1. Recycled Technology of Urban Road Construction Waste and
Miscellaneous Fill Used as Subgrade Fillings
Bingkang Liu a
Jinsong Fan b
Huimin Hu Lihui Fu
School of Civil Engineering, Hefei University of Technology, Hefei, China, 230009
a
liubingkang@hfut.edu.cn,
b
fanjinsong163@163.com
Key words: miscellaneous fill, subgrade fillings, CBR, cold recycling technology, soil modification
Abstract: Continuously increasing municipal renovation and roadbed excavation produced a
considerable quantity of solid waste each year such us miscellaneous fill mixed with broken pavement
fragments and brickbats, which resulted in a series of urban environment and soil resource issues.
Comprehensive recycling technology for soil modification to attain the strength requirements of
subgrade fillings from the related road construction codes is necessary and even urgent, since the
usual soil replacement is not the economic and environment-friendly method. The field investigation
and laboratorial tests hereby had been completed to confirm the application potentials of typical soil
specimens selected from different locations. The experimental results indicate some of specimens can
be reused as subgrade fillings in road construction immediately and others should be treated by soil
amelioration. To further certify the practicality of this recycled technology and broaden its range of
application, three typical reutilization and treatment techniques, cold recycling, soil solidification and
dynamic consolidation are introduced into engineering projects.
Introduction
Urban road excavation and pavement renovation always produced a large amount of construction
waste each year, which constituted a high proportion of the gross municipal solid waste (MSW) [1-2].
This kind of construction waste caused by road construction is usually composed of subgrade fillings
as major components with a handful of cement or concrete fragments and a little domestic garbage. It
is typical miscellaneous fill that is usually vaguely defined in the engineering as soil mixed by solid
wastes with different grain sizes and various natures. Its heterogeneous composition and uncontrolled
engineering performance determine its application difficulty in real construction. Recently, the most
common treatment method is soil replacement by discarding previous miscellaneous fill and then
instead bringing into qualified soil body used as subgrade fillings. Unfortunately, it will create more
problems in succession such us taking up more space, using more transport vehicles, consuming more
soil resource and inevitably contaminating urban environment or increasing construction cost. With
the noticeable increasing construction waste in urban area, correlative issues concerning
environmental pollution and resource-wasting should not be ignored.
In recent two decades, researchers and engineers at home and abroad completed a series of
experiments and practices to investigate the natures of this kind of miscellaneous fill mixed with
MSW, confirm its composition, and improve its road performance for reaching related construction
standards by means of proper treatment methods. Besides soil as principal part, systematic
experiments and tests by different research groups indicated that selected soil specimens usually
contained all kinds of MSW originated from those disused construction materials such us cracked
asphalt concrete or concrete blocks, brickbats, rubbles, metals, etc. and a small quantity of life solid
waste such us saw dust, plastic, or even paper scrap, glass and so on [3-4]. Meanwhile, its chemical
composition is more or less referring to silicate, oxidate, hydride, carbonate, sulphate and sulfide, etc
[5]. Despite the internal composition and ingredients proportion are so multifarious, most of them
have some common characteristics: thickness misproportion in soil layers, considerable deformation,
low strength, compressibility and collapsibility [6].
Based on the test, it also has several beneficial natures making it entirely possible to be applied in
the road renovation. Proper permeability, low frost heaving rate, unconspicuous plasticity and minor
Advanced Materials Research Online: 2011-05-17
ISSN: 1662-8985, Vols. 250-253, pp 3460-3464
doi:10.4028/www.scientific.net/AMR.250-253.3460
© 2011 Trans Tech Publications, Switzerland
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
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2. contractibility are both positive properties of materials which could be viewed as ideal subgrade
fillings. To achieve the recycling application, some other disadvantages and restrictions on further
application such us its uneven composition and non-uniformity of characteristics have to be overcome
by making detailed origin analysis, instituting rational classification systems with proper indicators.
For this purpose, this paper firstly completed a series of field investigation and experimental tests on
selected soil specimens, then acquired effective data to evaluate their roadbed performance, and
finally chose the appropriate construction equipments, procedures and construction technology,
which will be introduced as below.
Specimen Property Test
The field survey illustrated that miscellaneous fill mixed with MSW with complex composition from
one site to another generally originated by excavating old roadbeds. To acquire the rational soil
natures, four soil specimens (marked with A, B, C and D) were selected in separated areas of Hefei,
China. Fig. 1 shown below illustrates soil specimen in the original location.
