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Five slide presentation outline
1 – Title, presenter information and affiliation
2 – Overview of topic, include objective and motivation
3 – Main body of topic
4 – more body of topic
5 – Conclusions, main point(s) emphasized here
5.0 GEOSYNTHETICS IN ROADWAYS AND PAVEMENTS
5.1
INTRODUCTION
The most common use of geosynthetics is in road and pavement construction. Geotextiles increase
stability and improve performance of weak subgrade soils primarily by separating the aggregate
from the subgrade. In addition, geogrids and some geotextiles can provide strength through
friction or interlock developed between the aggregate and the geosynthetic. Geotextiles can also
provide filtration and drainage by allowing excess pore water pressures in the sub grade to dissipate
into the aggregate base course and, in cases of poor-quality aggregate, through the geotextile plane
itself.
In this chapter, each of the geosynthetic functions will be discussed and related to design concepts
and performance properties. Selection, specification, and construction procedures will also be
presented.
5.1-1 Functions of Geosynthetics in Roadways and Pavements
A geosynthetic placed at the interface between the aggregate base course and the subgrade
functions as a separator to prevent two dissimilar materials (subgrade soils and aggregates) from
intermixing. Geotextiles and geogrids perform this function by preventing penetration of the
aggregate into the subgrade (localized bearing failures) (Figure 5-1). In addition, geotextiles
prevent intrusion of subgrade soils up into the base course aggregate. Localized bearing failures
and subgrade intrusion occur in very soft, wet, weak subgrades. Subgrade intrusion can also
occur under long term dynamic loading due to pumping and migration of fines, especially when
open-graded base courses are used. It only takes a small amount of fines to significantly reduce
the friction angle of select granular aggregate. Therefore, separation is important to maintain the
design thickness and the stability and load-carrying capacity of the base course. Soft sub grade
soils are most susceptible to disturbance during construction activities such as clearing, grubbing,
and initial aggregate placement. Geosynthetics can help minimize sub grade disturbance and
prevent loss of aggregate during construction. Thus, the primary function of the geotextile in this
application is separation, and can in some cases be considered a secondary function for geogrids.
The system performance may also be influenced by functions of filtration and drainage (Table
1-1). The geotextile acts as a filter to prevent fines from migrating up into the aggregate due to
high pore water pressures induced by dynamic wheel loads. It also acts as a drain, allowing the
excess pore pressures to dissipate through the geotextile and the subgrade soils to gain strength
through consolidation and improve with time.
Geosynthetics in Roadways and Pavements
147
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Concept of geotextile separation in roadways (after Rankilor, 1981).
System perfonnance may also be improved through reinforcement. Geogrids and geotextiles
provide reinforcement through three possible mechanisms.
1. Lateral restraint of the base and subgrade through friction and interlock between the
aggregate, soil and the geosynthetic (Figure 5-2a).
2. Increase in the system bearing capacity by forcing the potential bearing capacity failure surface
to develop along alternate, higher shear strength surfaces (Figure 5-2b).
3. Membrane suppon of the wheel loads (Figure 5-2c).
When an aggregate layer is loaded by a wheel or track, the aggregate tends to move or shove
laterally, as shown in Figure 5-2a, unless it is restrained by the subgrade or geosynthetic
reinforcement. Soft, weak subgrade soils provide very little lateral restraint, so when the
aggregate moves laterally, ruts develop on the aggregate surface and also in the subgrade. A
geogrid with good interlocking capabilities or a geotextile with good frictional capabilities can
provide tensile resistance to lateral aggregate movement. Another possible geosynthetic
reinforcement mechanism is illustrated in Figure 5-2b. Using the analogy of a wheel load to a
footing, the geosynthetic reinforcement forces the potential bearing capacity failure surface to
follow an alternate higher strength path. This tends to increase the bearing capacity of the
roadway.
148
April 1998
, – – WHEEL LOAD
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Possible reinforcement functions provided by geosynthetics in roadways: (a)
lateral restraint, (b) bearing capacity increase, and (c) membrane tension
support (after Haliburton, et al., 1981).
Geosynthetics in Roadways and Pavements
149
A third possible geosynthetic reinforcement function is membrane-type support of wheel loads,
as shown conceptually in Figure 5-2c. In this case, the wheel load stresses must be great enough
to cause plastic deformation and ruts in the subgrade. If the geosynthetic has a sufficiently high
tensile modulus, tensile stresses will develop in the reinforcement, and the vertical component of
this membrane stress will help support the applied wheel loads. As tensile stress within the
geosynthetic cannot be developed without some elongation, wheel path rutting (in excess of 100
mm) is required to develop membrane-type support. Therefore, this mechanism is generally
limited to temporary roads or the first aggregate lift in permanent roadways.
