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PRELIMINARY CHARACTERIZATION OF EXCAVATED SOILS
COLLECTED AT A CDW LANDFILL OF SÃO PAULO CITY
Karen Kataguiri (1), Maria Eugenia Boscov (1), Cláudia Echevenguá Teixeira (2),
Sergio Cirelli Angulo (1)
(1) Escola Politécnica, Universidade de São Paulo, Brazil
(2) Institute for Technological Research of the State of São Paulo (IPT), Brazil
Abstract
A case study was conducted to characterize materials received in a construction and
demolition waste (CDW) landfill located in the Metropolitan Region of São Paulo. The study
consisted in: systematic collection of 35 samples of materials received in the landfill, random
selection of 8 among the 35 samples, sample mass reduction, preliminary visual classification
(predominantly soil or CDW) and basic geotechnical characterization. 38% of the samples can
be classified as excavated soil using particle size distribution and visual classification criteria,
whereas 62% of the samples were classified as mixtures of excavated soils and other CDW
types (especially cement based materials). Samples classified as mixtures soil-CDW were
divided in fractions of different grain size intervals and further analyzed by microscopy to
estimate the percentage of soil grains and cement based material. Results suggest that at least
49% of the material passing the 2 mm sieve could be recovered as soil; however, this fraction
of the samples (particles with diameter 2 mm) are not very significant compared to the bulk
sample: most of the material received is still a mixture of excavated soils and other CDW
mineral types. Therefore, the need of a proper segregation of these CDW types is necessary at
construction building sites, prior to final disposal, to ensure optimal recovery.
1. INTRODUCTION
Construction industry is responsible for generating more than 50% of the urban solid waste
mass in Brazil [1, 2]. Rocks, soils, ceramics, concrete, mortars, and other mineral-originated
materials account for about 90% of the mass of construction and demolition waste (CDW) in
this country [3].
According to previous authors study, in partnership with the Building Contractor Union of
the State of São Paulo (Sinduscon-SP), the volume of excavated soil during construction is on
average six times higher than other construction waste fractions. Densely urbanized areas in
continuous growth, as the Metropolitan Region of São Paulo (MRSP), require large
underground parking infrastructure, which tends to generate large amounts of surplus soils. In
European Countries, soil generation is about three to four times higher than that of the other
CDW components [4].
Even though representing a very significant fraction, little research has been conducted on
the soil fraction present in the CDW. Excavated soils are generally disposed of in inert waste
landfills, along with other CDWs. The practice of dumping soil in the few landfills for inert
waste available in MRSP reduces their lifetime. Most of this material could be reused on
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earthworks and trenchworks, vegetation replacement, paving, among other geotechnical uses,
or even as raw material for industrial products [5, 6, 7, 8]. While CDW has been employed as
aggregate for concrete and pavement construction, recovery of the dumped soil and the finer
fractions of CDW for geotechnical purposes is still to be explored.
This work aims to characterize and to establish technical and environmental criteria for
reuse of excavated soils and finer fractions of CDW for geotechnical applications, based on
laboratory tests carried out with samples collected at an inert waste landfill located in the
MRSP.
2. METHODOLOGY
The study consisted of two steps: a) sampling – systematic collection of 35 samples at an
inert waste landfill during eight weeks and random selection of 8 from the 35 samples, b)
characterization – visual classification (soil or mixture soil-CDW), particle size distribution
analysis, and determination of consistency limits for the fraction with diameter equal or lower
than 0.42 mm.
2.1 Sampling
C&D waste was collected in an inert waste landfill located in the south region of MRSP.
The landfill covers 360,000 m² and it is designed to receive 2,000 tons per day of mineral
C&D waste (concrete, mortar, ceramic, soils). The landfill receives material from
the southern region and part of the western region of São Paulo, as well as from other
neighboring cities: Embu, Embu Guaçu, Itapecerica da Serra, Santo Andre, São Bernardo do
Campo etc.
35 representative samples were collected from random trucks containing excavated soils
and other C&D materials. The program consisted of three collections per week (on Mondays,
Wednesdays and Fridays) during eight weeks, filling up a 40-L barrel per day, half in the
morning and half in the afternoon. Samples were collected at the moment of the material
discharge from the truck, from the scattered conical pile, with assistance of a tractor loader in
order to perform the collection at different positions of the scattered pile, preferably in the
middle and on the sides so as to obtain a more representative sample [9].
Among the 35 collected samples, 8 samples were randomly selected for conducting the
tests. Samples collection and mass reduction procedures were judicious to ensure samples
representativeness for the tests, considering the heterogeneity of the materials received at the
landfill [9].
2.2 Characterization
The initial material mass collected in each sample (about 40kg) was reduced to the
necessary masses for the tests using the riffle sample splitter (Jones type).
