STATENS VAG- OCH TRAFIKINSTITUT
National Swedish Road and Traffic Research InstituteTHE EFFECT OF SPRING THAW SUBGRADE CONDITIONS
ON THE LOAD CARRYING CAPABILITY OF FLEXIBLE
AIRFIELD PAVEMENTS IN COLD REGIONS
by
Bjiirn Orbom
REPORT No. 60 A Stockholm 1975
STATENS VAG- OCH TRAFIKINSTITUT
National Swedish Road and Traffic Research InstituteTHE EFFECT OF SPRING THAW SUBGRADE CONDITIONS
ON THE LOAD CARRYING CAPABILITY OF FLEXIBLE
AIRFIELD PAVEMENTS IN COLD REGIONS
by
Bjérn Orbom
REPORT N0. 60 A Stockholm 1975
VTI.
SUMMARY _
The bearing capacity of an flexible airfield pavement is influenced by many environmental factors but no doubt the greatest variations in this respect will occur in pavements on frost-susceptible subgrades, where deep frost penetration is normal in wintertime. The very high bearing capacity during the winter due to frost in the pavement and subgrade will be changed to a very low value at the spring thaw, when the tOp
layer of the subgrade is saturated.
In this paper a study was made on the seasonal variation of the strength of a normally designed flexible runway pavement in climatic conditions like those of Scandinavia. Based on results from falling weight deflectometer tests on two test sections with frostmsusceptible subgrade,
the strains on the bituminous pavement and the subgrade
were computed. The annual "failure ratios" for the
different seasons of the year and every applied load * then were calculated, the "failure ratio" being defined
as the number of strain applications caused by a
repeated invariable load in relation to the total number sf strain applications causing fatigue failure.
From the derived seasonal variation of the annual
failure ratio" for the pavement chosen, two important
conclusions can be drawn, viz (1) if the bearing
capa-city is to be expressed in one single safe load holding good for the whole year, the level of the load is to a 'very great extent determined by the comparatively high annual "failure ratio" for the spring thaw period, (2) by choosing a somewhat lower safe load for the spring thaw period, which for the circumstances in question is a small part of the year, it will be possible to
increase substantially the safe load for the rest of the. year. The last mentioned way of eXpressing the strength »of an airfield pavement resting on a frost susceptible
subgrade undoubtedly is more logical from a technical standpoint and will lead to a higher potential degree of utilization for flexible pavements in cold regions.
T A B L E
O F
C O N T E N T S
Page
I.
INTRODUCTION.
'
.
1
2.
SEASONAL CHANGES IN LOAD CARRYING
CAPA-BILITIES OF FROST-SUSCEPTIBLE SUBGRADES
2*
3.
PAVEMENT STRENGTH CALCULATION USING
SUBGRADE MEDIUM MODULUS
'
4
TABLE 1
.
v
4
FIGURE 1
.
-
I
6
TABLE 2 '
9
FIGURE 2 i
'
.
-
11
FIGURE 3
'
.
.
13
TABLE 3'
.
14
5.
CONCLUSIONS
15
REFERENCES ._ §.
.
'
16
JTI. Rapport Nr 60 AThe Effect of Spring Thaw Subgrade COnditions on the
Load-carrying Capability of Flexible Airfield'Pave~
ments in Cold Regions.
B érbom, National Swedish Road and Traffic Research
Institute, Stockholm, Sweden.
.1. INTRODUCTION
In regions subjected to deep frost action it is well known that the load carrying capability of flexible pavement structures is decreased during an annual period coincident with spring thaw. The degree of strength reduction is influenced by environmental factors, such as the frost-susceptibility of the subgrade soil and the depth of the ground water
table below the pavement surface. " _
Considerably reduced strength during spring has
caused road authorities in many countries to impose
reduced load limits for public roads during the critical Spring period. The axle load restrictions seem almost without exception to have been based on local experiencies, in spite of the fact that. last years suggestions based on scientific studies
were published (1) Of how to determine which roads
should be restricted to light axle loads during the critical period and.how.to decide the duration
of this period.
There is no doubt that imposing corresponding safe
__ load restrictions for airfield pavements subjected
- to the same environmental factors should be techni-cally sOund. In the Canadian engineering design manual for airport deve10pment a method is Specified for
quantitative determination of the spring reduction in~
the subgrade bearing value based on soil classification,VTI.
climatic conditions and the depth of the ground ~ water table, but apparently no corresponding
specifications have been published in the Northern Europeen countries on the subject of deep frost problems.
The object of this preliminary study was to make a calculation of the influence on the pavement
strength of the Spring thaw reduction in the subgrade bearing values for Scandinavian conditions. The
calculation is based on (a) published records of load deflection tests from pavements on frost susceptible subgrades and (b) data evaluated from a newly started Swedish limited research project,
sponsored by the Board of Civil Aviation, Sweden. The
results are tested on an airfield flexible pavement
construction typical of Swedish conditions and
designed for rather heavy traffic (ICAO-LCN 60).
2. SEASONAL CHANGES IN LOAD-CARRYING CAPA-BILITIES OF FROST-SUSCEPTIBLE SUBGRADES
Design of airfield pavements by elastic stress-strain analysis implies knowledge of-the mechanical pro-yperties of the pavement layers and the subgrade evaluated in terms of E-modulus and Poisson ratios.
