-
K = 1 0 m/s or less
Clay (<2060 m) content> 15 -20 % Plasticity Index I, (P1) > 7%
Minimum CEC of 10 meq (milliequivalants)/ 100 gr.Soil Leachate compatibility (no k- value increase)
Minimum thickness for MSW: 1.0 m, 0.6 m with geomembrane
Minimum thickness for industrial /toxic waste 3-4 m (15 m), alternatively Multiple Composite Liner Systems.
Table 2.8 Basic Characteristics of Liner Soils (Akter, 2007)
Type of Dry Dilatancy Plastiicity Toughness Remarks soils strength
Silt None to low Slow to None to Low or Lean clay is only rapid low thread can slightly plastic,
not be whereas fat clay is formed highly plastic.
Lean Medium to None to Low to Medium Dilatancy is
clay high slow medium increased in volume
Elastic Low to None to Medium Low to when soil is
silt medium slow medium compressed
Fat clay High to very None High High
2.7.1 Hydraulic Conductivity
A critical parameter of a landfill liner to isolate leachate from the subsurface environment is hydraulic conductivity. Of all the landfill liner parameters investigated by researchers, hydraulic conductivity has been received the most attention when designing and analyzing the performance of a landfill (Yanful et al. 1990, Fernandez and Quigley 1985 and Quigley et al. 1987). The hydraulic conductivity of soil generally decreases with an increasing amount of he fine grained soil. Lambe (1958) suggested that the soil with a flocculated structure (i.e. at dry of optimum moisture content) exhibits greater hydraulic conductivity than soil having an equal density and moisture content but a dispersed structure (i.e. at wet of optimum moisture content).
2.7.2 Moisture Content & Plasticity
Natural moisture content and plasticity should be carefully established as these are two key parameters in governing the ability of a soil to produce a well engineered and impermeable liner. For a given soil sample there is a unique compactive effort at which the density ceases to increase. The higher the moisture content, the lower the compactive effort beyond which no further increase in density occurs. A minimum Plasticity Index of 10% is normally required / stipulated as soils with a lower plasticity index are unlikely to achieve a sufficiently low permeability.
2.7.3 Plasticity Characteristics
Plasticity characteristics describe a material's ability to behave as a plastic or moldable material. Soils containing clay are generally categorized as plastic. Soils that do not contain clay are non-plastic and typically considered unsuitable materials for compacted clay liners, unless soil amendments such as bentonite clay are introduced. Plasticity characteristics are quantified by three parameters: liquid limit, plastic limit, and plasticity index. The liquid limit is defiuied as the minimum moisture content (in percent of oven- dried weight) at which a soil-water mixture can flow. The plastic limit is the minimum moisture content at which a soil can be molded. The plasticity index is defined as the liquid limit minus the plastic limit and defines the range of moisture content over which a
soil exhibits plastic behavior. When soils with high plastic limits are too dry during placement, they tend to form clods, or hardened clumps, that are difficult to break down during compaction. As a result, preferential pathways can form around these clumps allowing leachate to flow through the material at a higher rate. Soil plasticity indices typically range from 10 percent to 30 percent. Soils with a plasticity index greater than 30 percent are cohesive, sticky, and difficult to work with in the field. Common testing methods for plasticity characteristics include the methods specified in ASTM D-43 18, also known as Atterberg limits tests.
2.7.4 Percent Fines and Percent Gravel
Typical soil liner materials contain at least 30 percent fines and can contain up to 50 percent gravel, by weight. Common testing methods for percent fines and percent gravel are specified in ASTM D-422, also referred to as grain size distribution tests.5 Fines refer to silt and clay sized particles. Soils with less than 30 percent fines can be worked to obtain hydraulic conductivities below 1 x 10-7 cm/sec (4 x 10-8 in./sec), but use of these soils requires more careful construction practices. Gravel is defined as particles unable to pass through the openings of a Number 4 sieve, which has an opening size equal to 4.76 mm (0.2 in.). Although gravel itself has a high hydraulic conductivity, relatively large amounts of gravel, up to 50 percent by weight, can be uniformly mixed with clay materials without significantly increasing the hydraulic conductivity of the material. Clay materials fill voids created between gravel particles, thereby creating a gravel-clay mixture with a low hydraulic conductivity.
As long as the percent gravel in compacted clay mixture remains below 50 percent, creating a uniform mixture of clay and gravel, where clay can fill in gaps, is more critical than the actual gravel content of the mixture. Similar to gravel, soil particles or rock fragments also can create preferential flow paths. To help prevent the development of preferential pathways and an increased hydraulic conductivity, it is best to use soil liner materials where the soil particles and rock fragments are typically small e.g., 3/4 inches in
o
diameter). :f
€flg
171
S
p-i
Table 2.9: Typical Soil Properties (Akter, 2007) Soil Type Hydraulic
Conductivity K (cmls)
Total Porosity
ii (%)
Effective Porosity
n (%)
Bulk Density d (g/cm3)
Clayey i0 - 10' 40-60
0-5
1.2-1.8Silty 10' - i0
35-50
3-20Sandy 10 - 10' 20-50
10-35
1.3-1.9Gravelly 10 - 102
25-40
12-30 1.6-2.1Where a compacted clay liner functions as a bottom layer to a geosynthetic, gravel can cause puncturing in geosynthetic materials. Controlling the maximum particle size and angularity of the gravel should help prevent puncturing, as well as prevent gravel from creating preferential flow paths.