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Engineered synthetic liners on their own are not the ideal solution to protect land, water and living beings against landfill leachate pollution. Despite their impermeability, engineered liners are susceptible to fail during installation and after a few years of landfill operation, and have no attenuation properties. Conversely, natural clay liners can attenuate leachate pollutants by reactions of sorption, dilution, redox, biodegradation, precipitation and filtration; resulting in a decrease of the pollution load over time. Depending on the clay, significant differences exist in the shrinkage potential, sorption capacity, erosion resistance and permeability to fluids; which would affect the suitability and performance of the potential clay liner. Here, the physical, chemical, mineralogical and geotechnical characteristics of four natural clayey substrata were compared to discuss their feasibility as landfill liners. All the studied clayey substrata had favourable properties for attenuation of leachate pollutants, although different management options should be applied for each one. London clay (smectite-rich) is the best material based on the sorption capacity, hydraulic conductivity and low erodibility, but has the greatest susceptibility to excessive shrinkage and easily alterable clay minerals that partially collapse to illitic structures. Oxford clay (illite rich) is the best material for buffering acid leachates and degrading organic compounds. The Coal Measures Clays (kaoline-rich) have the lowest sorption capacity, but on the plus side they have the lowest plasticity and most resistant clay minerals to alteration by leachate exposure
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THE ROLE OF NATURAL CLAYS IN THE
SUSTAINABILITY OF LANDFILL LINERS
Mercedes Regadío *1, Jonathan A. Black 2 and Steven F. Thornton 1
1 Groundwater Protection and Restoration Group, Department of Civil and Structural Engineering, University
of Sheffield, Broad Lane, Sheffield, S3 7HQ, United Kingdom
2 Geotechnical Engineering Group, Department of Civil and Structural Engineering, University of Sheffield,
Mappin Street, Sheffield, S1 3JD, United Kingdom
* Corresponding author: Mercedes Regadío, mregadio@mregadio.com
ABSTRACT: Engineered synthetic liners on their own are not the ideal solution to protect land, water and
living beings against landfill leachate pollution. Despite their impermeability, engineered liners are
susceptible to fail during installation and after a few years of landfill operation, and have no attenuation
properties. Conversely, natural clay liners can attenuate leachate pollutants by reactions of sorption,
dilution, redox, biodegradation, precipitation and filtration; resulting in a decrease of the pollution load
over time. Depending on the clay, significant differences exist in the shrinkage potential, sorption
capacity, erosion resistance and permeability to fluids; which would affect the suitability and performance
of the potential clay liner. Here, the physical, chemical, mineralogical and geotechnical characteristics of
four natural clayey substrata were compared to discuss their feasibility as landfill liners. All the studied
clayey substrata had favourable properties for attenuation of leachate pollutants, although different
management options should be applied for each one. London clay (smectite-rich) is the best material
based on the sorption capacity, hydraulic conductivity and low erodibility, but has the greatest
susceptibility to excessive shrinkage and easily alterable clay minerals that partially collapse to illitic
structures. Oxford clay (illite rich) is the best material for buffering acid leachates and degrading organic
compounds. The Coal Measures Clays (kaoline-rich) have the lowest sorption capacity, but on the plus
side they have the lowest plasticity and most resistant clay minerals to alteration by leachate exposure.
Keywords: compacted clays, attenuation, landfill liners, leachate pollutants
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Evaluation of natural clayey substrata for the construction of sustainable landfill liners
AKNOWLEDGEMENTS
This project has received funding from the European Union’s Horizon 2020 research and innovation
programme under grant agreement No 743880. This work reflects only the author’s view, exempting the
community from any liability. The authors thank TerraConsult Ltd and Soil Hill Quarries Ltd for providing
the clay materials; technicians of Civil Structural Engineering and Chemical Biological Engineering for
general support (The university of Sheffield), J Cuevas and A Ortega for XRD analyses (Autonomous
University of Madrid); the German-Austrian-Swiss Clay Group (DTTG), Craig Fannin (TerraConsult Ldt)
and the Solid Waste Institute for Sustainability (SWIS) for insightful discussions.
1. INTRODUCTION
Leachates produced in municipal landfills constitute a health and environmental problem due to the
different pollutants present in these liquors. For this reason, liners are required to minimise offsite
migration of leachate. Two types of liners are currently used in modern landfills. Synthetic liners typically
made of HDPE, imply high technology and can be prone to failure within 10 years of service (Rowe and
Sangam, 2002). Compacted clay liners are puncture-resistant and natural materials, but can be unstable
and susceptible to cracking under repeated wetting and drying cycles (Louati et al., 2018; Yesiller et al.,
2000). Interestingly, clays have intrinsic reactivity properties which means they can biogeochemically
interact with pollutants in leachates to decrease their availability and potential hazards over time. This
reactivity is enhanced if rainwater is allowed to enter the waste, because then waste degradation would
be boosted, which would accelerate its stabilization (Allen, 2001). This results in shorter periods (1) of
risk of contamination and (2) of aftercare monitoring and, subsequently, lower landfill cost.
The low cost, ease of implementation and reactivity of compacted clay liners makes them more
attractive than synthetic geomembranes on their own in landfill liner systems. This is particularly important
in low-income countries where >90% of waste is openly dumped (Kaza et al., 2018). However, clays are
very diverse and the suitability of the clayey substratum as a potential liner must be properly evaluated.
This task is complex and, in order to avoid any adverse effects, must consider many factors with their
interactions such as: (1) mineralogy, (2) shrink/swell potential, (3) sorption capacity, (4) dispersive/erosion
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behaviour and (5) permeability to fluids. If the clay plasticity is too high, construction of the liner becomes
more difficult and the swelling/shrinking/cracking potential more significant as a failure mechanism.
In this study, the feasibility of four natural clayey substrata as landfill liners was evaluated. Their
physico-chemical, mineralogical and geotechnical characteristics were studied and the results were
discussed in terms of strengths and weaknesses as candidate materials for landfill liners. Finally, the
potential for attenuation of pollutants in leachate by each material was evaluated for sustainable waste
landfill applications. The aim was to characterise the relevant properties of the different clays in order to
identify those which are geotechnical stable and effective in pollutant attenuation for liner design. This is
essential to prevent pollution of the environment and protect human health from leachate spreading over
groundwater aquifers or adjacent rivers and lands.