5.1-2 Subgrade Conditions in which Geosynthetics are Useful
Geotextile separators have a 20 + year history of successful use for the stabilization of very soft
wet subgrades. Based on experience and several case histories summarized by Haliburton,
Lawmaster, and McGuffey (1981) and Christopher and Holtz (1985), the following subgrade
conditions are considered to be the most appropriate for geosynthetic use in roadway construction:

Poor soils
(USCS: SC, CL, CH, ML, MH, OL, OH, and PT)
(AASHTO: A-5, A-6, A-7-5, and A-7-6)

Low undrained shear strength
t f = Cu < 90 kPa CBR < 3 {Note: CBR as determined with ASTM D 4429 Bearing MR ::: 30MPa Ratio of Soils in Place (1994)} • • High water table High sensitivity Under these conditions, geosynthetics function primarily as separators and filters to stabilize the subgrade, improving construction conditions and allowing long-term strength improvements in the subgrade. If large ruts develop during placement of the first aggregate lift, then some reinforcing effect is also present. As a summary recommendation, the following geotextile functions are appropriate for the corresponding subgrade strengths: Undrained Shear Sub~rade Stren~th eBB. (kPa) 60 - 90 30 - 60 < 30 2-3 1-2 < 1 Functions Filtration and possibly separation Filtration, separation, and possibly reinforcement All functions, including reinforcement As the geosynthetic allows for subgrade improvement with time, AASHTO M288 has identified applications where the undrained shear strength is less than about 90 kPa (CBR about 3) as ISO April 1998 stabilization applications. From a foundation engineering point of view, clay soils with undrained shear strengths of 90 kPa are considered to be stiff clays (Terzaghi and Peck, 1967, P 30) and are generally quite good foundation materials. Allowable footing pressures on such soils equal 150 kPa or greater. Simple stress distribution calculations show that for static loads, such soils will readily support reasonable truck loads and tire pressures, even under relatively thin granular bases. Dynamic loads and high tire pressures are another matter. Some rutting will probably occur in such soils, especially after a few hundred passes (Webster, 1993). If traffic is limited, as it is in many temporary roads, or if shallow « 75 mm) ruts are acceptable, as in most construction operations, then a maximum undrained shear strength of about 90 kPa (CBR = 3) for geosynthetic use in highway construction seems reasonable. However, for soils that are seasonally weak (e.g., from frost heave) or for high fines content soils which are susceptible to pumping, a geotextile separator may be of benefit in preventing migration of fines. This is especially the case for permeable base applications. Even on firm subgrades, a geotextile placed beneath the base functions as a separator and filter, as illustrated in Figure 5-3. A greater range of geotextile applicability is recognized in the M288 specification (AASHTO, 1997) with a CBR ~ 3 the geotextile application is identified as separation. Further discussion of potential applicability of geotextiles on soils with CBR > 3 is presented in Appendix G of this manual, and the complete
M288 specification is presented in Appendix D .
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Geosynthetics in Roadways and Pavements
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5.2
APPLICATIONS
5.2-1 Temporary and Pennanent Roads
Roads and highways are broadly classified into two categories: permanent and temporary,
depending on their service life, traffic applications, or desired performance. Permanent roads
include both paved and unpaved systems which usually remain in service 10 years or more.
Permanent roads may be subjected to more than a million load applications during their design
lives. On the other hand, temporary roads are, in most cases, unpaved. They remain in service
for only short periods of time (often less than 1 year), and are usually subjected to fewer than
10,000 load applications during their services lives. Temporary roads include detours, haul and
access roads, construction platforms, and stabilized working tables required for the construction
of permanent roads, as well as embankments over soft foundations.
Geosynthetics allow construction equipment access to sites where the soils are normally too weak
to support the initial construction work. This is one of the more important uses of geosynthetics.
Even if the finished roadway can be supported by the subgrade, it may be virtually impossible to
begin construction of the embankment or roadway. Such sites require stabilization by dewatering,
demucking, excavation and replacement with select granular materials, utilization of stabilization
aggregate, chemical stabilization, etc. Geosynthetics can often be a cost-effective alternate to
these expensive foundation treatment procedures.
Furthermore, geosynthetic separators enable contractors to meet minimum compaction
specifications for the first two or three aggregate lifts. This is especially true on very soft, wet
subgrades, where the use of ordinary compaction equipment is very difficult or even impossible.
Long term, a geosynthetic acts to maintain the roadway design section and the base course
material integrity. Thus, the geosynthetic will ultimately increase the life of the roadway.
5.2-2 Benefits
Geosynthetics used in roadways on soft subgrades, may provide several cost and performance
benefits, including the following.
1. Reducing the intensity of stress on the subgrade and preventing the base aggregate from
penetrating into the sub grade (function: separation).
2. Preventing subgrade fines from pumping or otherwise migrating up into the base
(function: separation and filtration).