Particle size distribution of the bulk sample and consistency limits (liquid limit, plastic
limit and plasticity index) for the fraction with diameter 0.42 mm were evaluated according
to Brazilian standards ABNT NBR 6457: 1986, ABNT NBR 6459: 1984, ABNT NBR 6508:
1984, ABNT NBR 7180: 1988 and ABNT NBR 7181: 1988 [10, 11, 12, 13, 14], which are
similar to correspondent ASTM standards. Visual observation of the samples was conducted
to classify the samples according to the predominant material: (1) excavated soils and (2)
other CDW types (cement based materials – concrete and mortar – and red ceramic), which
were supposed to be a mixture of CDW and soil.
The next step was to determine the size fraction where soil particles were predominant in
the mixtures of CDW and soil. The mixtures of CDW and soil were sieved to the following
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particle size intervals: 2.0 to 1.2mm, 1.2 to 0.6mm, 0.6 to 0.4mm, 0.4 to 0.1 mm, and 0.1 to
0.075mm. A minimum representative mass was calculated for each particle size interval [9]
and the size fraction of each sample was reduced to this minimum representative mass using a
rotary sample splitter for fine particulates. The minimum representative samples of each
particle size interval were observed through a microscope equipped with a digital camera, and
objective zoom lenses up to 325x were used to obtain the corresponding images. In each
image, the grains were numbered and classified as cement based material (present in high
percentage in C&D waste – fig.1a) or soil (typical clay and silt material). The grains which
were not covered with or did not present flecks of cement paste were considered as soil
grains; however, this procedure has limitations, because ceramic particles (fig.1b) can be
visually similar to soil grains (fig.2a, 2b).
For grains with diameter larger than 2.0mm, it was possible to notice the main presence of
ceramic and cement materials by means of naked eye observation (fig.3).
Figure 1: (a) Cement and (b) ceramic based grains smaller than 0.6mm of C&D waste
observed in digital microscopy
Figure 2: Soil grains smaller than 1.2mm observed in digital microscopy
a b
B-5 B-12
B-19 B-22
B-4
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Figure 3: B-4, B-5, B-12, B-19 and B-22 sample grains larger than 2.0mm, mainly constituted
of ceramic and cement materials
3. PRELIMINARY RESULTS AND DISCUSSION
Fig.4 presents the particle size distribution of CDW landfill samples. B-7, B-15 and B-23
samples presented gravel fraction ranging between 7.2% and 23.0%, and fine fraction
( 0.075 mm) ranging between 31.4% and 48.6%. Samples B-4, B-5, B-12, B-19 and B-22
were preliminary and visually classified as mixed soil-CDW samples, with gravel fraction
ranging between 34.2% and 59.9%, and the fine fraction ( 0.075 mm) ranging between 9.2%
and 23.9%.
Figure 4: Particle size distributions of selected mineral CDW landfills samples
Particle size distributions of B-7, B-15 and B-23 presented higher percentage of fines than
the others (B-4, B-5, B-12, B-19, and B-22), being, in accordance with visual observation,
mainly constituted of excavated soils (fig.5). The B-4, B-5, B-12, B-19 and B-22 samples
were composed of a mixture of soils and other CDW types (especially cement based
materials, the most predominant fraction in this kind of waste – fig.6). According to these
results, only 38% of the studied samples can be considered as almost pure excavated soils;
most of them are mixtures of excavated soils and other CDW types.
0
10
20
30
40
50
60
70
80
90
100
0.000 0.001 0.010 0.100 1.000 10.000 100.000
Cu
mu
lati
ve
pa
ssed
ma
teri
al
(% g
/g)
Grains diameter (mm)
SAMPLES GRANULOMETRY
B-4
B-5
B-7
B-12
B-15
B-19
B-22
B-23
Clay Silt Fine Sand Medium Coarse Sand Gravel
Sieves (mesh) 200 100 50 40 30 16 10 4
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Figure 5: Preliminary visual classification of B-7, B-15, B-23 samples, mainly constituted of
excavated soils by visual observation
Figure 6: Preliminary visual classification of B-4, B-5, B-12, B-19, B-22 samples, constituted
of a mixture of excavated soils and other CDW (mainly cement based materials)
Table 1 presents consistency limits and plasticity indexes of the fraction with diameter
equal or smaller than 0.42mm. 2 of 3 samples (B-15 and B-23) mainly composed of
excavated soils presented this fraction ( 0.42 mm) with plasticity, B-15 showing plasticity
compatible with clay and B-23 with silt. 3 of 5 samples (B-4; B-12 and B-22) composed of a
mixture of excavated soils and other CDW types presented this fraction with some plasticity.
Samples were classified according to the Unified Soil Classification System [15] (Table 1).