2. MATERIALS AND METHODS
2.1 Materials
Four clayey substrata from the United Kingdom were studied: London Clay (LC), Oxford Clay (OC),
shallow and deep Coal Measures Clays (SCMC and DCMC) (Table 1). The LC originated from shale,
greensand, chalk, and lateritic soils during a sea level rise over the Northern Sea Basin. It is a silty to very
silty clay, slightly calcareous with disseminated pyrite. The OC, collected from the Peterborough Member,
contained many visible well-preserved fossils (vertebrate and invertebrate), particularly bivalves
(Meleagrinella). In contrast to the two previous clays, the Coal Measures Clays originated in fluvio-deltaic
environments and thus, also have a relatively high proportion of iron sulphides (pyrite, marcasite) and
gypsum, the latter following pyrite weathering. These Coal Measures Clays consist of interbedded clay,
shales, silt and sand, interstratified with coal.
Table 1. Information about the natural clayey substrata samples.
London Clay
(LC)
Oxford Clay from Peterborough
Member (OC)
Shallow Coal
Measures Clay
(SCMC)
Deep Coal Measures
Clay (DCMC)
Location
(UK)
North Essex
Northwest Buckinghamshire
West Yorkshire,
collected close to the
surface
West Yorkshire, collected
at a greater depth
Age
Eocene:
47.8 - 56.0 Ma
At the end of the Middle Jurassic:
164-166 Ma
Upper Carboniferous:
310 Ma
Upper Carboniferous:
310 Ma
Origin
Deep marine
sediments
Deep marine sediments
Fluvio-deltaic
sediments
Fluvio-deltaic sediments
Colour /
appearance
Uniform, firm,
brown colour
Grey colour with carbonaceous
shells and rootlets
Dark grey-dark brown
colour
Two-coloured: orange-
light brown and dark
brown
Selected
references
Fannin 2006; Kemp
and Wagner, 2006
Fannin 2006; Hudson and Martill,
1994; Scotney et al., 2012
Freeman, 1964,
McEvoy et al., 2016
Freeman, 1964, McEvoy
et al., 2016
Approximately 100, 73, 25 and 20 kg of LC, OC, SCMC and DCMC were recovered in-situ between
June and July 2018. The pore water and exchange complex compositions were analysed in several
subsamples that were not dried in the oven. The exchangeable cations were analysed from air-dried and
powder samples (agate mortar and pestle) after applying 3 cycles of 10 sec of ultrasonification. The
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elemental composition and mineralogy were determined after oven drying and grinding to a fine powder.
For geotechnical tests, the conglomerates were oven dried to remove residual moisture and the dry lumps
broken up until a particle size <2 mm. For that, with a rammer and several perforated screen trays fitted
in a CONTROLS sieve shaker (Model 15 d040/a1), the clays were first reduced up to maximum 20 mm
aggregate lumps. Next, the <2 mm particles were recovered separately and the 2 to 20 mm lumps were
discharged into a bench soil grinder (Humboldt Co) and broken up to <2 mm size. All results are
expressed on a dry mass basis.
2.2 Analyses
The concentrations of nitrogen, carbon, hydrogen and sulphur were analysed in duplicate samples
ground to ≤1 mm (0.005 g) using a Thermo Scientific FLASH 2000 Elemental Analyzer. The remaining
elements except oxygen and the halogens, were analysed using a Spectro-Ciros-Vision radial ICP-OES
instrument after acid digestions of 0.031 and 0.094 g at 150 °C. For that, 12 mL of aqua regia was applied
for 30 min and, twice 1 mL of HF for 10 min, and the resulting solution was eluted upto 50 mL with 1 mass
% HNO3. The mineralogy was determined by X-ray diffraction (XRD) using a Ni-filtered Cu Kα radiation
(k = 0.15406 nm) in a Philips X’Pert diffractometer, operating at 40 kV and 40 mA, with a step size of
0.016 and a speed of 2 s/step. The samples were prepared by sprayed random powder (after grinding
down to 1–3 μm) and by flat oriented slides (after obtaining the <2-μm fraction by dispersant and Stokes'
law). Each oriented sample was prepared from a suspension of 0.1 g of the clay-fraction in 2 mL of a
solution in three ways: (1) in water and air drying, (2) in water and 550 °C drying for 2 h and (3) in a
glycerol solution and air drying (Moore and Reynolds, 1997). The software PDF-4+ 2019 (version
4.19.0.1) and the database v. 4.1903 were used for the interpretation of the data. The content in organic
matter, sulphides, hydroxyls and carbonates phases, was interpretated complimentary to CHNS, ICP-
OES and XRD by termogravimetrical analysis (TGA). Replicates between 0.0150.030 g were heated
from 30 to 995 °C at a rate of 20 °C/min with a TGA 4000 Perkin Elmer under two atmospheras: N2 and
O2 gas (20 mL/min). The external specific surface area was measured in 0.20.5 g of degased material
(60 °C) by the Brunauer-Emmett-Teller (BET) method of nitrogen gas sorption at 77 K in Micromeritics
Tristar II 3020 and Beckman Coulter SA-3100. The material was prepared from 1 g of original sample
ground gently below 400 µm (at least 10 cycles) and discarding the fraction below 64 µm (Bertier et al.,
2016). The particle size distribution as volume percent was determined by the Malvern Mastersizer 3000®
(software version 3.62) assuming the refractive index and density of silica SIO2 (1.457 and 2.65 g/cm3).
Samples were dispersed in distilled water by stirring at 2500 rpm speed and ultrasonic treatment.
Measurements of 10 min duration were repeated in the same sample until the results were constant, to
then take their average.
