Chapter 4 Investigation of the effect of clay content and other factors on
4.3 Experimental procedures
apparent water content of modified sludge (at five levels of 26%, 28%, 30%, 32% and 34%) were considered. A total number of 25 experiments were conducted according to the L25 array proposed by DOE method. Then obtained results were evaluated by Signal to Noise Ratio (S/N) derived from the Taguchi approach that can identify the control factor settings and minimize the effect of noise on the response.
which refers to the ability of a soil to conduct water in geotechnical and geo-environmental engineering. Many factors affect soil permeability such as cracks and holes, and it is difficult to calculate representative values of permeability from actual measurements. A good study of soil profiles provides an essential check on such measurements. Observations on soil texture, structure, consistency, color, mottling, layering, visible pores and depth to impermeable layers such as bedrock and claypan form the basis for deciding if permeability measurements are likely to be representative [15].
The pore-water pressure, whether positive or negative, is an integral component of the stress state within the soil and consequently has a direct bearing on the shear strength and volume change behavior. In general, the size of the soil pores is of great importance with regard to the rate of infiltration (movement of water into the soil) and percolation (movement of water through the soil). Pore size and the number of pores are closely related to soil texture, structure, and soil permeability.
- Permeability variation according to soil texture:
Usually, the finer the soil texture, the slower the permeability, as shown below (Table 4-2 and Table 4-3):
Table 4-2 Relationship between soil texture and permeability
Soil Texture Permeability
Clayey soils Fine
From very slow to very rapid Loamy soils
Moderately fine Moderately coarse
Sandy soils Coarse
Table 4-3 Average permeability for different soil textures
Soil texture K [cm/hour]
Sand 5.0
Sandy loam 2.5
Loam 1.3
Clay loam 0.8
Silty clay 0.25
Clay 0.05
- Permeability variation according to soil structure
Structure may greatly modify the permeability rates shown above, as follows (Table 4-4):
Table 4-4 Relationship between soil structure and permeability Structure type Permeability
Platy - Greatly overlapping
From very slow to very rapid
- Slightly overlapping Blocky
Prismatic Granular
When soil is in unsaturated state, hydraulic conductivity becomes a function of the negative pore-water pressure in the soil. It is a complex processing. As a non-linear problem, it is difficult and meaningless to determine the permeability coefficient at unsaturated stage of soil. Therefore, experiments were carried out to determine the permeability with saturated soil. Figure 4-5 shows a sketch of water going through a soil structure. The knowledge of soil permeability is necessary for:
• Estimating the quantity of underground seepage.
• Solving problems such as pumping seepage water from construction exaction.
• Stability analyzing of earth structures and earth retaining walls subjected to seepage forces such as earth dam. Figure 4-6 shows paths of seepage through embankment dams.
Figure 0-5 Sketch of water go through soil structure
Figure 0-6 Paths of seepage through embankment dams [16] Figure 0-7 Apparatus of permeability test
Soil permeability is measured by hydraulic conductivity (k, cm/s) which is also known as permeability coefficient. The permeability coefficient can be determined in laboratory. Figure 4-7 is the permeability’s apparatus. Hydraulic conductivity of soils depends on several factors, such as:
• Fluid viscosity. Increasing water temperature causes a significant rise in the hydraulic conductivity of the soils.
• Property of pore water. The soil permeability is changed by changing of soil density and also its location below or under the water table.
• Void ratio. The increasing in the void ratio increases the area available for the flow hence increase the permeability’s soil.
• Degree of soil saturation. Higher degree of saturation, higher will be the permeability.
The permeability of modified sludge was studied and proposed some results that can clarify the importance of many factors on high level of resistance towards cracking and increase of permeability [17]. In theory, soil permeability follows Darcy’s Law [12][13]. In laboratory, there are two methods to investigate the permeability coefficient, which are constant-head and falling head tests. Constant head method is used for soil of much permeability and falling head method is used for soil of low permeability. The ASTM D2434 and ASTM D5084 standards are applied for testing permeability coefficient [14][15].
