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Background

Discuss about the Numerical Method in Geotechnical Engineering.

This article has a well-organized installation of some energy piles in two cites that were researched. Tests were done on thermal reponses, laboratory thermal test and thermal recovery. These sites are located at Carraigtwohill and Cork Docklands.

(Kempfert & Gebreselassie, 2006)provided a detailed description of the conditions of the ground at the adjacent site being referred as Dockland Site. The Dockland site has a distance that is not more than 150 m to the adjacent site. This was therefore used in the assumption of similar conditions of the ground. It was also confirmed that an observed resulting spoil was present during the operations drilling activities at the Dockland site. The conditions of the ground consisted of thin alluvium layer that was approximately 1-2m in depth underlined by gravel and sand that saturated too the bedrock at an approximate depth of 42.2m. Also, there was a layer of grey silty clay running form 15-25m underground. A representation in the graphical form of the strata of the soil at this site is depicted in the figure below (Madabhushi, et al., 2010).

The energy piles were put up to a 14.5m depth hence the bedrock which was speculated to be between 42.2 to 60m was not reached.

4 deep 14.5m piles were set up for the purpose of providing completion of media for the thermal test response. Also, for the purpose of understanding, the thermal test response explained the practical issues that associated with the setting up of the energy piles. Double-u and single-u shaped configuration of piping were put up to a distance of 14.2m below the ground onto concrete piles measuring 350mm and 250mm respectively. The major geological formation along the put up energy pile length had saturated sand and gravel expected to be having a thermal conductivity ranging 1.5W/mK – 5.02W/mK. The provided database in the energy from the ground heat exchanger tool was the Energy Erath Design that quoted a recommended thermal conductivity value being 2.4WmK in the saturated sand while 1.8W/mK being for alluvium. Saturated clay was set to be 1.6WmK. With the assumption that the profile of the soil along the 14.2m depth installed energy pile consisting of alluvium running 1.5m, clay running 0.5m and saturated gravel/sand running 12.2m, the expected averaged weight depth thermal conductivity formation was .31W/mK.

A polyethene piping having 40mm diameter with a thick wall of 3.7mm was used for putting up the installation. The used piping was connected to a central plunge bar before being inserted the energy pile making use of the pile installation from the CFA process of piling. The figure below shows the piping heat exchanger being installed set the Dockland site (Laloui & Donna, 2013).

Installation of Energy Piles in Cork Docklands

The figure below depicts a sketch of installed energy pile for the Carraigtwohill and Cock Docklands. In both double-u and single-u configured piping, the u-shaped pipes have a connection to the steel plunge bar (Spence & Kultermann, 2016)

In accordance with geology bedrock maps and QSI Quaternary, the site made of undifferentiated till that is underlain by formed limestone bedrock. These observations are also consistent with the borehole sites that are nearby that were performed for Cork gas Pipeline. This pipeline runs from west to the east direction with an approximate 2km distance south of Carraigtwohll. The records from the site investigating depict they are being primarily made of gravelly/sandy clay. Bedrocks in this area also are located below the ground level at 3 to 10 m. A representation of the Carraigtwohill soil strata is shown below (Wuttke, et al., 2016);

Here, 300m diameter energy piles are installed to run a depth of 6m.teh bedrock that was underlying did not get penetrated when drilling was being dined. Hence, every pile was installed into this geological formation while having saturated gravely/sandy clay. The thermal conductivity that was expected of the saturated sand and moist clay fell in the range 1.5W/mK – 5.02W/mK and 0.9W/mK – 2.22W/mK respectively. Hence the thermal conductivity that was expected of the present material found on Carraigtwohill site would have an estimate 2.2WmK or more.

Tests on thermal response – Line Source Method

The majorly used theory in evaluation in thermal response test is the line source method that is analytical that was developed coming from Kelvin’s theory in Line Source with the equation being;

Data can then be evaluated to a leveleled10% accuracy when the criterion of time is lower to satisfy the equation below;

When in need of detailed theory’s overview, the analytical development of the line source method could be used.

