Assessment of Hydraulic Conductivity of Soil
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Hydraulic Conductivity Soil
Chapter 1 Introduction
Hydraulic conductivity or permeability of a soil is one important soil properties used in geotechnical engineering. It can be seen from the difficulty in measuring accurate and reliable values of hydraulic conductivity. Hydraulic conductivity of soil is basically the capacity of water to let water to pass through the pores or voids in the soil.
There are many methods developed in order to measure the hydraulic conductivity of soil; both laboratory and in-situ field methods. Some of the common laboratory methods are the constant-head test and falling head test. On the other hand, the common in-situ field methods are pumping well test, borehole tests (e.g. slug test, variable head test), infiltrometer tests and using porous probes (BAT permeameter). All these in-situ field test methods were used to measure the hydraulic conductivity of subsoil for both saturated and unsaturated media.
One other in-situ field measurement method that has been introduced is the Two-Stage Borehole (TSB) test, also known as the Boutwell permeameter test. This testing method is commonly used to test a low hydraulic conductivity soil such as compacted clay liner used in landfill barrier system or covers used at waste disposal facilities, for canal and reservoir liners, for seepage blankets, and for amended soil liners.
The advantage of using this method is that it can be used to measure both the vertical and horizontal hydraulic conductivity values of soil, kv and kh respectively. One other advantages of using this method is that it can be used to measure the rate of infiltration of water or other fluid into a large mass of soil which can represent the tested site. However, the application of the TSB/Boutwell permeameter test for natural soil or other soils having a higher permeability value has been limited.
This report will discuss the theory behind the TSB/Boutwell permeameter test and the application of this method on natural soil. The methodology of this test will also be included in this report. In addition to the standard TSB setup, this report will also discuss the modification made to the standard TSB test which can be easily and quickly installed in shallow boreholes for subsequent testing. The methodology and results from the modified setup will also be included. The results from both the standard and modified setup will then be compared.
The objectives of this project is summarised into four stages. In the first stage, the objective is to measure the hydraulic conductivity of the soil using the standard TSB/Boutwell permeameter setup. The second stage involves the modification of the standard TSB/Boutwell Permeameter setup. The aim is to obtain a simple installation setup which can be easily and quickly installed in shallow boreholes for subsequent testing.
In the third stage, the objective is to test the modified TSB/Boutwell Permeameter test in the field. This is done by carrying out a series of tests in varied subsurface media at the assigned site location. The results from both the standard and modified TSB/Boutwell Permeameter test will be compared.
The last stage of the project consists of particle size analysis of the soil obtained from site. The results from the two setups will again be compared to the hydraulic conductivity values obtained from the derivation of the Particle Size Distribution curves.
The tasks that are done in this project include:
The review of TSB/Boutwell Permeameter methodology
Developing the modify TSB/Boutwell Permeameter
Completion of field tests using the TSB/Boutwell Permeameter
Collection of soil samples and subsequent particle size analysis
Chapter 2 Literature Review
2.1 Soil Water
Soils are consists of separate solid particles. The pore spaces between the solid particles are all interconnected which mean that water is free to flow through these interconnected pore spaces (Whitlow, 2001). The water will flow from a higher pore pressure point to a lower pore pressure point. The pressure of the pore water is measure relatively to the atmospheric pressure. The level in which the pressure is zero (i.e. atmospheric) is defined as the water table (Craig, 2004). The soil above the water table is assumed to be unsaturated and the soil below the water table is assumed to be fully saturated. The level of water table changes in relation with climate conditions and can also be affected by any constructional operations (Craig, 2004).
It is usual to express a pressure as a pressure head or head which is measured in metres of water when considering water flow problems. According to Bernoulli's equation, the total head at a point in flowing water can be given by the sum of three head components; pressure head (u/γw), velocity head (v2/2g) and elevation head (Z). This relationship is illustrated in the equation below:
where; h = total head
u = pressure
v = velocity
g = acceleration due to gravity
γw = unit weight of water
Z = elevation head
However, since the seepage velocities in the soil are so small due to the high resistance to flow offered by the granular structure of the soil, the velocity head is often omitted from the equation (Whitlow, 2001). The total head at any point is then can be adequately represented by:
In saturated conditions, the one-dimensional water flow in soil is governed by the Darcy's Law, which states that the velocity of the groundwater flow is proportional to the hydraulic gradient:
where; v = velocity of groundwater flow = flow/area (q/A)
k = coefficient of permeability or hydraulic conductivity (constant)
i = hydraulic gradient = head/length (h/L)
The empirical validity of Darcy's Law depends heavily on the hydraulic conductivity, k, which must be carefully determined so that it can represent the soil mass (Azizi, 2000). The different practical methods that can be used to measure the hydraulic conductivity will be discussed in Section 2.3.
It is important to study the flow of water through porous media in soil mechanics. This is necessary for the estimation of underground seepage under various conditions, for investigation of problems involving the pumping of water for underground constructions, and for making stability analyses of retaining structures that are subjected to seepage forces (Das, 2006).
Hydraulic Conductivity (Coefficient of Permeability)
Hydraulic conductivity, k, of a soil is the capacity of the soil to allow water to pass through it. The value of hydraulic conductivity is often used to measure the resistance of a soil to water flow. Hydraulic conductivity has units of length divided by time. The most common unit used of measurement is meter per second (m/s). Although hydraulic conductivity has the same unit as those to describe velocity, it is not a measure of velocity (Coduto, 1999).