The pore water chemistry was obtained by mixing 20 mL of deionized water to 10 g of wet clay at room
temperature. Sample pH was measured with a pH glass electrode in the water:clay mixture after settling
for 24 h and shaking prior to analyses. The solution was then centrifuged and filtered (0.45 m) to
measure the EC with a 4-mm sensor. The soluble anions and cations were analysed by ion
chromatography (Dionex ICS-3000), the alkalinity by titration with H2SO4 (HACH digital titrator) and the
carbon soluble species by TOC-V-CSH analyser (Shimadzu ASI-V). The cations in the exchange complex
were determined as the difference between those extracted with a 1.26 M SrCl2 solution (80 mL) minus
the soluble ones extracted with water (80 mL) after shaking with 5 g of clay for 10 minutes (Edmeades
and Clinton, 1981). Due to the high ionic strength of the SrCl2 solutions, sodium, potassium, calcium and
magnesium in these extracts were analysed by atomic absorption spectroscopy (HITACHI Polarized
Zeeman Z2300), whereas ammonium was analysed by atomic emission spectroscopy. LaCl3 was added
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at 20% to standards and samples for calcium and magnesium analyses. The cation-exchange capacity
(CEC) was determined by copper complex with Cu-triethylenetetramine at pH 78, with a photometer at
a wavelength for maximum extension of 579 nm (Holden et al., 2012; Stanjek and Künkel, 2016).
To study the consistency and engineering behaviour of the materials, the clay samples were hydrated
with different amounts of water for a period of 24 hours in sealed plastic bags prior to index property tests
(Head, 2006). The consistency was studied in the <425-m fraction (250 g) by the determination of two
specific water (or moisture) contents: the liquid limit (water content that separates the plastic and liquid
states) and the plastic limit (water content that separates the semi-solid and plastic states). The change
of clay consistency from plastic to liquid state was determined by the free-falling cone test at a penetration
of 20 mm into the wet sample, with duplicates differing ≤0.5 mm (BS 1377:2:4.3:1990). The change of
clay consistency from semi-solid to plastic state was determined by manual rolling wet samples (20 g)
until threads of 3-mm diameter begin to crumble, with four replicates differing ≤2% moisture content of
their plastic limit (BS 1377:2:5.3:1990 and ASTM D 4318,15). To know the range of water content in which
the clayey material has a plastic consistency, the plasticity index was calculated as the difference between
the liquid and the plastic limits (Head, 2006). All actual moisture contents were determined by mass %
dry basis (Eq. 1) after oven drying 510 g of material (105 °C, 48 h) with duplicates that differ ≤0.5%
  
  (1)
where  is the moisture content dry basis (%), mw is the mass of wet sample before moisture removal
(g), md is the mass of sample after drying (g).
The optimal condition of the clays at which the susceptibility to settlement is reduced was studied by
applying the same compactive effort in different hydrated samples (240540 mL water in 16001800 g
clays). The compaction was into a mould of 105-mm diameter and 115.5-mm high, in three layers
subjected to 27 blows each one, by a 2.5 kg rammer of 50-mm diameter that drops from a height of 300
mm (BS 1377:4:3.3:1990). The optimum moisture content (OMC) was selected on the basis of the
maximum dry (bulk) density (MDD) after the compaction. To calculate the particle density of the solids,
the specific gravity () was obtained by triplicates at 20 C, in desiccated materials of <2 mm size (50 g)
using air-dried pycnometers of 50 mL (BS 1377:2:8.3:1990 but 20 instead of 25 C). The porosity (
)
was therefore calculated following Eq. 2:
 
 

  In the optimal condition:
 


  (2)
where
 is the volume of the air (mL),  is the volume of the water (mL),
 is the total volume
(1000 mL, mould),
 is the volume of the solid particles (mL), and  the dried mass of sample
at the OMC (g).
Air void saturation lines () were drawn as a function of dry bulk density () relative to the moisture
content ratio (Eq. 3). The specific value for the air void saturation after compaction at the optimal
conditions () was calculated with the and MC equal to MDD and OMC ratio, respectively (Eq.
4)

 (3)
where is the dry bulk density (g/cm3),  is the air void saturation in relation to the total volume (ratio),
is the specific gravity,  is the density of water (1 g/cm3) and
MC
is the moisture content ratio.
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    
    (4)
3. RESULTS AND DISCUSSION
3.1 Elemental composition and mineralogy
The four clayey substratum were mainly composed of silicium and aluminium, followed by iron,
potassium, hydrogen and magnesium (Figure 1). This agreed with the expected high presence of clay
minerals, potentially higher in Coal Measures Clays (see below). Silica, potassium, titanium, and
especially magnesium were higher in LC than in the rest. OC was notable for its high content in calcium,
carbon, sulphur (g/kg) and strontium (mg/kg), with lower content in silica, iron and phosphorus. The Coal
Measures Clays were notable for their high concentration of aluminium, nitrogen and manganese,
whereas the concentration of calcium, sodium, potassium, strontium and boron were the lowest within the
four samples. In the case of SCMC, there was more carbon, nitrogen and sulphur than in DCMC (Figure
1).
Figure 1. Elemental composition of London Clay (LC), Oxford Clay (OC), shallow Coal Measures Clay (SCMC)
and deep Coal Measures Clay (DCMC).
The differences between the clayey substrata are due to their mineralogy and origin. All samples
contain smectite, illite, kaolinite and chlorite but in different proportions (Figure 2). Smectite was most
important in LC, illite together with kaolinite in OC and kaolinite (followed by illite) in the Coal Measures
Clays. To a lesser extent, phlogopite mica was detected in LC, chlorite in both LC and OC, and
interstratification of illite/smectite (mixture of layers) in OC and the two Coal Measures Clays. Smectite is
often found interstratified with illite and in mixtures with chlorite and sometimes kaolinite in deep sea
marine sediments, as for LC and OC but not SCMC and DCMC which have a fluvio-deltaic origin. Another
indication of the presence of smectite is the high silica-to-aluminium ratio, especially in LC (Weaver and
Pollard, 1973).
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Figure 2. Sheet-silicate mineralogy of London Clay (LC), Oxford Clay (OC), shallow Coal Measures Clay
(SCMC) and deep Coal Measures Clay (DCMC). Water: water and air drying preparation, GL: glycerol and air
drying preparation, 550 °C: water and 550 °C drying preparation. S: smectite d001 reflection under GL preparation,
C (chlorite) and I (illite) d001 reflections under all three preparations. K: kaolinite d001 reflection under water and GL
preparations.
