PAVEMENT
DESIGN AND EVALUATION GRAPHS PROVIDED BY
Notes:
1) The pavement design and evaluation graphs included in this Appendix
are based on the same aircraft characteristics (track, wheel base, standard
tire pressure) as those used to calculate the ACN.
2) The weights shown in the graphs represent static loads on the main
undercarriage leg.
3) The rigid pavement graphs assume that the tire pressure remains
constant at the value q shown in the graphs. Should the actual tire pressure q
be different from q o, proceed as follows:
Where h is the thickness sought for pressure q
h° is the thickness read on the graph drawn up for pressure q°.
5) Figures A3-1 to A3-10 are provided as examples.
Graphs for all types of aircraft are available on request from:
MINISTERE DES TRANSPORTS
Direction General de 1'Aviation Civil Service Technique des Bases
Aeriennes
246, rue Lecourbe - 75732 PARIS CEDEX 15 – FRANCE
BACKGROUND
INFORMATION ON THE UNITED STATES PRACTICE FOR THE DESIGN AND EVALUATION OF
PAVEMENTS
1. Prior FAA method of soil classification
1.1 Background
The FAA method of soil classification which was used prior to the
adoption of the Unified Soil Classification System is presented in this
Appendix. The reason for including the method in this Appendix is that many
past records contain references
to the FAA method and this Appendix allows the reader to converse in
the FAA classification method.
1.2 Soil classification
a)
While the results of individual tests indicate certain physical
properties of the soil, the principal value is derived from the fact that,
through correlation of the data so obtained, it is possible to prepare an engineering
classification of soils related to their field behavior. Such a classification
is presented in Figure A4-1.
b)
The soil classification requires basically the performance of three
tests -- the mechanical analysis, determination of the liquid limit, and
determination of the plastic limit. Tests for these properties have been
utilized for many years as a means of evaluating soil for use in the
construction of embankments and pavement subgrades. These tests identify a
particular soil as having physical properties similar to those of a soil whose
performance and behavior are known. Therefore, the test soil
can be expected to possess the same characteristics and degree of
stability under like conditions of moisture and climate.
c)
As can be discerned from Figure A4-l, the mechanical analyses provide
the information to permit separation of the granular soils from the
fine-grained soils, whereas the several groups are arranged in order of
increasing values of liquid limit and plasticity index. The division between
granular and fine-grained soils is made upon the requirement that granular
soils must have less than 35 per cent of silt and clay combined. Determination
of the sand, silt, and clay fractions is made on that portion of the sample
passing the No. 10 sieve because this is considered to be the critical portion
with respect to changes in moisture and other climatic influences. The
classification of the soils with respect to different percentages of sand,
silt, and clay is
shown in Figure A4-2.
1) Group E-1 includes well-graded, coarse, granular soils that are
stable even under poor drainage conditions and are not generally subject to
detrimental frost heave. Soils of this group may conform to well-graded sands
and gravels with little or no fines. Tf frost is a factor, the soil should be
checked to determine the percentage of the material less than
2) Group E-2 is similar to Group E-1 but has less coarse sand and may
contain greater percentages of silt and clay. Soils of this group may become
unstable when poorly drained as well as being subject to frost heave to a
limited extent.
3) Groups E-3 and E-4 include the fine, sandy soils of inferior
grading. They may consist of fine cohesion less sand or sand-clay types with a
fair-to-good quality of binder. They are less
stable than Group E-2 soils under adverse conditions of drainage and
frost action.
4) Group E-5 comprises all poorly graded soils having more than
35 per cent but less than 45 per cent of silt and clay combined. This
group also includes all soils with less than 45 per cent of silt and clay but
which have plasticity indices of 10 to 15. These soils are susceptible to frost
action.
5) Group E-6 consists of the silts and sandy silts having zero-to-low
plasticity. These soils are friable and quite stable when dry or at low
moisture contents. They lose stability and become very
spongy when wet and, for this reason, are difficult to compact unless
the moisture content is carefully controlled. Capillary rise in the soils of
this group is very rapid; and they, more than soils of any other group, are
subject to detrimental frost heave.
6) Group E-7 includes the silly clay, sand clay, clayey sands, and
clayey silts. They range from friable to hard consistency when dry and are
plastic when wet. These soils are stiff and dense when compacted at the proper
moisture content. Variations in moisture are apt to produce a detrimental
volume change. Capillary forces acting in the soil are strong, but the rate of
capillary rise is relatively slow and frost heave, while detrimental, is not as
severe as in the E-6 soils.
7) Group E-8 soils are similar to the E-7 soils but the higher liquid
limits indicate a greater degree of compressibility expansion, shrinkage, and
lower stability under adverse moisture conditions.
8) Group E-9 comprises the silts and clays containing micaceous and
diatomaceous materials. They are highly elastic and very difficult to compact.
They have low stability in both the wet and dry state and are subject to frost
heave.