Importance of Hydraulic Conductivity
Hydraulic conductivity is a very important parameter in geotechnical engineering or in determining the widespread of contamination. This can be seen in the difficulties in measuring it. This is because hydraulic conductivity can varies from one point in a soil to another, even with small changes in the soil characteristics. It is also, as mentioned in the previous section, influenced by the viscosity and unit weight of the fluid flowing through the soil. Hydraulic conductivity is also dependent to the direction of flow which means that the vertical hydraulic conductivity would not be the same as the horizontal hydraulic conductivity. This condition of the soil is said to be anisotropic. Studies that have been made indicate that the value of vertical hydraulic conductivity (Kv) of a soil is usually higher than the horizontal hydraulic conductivity (Kh) in one or two order of magnitude (Chen, 2000).
Some applications in which information on hydraulic conductivity is very important are in modelling the groundwater flow and transportation of contaminants in the soil. Hydraulic conductivity data of a soil is also important for designing drainage of an area and in the construction of earth dam and levee. In addition, it is very important in tackling most of the geotechnical problems such as seepage losses, settlement calculations, and stability analyses (Odong, 2007).
Factors Affecting Hydraulic Conductivity
The hydraulic conductivity of a soil depends on many factors. The main factor that affecting the value of hydraulic conductivity is the average size of the pores between particles in the soil, which in turn is related to the distribution of particle sizes, particle shape and roughness, pore continuity, and soil structure (Craig,2004). In general; the bigger the average size of the pores, the higher the value of hydraulic conductivity is.
The value of hydraulic conductivity of a soil that has a presence of small percentages of fines will be significantly lower than the same soil without fines. In the other hand, the presence of fissures in clay will result in a much higher value of hydraulic conductivity compared to that of unfissured clay (Craig, 2004).
The range of the hydraulic conductivity value is very large. Table 1 below illustrates the range of hydraulic conductivity which differs from one soil type to another which is mainly due to the different average size of the pores between the soil particles.
Table 1 Range of hydraulic conductivity values (m/s) with different soil type (Whitlow, 2001)
Very good drainage
Fissured and weathered clays
Clay silts (>20% clay)
The hydraulic conductivity is also dependent to viscosity and density of water in which both are affected by temperature. It is therefore conclude that the value of hydraulic conductivity will then be affected by changes in temperature. Theoretically, it can be shown that for laminar flow and saturated soil condition the relationship between temperature and hydraulic conductivity:
Where; γw= unit weight of water
η = viscosity of water
K = absolute coefficient (units m2). This value is dependent on the characteristic of the soil skeleton.
Since most of the laboratory graduations were standardised at 20C, the value of hydraulic conductivity at this temperature is taken as 100% (Craig, 2004). Other value of hydraulic conductivity at 10C and 0C are 77% and 56% respectively (Craig, 2004).
Hydraulic Conductivity Tests
Most of the tests for measuring hydraulic conductivity measured one average value of hydraulic conductivity. However, some tests measured both the vertical and horizontal hydraulic conductivity values to obtained more accurate estimation. There are numbers of experiments and test that can be done to measure the hydraulic conductivity of a soil. These tests to measure the hydraulic conductivity can be done both in the laboratory and in the field. The following sections will briefly discussed the most common laboratory and in-situ tests practiced today to measure the hydraulic conductivity of a soil.
Although with all the various tests developed to measured the hydraulic conductivity, there are uncertainties arise on how the soils that being tested represent the whole soil condition at the site of interest. It is therefore a good practice to perform different tests and comparing the results obtained.
Laboratory Permeability Tests
One problem with laboratory tests is that the samples collected do not adequately represent the detailed conditions of the soil, e.g. fissures, joints or other characteristics in the site of interest. Even with carefully conducted tests and good sampling techniques, it is impossible to obtain a very accurate result. The results typically have a precision of about 50% or more (Coduto, 1999). It is therefore important to take this into consideration if any construction activities or contamination remediation operations to be perform at the site of interest.
Constant Head Permeability Test
The constant head test is used to measure the hydraulic conductivity of more permeable soils such as gravels and sands which have a hydraulic conductivity value of 10-4 m/s (Whitlow, 2001). The equipments used for this test is called a constant head permeameter. A schematic illustration of this equipment is shown in Figure 2.1.
The constant head permeameter was developed base on the basic idea of Darcy's Law (Equation 3). The soil sample is contained in a cylinder of cross-sectional area A. Continuous water supply is let to flow from a tank to the sample to maintain a constant head. The water that flow through the sample is collected in a collection jar or container and the discharge through the sample is measured by calculating the volume of the water in the collection container over a period of time t.
Figure 2.1 Schematic diagram of Constant Head Permeameter (www.geology.sdsu.edu)
The hydraulic conductivity, k of the tested soil is then calculated by:
From equation 3:
Where; Q = the discharge through the sample (m3/s)
L = the length of the sample (m)
A = cross-section of the sample (m2)
h = hydraulic head (m)
The above diagram shows a simple setup of the constant-head permeameter. Other setup is also available which make use a pair of standpipes to measure the pore pressure and potential at two points. This is illustrated in Figure 2.2 below. Although both the setups are different, it makes used of the same concepts; Darcy's Law.