3. Preventing contamination of the base materials which may allow more open-graded, freedraining aggregates to be considered in the design (function: filtration).
4. Reducing the depth of excavation required for the removal of unsuitable subgrade materials
152
April 1998
5.
6.
7.
8.
9.
(function: separation and reinforcement).
Reducing the thickness of aggregate required to stabilize the subgrade (function:
separation and reinforcement).
Reducing disturbance of the subgrade during construction (function: separation and
reinforcement) .
Allowing an increase in subgrade strength over time (function: filtration).
Reducing the differential settlement of the roadway, which helps maintain pavement
integrity and uniformity (function: reinforcement). Geosynthetics will also aid in
reducing differential settlement in transition areas from cut to fill. {NOTE: Total and
consolidation settlements are not reduced by the use of geosynthetic reinforcement.}
Reducing maintenance and extending the life of the pavement (functions: all).
Geosynthetics are also used in permanent roadways to provide capillary breaks to reduce frost
action in frost-susceptible soils, and to provide membrane-encapsulated soil layers (MESL) to
reduce the effects of seasonal water content changes on roadways on swelling clays.
5.3
POSSmLE FAILURE MODES OF PERMANENT ROADS
Yoder and Witczak (1975) define two types of pavement distress, or failure. The first is a
structural failure, in which a collapse of the entire structure or a breakdown of one or more of the
pavement components renders the pavement incapable of sustaining the loads imposed on its
surface. The second type failure is a functional failure; it occurs when the pavement, due to its
roughness, is unable to carry out its intended function without causing discomfort to drivers or
passengers or imposing high stresses on vehicles. The cause of these failure conditions may be
due to excessive loads, climatic and environmental conditions, poor drainage leading to poor
subgrade conditions, and disintegration of the component materials. Excessive loads, excessive
repetition of loads, and high tire pressures can cause either structural or functional failures.
Pavement failures may occur due to the intrusion of subgrade soils into the granular base, which
results in inadequate drainage and reduced stability. Distress may also occur due to excessive
loads that cause a shear failure in the subgrade, base course, or the surface. Other causes of
failures are surface fatigue and excessive settlement, especially differential of the subgrade.
Volume change of subgrade soils due to wetting and drying, freezing and thawing, or improper
drainage may also cause pavement distress. Inadequate drainage of water from the base and
subgrade is a major cause of pavement problems (Cedergren, 1987). If the subgrade is saturated,
excess pore pressures will develop under traffic loads, resulting in subsequent softening of the
subgrade. Under dynamic loading, fines can be literally pumped up into the subgrade or base.
Geosynthetics in Roadways and Pavemellts
153
Improper construction practices may also cause pavement distress. Wetting of the subgrade during
construction may permit water accumulation and subsequent softening of the subgrade in the rutted
areas after construction is completed. Use of dirty aggregates or contamination of the base
aggregates during construction may produce inadequate drainage, instability, and frost
susceptibility. Reduction in design thickness during construction due to insufficient subgrade
preparation may result in undulating subgrade surfaces, failure to place proper layer thicknesses,
and unanticipated loss of base materials due to subgrade intrusion. Yoder and Witczak (1975)
state that a major cause of pavement deterioration is inadequate observation and field control by
qualified engineers and technicians during construction.
After construction is complete, improper or inadequate maintenance may also result in pavement
distress. Sealing of cracks and joints at proper intervals must be performed to prevent surface
water infiltration. Maintenance of shoulders will also affect pavement performance.
As indicated in the list of possible benefits resulting from geosynthetic use in permanent roadway
systems (section 5.2-2), properly designed geosynthetics can enhance pavement performance and
reduce the likelihood of failures.
5.4
ROADW AY DESIGN USING GEQTEXTILES
Certain design principles are common to all types of roadways, regardless of the design method.
Basically, the design of any roadway involves a study of each of the components of the system,
(surface, aggregate base courses and subgrade) detailing their behavior under traffic load and their
ability to carry that load under various climatic and environmental conditions. All roadway
systems, whether permanent or temporary, derive their support from the underlying subgrade
soils. Thus, the geotextile functions are similar for either temporary or permanent roadway
applications. However, due to different performance requirements, design methodologies for
temporary roads should not be used to design permanent roads. Temporary roadway design
usually allows some rutting to occur over the design life, as ruts will not necessarily impair
service. Obviously, ruts are not acceptable in permanent roadways. In the following two
sections, recommended design procedures for both temporary and permanent roads are presented.
Our permanent road and pavement design basically uses geotextiles for the construction or
stabilization lift only; the base course thickness required to adequately carry the design traffic
loads for the design life of the pavement is not reduced due to the use of a geotextile. There is
some evidence, however, that suggests a geogrid placed at the bottom of the aggregate base may
permit a 10 to 20% base thickness reduction, as noted in Appendix 0, Recent Roadway Research.
154
April 1998

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