Quartz and feldspars were the most important phases in all materials. These are accompanied by
oxides except in OC, which mainly contained calcium carbonate and iron sulphide (calcite and pyrite).
Also relatively high levels of sulphides were found in SCMC, whereas, there were fluorides and oxide-
fluoride in DCMC (Figure 3). The highest mass loss due to dehydroxylation (connected with the mass of
clay minerals) was in Coal Measures Clays (59 %), while the highest mass loss due to organics,
sulphides and carbonates phases corresponded to OC (3, 4 and 5% respectively). Particulate organic
matter as an associated material in these clays was especially high in OC in the form of disseminated
organic matter, coarse lignite fragments and vertebrate and invertebrate fossils. Particulate organic matter
is important for the attenuation of contaminant molecules by sorbing them to its surface or fostering
microbial communities that would breakdown the contaminants to less toxic or nontoxic compounds
(biodegradation).
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Figure 3. Left: Global mineralogy by sprayed random powder. Right: TGA curves with mass relative to the mass
after dehydration: (a-b) Mass loss due to organic matter, (b-c) Mass loss due to sulphides, (c-d) Mass loss due to
dehydroxylation, (d-e) Mass loss due to carbonate decomposition and (e-end) decomposition, recrystallisation.
3.2 Pore water and exchangeable cation compositions
The LC had the highest natural moisture content followed by OC (39 and 25 ± 2%, respectively),
whereas SCMC and DCMC had the lowest values (10 and 12 ± 1%). This indicates a decreasing water
absorption and porosity from LC > OC > Coal Measures Clays. The pore water composition of the clayey
materials also showed some differences. OC was the most basic material due to the presence of calcium
carbonate phases (pH 9.0) and LC was slightly basic with a pH of 7.3. In contrast, SCMC and DCMC
were acidic with pH of 3.8 and 5.4, respectively (water/clay suspensions 2:1). The acidity in the latter case
most likely arises from oxidation of pyrite in the upper, weathered zone, producing a low pH, gypsum and
amorphous iron oxides as by-products. Acconcordingly, alkalinity was only present in OC (10.3 mmol/kg
as CaCO3) and LC (1.7 mmol/kg as CaCO3). The absence of calcite in Coal Measures Clays explains
their poor acid neutralization capacity. The total dissolved ion content, in terms of electrical conductivity,
was between 13 mS/cm in the filtered pore water extracts (liquid to solid ratio 2:1), except for DCMC,
which was only 0.1 mS/cm and for OC, which was 5 mS/cm. The predominant soluble anion in all samples
was sulphate (SO42−), mainly balanced by calcium (Ca2+) and sodium (Na+) in both LC and OC, by
magnesium (Mg2+) and Ca2+ in SCMC and mostly Na+ in DCM. In all cases the concentration of potassium
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(K+) was very low and ammonium (NH4+) was not detected.
The surface of soil particles is critical for the chemical reactions, sorption, colloid filtration, and
transport of contaminants. As expected, the exchangeable cations in the negatively charged sites of the
clays and particulate organic matter were the same as those most abundant in the pore water. The sum
of exchangeable cations often exceeded the total charge of the clay (CEC), due to high concentrations of
Ca2+. The Ca2+ was released by dissolution of carbonate minerals in the extracting solution instead of
replacement of the exchangeable cations. Thus, the CEC was measured directly instead of estimating
this from the sum of exchangeable cations. The CEC at pH 78 decreased from LC (26 cmol+/kg) < OC
(16 cmol+/kg) < Coal Measures Clays (13 cmol+/kg). All clayey materials and especially OC contained
particulate organic matter (1 3 wt%) which also has a large surface area in addition to the CEC.
Remarkably, the pore water of OC contained 10 mg/g of soluble carbon. The external specific surface
area increased from LC < OC < DCMC < SCMC (9.281 ± 0.305, 12.759 ± 3.556, 31.030 ± 1.037, 53.735
± 0.967 m2/g). The highest values corresponded to the material with the highest amount of hydrous
aluminium phyllosilicates minerals (Coal Measures Clays), followed by the material with the highest
amount of particulate organic matter (OC). However, the total surface area (external plus internal) of LC
is expected to be the largest because of its content on expandable clay minerals.
3.3 Consistency and compaction properties
The moisture contents at plastic and liquid limits (PL and LL) were determined to identify clays
susceptible to dispersion and excessive shrinkage in the field. These parameters are also useful to
distinguish between silt- and clay- size, and organic or inorganic character. All clays have a LL >20%,
confirming that they are cohesive materials (no sands). LL and PI decreased from LC > OC > Coal
Measures Clays, consistent with the dominant sheet silicate in each material (smectite > illite(kaolinite) >
kaolinite). LC and OC were fat (greasy) clays of high plasticity, high toughness and high to very high dry
strength Figure 4). The high plasticity of LC was notable for the broad range of water contents at which
this clay has plastic consistency (from 28 to 78%, i.e., PI = 50%), twice that of OC, the next in second
place. This is due to the presence of smectite (expandable clay mineral) and the more clay-size particles
expected in LC than in the other clays which have a higher silt content. The majority (90% by volume) of
the particles in LC had a diameter < 62 m, whereas in OC, SCM and DCM were 764, 675 and 253 m
as measured by laser diffraction. Smectites sorb larger quantities of water that decrease the soil strength,
causing destructive landslides and slope failure (Borchardt 1977; Wagner 2013; Yalcin 2007). To ensure
landfill liner stability, clays should have PIs of 1530% (25% is good) and clays with PI >40% should not
be used on their own. The two Coal Measures Clays gave similar results: lean clays of intermediate
plasticity, medium toughness and high dry strength. The "A-line” on the plasticity chart (Figure 4) denotes
the empirical boundary between inorganic materials and clays (above the line) and organic clays and
clastic silts (below it). Both Coal Measures Clays and especially OC fell in the vicinity of organic silts, LC
being noticeable for being a more inorganic clay. The dark grey colour of the former also reflects the
higher particulate organic matter content.