Figure 2.2 Alternative setup of Constant Head Permeameter (Whitlow, 2001)
Falling Head Permeability Test
The falling head test is used to measure the hydraulic conductivity of less permeable soils such as fine sands, silt and clay. The water flow resistance in these types of soil are very high which unable to measure accurate measurements of hydraulic conductivity if used with constant head permeameter. Undisturbed samples are required to perform laboratory test to measure the hydraulic conductivity of a soil. However, a small degree of disturbance of the sample is accepted as it is very hard to obtain a perfect undisturbed sample. An undisturbed sample can be obtained usually using a U100 sample tube or a core-cutter tube (Whitlow, 2001).The schematic illustration of the falling head test setup is shown in Figure 2.3.
Figure 2.3 Laboratory setup of falling head test (Whitlow, 2001)
The sample is place in a cylinder container with a wire mesh and gravel filter at both end of the cylinder. The base of the cylinder is left to stand in a water reservoir fitted with a constant level overflow. At the other end, which is the top of the cylinder, it is connected to a glass standpipe of known diameter (Whitlow, 2001). These standpipes are then filled with de-aired water and it is allow to flow through the soil sample. The height of the water in the standpipe is measured at several time intervals. The test is then repeated using standpipes of different diameters.
It is a good practice to take note of the initial and final unit weight and water content of the sample to get additional information about the properties of the sample (Whitlow, 2001). The hydraulic conductivity of the sample is then calculated from the results obtained from the tests. The Darcy's Law concept is still used in determining the hydraulic conductivity. The derivation of the hydraulic conductivity for the falling head test is done as follow (Whitlow, 2001).
Deriving from Equation 3:
With reference to Figure 2.3, if the level of the water in the standpipe fall dh in a time of dt the flow, q will be
and the hydraulic gradient, i
Where; a = cross-sectional area of the standpipe
A = cross-sectional area of the sample
When equation 6 is rearranged and integrated, the final equation to calculate the hydraulic conductivity is given as
Particle Size Analysis
Particle size analysis is commonly used to classify the physical properties of the soil being tested. This testing method is used for both soil science and engineering purposes (Keller and Gee, 2006). In context of engineering purposes, it is commonly used to define the particle size distributions of the soil. The data obtained from the particle size distributions can then be used to estimate the pore-size classes needed in calculating the hydraulic properties of the soil such as hydraulic conductivity (Keller and Gee, 2006).
There are various methods of measuring particle size analysis. Traditional methods include sieving, hydrometer and pipette. Other new techniques are also been developed; one example is laser-diffraction techniques (Eshel et al, 2004). However, particle size analysis is dependent on the technique used for defining the particle size distribution. It is therefore a common practice to do more than one method to define the particle size distribution (Keller and Gee, 2006). The results from all the different methods can then be compared to obtain more representative result.
For the traditional particle size analysis methods, two separate procedures are used in order to obtain wider range of particles sizes (Head, 1980). The two procedures are sieving and sedimentation procedures (hydrometer or pipette method). Sieving is used to categorise large particle such as gravel and coarse sand. The particles can be separated into different size ranges using a series of standard sieves. For the finer particles such as silt and clay, sedimentation procedure is used (Head, 1980).
Once the particle size distribution is defined from the particle size analysis, the hydraulic conductivity of the tested soil can then be estimated using a number of established empirical equations. However, the applicability of the above equations depends on the type of soil that is being tested. The following paragraphs summarised several empirical equations from previous studies (Odong, 2007).
Where; g = acceleration due to gravity
v = kinematic viscosity
n = porosity of the soil
d10 = grain size in which 10% the sample is finer than
The estimation of the hydraulic conductivity from these equations required information on the kinematic viscosity v and porosity n of the soil. The kinematic viscosity can be calculated by:
Where; = dynamic viscosity
Ρ = density of water
The porosity n can be calculated using the empirical relationship below:
Where U is the coefficient of grain uniformity and is given by:
The values of d60and d10 can be obtained from the particle size distribution. d60and d10 represent the grain size for which 60% and 10% of the sample respectively is finer than.
In-situ Field Permeability Tests
Due to the problems associated with reliability and laboratory tests, as mention in Section 2.3.1, field methods of measuring the hydraulic conductivity should be used to obtain more accurate and reliable measurements. In the field test, the soil disturbances is kept to a minimum level and they usually involves the testing of larger, more representative samples. Although, in term of cost and time, field measurement method is more expensive, it will as well provide more reliable measurement of hydraulic conductivity when dealing with a wide range of soil macro-structural characteristics. Other more economic option of field measurement can also be done. Such example is by performing borehole test, provided the pumping observation sequences are carefully planned and controlled (Whitlow, 2001).
Well Pumping Tests
This method is more suitable if used to measure hydraulic conductivity in homogenous coarse soil strata (Craig, 2004). The procedure involves the measurement of water that is being pumped out of a well at a constant rate, then observing the effect of these pumping activities to the drawdown of the groundwater level at other wells. The diameter of the well is normally at least 300mm and penetrates to the bottom of the stratum under test (Craig, 2004). The pumping rate and the groundwater levels in two or more monitoring wells are then recorded. The analysis of the results depends whether the aquifer is confined or unconfined.
Well pumping test in a confined aquifer
In confined aquifer the permeable stratum is squeezed in between two impermeable layers. This is illustrated in Figure 2.4 below. To perform the test, the pumping rate must not be too high to reduce the level in the pumping well below the top of the aquifer. The interface between the top aquifer and the overlying impermeable stratum therefore forms the top stream line (Whitlow, 2001).