To effectively evaluate the application potentials of selected soil specimens in real recycle projects,
raw specimens should be separated firstly with different particle diameters. The precise statistic result
is listed in Table 1 to tell the proportion of solid waste which has particle diameter over 6mm in gross
weight of each soil specimen. Table 1 and Fig.2 show us some detailed materials natures of soil
specimens A-D (main soil body with particle diameters less than 6mm) on the basis of the Test
Methods of Soils for Highway Engineering (JTG E40-2007) which provides specific experiment
items and testing procedures.
Table 1 Materials properties testing results
Specimens A B C D
Proportion of Solid Waste(in gross weight)* % 20.3 2.3 53.5 4.4
Natural Moisture Ratios % 17.1 18.2 18.7 14.5
Free Swelling Ratio % 44 23 39 48
Plastic Limits 20.3 19.8 22.6 20.1
Plasticity Index 24.2 15.1 19.8 32.2
Optimum Moisture Content % 13.0 13.2 14.5 15.0
Maximum Dry Density g/cm3
1.80 1.94 1.80 1.85
CBR % 4.28 15.5 9.61 7.76
*Define 6mm (Diameter) as the limit between solid waste blocks and soil
Fig.1 Soil specimen in original position Fig. 2 Soil specimen size distribution curves
A three-level classification system of miscellaneous fill mixed with MSW is proposed carefully for
the purpose of analyzing materials origin, confirming proportion of ingredients and evaluating
application potentials. These three classes with related indicators and specifications are illustrated in
Table 2.
Advanced Materials Research Vols. 250-253 3461

3. Modification Experiment
Based on the property tests of specimens shown above, the soil specimen A can not satisfy the
construction codes and therefore the proper soil modification seems necessary. The first-level lime
(CaO&MgO≥60% of gross weight) had been chosen as inorganic binder to improve specimen A’s
road performance. Compaction test and unconfined compressive strength test were selected to
evaluate the modification effects.
Table 2 Evaluation of soil specimens based on the three-level classification system
Third Class(soil properties )
Symbol
Classification
indicators
Symbol
Classification
indicators
Classification descriptions
A J SK
weak swelling soil(Free
Swelling Ratio<65%); low
liquid limit clay (δef <50)
;CBR<5%
Not qualified for
road embankment
or roadbed fillings
B J SK
weak swelling soil(Free
Swelling Ratio<65%) ;low
liquid limit clay (δef <50)
;CBR>8%
Qualified for road
embankment and
roadbed fillings
C J SK
weak swelling soil(Free
Swelling Ratio<65%) ;low
liquid limit clay (δef <50)
;CBR>8%
Qualified for road
embankment and
roadbed fillings
D J TT
weak swelling soil(Free
Swelling Ratio<65%);high
liquid limits clay (δef >50)
;CBR 5%－8%
Qualified for road
embankment
J－construction
waste
S－life waste
G－industrial
waste
H－mixed waste
SK－ massive rocks
content≥50% of
gross weight
TT－ massive rocks
content ＜ 50% of
gross weight
Massive rocks are
defined as soil
particles with size
diameter over 6 mm.
Road performace
evaluation*
Specimens
First Class(origins)
Second Class(proportion of
solid waste)
* According to Specifications for Design of Highway Subgrades (JTG D30-2004)
Compaction Test
The first-level lime had been added into specimen A by different sets with proportions of 5%, 7% and
8%. Fig.3 describes the optimum moisture content slightly raises and the maximum dry density
slowly reduces with increasing lime contents, which manifests the maximum dry density becomes
insensitive to the change of moisture contents. This new trend of diminished maximum dry density by
increasing lime proportion also demonstrates this modification method effectively broaden the
applicable scope of soil materials for qualified strength. It means that feed with same amount of water,
soil specimens become more compact after soil modification with lime than before.
Fig.3 Optimum moisture content & maximum dry density changes
3462 Advanced Building Materials

4. Unconfined Compressive Strength Test
Unconfined compressive strength test can be used to detect the subgrade fillings strength. It made
three groups of 7 cylinder specimens with different sets of lime admixture (5%, 7%and 8%) to
determine their 7 days strength. According to Technical Specifications for Construction of Highway
Roadbases (JTJ 034-2000), experiment results and evaluation are summarized in Table 3.
Table 3 Experimental results and evaluation on specimen A mixed with lime
Highways&First
Class Roads
Other
Roads
Highways&First
Class Roads
Other
Roads
5% 0.79 0.125 × √
7% 0.95 0.089 √ √
8% 0.82 0.101 √ √
Evaluation
≥0.8 0.5-0.7
Lime
Content
Average
Intensity
Value
(Mpa)
Deviation
Coefficient
of 7 cylinder
specimens
Cv
Required Minimum
Intensity(Mpa)
Experiment results and evaluation indicate lime as additive in specimen A can effectively improve
its road performance. They also recommend that 7% seems to be the optimum adding proportion of
lime. It also reveals the application potential for similar inorganic binder materials.