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Figure 4. Soil classification and plasticity evaluation of London Clay (LC), Oxford Clay (OC), shallow Coal
Measures Clay (SCMC) and deep Coal Measures Clay (DCMC). L, I, H denote low, intermediate and high
plasticity. O denotes organic character. C denotes clayey and M denotes silty and/or sandy (Unified Soil
Classification System, USCS). U-line is the upper bound of PI for natural soils defined by the two equations: PI =
7 if LLs ≤16, and PI = 0.9 (LL 8) if LLs >16.
All materials had clays with intermediate to high plasticity, confirming the absence of dispersive clay
fines. Dispersive clays resemble normal clays, but can be highly erosive and susceptible to severe
damage or failure. Soils of high plasticity silt (MH in the USCS classification, Figure 4) and smetite-rich
materials rarely contain dispersive clays. In the case of LC, smectites are responsible for the adhesion
forces between particles, which helps to prevent dispersion and thus soil erosion. Dispersive clays
typically occur in soils classified as CL, sometimes also ML, CL-ML and CH lacking of smectite (Figure
4). Clays are normally compacted for placing and constructing the clay liners because it increases the
shear strength and bearing capacity, which limits future settlement. By compacting, the void ratio and
permeability decrease, which consequently reduces the leakage of landfill leachate and seepage of
groundwater flow. In addition, variations in volume change are less pronounced, and thus clays are less
susceptible to cracking that would offer preferential flow paths for leachate mobilisation. To optimise this,
clays should be compacted close to the OMC, which is the quantity of water necessary to achieve the
maximum dry (bulk) density. Under the same compacted effort, the OMC decreased from LC > OC > Coal
Measures Clays while the maximum dry (bulk) densities followed the inverse sequence (Figure 5). This
sequence agrees with the one for air void saturation after compaction at the optimal conditions and the
one for porosity, and with the higher plasticity of LC, followed by OC, which can accommodate more water
to achieve their maximum dry densities than the Coal Measures Clays. Generally, soils dominated by
clay-sized particles exhibit the highest OMCs whereas silt-rich soils have medium values and sandy
materials have the lowest values. Accordingly, OC and LC exhibited the highest OMC (25-29% with
maxima dry bulk densities of 1.44-1.46 g/cm3, heavy clays) whereas the Coal Measures Clays the lowest
(17% with maxima dry bulk densities of 1.78-1.80 g/cm3, sandy-clay materials). The particle density of
the solids, in terms of specific gravity, decreased from LC > Coal Measures Clays > OC due to the highest
amount of particulate organic matter of the last one (Figure 5). Both Coal Measures Clays have very
similar consistency and engineering behaviours (Figure 4 and Figure 5).
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Figure 5. Comparison of compaction curves under Proctor BS 1377:4:3.3:1990 to estimate the (optimum)
moisture content at which the maximum dry bulk densities occur. Air void saturation lines (v% of the total volume)
at the optimal conditions. LC: London clay, OC: Oxford clay, SCMC: shallow Coal Measures Clay and DCMC:
deep Coal Measures Clay (specific gravities and porosities between brakets).
3.4 Evaluation as landfill liners
Based on the previous analysis, the feasibility of the four natural clayey substrata to attenuate landfill
leachate is discussed below. Although its composition varies, landfill leachate always contains high
concentrations of sodium, potassium, bicarbonate and chloride, with significant ammonium and organic
compounds. The heavy metal content is generally relatively low, often at not major concern and limited
to chromium, nickel and zinc (Aucott 2006; Kjeldsen et al., 2002). In addition to the low permeability that
they provide, compacted clays can attenuate these leachate pollutants by sorption, dilution, redox
transformations, biodegradation, precipitation and filtration (Allen, 2001; Griffin et al., 1976). Attenuation
here refers to a reduction of the mass of pollutants by naturally occurring processes (Regadío et al.,
2015). These attenuation processes occur simultaneously and can affect more than one leachate
pollutant. By sorption, pollutants are attached to mineral phases or particulate organic matter by a physical
or chemical process and encompasses ion exchange, adsorption, absorption and chemisorption. By
redox transformations, organic and metal compounds are converted into less toxic or immobile forms by
electron transfer reactions. By biodegradation, organic pollutants are chemically decomposed by
microorganisms. By precipitation, metallic pollutants become less bioavailable or mobile. By filtration,
larger pollutants such as metal-organo complexes in the leachate, remain physically trapped within the
liner fabric.
For sorption, the CEC is very important in clay minerals and particulate organic matter. In this case,
cations are sorbed from the pore water by clays to neutralize the negative charge created by unbalanced
substitutions of their structural cations. Naturally sorbed cations can be replaced by cationic pollutants in
the leachate. In the case of ammonium and potassium from leachate, illites (OC, Coal Measures Clays
and LC) have high affinity for selective sorption of these species due to their size compatibility with the
interlayer (exchange) sites in the clay lattice (Griffin et al., 1976). Smectites (LC) also fix these cations
but this destabilizes smectitic minerals resulting in illitization, i.e., partial collapse of smectites with their
1516 26 29 38
1.25
1.43
1.47
1.76
1.80
7.6% air
with Proctor compaction BS 1377: 4:1990: 3.3
Bulk dry density, g/cm3
Moisture content, %db
SCMC (2.67, 33 v%)
DCMC (2.67, 33 v%)
OC (2.59, 44 v%)
LC (2.73, 48 v%)
(specific gravities between brackets)
15 16
3% air 2% air
6.5% air
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subsequent conversion into illite. In the case of larger or organic cations or organometallic complexes in
leachate, smectites sorbed these species preferentially relative to smaller, inorganic or uncomplexed
metals. This is because for the same valence these weakly hydrated cations are the easiest to sorb in the
exchange sites than stronger hydrated small cations (Teppen and Miller, 2006), and only smectites
provide an exchanger interlayer space large enough to accommodate them. CEC varies with pH,
generally increasing with pH due to the development of greater negative charge (average pH in leachate
is 78). Smectite, along with vermiculite (interlayer Mg), provide high CEC, while illites provide mid-range
values and kaolinite very low values. Thus the capacity to reduce the concentration of cationic pollutants
in leachate by cation exchange reactions decreases from LC > OC > Coal Measures Clays. Anion sorption
(bicarbonate, chloride, sulphate from leachate) is less efficient and very similar for the different clay
minerals (kaolinite, smectite). It occurs at OH ions exposed on the mineral edges and is enhanced by
positively charged iron-oxide colloids (LC and Coal Measures Clays) associated with clays
(Raymahashay, 1987). Bicarbonate is the major compound of leachate and largely determine the acid-
base neutralisation potential of the system. This is important where carbonate minerals are scarce (Coal
Measures Clays) to attenuate acidic episodes such as oxidation of sulphides (pyrite) by infiltrating
rainwater or acetogenic leachate (from fresh waste). Chloride can be attenuated to a lesser extent by
retention in apatite crystals and in clay minerals (Koutsopoulou and Kornaros, 2010), but mainly diffuses
through the clay liner, together with sodium and the cations displaced from the exchange sites of clays
(usually Ca2+ and Mg2+ substituted by NH4+, K+) (Regadío et al., 2012). These elements are diluted by the
receiving groundwater and are generally not a problem due to its non-toxicity even at high concentrations.