9) Group E-10 includes the silty clay and clay soils that form hard
clods when dry and are very plastic when wet. They are very compressible,
possess the properties of expansion, shrinkage,
and elasticity to a high degree and are subject to frost heave. Soils of
this group are more difficult to compact than those of the E-7 or E-8 groups
and require careful control of moisture to produce a dense, stable fill.
10) Group E-11 soils are similar to those of the E-10 group but have
higher liquid limits. This group includes all soils with liquid limits between
70 and 80 and plasticity indices over 30.
11) Group E-12 comprises all soils having liquid limits over 80
regardless of their plasticity indices. They may be highly plastic clays that
are extremely unstable in the presence of moisture, or they may be very elastic
soils containing mica, diatoms, or organic matter in excessive amounts.
Whatever the cause of their instability, they will require the maximum in
corrective measures.
12) Group E-13 encompasses
organic swamp soils such as muck and peat which are recognized by examination
in the field. In their natural state, they are characterized by very low
stability and density and very high moisture content.
1.3 Special conditions affecting fine-grained soils
a) A soil may possibly contain certain constituents that will give test
results which would place it, according to Figure A4-
b) Soils with plasticity indices higher than corresponding to the
maximum liquid limit of the particular group are not of common occurrence. When
encountered, they are placed in the higher numbered group as shown in Figure
A4-3. This is justified by the fact that for equal liquid limits the higher the
plasticity index, the lower the plastic limit (the plastic limit is the point
when a slight increase in moisture causes the soil to rapidly lose stability).
1.4 Coarse material retained on No. 10 sieve
Only that portion of the sample passing the No. 10 sieve is considered
in the above-described classification. Obviously, the presence of material
retained on the No. 10 sieve should serve to improve the over-all stability of
the soil. For this
reason, upgrading the soil from 1 to 2 classes is permitted when the
percentage of the total sample retained on the No. 10 sieve exceeds 45 per cent
for soils of the E-1 to E-4 groups and 55 per cent for the others. This applies
when the coarse fraction consists of reasonably sound material which is fairly
well graded from the maximum size down to the No. 10 sieve size. Stones or rock
fragments scattered through a soil should not be considered of sufficient
benefit to warrant upgrading.
1.5 Sub grade classification
a) For each soil group, there are corresponding sub grade classes.
These classes are based on the performance of the particular soil as a sub
grade for rigid or flexible pavements under different conditions of drainage
and frost. The sub grade class is determined from the results of soil tests and
the information obtained by means of the soil survey and a study of climatologically
and topographical data. The sub grade classes and their relationship to the
soil groups are shown in Figure A4-4. The prefix "F" indicates sub
grade classes for flexible pavements. These sub grade classes determine the
total pavement thickness for a given aircraft load. A brief description of the
classes will be presented here.
b) Sub grades classed as Fa furnish adequate sub grade support without
the addition of sub-base material. The soil's value as a sub grade material
decreases as the number increases.
c) Good and poor drainage refer to the subsurface soil drainage.
1) Poor drainage is defined for the purpose of this manual as soil that
cannot be drained because of its composition or because of the conditions at
the site. Soils primarily composed of silts and clay for all practical purposes
are impervious; and as long as a water source is available, the soil's natural
affinity for moisture will render these materials unstable. These fine-grained
soils cannot be drained and are classified as poor drainage as indicated in
Figure A4-
2) Good drainage is defined as a condition where the internal soil
drainage characteristics are such that the material can and does remain well
drained resulting in a stable subgrade material under all conditions.
d) There is a tendency to overlook the detrimental effects of frost in
pavement design. The effects of frost are widely known; however, experience
shows that all too often pavements are damaged or destroyed by frost that was
not properly taken into account
in the design. Most inorganic soils containing 3 per cent or more of
grains finer than
1) No frost should be used in the design when the average frost
penetration anticipated is less than the thickness of the pavement section.
2) Frost should be used when the anticipated average exceeds the
pavement sections. The design should non-frost susceptible material below the
required
minimize or eliminate the detrimental The extent of the sub grade
protection
and the surface and subsurface environment at the site. frost
penetration consider including sub-base to frost effect on the sub grade.
needed depends on the soil
2. Development of pavement design curves
2.1 Background.
a) The pavement design curves presented in Chapter 4, 4.4 of this
manual were developed using the California Bearing Ratio (CBR) method for
flexible pavements and the Westergaard edge loading analysis for
rigid pavements. The curves are constructed for the gross weight of the
aircraft assuming 95 per cent of the gross weight is carried on the main
landing gear assembly and 5 per cent of the gross weight is carried on the nose
gear assembly. Aircraft traffic is assumed to be normally distributed across
the pavement in the transverse direction. Pavements are designed on the basis
of static load analysis. Impact loads are not considered to increase the
pavement thickness requirements.
b) Generalized design curves have been developed for single, dual, and
dual tandem main landing gear assemblies. These generalized curves do not
represent specific aircraft but are prepared for a range of aircraft
characteristics which are representative of all civil aircraft except wide
body. The aircraft characteristics assumed for each landing gear assembly are
shown in Table A4-1, A4-2 and A4-3.