Figure 2.4 Pumping test in confined aquifer (Azizi, 2000)
Figure 2.4 illustrates the arrangement of the pumping well and two other monitoring wells. Two assumptions were made at this point; the piezometric surface is above the upper surface of the aquifer and the hydraulic gradient is constant at a given radius (Whitlow, 2001). In steady state condition, the hydraulic gradient through an elemental cylinder with radius r from the well centres estimated as follow:
where; dr = thickness
h = height
The area in which the water flow, A:
where; D = the thickness of the aquifer
Substituting the area A into the Darcy's Law (Equation 4) will give;
And therefore the hydraulic conductivity is:
In the case that the piezometric level is above ground level, where the water level inside the well inserted into the confined aquifer rises above the ground level, this scenario is called Artesian conditions (Azizi, 2000). This is illustrated in Figure 2.5.
Figure 2.5 Artesian conditions (Azizi, 2000)
Well pumping test in unconfined aquifer
An unconfined aquifer is a free-draining surface layer that allows water to flow through the surface. The permeable stratum is not overlain by an impermeable layer. The piezometric surface is therefore in the same level of the water table. This is illustrated in Figure 2.6 below. The surface layer permeability is very high, thus allowing the water table to fluctuate up and down easily.
Figure 2.6 Pumping test in an unconfined aquifer (Whitlow, 2001)
Under steady state pumping conditions, the hydraulic gradient i at a given radius is assumed to be constant in a homogenous media. Homogenous unit is where the properties at any location are the same. For instance, sandstone has grain size distribution, porosity and thickness variation within a very small limit (Fetter, 2001). With reference to the arrangement of pumping well and two monitoring wells in Figure 2.6 above, the hydraulic conductivity can be determine by:
Deriving from Equation 3:
Hydraulic gradient i is
And area through which the water flow,
Thus, hydraulic conductivity for an unconfined aquifer (after integrating the above equation) is
Borehole Permeameter Tests
There are many borehole tests developed to determine the hydraulic conductivity of a soil. The most common in-situ borehole tests are as follow:
Two-stage borehole test/ Boutwell Permeameter
Variable head test
In-situ constant head test
Slug test is one of the cheapest in-situ field methods to determine the hydraulic conductivity of a soil. The procedure of this test involves the rapid adding or removing a 'slug' or water into a monitoring well. The 'slug' can be of anything that can displace the volume of the water in the well, e.g. water, plastic tubing capped at both ends, and other material of known volume and can fit into the monitoring well. The rate of rise and fall of the groundwater level is then observed until it reaches an equilibrium state.
In a variable head test, a slug is introduced into the monitoring well by either adding in a measured volume of water into the well or other materials mentioned earlier. The rate of water level fall is then measured in time. This is called falling head test. The water can also be removed out from the well by using a bailer or a pump. The rate of water level rise is then measured with time. This is called a rising head test. Depending on the properties of the aquifer and the soil, and the size of the slug used the water can either returns to its original water level before the test quickly or very slowly. For instance, if the porosity of the soil is high then the water level will returns very quickly to its original water level before the test is done.
There is also the constant head test. In this test the water level or head is maintained throughout the test at a given level. This is done by adjusting and measuring the flow rate of the water at intervals from start to the end of the test (Whitlow, 2001). The constant head test is said to give more accurate results, provided the water pressure is controlled so that it would not cause fracturing or other disturbance to the soil (Whitlow, 2001). There are several assumptions made for this test:
The soil is homogenous, isotropic, uniformly soaked
Soil does not swell when wetted
The expressions use to calculate the hydraulic conductivity for the above tests depend on whether the stratum is unconfined or unconfined, the position of the bottom of the casing within the stratum and details of the drainage face in the soil (Craig, 2004). The horizontal hydraulic conductivity is tend to be measured if the soil is anisotropic with respect to permeability and if the borehole extends below the bottom of the casing. On the other hand, the vertical hydraulic conductivity is often measured if the casing penetrates below soil level in the bottom of the borehole (Craig, 2004). The following expressions are all recommended in BS 5930 to calculate the hydraulic conductivity (Whitlow, 2001).
For variable head test:
For constant head test:
Hvorslev's time lag analysis
Gibson's root-time method
where; A = cross-sectional area of the standpipe or borehole casing
F = and intake factor dependent on conditions at the bottom of the borehole. The value for F can be obtained from Figure 6 of BS 5930 (BS 5930, 1999).
T = basic time lag. Figure 7 and 8 of BS 5930 (BS 5930, 1999).
H1, H2 = variable heads measured at elapsed times of t1 and t2 respectively
Hc = constant head
q = rate of inflow
= steady state of inflow, obtained from Figure 10 of BS 5930 (BS 5930, 1999).
Two-Stage Borehole (TSB) test/ Boutwell Permeameter
This project involves the use of this measurement technique. It is one of the borehole permeability tests and can be used in both saturated and unsaturated region. This method is first developed by Professor Gordon P. Boutwell as a relatively quick and simple way to calculate the effectiveness of compacted soil liner construction techniques (www.erosioncontrol.com). This test method may also be utilised for compacted fills or natural deposits, above or below the water table.