Sulphate in leachate is attenuated by anaerobic microbial reduction, a common redox process in landfills
(Batchelder et al., 1998), supporting metal sulphide precipitation in the liner and low concentrations of
sulphate in leachate.
Biodegradation is also accompanied by changes in redox potential in the landfill, which results in
transformation of organic and inorganic species by reactions under aerobic and a range of anaerobic
conditions. Depending on the specific redox conditions in the landfill and liner (aerobic, nitrate-reducing
or sulphate-reducing), one or other organic compounds are biodegraded. The concentration of the
oxidising agents and their reduced species in the leachate indicates the redox conditions (Taylor and
Allen, 2006). All the studied clayey substrata content redox-sensitive species, the most important being
pyrite in OC and SCMC and iron oxides in LC, SCMC, DCMC. Differences in clay minerals have a minor
effect on the biodegradation of organic pollutants than on the (already discussed) sorption of inorganic
pollutants. This is because organic pollutants are attenuated mainly by anaerobic biodegradation rather
than by sorption to clay minerals (Adar and Bilgili, 2015). Their decomposition to ultimately water,
methane and carbon dioxide, depends on the establishment of an appropriate microbial population. The
native particulate organic matter of clay materials is essential to support in situ biological activity in liners,
with the highest presence found in OC (followed by SCMC), sampled from a fossiliferous location (Martill
et al., 1994). Anaerobic degradation in the liner is also sustained by the microbial inoculum in the leachate
itself.
The attenuation of heavy metals in leachate is associated with particulate organic matter and mineral
phases, including clay minerals in liners through sorption, redox transformation and precipitation
processes (Fannin 2006). These processes are supported by specific mineral phases such as sulphate-
bearing species (pyrite, gypsum), iron and manganese oxides and oxyhydroxides and clays (mainly
smectite and illite) (Fisher and Hudson, 1987). The studied materials all contain a high content of clay
minerals which assists retention of heavy metals. OC and SCMC contain pyrite, and LC, SCMC, DCMC
contain iron/metal oxides. The high native particulate organic matter content of OC favours the sorption
of metals, whereas the dissolved organic compounds of leachate favours the formation of metal-organo
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complexes. Despite the fact that metal-organo complexes are dissolved in leachate and therefore mobile,
they can be attenuated by filtration due to their larger size (Christensen et al., 1996; Gregson et al., 2008).
Regarding hydraulic conductivity, the four clayey substrata all meet the relevant design criteria and
are candidates for use in a landfill liner. Materials with at least 20% LL, 7% PI, 30% fines and 15% clays
have hydraulic conductivities below 10-9 m/s, the maximum regulatory criterion in the EU and most
countries (Benson et al., 1994). Coal measures Clays were the easiest to compact until negligible air is
present in their voids, which is convenient for low leachate permeability of the liner.
LC is the only one that contains significant amounts of smectites among its clay minerals. These are
expandable sheet silicates with desirable properties such as erosion resistance, low permeability and the
best ability to attenuate pollutants. Thus, they have been used globally to improve landfill liners (Ruiz et
al., 2012). A key limitation is that smectites have high plasticity and may shrink most when in contact with
leachates. This can induce instability and cracks in compacted clays and increase leakage through liners
(Borchardt 1977; Wagner 2013; Yalcin 2007). This risk can be reduced by compaction and by addition of
sand (Tanit and Arrykul 2005; Varghese and Anjana, 2015). A further drawback of smectites is their
alteration to newly formed illite or even kaolinite, resulting in much less chemo-mechanical stable
materials than originally kaolinite-rich and illite-rich samples not derived from smectite (Zhao et al., 2007).
This illitization at the expense of the smectite content would occur after ammonium and potassium
sorption from the landfill leachate (Regadío et al., 2015). Its consequent impact on the clay CEC due to
the decrease of the smectite component would be a minor impact, since in these cases CEC normally
decreases by ≤10%.
4. CONCLUSIONS
The performance of four natural clayey substrata as potential landfill liners was assessed by measuring
their physico-chemical properties and stability and alterability upon contact with leachate. All materials
can achieve a low hydraulic conductivity to meet relevant liner design specifications and minimise
leachate migration. The attenuation of pollutants in leachate depends on the pollutant species and liner
mineralogy. Potassium, ammonium, (dissolved) organic compounds and heavy metals (chromium, nickel
and zinc) are the most representative leachate pollutants, according to their concentration, toxicity or
persistence. These compounds are mainly attenuated in the clayey materials by anaerobic
biodegradation and sorption mechanisms, especially cation exchange. Chloride and sodium in leachate
and native cations released from exchange sites after sorption of pollutants can be diluted by rainfall and
groundwater without harmful effects.