2.2 Flexible pavements
a) The design curves for
flexible pavements are based on the CBR method of design. The CBR is the ratio
of the load required to produce a specified penetration of a standard piston
into the material in question to the load required to produce the same
penetration in a standard well-graded, crushed limestone. Pavement thicknesses
necessary to protect various CBR values from shear failure have been developed
through test track studies and observations of in-service pavements. These
thicknesses have been developed for single wheel loadings. Assemblies other
than single wheel are designed by computing the equivalent single wheel load
for the assembly based on deflection. Once the equivalent single wheel is
established, the pavement section thickness can be determined from the
relationships discussed above.
b) Load repetitions are indicated on the design curves in terms of
annual departures. The annual departures are assumed to occur over a 20-year
life. In the development of the design curves, departures are converted to
coverages. For flexible pavements, coverage is a measure of the number of
maximum stress applications that occur on the surface of the pavement due to
the applied traffic. One coverage occurs when all points on the pavement
surface within the traffic lane have been subjected to one application of
maximum stress, assuming the stress is equal under the full tire print. Each
pass (departure) of an aircraft can be converted to coverages using a single
pass-to-coverage ratio which is developed assuming a normal distribution and applying
standard statistical techniques. The pass-to-coverage ratios used in developing
the flexible pavement design curves are given in Table A4-4. Annual departures
are converted to coverage by multiplying by 20 and dividing that product by the
pass-to-coverage ratio given in Tables A4-4. Figure A4-5 shows the relationship
between load repetition factor and coverage. The pavement section thickness
determined in accordance with a) above is multiplied by the appropriate load
repetition factor (Figure A4-5) to give the final pavement thickness required
for various traffic levels.
2.3 Rigid pavements
a) The design of rigid airport pavements is based on the Westergaard
analysis of an edge loaded slab resting on a dense liquid foundation. The edge
loading stresses are reduced by 25 per cent to account for load transfer across
joints. Two different cases of edge loading are covered by the design curves.
Figures 4-46 to 4-54 of Chapter 4 assume the landing gear assembly is either
tangent to a longitudinal joint or perpendicular to a transverse joint,
whichever produces the largest stress. Figures 4-56 to 4-62 of the same chapter
are for dual tandem assemblies and have been rotated through an angle to
produce the maximum edge stress. Computer analyses were performed for angles
from 0 to 90 degrees in 10-degree increments. Single and dual wheel assemblies
were analysed for loadings tangent to the edge only as the stress is maximum in
that position. Sketches of the various assembly positions are shown in Figure
A4-6.
b) Fatigue effects are taken into consideration by converting traffic
to coverages. The coverage concept provides a means of normalizing pavement
performance data which can consist of a variety of wheel sizes, spacings and
loads for pavements of different cross sections. For rigid pavements, coverage
is a measure of the number of maximum stress applications occurring within the
pavement slab due to the applied traffic. One coverage occurs when each point
in the pavement within the limits of the traffic lane has experienced a maximum
stress, assuming the stress is equal under the full tire print. Each pass
(departure) of an aircraft can be converted to coverages using a single
pass-to-coverage ratio which is developed assuming a normal distribution and
applying standard statistical techniques. The pass-to-coverage ratios used in
developing the rigid pavement design curves are given in Table A4-5. Annual
departures are converted to coverages assuming a 20-year design life. Coverages
are determined
by multiplying annual departures by 20 and dividing that product by the
pass-to-coverage ratio shown in Table A4-5.
c) After the conversion of departures to coverages, the slab thickness
is adjusted in accordance with the fatigue curve developed by the Corps of
Engineers from test track data and observation of in-service pavements. The
fatigue relationship is applicable to the pavement structure; i.e., the slab
and foundation are both included in the relationship. The thickness of pavement
required to sustain
5 000 coverages of the design loading is considered to be 100 per cent
thickness. Any coverage level could have been selected as the
100 per cent thickness level as long as the relative thicknesses for
other coverage levels shown in Figure A4-7 is maintained.
d) Pavement thickness requirements for 5 000 coverages were computed
for various concrete strengths and subgrade moduli. Allowable concrete stress
for 5 000 coverages was computed by dividing the concrete flexural strength by
1.3 (analogous to a safety factor). The pavement thickness necessary to produce
the allowable concrete stress for 5 000 coverages is then multiplied by the
percentage thickness shown in Figure A4-7 for other coverage levels.
3. Prior FAA pavement evaluation curves
3.1 To facilitate the pavement
evaluation policy described in Chapter 4, 4.4.27.2 the evaluation curves used
by the FAA previously are reproduced as Figures A4-8 to A4-21 of this Appendix.
Appendix 5
ACNS FOR SEVERAL AIRCRAFT TYPES