Table 1: Consistency limits and plasticity indexes of mineral CDW landfill fraction
( 0.42 mm) samples
Sample Liquid
Limit (%)
Plastic
Limit (%)
Plasticity
Index (%) Unified Soil Classification System
B-4 29 21 8 No applicable (mixed material)
B-5 28 * * No applicable (mixed material)
B-7 27 * * Well-graded Sand (SW)
B-12 32 19 13 No applicable (mixed material)
B-15 30 21 9 Clayey Sand (SC)
B-19 * * * No applicable (mixed material)
B-22 27 18 9 No applicable (mixed material)
B-23 33 27 6 Silty Sand (SM)
B-7 B-15 B-23
B-19 B-22
B-12 B-4 B-5
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(*) fine fraction with no plasticity
Table 2 summarizes the counting of cement and soil grains. The digital microscopy image
criteria to determine the presence of cement material could not be applied for grains smaller
than 0.4 mm due to the difficulty to differentiate the cement paste grains from some feldspar
minerals.
For B-4 and B-12 samples (mixed soil-CDW samples with fraction with diameter
0.42mm showing some plasticity), separation of excavated soils (silt, clay and sand) was
possible for particle size fraction smaller than 2 mm (fig.7 and fig.8). Excavated soil
separation according to particle sizes was not possible for the other mixed samples, confirmed
by Fig. 9, 10 and 11.
Table 2: Composition classification (soil and cement based grains, smaller than 2mm) of
different particle size fractions of the mixed samples by digital microscopy
Sample Preliminary Visual
Classification
Particle Size
(mm)
Cement Fraction
(%)
Soil Fraction
(%)
B-4 Excavated soils with
other CDW types
2.0 – 1.2 49 51
1.2 – 0.6 65 35
0.6 – 0.4 61 39
B-5 Excavated soils with
other CDW types
2.0 – 1.2 84 16
1.2 – 0.6 89 11
0.6 – 0.4 98 2
B-12 Excavated soils with
other CDW types
2.0 – 1.2 37 63
1.2 – 0.6 45 55
0.6 – 0.4 53 47
B-19 Excavated soils with
other CDW types
2.0 – 1.2 92 8
1.2 – 0.6 92 8
0.6 – 0.4 91 9
B-22 Excavated soils with
other CDW types
2.0 – 1.2 98 2
1.2 – 0.6 92 8
0.6 – 0.4 96 4
Figure 7: Digital microscopy image of B-4 sample for particle size ranging from (a) 2.0 to 1.2
mm (b) 1.2 to 0.6 mm, (c) 0.6 to 0.4 mm.
a b c
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Figure 8: Digital microscopy image of B-12 sample for particle size ranging from (a) 2.0 to
1.2 mm, (b) 1.2 to 0.6 mm, (c) 0.6 to 0.4 mm.
Figure 9: Digital microscopy image of B-5 sample for particle size ranging from (a) 2.0 to 1.2
mm, (b) 1.2 to 0.6 mm, (c) 0.6 to 0.4 mm.
Figure 10: Digital microscopy image of B-19 sample for particle size ranging from (a) 2.0 to
1.2 mm, (b) 1.2 to 0.6 mm, (c) 0.6 to 0.4 mm.
Figure 11: Digital microscopy image of B-22 sample for particle size ranging from (a) 2.0 to
1.2 mm, (b) 1.2 to 0.6 mm, (c) 0.6 to 0.4 mm.
a b c
a b c
a b c
a b c
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4. CONCLUSIONS
This study presents a preliminary characterization of samples collected at an inert waste
landfill located in the Metropolitan Region of São Paulo (MRSP). 38% of the studied samples
can be classified as excavated soils using particle size distribution and visual classification
criteria, whereas 62% of the samples were classified as mixtures of soils and other CDW
types (especially cement based materials).
Two from the three excavated soil samples presented fraction with diameter 0.42 mm
with some plasticity, what was also observed in three of the five samples constituted of a
mixture of excavated soils and other CDW types. The mixtures, on the other hand, had a low
percentage of this fraction. The visual classification in excavated soil or mixtures soil-CDW,
therefore, does not provide a consistent indication of the plasticity of the fines.
Digital microscopy was used to classify the composition of mixtures in soil grains or
cement based materials for different particle size intervals. Results suggest that it is possible
to obtain material with predominance of soil grains sieving the mixtures soil-CDW through a
2 mm mesh sieve, a recovery of at least 49% of soil was possible in this case. As this material
still contains a percentage of cement, geotechnical tests will be carried out to assess the
geotechnical properties of the recovered material.
Preliminary investigations in this study shows that, using visual and particle size
distributions criteria, a recovery of at least 49% of excavated soils in samples collected in the
investigated inert waste landfill is possible. However, most of the material received is still a
mixture of excavated soils and other CDW mineral types. Therefore, the need of a proper
segregation of different CDW types is necessary at the construction building sites, prior to
final disposal, to ensure optimal recovery.
ACKNOWLEDGEMENTS
We gratefully acknowledge Institute for Technological Research of the State of São Paulo
(IPT) for supporting “Programa Novos Talentos” fellowship, and Building Contractor Union
of the State of São Paulo (Sinduscon-SP) for the technical and financial support provided for
research about construction waste generation indicators. We also acknowledge IPT and
Intercement for providing mineral CDW landfill samples used in this characterization study.
REFERENCES
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