The purposes of this test were to investigate the effects of adding cement and RS fibers on permeability coefficient and to modify the optimum functions of RS content and cement content with combining effects of strength-strain, durability, and permeability target values. Furthermore, an empirical function was withdrawn to estimate the permeability coefficient.
4.3.1 Permeability procedures
Imitated sludge, fly ash, and alkaline solution were applied. Figure 4-8 shows testing procedure and described as follows.
• Mixing the imitation sludge with fly ash and alkaline solution to make modified-sludge and cure it at 20 ± 30C until it reaches the target for apparent water content.
• Make specimens (5cm in diameter and 5.1cm in height) by compaction method. The specimens were compacted with four layers (5 times for the first layer, 10 times for the second layer, 10 times for the third layer, and 20 times for the final layer), then cure the specimens for 7 days at 20 ± 30C.
• Soak the specimens in water for 1 day at 20 ± 30C. Measure temperature of the water which are going to use for the test.
• Carry out the falling head test.
Mixing to make imitated sludge
Curing at 20 ± 30C 3 days
Making specimen Curing at 20 ± 30C
7 days
Testing with permeability apparatus
Soaked in water at 20 ± 30C
1 day Sludge
Cement Rice
straw
Figure 0-8 Permeability testing procedure
According to Darcy’s law, the permeability coefficient at 150C, K15, is bellowed and viscosity of coefficient is in Table 4-5:
T 1
15 10
15 2
2.3 log
= aL h
K At h
Where A: cross sectional area of sample, L: length of sample, a: cross sectional area of scale tube, t: experimental time, h1,2: water head before and after t time.
Table 0-5 ƞT/ ƞ15 coefficient
T (0C) 0 1 2 3 4 5 6 7 8 9
0 1.575 1.521 1.470 1.424 1.378 1.336 1.295 1.255 1.217 1.181 10 1.149 1.116 1.085 1.055 1.027 1.000 0.975 0.950 0.925 0.925 20 0.880 0.859 0.839 0.819 0.800 0.782 0.764 0.748 0.731 0.715 30 0.700 0.685 0.671 0.657 0.645 0.632 0.620 0.607 0.596 0.584 40 0.574 0.564 0.554 0.544 0.535 0.525 0.517 0.507 0.498 0.490
4.3.2 Design of experiment
Four main factors for the Taguchi experimental design were: “geopolymer content”, “clay content”, “initial water content”, “apparent water content” while each implemented at five different levels. The variation levels of each parameter has been
Table 4-6 Mixture design with results of failure strength, failure strain and moist density
No. Geopolymer
content [%] Clay content [%] Initial water
content [%] Apparent water content [%]
T1 5 0 40 26
T2 5 30 50 28
T3 5 60 60 30
T4 5 70 70 32
T5 5 100 80 34
T6 10 0 50 30
T7 10 30 60 32
T8 10 60 70 34
T9 10 70 80 26
T10 10 100 40 28
T11 15 0 60 34
T12 15 30 70 26
T13 15 60 80 28
T14 15 70 40 30
T15 15 100 50 32
T16 20 0 70 28
T17 20 30 80 30
T18 20 60 40 32
T19 20 70 50 34
T20 20 100 60 26
T21 25 0 80 32
T22 25 30 40 34
T23 25 60 50 26
T24 25 70 60 28
T25 25 100 70 30
Table 4-7 The introduced levels for each factor in experiment design
Factor Description Level 1 Level 2 Level 3 Level 4 Level 5
A Geopolymer
content [%] 5 10 15 20 25
B Clay content [%] 0 30 60 70 100
C Initial water
content [%] 40 50 60 70 80
D Apparent water
content [%] 26 28 30 32 34
presented in Table 4-7. The L25 array is designed (Table 4-6) that is suggested by Taguchi method for four factors of five levels.