All studied clayey materials are useful for the attenuation of leachate pollutants in sustainable waste
landfills. However, different management options should be applied depending on the clayey material. LC
is the best material based on the sorption capacity and erosion resistance. However, LC has a large
plasticity (high susceptibility to excessive shrinkage) and easily alterable smectite clay minerals that
partially collapse to illitic structures. Illitization has less impact on the CEC of the liner than on its chemo-
mechanical stability and could be countered by compacting and mixing LC with sands. OC is also plastic
but to a lesser extent, with an acceptable plastic index. This substratum has a significant sorption capacity
and is the best material for buffering acid leachates (native calcite) and degrading organic compounds.
On the negative side, Coal Measures Clays have the lowest sorption capacity and null neutralization
power. However, they have the lowest plasticity and the most resistant clay minerals (kaolinite
accompanied by illite) to alteration by exposure to leachate. In addition, both Coal Measures Clays are
the easiest to compact until no or negligible air voids, which favours low hydraulic conductivities. SCMC
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contained sulphate-bearing species (resulting from oxidation of pyrite) that enhance the retention of heavy
metals. DCMC had very low mineral phases or inorganic salts that are readily dissolved in water, which
is advantageous because of less mobilization of leachable salts from the liner itself. LC and Coal
Measures Clays have associated iron/metal oxides and oxyhydroxides that can enhance anion exchange
and the removal of metals by sorption. The redox-sensitive species pyrite (OC and SCMC) and iron oxides
(LC and Coal Measures Clays) can enhance the removal of metals by redox transformation and
precipitation. This presence of pyrite and iron oxides also determine to a large extent the acid-base
neutralisation potential, together with native carbonates in OC and bicarbonates in the leachate.
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... However, the void saturation achieved with water is lower for the London and Oxford clays compared with the Coal Measures Clay, decreasing the overall air content from 8 and 7 to 2 vol% respectively, and thus the permeability to fluids. The physico-chemical, mineralogical and geotechnical properties of the clays related to their feasibility as landfill liners are described in more detail in Regadío et al. (2020). ...
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A new approach in landfill liner design which combines hydraulic containment of leachate with contaminant attenuation to improve the performance of these environmental control systems at landfills is described. The idea is to re-use readily available industrial waste residues (construction and biomass waste) as additives for natural clay liners, wherein the additives have specific properties which enhance the attenuation of contaminants by the mixture. The aim is to (1) evaluate the contaminant attenuation capacity of these mixtures, (2) develop design guidelines to construct liners for waste containment systems and similar applications, and (3) interpret their performance using numerical modelling. This is evaluated in permeation studies using a geotechnical centrifuge, which enables the performance of liner compositions to be tested for representive time-scales (100 years), pressures and temperatures at realistic experimental time-scales of days-weeks in the laboratory. The permeation experiments include liner compositions flushed with leachate to deduce contaminant transport and attenuation mechanisms, followed by rainwater to assess the potential for release of attenuated contaminants. This experimental methodology is illustrated with depth profiles from permeation studies conducted on different clay-additive compositions. The concept will be applicable for liner design at other waste disposal facilities and is a timely improvement which addresses the problem of managing large quantities of industrial residues. Instead of disposal these can be recycled as an additive in host clay to construct these liners, thus conserving natural resources (clay) and reducing construction costs. It also provides an effective and more environmentally sustainable basis to reduce risks from leachate leakage.
... However, the void saturation achieved with water is lower for the London and Oxford clays compared with the Coal Measures Clay, decreasing the overall air content from 8 and 7 to 2 vol% respectively, and thus the permeability to fluids. The physico-chemical, mineralogical and geotechnical properties of the clays related to their feasibility as landfill liners are described in more detail in Regadío et al. (2020). ...
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A new approach in landfill liner design which combines hydraulic containment of leachate with contaminant attenuation to improve the performance of these environmental control systems at landfills is described. The idea is to re-use readily available industrial waste residues (construction and biomass waste) as additives for natural clay liners, wherein the additives have specific properties which enhance the attenuation of contaminants by the mixture. The aim is to (1) evaluate the contaminant attenuation capacity of these mixtures, (2) develop design guidelines to construct liners for waste containment systems and similar applications, and (3) interpret their performance using numerical modelling. This is evaluated in permeation studies using a geotechnical centrifuge, which enables the performance of liner compositions to be tested for representive time-scales (100 years), pressures and temperatures at realistic experimental time-scales of days-weeks in the laboratory. The permeation experiments include liner compositions flushed with leachate to deduce contaminant transport and attenuation mechanisms, followed by rainwater to assess the potential for release of attenuated contaminants. This experimental methodology is illustrated with depth profiles from permeation studies conducted on different clay-additive compositions. The concept will be applicable for liner design at other waste disposal facilities and is a timely improvement which addresses the problem of managing large quantities of industrial residues. Instead of disposal these can be recycled as an additive in host clay to construct these liners, thus conserving natural resources (clay) and reducing construction costs. It also provides an effective and more environmentally sustainable basis to reduce risks from leachate leakage.
Preprint
Full-text available
A new approach in landfill liner design which combines hydraulic containment of leachate with contaminant attenuation to improve the performance of these environmental control systems at landfills is described. The idea is to re-use readily available industrial waste residues (construction and biomass waste) as additives for natural clay liners, wherein the additives have specific properties which enhance the attenuation of contaminants by the mixture. The aim is to (1) evaluate the contaminant attenuation capacity of these mixtures, (2) develop design guidelines to construct liners for waste containment systems and similar applications, and (3) interpret their performance using numerical modelling. This is evaluated in permeation studies using a geotechnical centrifuge, which enables the performance of liner compositions to be tested for representive time-scales (100 years), pressures and temperatures at realistic experimental time-scales of days-weeks in the laboratory. The permeation experiments include liner compositions flushed with leachate to deduce contaminant transport and attenuation mechanisms, followed by rainwater to assess the potential for release of attenuated contaminants. This experimental methodology is illustrated with depth profiles from permeation studies conducted on different clay-additive compositions. The concept will be applicable for liner design at other waste disposal facilities and is a timely improvement which addresses the problem of managing large quantities of industrial residues. Instead of disposal these can be recycled as an additive in host clay to construct these liners, thus conserving natural resources (clay) and reducing construction costs. It also provides an effective and more environmentally sustainable basis to reduce risks from leachate leakage.
