shotcrete Total fill pumped 2,500 m 3 2,044 m 3 1,778 m 3 Filling sequence Single stage Two stage Twodaysplugcure Two stage Sevendaysplugcure Fill pausing time 8 hours 15 hours 1.5 hours Total filling time 58 hours 54 hours 49 hours Fill completion 2.8 days 5 days 9 days Avg. fill rate 42 m 3 /h 37 m 3 /h 36 m 3 /h Avg. fill rise rate 0.35 m/h 0.35 m/h 0.35 m/h Slump 192 mm 192 mm 195 mm Cement 7.43% 7.40% 7.05%

6 238 The first instrumented stope was a primary transverse open stope exposing ore pillars on the north and south sidewalls. The stope was vertical in shape and was accessible from the over cut sill drift. The stope had an average width of 6.2 m, length of 19.5 m and height of 19.5 m. About 5,000 tonnes of ore were mucked from this stope in a period of 12 days. This stope was filled at a fill rise rate of 0.35 m per hour (8 m per day) continuously in a single stage. The second test was conducted in a small blind secondary stope exposing paste fill at the back and on the sidewalls. The stope width was 8 m; the height was 17 m and the length was 13.6 m. The total CMS stope volume was 2185 m 3. Stope production was completed in eight days and about 3,000 tonnes of ore were mined. This stope was filled in two stages; stage one included 7 m high plug fill and stage two starting after 2 days plug cure time. Fill rise rate was 0.35 m per hour (8 m per day). The third instrumented stope was a second part of a long primary transverse blind stope exposing ore pillars on the north and south sidewalls and paste fill on the footwall side. The stope had an average width of 7.0 m, length of 14.9 m and height of 20.0 m. This test stope was also filled in two stages; stage one included 7 m high plug fill and stage two starting after 7 days plug cure time. Fill rise rate was 0.35 m per hour (8 m per day). 3. Pressure monitoring results 3.1. Pressure results Table 2 lists a comparison of key parameters measured during all three tests. The peak pressures values were normalized maximum values recorded during filling. In all three tests the filling rate was set at 0.35 m/h for direct comparison. Normalized barricade pressures for all three tests are plotted in Figure 3 through to Figure 7. Although the tailings, the slump, the cement content and the vertical fill rise rates were the same for all three tests the measured barricade pressures were different for each test. Test results indicate the following: Measured paste fill loads acting on the shotcrete barricades vary between 50 kpa to 100 kpa. Maximum pore water pressure of 100 kpa was recorded in Test 1 with continuous filling. Peak vertical pressures measured inside stope are higher than horizontal fill pressures. Lateral loads acting on the barricades are lower than horizontal fill pressures inside stope. During plug cure time total pressures initially drop with time and then increase indicating possible temperature effects on the readings. Pore water pressure drops as results of cement hydration and consolidation.

7 239 Summary of pressure monitoring test results Podsumowanie wyników prób monitorowania ciœnienia TABLE 2 TABELA 2 Measured Parameters Test 1 Test 2 Test 3 Stope name S860 S15 S820 N07 S880S13-P2 Stope type Open Blind Blind Peak barricade pressure 98 kpa 50 kpa 72 kpa Peak vertical pressure Not measured 90 kpa 90 kpa Peak horizontal pressure Not measured 75 kpa 90 kpa Peak pore water pressure 100 kpa 53 kpa 53 kpa Peak temp. barricade C (+15 C) C (+11 C) C (+11 C) Peak temp. stope C (+17 C) C (+16 C) C (+16 C) End of geostatic loading 10 hours (3.5 m) 6 hours (2.1 m) 4 hours (1.4 m) Start of arching 20 hours (7.0 m) 12 hours (4.2 m) 18 hours (6.3 m) Fig. 3. Paste fill pressure results for Test 1 Rys. 3. Wyniki wype³nienia ciœnienia dla Próby 1

8 240 Fig. 4. Paste fill pressure results for Test 2 Rys. 4. Wyniki wype³nienia ciœnienia dla Próby 2 Fig. 5. Paste fill pressure results for Test 3 Rys. 5. Wyniki wype³nienia ciœnienia dla Próby 3

9 241 Fig. 6. Comparison of barricade pressure results for all three tests Rys. 6. Porównanie wyników ciœnienia dla wszystkich trzech prób Analysis of the results indicates the following common pressure development process: Initially measured pressures are equal and correspond to geostatic overburden load. This is observed during the first 4 to 10 hours corresponding to fill heights of 1.4to3.5m. Later pressures deviate from the geostatic pressures in response to dissipation of pore water pressure as a result of cement hydration. The fill gains cohesive shear strength with time as cement hydration continues. Continued filling pushes paste fill solids together creating effective stresses within fill mass, which leads to development of fill arching. Development of full arching starts after 12 to 20 hours corresponding to a fill height of 4.2to7.0m Temperature measurements Each pressure instrument had a built in thermistor which measured temperature variations during and after filling. Figures 13 to 15 show the temperature profiles in the paste fill for all three tests. The following general conclusions can be made about the temperatures in paste fill: Except for the first test, the paste fill temperature inside the stope is higher than the one at the barricade. The rate of fill temperature increase at the barricade is lower than that measured inside the stope.