TSB test or Boutwell permeameter is usually used to measured the hydraulic conductivity for a low permeability media such as natural clay liner used in landfill barrier system or other material with hydraulic conductivity value less than or equal to 1x10-5 m/s (ASTM, 1999). This test method involves two-stage falling head test using infiltrometer installed inside the ground. The infiltrometer is basically a standpipe used to measure the fall in the water level in the borehole as the water dissipated into the soil.
In both stages of the TSB test, the rate of flow in which water flow into the soil through a sealed, cased borehole is measured usually using a standpipe in a falling-head test procedure. In stage 1 of the TSB test, the bottom of the casing is in the same level with the bottom of the borehole. The casing will prevent the water to dissipate horizontally. This will ensure the maximum effect of the vertical hydraulic conductivity KV. In stage 2 of the TSB test, the borehole is extended below the bottom of the casing. This will make the water to dissipate both vertically and horizontally. Both the vertical and horizontal hydraulic conductivity values, KV and KH respectively, can be calculated. The setup for TSB is illustrated in Figure 2.7 below. The value obtained from this test is only the limiting hydraulic conductivity values K1 and K2. The actual value of KV and KH are then calculated from these limiting values.
Figure 2.7 Setup for two-stage borehole test (a) Stage 1 (b) Stage 2
This method covers field measurement of limiting values for both vertical and horizontal hydraulic conductivities of porous materials the two-stage cased borehole technique. These limiting hydraulic conductivity values are the maximum possible value for the vertical direction and the minimum possible value for the horizontal direction (ASTM, 1999). The methodology of this method will be further discussed in the following chapter.
Chapter 3 Methodology
This chapter will discuss the methodology and equipments used to perform both the standard and modified setup of the TSB /Boutwell Permeameter test. This chapter will also discuss the methodology of the laboratory test for the particle size analysis. The site background on which the tests are done will also be included in this chapter. The results from both the standard and modified TSB/Boutwell tests will then be compared to that of the results obtained from the soil particle size analysis.
3.2 Site background and condition
The tests were performed at the Quad of the David Kier Building, Queen's University Belfast. The site can be access from the Stranmillis Road or the Malone Road. Figure 3.1 below shows the satellite image of the site.
Figure 3.1 Satellite image of the site (Google Map)
With reference to Figure 3.1, the site is found to be sloping downward. The site is surrounded by the David Kier building. There are three big trees grew within the quad. There is a big excavation done in the site during its construction in 1962. The soil at the site might consist of man-made fill. However, the soil below 1 or 2 metre might represent the natural soil of the site.
3.3 Field test of the standard TSB/Boutwell Permeameter test
As mention earlier in the literature review, the TSB test or Boutwell Permeameter was used to measure the limiting values of the vertical and horizontal hydraulic conductivity. For the case of the vertical direction, the maximum possible value is taken. On the other hand, the minimum possible value is taken for the horizontal direction.
Both the two stages in the TSB test involve the measurement of the flow rate of water flow into the soil through a sealed, cased borehole. Grout was usually used for sealing the borehole and the casing used is commonly a plastic PVC pipe. In Stage 1 the casing extended to the bottom of the borehole to prevent the water dissipates into the soil horizontally. Water will only flow into the soil through the bottom of the borehole. This will enable for the measurement of the maximum effect of the vertical hydraulic conductivity (Kv).
In Stage 2 of the TSB test, the length of the borehole is extended. The length of the extended borehole depends on the diameter (D) of the casing (see Figure 2.7b). Standard values that were usually used are 1.0D, 1.5D and 2.0D (Daniel, 1989). Water will now be able to flow into the soil through the side wall and bottom of the borehole. This will enable the measurement of the maximum effects of both Kv and Kh hydraulic conductivity.
The standard TSB/Boutwell Permeameter test method that was done in this project will be slightly different from the standard test method in the ATM International: Designation D6391-99. More of this will be discuss in sub-section 3.4.3.
However, note must be taken that the direct results obtained from the field test are not the actual value of Kv and Kh. It is only the measurement of the limiting values of K1 and K2. The actual values of Kv and Kh are then calculated from these limiting values.
This section will list down the equipments used during the whole TSB test. Below are all the equipments.
Hand auger - used to drill the borehole.
Reamer - used to ream the bottom of the borehole to a level plane.
Borehole casing - consists of watertight plastic PVC tubing. The bottom end of the casing is not capped.
Dip meter - to measure the head in the standpipe manually. It has a sensor at the very end of it. This sensor will detect any presence of water in the borehole.
Stopwatch - to measure the time taken for any fall in head in the borehole. Readable to 1 second.
Datalogger - This will automatically measures the heads in the borehole.
220.127.116.11 Borehole and casing setup
This is the most important steps in the whole procedure. Extra care must be taken to when doing this stage. The borehole is first drilled using the hand auger. The drilling procedure must be done in the direction perpendicular to the ground surface. This is to ensure that the borehole will not be drill at an angle to the ground surface.
No drilling fluid was used during the drilling. The diameter of the drilled borehole was 7cm. The bottom of the borehole is then ream and smoothed using a flat auger. The bottom of the borehole must be smooth and flat to ensure proper measurement during the test.
The casing is then inserted into the borehole after the borehole preparation was done. The casing used was 5.5cm in diameter. The casing was set parallel to the axis of the borehole and centered by hand. The depth from the top and the bottom of the casing was then measured. The casing is extended 5cm above the ground surface .The borehole is then sealed using a mixture of bentonite and water.