Thermal response – Geothermal properties Model of Measurement

This method is an advanced method that can evaluate thermal response. GPM model was produced for the purpose of analyzing thermal properties for shorter test concerning thermal response by use of parameter response estimation method. This method has been suggested by many researchers but has not been widely used in industrial or academic purposes. In both industrial and academic purposes, the GPM however also makes use of the line source methodology which is a norm since it is simple for application. (Kempfert & Gebreselassie, 2006)has a presentation of an analyzed tested data of thermal response from 50-hour tests that were performed on two sites whose result were compared against the yearlong operation data that were respectively derived from the sites (Craig, 2004). The thermal conductivity value that was able to be produced by the GPM was 2 and 4% respectively. This is a statement that GPM method is an example of methods that determine the thermal conductivity of the ground form the tests performed. These tests have their results analyzed by use of line sauce methodology, GPM methodology as well as two-variable parameter fit method. The values of thermal conductivity ranged from 1.75W/mK - .35W/mK being calculated with three methods. Earlier research showed that standard analysis prefers GPM methodology. Hence it was the best practical method for data analysis (Serrador, 2014). However, when investigated further, the method’s applicability in short duration or merited energy lie test. The GPM model has its operation done by getting the finite-difference solutions of the Fourier equation. The backfill material of the borehole such as concrete or grout, as well as the geometric pipe arrangement, are modelled in effect radius of single pipes. The purpose of this approach enables the solving of Fourier equation by making an assumption of the field temperature along the borehole’s heat exchanger vertical axis to be constant. Also. The heat exchanger is supposed to have a radius that is constant about the vertical axis (Manassero, et al., 2013).

Installation of Energy Piles in Carraigtwohill

The key observation is between the Fourier’s laws that describes the transfer of heat by conduction. Darcy’s law describes the flow of groundwater. These two equations are presented in the table below that put them into a comparison;

An aquifer’s hydrological characteristic can be obtained by the conduction of pumping test. For the purpose of performing the pumping test, water has to be pumped from a well with the resulting groundwater level fall at nearby well observation has to be monitored by using piezometer. The completion of the pumping test ends with the pump being shut down and the observation well water level gradually rising until the initial level is reached. Information from the pumping test are sources of providing independent result check of the pumping tests that follow. The collected data has been stated to be more reliable since the process of recovery happens at a constant rate while the discharging becomes difficult in achieving (Murgul & Popovic, 2017).

The figure below represents the typical plot of test data from pumping recovery test and pumping rest. Pre-pumping period of measurement comes before the test to make the regional groundwater level effects be recorded. The pumping length is supposed to be varying depending on the test objectives, the aquifer type, position of the boundaries and the level of accuracy required. Generally, the test length is obtained when there is the satisfaction of data quantity to the adequate particular test purpose. There resulting trend line produced on the pumping test information is used in extrapolating the probable pumping outcome. The difference that occurs in the collected test data on the pumping and the data on the extended pumping for each recovery point turned.

The relation between pumping/pumping recovery test and thermal recovery test/thermal response can almost be seen immediately. The start of the heating period has a correspondence to a steep original temperature increase of the water that is circulating around the piping, following the steady temperature increase state that develops in the increase of temperature rate controlled by geological thermal conductivity surrounding the BHE. Once the test on thermal response ends, the heaters get turned off and the fluid yet continues circulating (Fang, 2013). The temperature of the fluid then starts going back to a value near the original standing temperature. The answer to the difference in temperature after the end of the test is the imparted energy from the pump in small amounts. This additional energy can be evaluated and included into the tests on thermal recovery and thermal response (Hetnarski & Eslami, 2008).

Tests on Thermal Response

The figure below indicates a trend line figure that can be eventuated in case the test on thermal response continues the values can be calculated in a similar manner in the pumping test. Test data on thermal recovery differ with thermal response in each respective point with the result coming from the reduced power to heater switching off. There can be a plot of the calculated recovery on a temperature recovery versus time natural log with evaluation by use of line source methodology.

The thermal recovery and response tests were done using rig thermal response and operation of fundamental description in an approximate 12 months after construction of the pile. The distribution of temperature along the energy pile’s length as well as the varying distances and other elements suggest the researchers assumed an analyzed line source method (Tomlinson & Woodward, 2014). The use of line source methodology in gathering data on tested energy piles on 300mm as well as 450mm diameters.

Before the start of thermal recovery and thermal response tests, the ground temperature that remains undisturbed along the pile/borehole has to be established. Use of temperature probe in reading deep positions’ temperature was done on the energy pile. The result is a measure of the averagely undisturbed temperature of the ground at 12.8 0. The leftmost lines running vertically are the average temperatures (Brown, 2014).

Depth measurement of temperature was found to be 5.5 after the completion of the test and noted down. The figure below has an indication of a confirmation of numerous investigation sites indicate the existence of soft layers of clay at 6m below the ground level. There also is an indication of return rate of natural temperature of the soil. This is relative to return rate to the natural temperature of the ground. One reason for such features is the existence of material with low material conductivity. The return rate of temperature to natural conditions is increased 6m below the ground level. When the response is very high the produced test on thermal response can be eliminated faster.