Article
Full-text available
Several geotechnical structures constructed with fine soils submitted to wetting and drying (W-D) cycles face a variety of disorders. This is the case of some roads and slopes in the North of Tunisia where the upper soil layers are silty and clayey. Due to desiccation cracks, preferential pathways for rainfall infiltration and contaminant transport are created. In fact, the permeability varies significantly considering the evolution of desiccation cracks during W-D cycles. In the current research work, we developed an equipment of 150 × 150 × 50 mm, to measure the clay permeability during W-D cycles taking into consideration the influence of initial soil density. In addition, the role of the micro-structural evolution during the W-D cycles was investigated. The permeability was predicted using a proposed fractal model, based on the evaluation of the morphological parameters of cracks and the saturated permeability before crack development. The predicted and measured values of saturated hydraulic conductivity were found in good agreement. With cracking, the permeability can be increased to a limit value and the impact of the initial density becomes insignificant after seven W-D cycles.
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This paper presents a study assessing the possible application of seven clay substrates of various particle compositions and plasticity, sampled locally in rural regions, as materials allowing affordable construction of the waste landfill liners, which meet the main principles of sustainability, utilize locally available materials and limit the environmental threats posed by landfill leachate to water, public health and arable land. The researched substrates were tested according to their long-term sealing properties by their saturated hydraulic conductivity after compaction, swelling and shrinkage characteristics and ability to sustain their sealing capability after repeated drying and rewetting. The basic characteristics of soils were determined by the standard methods. Saturated hydraulic conductivity after compaction and after repeated shrinking and swelling were tested in laboratory falling head permeameters. Shrinkage characteristics were based on dimensionless indicators of the geometry and linear extensibility. The obtained results showed that the tested clay substrates were found applicable to construction of compacted clay liner for sustainable waste landfill. The environmental sustainability of a local, rural waste landfill, isolated by compacted earthen liners utilizing local materials is, in our opinion possible, but strongly related to the compaction parameters applied during liner construction for the given clay substrate.
Conference Paper
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The present paper aims at investigating the effect of water salinity on engineering properties of fine-grain soil. Saline water used in this study was obtained from half saline water of Ajichay River (with TDS more than 1877 ppm) and saline branch of Korchay (with TDS more than 97000 ppm) located in Northwestern of Iran. Fine-grain soil studied is from Korchay dam core. Due to the high hardness of water in this river, the feasibility of using saline water as the water required in clay core of the dam process has been studied in this research. The experiments were performed including Atterberg limits, standard compaction, consolidation, direct shear and dispersion. These tests were performed by using distilled water, half saline water and saline water on reservoir materials. Due to low percentage of clay minerals in the soil, the changes in engineering soil properties as salinity increases are negligible. However, the use of saline water reduces Atterberg limits, compression index (Cc), swell index (Cs(, coefficient of compressibility (av), coefficient of volume compressibility (mv) and causes slight increase in soil shear strength parameters. Despite the high percentage of sodium in half saline and saline water, dispersion degree of soil is ND2 in pinhole test
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Human activity inevitably produces waste materials that must be managed. Some waste can be reused. However many wastes that cannot be used beneficially must be disposed of ensuring environmental safety. One of the common methods of disposal is landfilling. The most common problems of the landfill site are environmental degradation and groundwater contamination caused by leachate produced during the decomposition process of organic material and rainfall. Liner in a landfill is an important component which prevent leachate migration and prevent groundwater contamination. Earthen liners have been widely used to contain waste materials in landfill. Liners and covers for municipal and hazardous waste containment facilities are often constructed with the use of fine–grained, low plasticity soils. Because of low permeability geosynthetic clay liners and compacted clay liners are the main materials used in waste disposal landfills. This paper summaries the important geotechnical characteristics such as hydraulic conductivity, liquid limit and free swell index of geosynthetic clay liner and compacted clay liner based on research findings. This paper also compares geosynthetic clay liner and compacted clay liner based on certain criteria such as thickness, availability of materials, vulnerability to damage etc.
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An important component of modern landfills is the liner system for the prevention of leachate contamination of surrounding ground. Among landfill liner systems, geosynthetic clay liner (GCL) has gained widespread popularity across the world because of its lower hydraulic conductivity as well as its ability to self-heal local damage, which is almost unavoidable in the field. Over the past few decades, numerous studies have been conducted to examine the performance of GCLs, particularly in regard to hydraulic conductivity, chemical compatibility, water-swelling, selfhealing capacity, diffusion characteristics, gas migration, and mechanical behavior. In this paper, a brief introduction on modern GCL products is firstly given. Subsequently, the main findings of previous publications on the critical properties influencing the long-term performance of GCLs are summarized in a comprehensive manner. Finally, further research perspectives on polymer-treated GCLs are presented. This paper provides general insights that help readers gain a state-of-the-art overview of GCLs and trends for future development.
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The use of Pervious Concrete (PC) increased in the last years as an alternative to solve the run-off problem. PC shows a high percentage of empty spaces/gaps, which vary from 10 to 35%, facilitating the flow of rain and water through its structure. PC presents higher k permeability coefficient compared to conventional concrete. Permeability is the main property of PC, although there is no standardized method that guarantees the correct and precise measurement of such property in laboratory conditions. Currently, two main methodologies are used to assess the permeability coefficient: the falling head and the constant head permeability tests. In that regard, the American Concrete Institute recommends the use of the first method, although no comparison was done between them. Furthermore, the recommendations do not consider the use of sustainable aggregates during the production of the PC. In this study, the permeability tests were explained and used to assess the permeability coefficient of different mixes of PC produced with sustainable aggregates. The experimental results were used to analyze the relationship between the porosity and the permeability of concrete and compare the performance of the falling head and constant head permeability tests. The study presents the advantages of performing the constant head permeability test to assess the permeability of the PC.
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Study to develop reliable means for predicting potential expansion characteristics of clays from classification test data is described; it is shown that well-defined relationship can be established between percentage of clay sizes present in soil, activity of clay, and percentage of swell under 1-psi surcharge of sample compacted at optimum water content to maximum density in Standard AASHO Compaction Test; charts developed from data.