The bentonite mixture extended 2-3cm to the ground surface. Extra attention was given to make sure no bentonite spill into the casing. The bentonite was then let to dry (hydration period). The hydration period take approximately atleast 12 hours. The top of the casing was covered to prevent any rainfall, surface runoff or other debris to enter the borehole.
In the ASTM International, a flow system and a standpipe should be installed on top of the borehole casing. However, in this project both the flow system and standpipe will not be used. Usually the TSB/Boutwell Permeameter test was conducted to measure the hydraulic conductivity of compacted clay liner which has a very low hydraulic conductivity value. This means that water will flow very slowly into the soil. Since there will only be a very small changes of water level in the borehole for testing a compacted clay liner over a period of one hour, the standpipe will be very useful to measure this difference of water level. However, this will not heavily affect the results of the test.
18.104.22.168 Stage 1
At the start of the test, sock should be inserted into the bottom of the borehole. The purpose of the sock is to protect the soil at the bottom of the casing from disturbance when water is introduced into the casing (ASTM, 1999). However, due to the unavailability of this item, the sock was not used during the whole project. Water is then introduced into the casing slowly to prevent any damage to the soil exposed at the bottom of the casing.
The borehole was filled with water until the water level is just above the ground level. This is to check whether the borehole seal is working properly. If there is water flowing out through the seal, this mean that the seal is not working properly. In this case the sealing procedure should be repeated.
There are two ways how the rates of water flow into the soil can be measured; (i) manually using a dipmeter and a stopwatch and, (ii) automatically using a datalogger. The measuring procedure is similar to that is apply for falling-head test. For taking the manual reading, the initial water level was first measured using the dipmeter. Record this reading and mark it as time zero. This is when the test is started. Measurements of the water level in the casing were then taken at a given time interval (e.g. for every 5 minutes).
In the case of taking the reading automatically, datalogger was used. The datalogger was first set using a computer. The datalogger has the ability to calculate the water depth in the borehole by measuring the atmospheric pressure in the borehole. It also can also measure the water temperature in the borehole. Once the test is terminated, the data in the datalogger is downloaded into a computer.
Since the test was conducted on natural soil, the flow of water into the soil is expected to be higher than the test conducted on compacted clay liner. This will mean that water level in the borehole will drop quickly. The test will be terminated once the water in the borehole fully infiltrate into the soil.
The data collected from this test is then use to calculate the vertical limiting conductivity (K1) of the soil. The following Hvorslev's equation is used to calculate the value of K1 (Daniel, 1989). With reference to Figure 2.7(a) K1 is calculated as follow:
where; d= the diameter of the standpipe
D = the diameter of the borehole casing
t1 = time at the start of the test
t2 = time at the end of the test
H1 = head at the start of the test
H2 = head at the end of the test
Note must be taken that since the standpipe is not used during the setup, the value of d will then be equal to the value of D. The values of log K1 is then plotted against log time.
22.214.171.124 Stage 2
The Stage 2 of this test can be perform when the Stage 1 have been done. Make sure the water in the borehole from Stage 1 has fully infiltrated into the soil. The borehole is then extended to the specified depth using a smaller diameter hand-auger. The length of the extended borehole (L) depends on the geometry factor to be used. Values that are commonly used are 1.0D, 1.5D and 2.0D; where D is the diameter of the borehole casing (Daniel, 1989). In this project, the default length of the extended borehole used is 1.5D. Water was then introduced into the borehole.
The Stage 2 test is then started. It is of the same procedure as the test made in Stage 1 where the rate of water flow into the soil in the borehole is measured with respect to time. As in Stage 1, the test will be terminated once the water in the borehole fully infiltrate into the soil.
With reference to Figure 2.7(b), the horizontal limiting conductivity, K2 can then be calculated using the following Hvorslev's equation (Daniel, 1989):
As in Stage 1, since no standpipe is used during the setup, the value of d will be equal to the value of D. The values of log K2 is then plotted against log time.
3.4 Field test of the modified TSB test / Boutwell Permeameter
The basic idea of the modified TSB test/Boutwell permeameter setup was to obtain a more simple installation setup which can be easily and quickly installed in shallow boreholes for subsequent testing. The standard TSB test/Boutwell permeameter setup, although proven was effective in measuring the hydraulic conductivity of the tested soil, is considered to be of much works. This is mainly because the person who is doing the test have to drill the same borehole twice; once during the Stage 1 and once during the Stage 2. The modified setup of the TSB test/Boutwell permeameter suggested in this report involved only involved a single drilling procedure. The detail setup procedure will be discussed in section 3.5.3.
All the equipments used for the modified setup was similar to the standard setup. The only different equipment used in the modified setup was the borehole casing. In the standard setup, only one plastic PVC tubing (not capped both ends) was used. In the modified setup, two plastic PVC tubing (not capped both ends) of different diameter sizes were used. The diameter of the bigger pipe and smaller pipe is 5.0cm and 4.5cm respectively.
Both the tubes are connected together in a way that it can be adjusted to two different lengths by sliding the smaller size diameter tube into the larger size diameter tube. The connection between the two pipes is ensured to be watertight so that water will not leak through the pipe during the testing stage. An O-Ring (Figure 3.2) was used for this purpose. The O-Ring is inserted in between the bigger size pipe and the smaller size pipe.