10 242 Fig. 7. Measured paste fill temperature profile for Test 2 Rys. 7. Profil zmierzonej temperatury wype³nienia dla Próby 2 The fill temperature inside the stope increases by about 16 degrees over five days. The fill temperature decrease after five days. The rate of temperature drop is slower and similar in magnitude both at the barricade and inside the stope. Furthermore, ignoring the effect of nearby mining the following temperature effects were observed: During plug cure time pressures increase in response to temperature increase. After filling is completed pressures drop in response to temperature drop. Measured paste fill uniaxial compressive strength values are typically less than 100 kpa at five days cure age. The temperature effects during the first five days will therefore be limited due to low deformation modulus of paste fill compared pressure cell stiffness. Measured pressure readings reported in this paper are therefore not corrected for temperature effects. Conclusions Based on the pressure monitoring test results described in this paper, the following general conclusions can be drawn: Despite the same fill recipes and fill rise rates used, barricade pressures were different. The is attributed to different stope size and filling sequences. Continuous filling with no cure time leads to higher barricade pressures. This is due to the higher pore water pressure and slower pore water pressure dissipation with ongoing filling.

11 243 At the same fill rise rate, staged filling results in lower barricade pressure. This is attributed to the higher fill strength in the plug fill. The rest times enables pore water pressure dissipation and longer fill cure times. The weight of ongoing filling is therefore partially distributed to the stope sides as a result of arching. Temperature changes during cement hydration can have an impact on the pressure readings. Test results indicate that an increase in temperature increases pressure and a decrease in temperature decreases pressures. Based on the pressure monitoring tests, the following filling and placement schedule were developed: Fill stopes in two separate stages. Limit the fill rise rate to 8 m per day (0.35 m per hour). The initial filling comprise 7 m plug fill cured for 7 days. Use higher cement for plug fill irrespective of primary or secondary stope. Fill the rest of stope continuously. The pressures generation process is very complex and site specific. Paste fill pressures depends on many factors such as tailings properties (type, particle sizing and specific gravity), paste fill recipe (slump, solids content, and cement type and content), filling rate, filling placement sequence and stope size and geometry. These pressure monitoring results and conclusions drawn should therefore be applied to other paste fill operations with some degree of caution. The author is grateful to the management of AMC Consultants Pty Ltd for the motivation and help during the preparation of this paper. MONITOROWANIE CIŒNIENIA BARYKADY W MATERIALE PODSADZKI S³owa kluczowe Wype³nienie, podsadzka, torkret, barykady, przegrody, monitorowanie ciœnienia, awaria barykady, komórki ciœnienia ca³kowitego, piezometry Streszczenie Mimo, e jest to dojrza³a i dobrze sprawdzona technologia, pocz¹tkowe zachowanie umieszczonego wype³nienia i mechanizm powstawania ciœnienia w wype³nieniu i powsta³e obci¹ enia dzia³aj¹ce na barykady nie s¹ dobrze zrozumiane. Operatorzy kopalni zwykle zapo yczaj¹ projekty barykad i procedury wype³nienia z innych kopalni bez uwzglêdniania specyficznych okolicznoœci. Wskutek tego, na ca³ym œwiecie nadal zg³aszane s¹ awarie barykad podczas wykonywania wype³nieñ. Bezpieczne i efektywne wykonanie wype³nienia wymaga szczegó³owego rozumienia charakterystyki wype³nienia od etapu produkcji do ostatecznej ekspozycji wype³nienia. Opracowanie przedstawia szczegó³y przyrz¹dów terenowych i programu monitorowania dla oceny obci¹- eñ wype³nienia dzia³aj¹cych na barykady i dla zrozumienia mechanizmu powstawania ciœnienia w masie wype³niaj¹cej.

12 244 BARRICADE PRESSURE MONITORING IN PASTE BACKFILL Key words Paste fill, backfill, shotcrete, barricades, bulkheads, pressure monitoring, barricade failure, total pressure cells, piezometers Abstract Despite being a mature and well proven technology, the early age behaviour of placed paste fill and the mechanism of pressure development within paste fill and the resultant loads acting on the barricades are not well understood. Mine operators usually borrow barricade designs and fill placement procedures from other mines without due consideration to their particular circumstances. Consequently, barricade failures continue to be reported in paste fill operations around the world. Safe and efficient placement of paste fill requires a detailed understanding of paste fill characteristics from the production stage to the final fill e posures. This paper presents details of a field instrumentation and monitoring program to ascertain the paste fill pressure loads acting on the barricades and to understand the pressure development mechanism within paste fill mass.