This observation is confirmed by inspection of investigation site presented data as per (Hetnarski & Eslami, 2008)who shows clear plots resistance  of CPTU cone, as well as the ratio of CPTU friction with depth  in that layers that are soft, approximately exist 6m deep in the ground.

The figure that follows has the results from tests on thermal response done on 250mm diameter pile pipes made of single-u installation. The black line on the graph depicts the energy pile that is injected with fluid temperature. This is denoted as T(down). The dashed line is a representation of the temperature of returning fluid.  Denoted as T(Up) while the grey line is a representation of flow rate. The flow rate remained turbulent all through the test procedure. (Brown, 2014)suggests heat rates input between 30W/m in formations having low conductivity while 80W/m in formations having high conductivity (Han & Alzamora, 2011). After using net rate input of 3kWfor evaluating the thermal ground properties at 14.2m deep produced a thermal response that was due to protection from damage to the system in the study due to overheating. The profile of return and injected fluid T(Up) and T(Down) respectively are shown in the figure.

Thermal Response - Geothermal properties Model of Measurement

The same figure above shows a reduced flow progression of the test on thermal flow. It is a belief that the reduced flow may arise due to borehole thermal expansion heat exchanger pipe system as the test is conducted. The pressure of the system therefore reduces. Another cause of the reduced flow is small leakage in the system’s closed loop resulting in an increased difficulty in pumping. However, the fluid used was not noticed to be leaking on site.

When evaluating the different portions of the environment tests on thermal response leads to use of line source methodology. The continual calculated reduction in thermal conductivity in every evaluation showed that the steady-state conditions were not fully achieved. The lower time section is to be used in analyzing line source methodology. The thermal diffusivity of the ground was calculated by sectioning the obtained thermal conductivity from test results with the use of soil volumetric capacity of heat taking the database from EED (Linden, et al., 2013).

Grouped test of thermal response test was done on Carraigtwohill by two-pile connection in series. Hence there is an increased depth of the geothermal activity of the tested heat exchanger. The figures below depict the results from the test on the thermal response that makes a visual profile. There is a sharp average increase in circulating fluid in early stages during heating up of backfill material. This is followed by stabilization of increase of temperature with time. The test on thermal response lasted 10 hours after the electrical heat resistance being turned off.

The figure that follows is an indication of measured data that had to be analyzed in a similar fashion as Cork Dockland. The result is a 2.60W/mK thermal conductivity the values that were measured was 2.87W/mK thermal conductivity from the thermal response while 2.60W/mK was gotten form testing thermal recovery. Although these results are similar, the little difference that arises comes from the shortened duration of tests. This was necessitated by combining shallow geothermal active depth of the 12m energy piles with least UCD TRT heating power of the rig (Hicks, et al., 2014).

The results got from the Cock Dockland and Carraigtwohill sites had been analyzed with the use of both GPM method and line source method. The most used method was the line source method that allows analysis of tests on the thermal response. The duration of the test on the Carraigtwohill and Cork Dockland sites were 10 hours and 13 hours respectively with necessary minimum UCD test on thermal response heating power (Manzanal & Sfriso, 2015).

Comparison between Pumping Test Data and Thermal Response/Recovery Test Data

The results able to be obtained from the GPM and line source method analyses of the sites are put in the table below. Carraigtwohill site had values that were due to the sued GPM and line source method that was agreeable. However, Cork Dockland produced diverging results that are complicated due to the flow of groundwater regime (Hetnarski & Eslami, 2008).

Conclusion

The purpose of presenting the article on this research is to produce an analyzed documentation of performed thermal test on foundations of energy piles at two sites. These sites were Cock Docklands and Carraigtwohill. However, the least injectable power from the UCD rig test on thermal response coupled with energy piles with shallow depth. However, the testes o thermal response had to be curtailed to shorter lengths than the most used procedures in the industry. For the purpose of providing further confidence values in thermal conductivity, standard industry calculated values, the analytical method was used. These analytical methods were supplemented by tests done in the laboratory on the soil samples that were obtained from Cork Dockland site. The results were in in agreement with the Cork Docklands posing a complication in this groundwater regime of flow of operation. Further research had to be done to a make possibility of defining the extent, effect and characteristics of the groundwater flow. The most used evaluation of test in thermal response was the line source methodology with a recommendation of GPM methodology (Liu & Evett, 2013).

References

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