Figure 3.2 Different sizes of O-Ring (supaseal.co.uk)
126.96.36.199 Borehole and casing setup
The drilling borehole and casing setup procedure in the modified setup was very similar to the standard setup. The drilling procedure must be done in the direction perpendicular to the ground surface. No drilling fluid was used during the drilling. In the modified setup, the borehole was drilled to a much deeper depth compare to the standard setup. The bottom of the borehole is then ream and smoothed using a flat auger. The bottom of the borehole must be smooth and flat to ensure proper measurement during the test.
The modified casing is then inserted into the borehole after the borehole preparation was done. The casing was set parallel to the axis of the borehole and centered by hand. The maximum length of the modified casing is used. The depth from the top and the bottom of the casing is then measured. For the modified TSB/Boutwell permeameter setup, two sealing methods are used. From the bottom of the larger pipe to half of its length, a geo-synthetic clay liner is used.
For the remaining half of the larger pipe length a mixture of bentonite powder and water is used. This is illustrated in Figure 3.3 below. Before inserting the bentonite into the borehole, the geo-synthetic liner must first be wetted. Once inserted the bentonite is then let to dry. The hydration period take atleast 12 hours. The top of the casing is covered to prevent any rainfall, surface runoff or other debris to enter the borehole.
Figure 3.3 Schematic illustration of Stage 1 of the modified TSB/Boutwell Permeameter test setup
Geo-synthetic clay liner
4.5cm Ñ„ pipe
5.0cm Ñ„ pipe
188.8.131.52 Stage 1
Figure 3.3 is a schematic illustration of the Stage 1 of the modified TSB/Boutwell Permeameter test setup. Water is first introduced into the casing slowly to prevent any damage to the soil exposed at the bottom of the casing. Once the water inside the casing is almost full, the water depth is noted. This is the initial water depth. The Stage 1 can now be started. The idea of measuring the rate in which the water level in the casing dropped is still the same as in the standard TSB/Boutwell permeameter test setup. A datalogger is used to measure the water depth in the borehole. The test is terminated once the water in the casing fully infiltrated into the soil.
The data from the datalogger is then used to calculate the vertical limiting conductivity, K1 of the soil using Equation 21. The values of log K1 is then plotted against log time.
184.108.40.206 Stage 2
In stage 2 of the standard setup, the borehole is extended using the hand auger. However, in the modified setup instead of drilling the borehole, the smaller diameter tube is raised up by sliding it upward for approximately 1.5 times the diameter of the larger pipe. Since the diameter of the smaller pipe is 5.0cm, therefore the length L will be approximately 7.5cm. The smaller pipe is then slide 7.5cm upward. This will then creates a similar scenario as in the Stage 2 of the standard setup with the extension of the borehole. Figure 3.4 illustrates the Stage 2 of the modified TSB/Boutwell permeameter test setup.
Figure 3.4 Schematic illustration of Stage 2 of the modified TSB/Boutwell Permeameter test setup
1.5 x 5.0cm ≈ 7.5cm
Geo-synthetic clay liner
5.0cm diameter pipe
4.5cm diameter pipe
Water is then introduced slowly into the casing. The initial water depth inside the casing is noted. It is the same procedure as the test made in Stage 1 where the measurement of the rate of water level dropped in the casing with respect to time is recorded. The only different factor was that both the horizontal and the vertical hydraulic conductivity will take effect. The Stage 2 test is terminated once the water has fully infiltrated into the soil. The data from the datalogger is then used to calculate the horizontal limiting conductivity, K2 of the soil using Equation 22. The values of log K2 is then plotted against log time.
3.5 Calculating horizontal and vertical hydraulic conductivity
The horizontal and vertical hydraulic conductivity, Kh and Kv respectively, of the tested soil can be calculated from the limiting hydraulic conductivity values of K1 and K2. However, the anisotropy condition of soil must first be taken into account when calculating Kv and Kh by relating the ratio K2/K1 to the degree of anisotropy, m.
A value of m must first be selected, where m is defined as follow:
The corresponding value of K2/K1 is then calculated by using the following equation:
Values of K2/K1 are plotted against values of m and L/D and are shown in Figure 3.5 (Daniel, 1989). The value of m that corresponds to the actual K2/K1 value can be determined from this graph. Once the value of m is obtained, the values of Kh and Kv can then be calculated using the following expressions:
3.6 Particle Size Analysis
Particle size analysis, as mention earlier, is the common method practiced to classify the physical properties of the tested soil. There is a very wide range of particle sizes that can be encountered in soil. The main purpose of performing the particle sieve analysis is to define the particle size distributions of the soil. In addition, a number of engineering properties such as permeability, frost, susceptibility and compressibility are related directly or indirectly to particle size characteristics (Whitlow, 2001).
There are many ways in which soil analysis can be done. However, the most common and cheapest method is using the sieve analysis. In the case of coarse soil, where the fine particles have been removed or were absent, a dry sieve analysis is done. Here a representative amount of soil sample is passed through a series of standard size sieves arranged in descending order. The weight of soil retained on each individual sieve is determined and the cumulative percentage of the sub-sample weight passing each sieve is calculated.
In the case of the soil sample contains fine particles, a wet sieving is first carried out to remove these. Sedimentation test using hydrometer can be done to further classify the particle size distribution in the fine particle fraction.
Listed below are all the equipments used during the particle sieve analysis.