Utilization and Treatment Technology
Cold Recycling Technology. Cold recycling technology is a typical technique for pavements and
roadbeds reconstruction. It usually crushes the discarded road construction waste into pieces within
limited particle sizes, adds some new or fine aggregate or inorganic binder as supplements, and then
completes a set of technical process such us milling, crushing, mixing, paving and compacting into
forms. It has some more obvious advantages than traditional technics. Saving construction material by
recycling waste, simplifying the construction process by accomplishing each process continuously,
protecting environment by simplifying transportation sections are several significant strong points.
ALLU Screener Crusher made by ALLU Finland Ltd. can be fixed on the common excavators for
solid waste separation, crush, grind, aeration, mixing, feeding and loading (shown in Fig.4).
Material crushing Material blending
Fig. 4 ALLU Screener Crusher
Soil Solidification. Soil stabilizers are usually divided into Calcium-Based Stabilizer (CBS) and
Non-Calcium-Based Stabilizer (NCBS), which can stimulate soil activity, ameliorate soil
microstructure and improve soil strength. CBS contains lime, cement, fly ash and slag as well as
NCBS refers to ionic soil stabilizer (ISS, Condor SS, Roadpacker, etc.), enzyme(CMC2,
Perma-Zyme, etc.), polymer(Road Oyl, Consoild 444,etc.) and so on.
Advanced Materials Research Vols. 250-253 3463

5. Various sorts of soil stabilizers have different modification mechanisms. Taking lime as
Calcium-Based Stabilizer for example, the procedures of ion-exchange, carbonation, crystallization
occur in succession and interact simultaneously. Lime stabilized soil is the common product of lime
solidification based on the main chemical reaction presented as Equation.1.
xCa(OH)2+SiO2+(n-x)H2O→xCaO·SiO2·H2O (1)
The main product of this chemical reaction is CSH (calcium silicate hydrate) which generates
cementation to bond minor particles with soil particles. The colloidal particles of CSH can also fill up
interspaces between soil particles for enhancing compactness and strength of soil body dramatically.
The working mechanism of polymer stabilizer is physical but not chemical. It commonly does
not change the inner structure of soil microstructure, but does concentrate and solidify the soil
particles to yield strong adherence by enclosing single soil particle.
Heavy-tamping Method. Heavy-tamping method initially invented by Mena Technology
Company in France with another name dynamic consolidation method usually hoists the heavy punch
up to 8-25m and then makes it in free fall to press the top-soil repeatedly. It can transform kinetic
energy from heavy punch into shock wave and dynamic stress in the deep soil body, force the soil
particles to realign and become compact. Simple construction process, low cost and ideal effect make
it previous in engineering projects. It also can improve the bearing capacity, compression modulus,
increase dry density, reduce the void ratio, and eliminate the collapsibility, expansibility or
liquefaction. Heavy-tamping method has been certified with broaden applications for different soils.
For miscellaneous fill used as subgrade and ground fillings, the compressibility of roadbed and
building foundation can reduce by 20～100% and strength can rise by 200～500%. It is not easy to
conclude the mechanisms in one single theory. Nowadays, dynamic consolidation theory, vibration
wave compaction theory and solid micromechanics are both recommended as acceptable theories.
Conclusion
In order to explore the application potentials of miscellaneous fill with solid waste caused by road
excavation and renovation, specimens A, B, C and D are selected to be tested to acquire their material
properties. The comprehensive evaluation shows us it is entirely possible to reutilize most of them as
subgrade fillings directly or just ameliorate them by some methods at the guidance of related road
construction codes and regulations. In engineering projects, cold recycling technology, soil
modification with additives and heavy-tamping method are recommended as several effective ways to
implement recycling and utilization technology.
Acknowledgement
The authors are grateful for support received from the Hefei Administration of Key Construction
Projects and Hefei University of Technology (research item: “Comprehensive Utilization and
Technical Research on Urban Road Construction Waste and Miscellaneous Fill”, 2008-2009).
References
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3464 Advanced Building Materials

6. Advanced Building Materials
10.4028/www.scientific.net/AMR.250-253
Recycled Technology of Urban Road Construction Waste and Miscellaneous Fill Used as Subgrade
Fillings
10.4028/www.scientific.net/AMR.250-253.3460
DOI References
[1] Tapan Narayana: Waste Management, Vol. 29(2009), p.1163–1166.
10.1016/j.wasman.2008.06.038
[5] Z. Salem, K. Hamouri, R. Djemaa, K. Allia: Desalination, Vol. 220(2008), p.108–114.
10.1016/j.desal.2007.01.026