Flowmeter Tests in Pak Kong Slope
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Flowmeter Tests in Pak Kong Slope (2nd draft) Dr Jimmy Jiao Department of Earth Sciences The University of Hong Kong September 13, 2007 Flowmeter Tests in Pak Kong Slope Contents 1. Introduction ................................................................................................................. 3 2. About the site and boreholes ....................................................................................... 3 3. Basics of a flowmeter test ........................................................................................... 5 3.1 Equipment .............................................................................................................. 5 3.2 Theory .................................................................................................................... 6 4. Test and results ........................................................................................................... 9 4.1. Tests in PH3 .......................................................................................................... 9 4.1.1 Field procedure ............................................................................................ 9 4.1.2 Data analysis .............................................................................................. 10 4.1.3 Estimation of K profile ............................................................................... 12 4.2 Tests in PH4 ......................................................................................................... 14 4.2.1 Field procedure .......................................................................................... 14 4.2.2 Data analysis .............................................................................................. 15 4.3.3 Estimation of K profile ............................................................................... 15 5. Conclusions ................................................................................................................... 16 References ......................................................................................................................... 18 Appendices ........................................................................................................................ 40 2 1. Introduction The Department of Earth Sciences in The University of Hong Kong was invited to conduct flowmeter tests in two drillholes installed in the Pak Kong slope as part of the ground investigation for landslides coordinated by Maunsell Geotechnical Services Limited. This report presented the field measurements, data interpretation and findings of the flowmeter tests. 2. About the site and boreholes The site is located at Slope No. 8SW-C/CR175, near Pak Kong Water Treatment Works at Sai Kung (Figures 1 and 2). The site is underlain by bedrock of coarse ash crystal tuff. More information on site geology can be found in the report by Maunsell Geotechnical Services Limited (2005). Two boreholes PH3 and PH4 were drilled by DrilTech Ground Engineering Ltd. Drilling of the holes was carried out using hydraulic rotary drilling rigs with air foam as flushing medium (DrilTech, 2007). The initial diameter of the borehole was 4.5 inch. To enlarge the hole to fit the flowmeter, the holes were reamed by core barrel to increase the diameter to be?. A PVC pipe with 107 mm internal diameter (the lower limit of the diameter which the flowmeter can work) and with 6 mm perforation at 50 mm intervals in a staggered arrangement was installed in each hole (DrilTech, 2007). The PVC pile has screens from 3 m below the top of the hole all the way down to the bottom. Details of the progress of installing PH3 and PH4 are outlined in Table 1. PH4 is located on a relatively flat area on a road side. PH3 is located on the steep slope of a road site. Some details of the two holes are shown in Table 2. The distance between the two drillholes is 47.11 m. Each borehole has fill, saprolite and bedrock. The 3 fill in both drillholes is about 1.80 m. The bedrock is located at about 22.05 m below the ground level (measured by DrillTech) in PH3 and 21.50 m below the ground level in PH4 (DrillTech, 2007). Table 1 Progress of installing PH3 and PH4 Activity PH4 PH3 Start End Start End Drilling March 2 March 16 April 2 April 22 Reaming March 12 April 20 April 27 May 16 Well screen installation April 21 April 24 May ? May ? Well flushing June 29 June 30 June 26 June 28 A trial flowmeter test was carried out in PH4 on May 17. It was then suggested that well development should be conducted in the drillholes after screen installation. Following the installation of a monitoring well or any well for hydraulic-test purposes, it is critical to perform a procedure called well development. The purpose of well development is to ensure removal of fines from the vicinity of the well screen which are created in the drilling processes. This allows free flow of water from the formation into the well. Development of a well should occur as soon as it is practical after installation (USEPA, 1994). Well development is a sophisticated job. If the development process is too vigorous, it may remove fine material beyond the edge of the borehole and then increase artificially the hydraulic conductivity of the aquifer (Fetter, 2001). Figure 3 demonstrates the flushing procedure used in the development of the two drillholes used by DrillTech: 1) Feed compressed air through to base of hole and push the in-situ water out of the hole; 2) Pour normal water into the base of hole and feed compressed air; 3) Continuously repeat the above steps for two to three days until water seeping into the borehole becomes clean. The flushing in the two holes was carried out between June 26 and 29 (Table 1). Figure 4 shows the well structure after the completion of the installation. The ground levels of PH3 and PH4 are 65.94 and 67.07 mPH, respectively. The ground level of PH4 is the same as the level of the surface of the concrete box over the borehole, but 4 ground level of PH3 is lower than level of the surface of the concrete box (Figures 4 and 5). The difference is estimated to be about 0.3 m. All the downhole measurements of this study used the top of the concrete box as the ground level or zero point of the well depth. If the data from PH3 are used to compare with the results from other hydraulic conductivity studies, the difference of 0.3 m should be noted. Table 2 Some details of the two boreholes PH3 and PH4 (from DrillTech (2007) except water-level data) PH3 PH4 Ground level (mPD) 65.94 67.07 Easting 845124.09 845087.67 Northing 826749.12 826779.01 Depth of the hole 43.87 37.16 Residual soil bottom level (mPD) 61.14 63.37 Completely to highly decomposed rock (mPD) 43.89 45.57 Moderately decomposed or less decomposed 43.89? 45.57 ? rock (top level, mPD) End of hole (mPD) 22.07 29.91 Water-level depth from top of the concrete box 17.11 m at 12.46 m at 9:40am, 5/7 9:40 am, 3/7 3. Basics of a flowmeter test 3.1 Equipment A borehole flowmeter can be used to measure the ambient (natural) flow in boreholes; it also enables one to determine the flow distribution in the borehole under pumping or injection condition, or so-called stressed well conditions. These data allow to determine relative aquifer hydraulic conductivity as a function of depth or identify fracture locations depending on the type of aquifer as described by Molz et al. (1994). If 5 the average hydraulic conductivity of the aquifer is estimated by a pumping or injection test or any other aquifer tests, the hydraulic conductivity of individual layers or zones can be estimated. The flowmeter type used for this study is heat-pulse flowmeter (Model HFP-2293) with a measuring range of 0.03 to 1.0 gallon/min (or 0.113 to 3.785 L/min). It was produced by Mount Sopris Instrument Co., Inc. USA. The flowmeter is operated by MGXII, a geophysical logging system. The flowmeter measures the direction and rate of vertical flow in the borehole. Accurate measurements require sufficient time between readings for the area around the tool to stabilize. At least three readings are required per interval in order to obtain a reasonably reliable average flow rate. The flowmeter is supplied with PC based software, centralizers and a range of diverters for 4 inch through 8 inch (101.6 mm through 203.2 mm) boreholes. The measurements occur in the probe, and the water flow outside the flowmeter will be blocked by diverters. The flowmeter is of 122 cm in length and 4.1 cm in diameter. Diverters are devises that divert flow in the borehole through a column where the measurement is taken by the tool. The fluctuation of water level in the well during the entire test period should be monitored when the flow is measured by the flowmeter under ambient or stressed flow conditions. The water level information can be used to assist the interpretation of flowmeter data, and under stressed well condition the water level data can be used to estimate the average hydraulic conductivity of the aquifer. For the Pak Kong site, a computer-operated transducer called miniTROLL 9000 produced by In-Situ Inc., USA was used to monitor the water level. Water level was also measured manually from time to time so that the results of the transducer could be cross checked. The manually-measured data can be used as a backup in case that the transducer goes wrong for any unexpected reason. 3.2 Theory 6 A pump is placed in the borehole and operated at a constant flow rate Q. When pseudo steady state behavior is obtained, the flowmeter