1000ml measuring cylinder
Dry sieve analysis:
Series of different sizes sieve
Weighing machine - readable to 0.1g
220.127.116.11 Hydrometer test
There are several steps required to be done first before actually doing the hydrometer test. The sample is first dried oven at 105-110C for a day. Once the sample is dried, take a mass of the dried sample and record the mass. The weight sample is then mixed with 100ml of sodium hexametaphosphate solution. This solution can be prepared by dissolving 33g of sodium hexametaphosphate and 7g of anhydrous sodium carbonate in distilled water to make 1L of the solution (BS 1377-2, 1990). The purpose of mixing the sample and the solution is to break down the bonding between the different types of soil in a sample.
The amount of sample to be mix with the solution depends on the type of the soil. For sandy soil a mass of 100g of the sample is appropriate. For silty and clayey soil, a mass of 50g and 30g respectively are adequate (BS 1377-2, 1990). Once the sample is mixed with the solution, it is left for another day.
The next step is to wet sieve the sample through a standard 63μm sieve. The sample is put into a 1000ml measuring cylinder through a 63μm sieve. The particles that passed through the 63μm sieve will be retained in the cylinder. A conical flask is used to ensure all the particles passing through the 63μm sieve will be retained in the cylinder. Water is then added into the cylinder until it reaches the 1000ml graduation mark. The cylinder is then put into the water bath and left for a day. This is to ensure no temperature variation for testing multiple samples. The particles that are retained in the 63μm sieve will be oven dried again for a day at 105-110C and will be subjected to dry sieve analysis.
Before starting the hydrometer test, the sample in the cylinder is shocks thoroughly making sure no particles are left settling on the bottom of the cylinder. After shaking the cylinder, immediately put the cylinder back into the water bath. At the same instant start the stopwatch and the test is started. Put the hydrometer into the cylinder and record the hydrometer reading at the upper rim of the meniscus. Readings are taken at 0.5min, 1 min, 2min, 4min, 8min, 15min, 30min, 1hr, 2hrs, 4hrs, 8hrs and 24hrs after the start of the test.
18.104.22.168 Dry sieve analysis
For the particles that are retained at the 63μm sieve during the wet sieve, it is subjected to dry sieve analysis. The particles are first weight after they are dried oven. Record the mass of the retained particles. The particles are then passed through a series of standard size sieve arranged in descending order. For this test sieves of size 5mm, 3.35mm, 2.36mm, 1.18mm, 0.6mm, 0.425mm, 0.3mm, 0.212mm, 0.15mm, 0.063mm and 0.00mm are used.
The particles inside the series of sieve are then shock for about 10-15mins using an automatic shaker. Record the mass of the particles that is retained on each sieve. The data from both the hydrometer test and the dry sieve analysis are then used to define the particle size distribution of the sample.
3.6.4 Calculating hydraulic conductivity
The hydraulic conductivity of the soil can be calculated once the particle size distribution is defined. From the particle size distribution, take the sizes of the particle that correspond to d10 and d60. These two values are then put into Equation 14 to calculate the coefficient of grain uniformity U. The porosity n of the soil can be calculated by empirical relationship with U (Equation 13).
For the purpose of comparison, different empirical equations are used to calculate the hydraulic conductivity of the tested soil. Below are all the empirical equations used:
Hazen's equation (Equation 8)
Kozeny-Carman's equation (Equation 9)
Breyer's equation (Equation 10)
Slitcher's equation (Equation 11)
The values of n, d10 and kinematic viscosity v are inserted into the above empirical equations to calculate the hydraulic conductivity of the tested soil.
Chapter 4 Results
4.1 Borehole locations
A total of 6 boreholes are installed during the project; three boreholes using the standard TSB/Boutwell permeameter setup and three boreholes using the modified setup. For the standard setup, the boreholes are numbered BH1, BH2 and BH5. For the modified setup, the boreholes are numbered as BH3, BH4 and BH6. The locations of each borehole are shown in Figure 4.1 below. The red circles indicate the borehole with the standard setup whereas the yellow triangles indicate the boreholes with modified setup. The results from all the tests; both from field and laboratory test will be included in the following sub section.
Figure 4.1 Locations of the boreholes (Google Map)
4.2 General description of the soil
The maximum depth of the borehole used in the test is 1.5m. For the first 0.4m depth, the soil consists of dark brown colour soil. For the remaining of the depth to 1.5m, the soil is reddish-brown in colour predominantly sand and silt. This is as expected for man-made fill since sand is one of the most abundance soils in the on the Earth. This is common for all the boreholes on the site. Small gravels are found occasionally at different depth in all the boreholes. The average soil description of all the boreholes is shown in Figure 4.2 below.
Figure 4.2 Average soil description of the site
Top soil. Dark brown in colour. Consist mainly of sand and some small gravel.
Reddish-brown soil. Mixture of sand and silt. Silty sand.
Reddish-brown soil. Silty sand with a very little amount of clay.
4.3 Standard TSB test/Boutwell Permeameter
All the results from the field and laboratory results for the standard TSB/Boutwell permeameter test will be presented in this sub-section. Refer to Appendix for the full data on the result for both field and laboratory tests. For the field TSB/Boutwell permeameter test, all the measurements were taken automatically using the datalogger..
4.3.1 Field test
The summary of all the results done with the standard setup, obtained from the field test, is given in Table 2. More detailed results from the field test is given in the following sub-section.
Table 2 Summary of the field standard TSB/Boutwell permeameter test
4.8 x 10-7