The seasonal variation of the elastic properties
of the bitumen bound tOp layers of the pavement dueto Changes in air temperature has been well examined (2)
(3). The data published on the seasonal variation of the elastic properties of subgrade soils due to changes in water content and temperature are less eXtensive. Of great value in this respect is, how ever, two reports of recorded test load deflections througout the seasons in cold regions, one from Canadian Good Roads Association (4) and the other
from Highway Research Borad (l).
VTI.
The very high load carrying'ez-frost-susceptible subgrade-so;»
decrease dramatically due to antwiwt
ice lenses as frost leaves the ground. During the springlthat
mechanical point of View the 9;:
as consisting of three layers prOperties, an upper thawing ;
of the E-modulus and a graduaiiw
an intermediate frozen layer Law of E modulus and a decreasing layer of semi-infinite extent corresponding to the soil in us
I..
,... ,4.
Ni
A
,\.g.._,
with the water content in quest;
As a first approximation by t1 pavement bearing capacity the
"N
mentioned werey however, treat * foundation with a medium valux
By this simplified assumption
use the above mentioned Canadgay (4) to derive and compare the
E-modulus during the spring tiuw and the summer fall period, (t;
The relation derived between values of the foundation medii
certain frost susceptible subw;a ¢ used for the bearing capacity
field pavements. Rapport Nr 60 A 9.3 a~frozen m w:;.lrtime will 7,,v'melting 'ger of the 3 ~ DEL a
'uw way he considered,
arent iowvvalue zing.thickness, in; a tvgh value» W & g nd a lower «39 ggw wmodulus_ lcuf"7 on of the vade layers ~¢ogeneous $9.:vtiéen_values' medium pegrug~gmin.value) rm"... , - , i .1. dal wj Vespectively. * wm»;and»maximum " ;;:1:§-I,a;: if; 77?. ( Va 1. i d f or
Canada, was then ix gxr As
for.air-MEDIUM MODULUS'
PAVEMENT STRENGTH CALCULATION USING SUQGRADE
The study of the Canadian test load resulte published was performed by means of a computer program (CHEVRON).
' The calculated values of the medium E-modulus (static modulus) at spring thaw based on the peak deflection
was found to be 17 .40 % of the Emmodulus for the
summerufall period.
Table l. Design data for an airfield pavement on froat
susceptible subgrade, (l layered semi~infinite)._
A
Semi~infinite
'VTI. Rapport Nr 60~ZX
. Design period through.the year, 1 2 3 . . ' 5
- 2
a m
-
.No
A1t(a)A1t(b)Alt(a) 1t(b)
Duration of design period, month 2 3/4 31/4
3
' 2
1
2
_
2 .
Bitum.pavement temp, 0C 25.0 18.2 6.5 6.5 5.0- 5.0 0 v Dynamic E~modolus (kp/cmz)
RA ?72Z/ _ P0103008 Ra io v ~ ». _ w
0 - z~ / Bituminous concrete 23000 38000. 92000-92000 100000 100000 >100000
/éé; graVel ' .
«i j;.°;.15 cm Base course
5000
5000
5000 5000
1 500 1 500>>500
a ' o, Gravel material 0.35 0.35 0.35 0.35 0.35 0.35 v
- -'75 cm; Subbase
2500 2500. -2500
2500
750
750 >>2500
' . . 0.35 0.35 00.35 0.35 0.35 0.35 .. . .. Sandy materzal . 2/57/1202)» ' -Frostw usceptible 1000 1000 1000 1000 300 300 >1000 Subgrade 0.35 0.35 0.35 . 0.35 0.50 0.50 lwlayered ' 'These results were applied to an airfield pavement
construction on frost susceptible subgrade (Table l).
The following analytical Calculation of the effect of the subgrade spring thaw on the bearing capacity
of the pavement was based on the theory of elasticity,
using the CHEVRON computer program. The load acting
on the pavement surface was assumed to be an ESWL
with circular contact area and tire pressure of
10 kp/cmz. The dynamic modulus of the various
layers were asSumed to be of constant value during
the design periods into which the year was divided. The values are dependent on the temperature (bituminous layers) (2) (3) and water content (unbound pavement Vlayers and subgrade). The decrease of the subgrade
medium dynamic modulus during spring thaw was assumed to correspond with the decrease mentioned found for Static modulus, an assumption which may be reasonable.
The radial tensile strain at the lower interface of
the bituminous layer and the vertical strain at the top of the subgrade were then calculated. From the relation between the bitumionus layer strain and the" number of load application to failure used byKingham (2), the "failure ratio" per year for every design period and applied load was calculated.
Failure ratio" here was defined as the number of
strain applications caused by a repeated invariable load in relation to (in per cent) the allowable total ': number of strain applicatiOns causing fatigue failure.
In the same way the "failure ratio", referring to the subgrade strains and based 0n the AASHO Road TeSt
criteria (5) was calculated. In all load cases
in-vestigated_the "failure ratio" was critical for the bituminous layer and not for the subgrade.
From the values of subgrade medium modulus given in Table 1 it can_be seen that, at the bearing capacity
calculation, a subgrade modulus reduction to 30 6
of the value for summer conditions was used. This
VTI . 6. t 18 t -pe ri od -. M o n t h A .S pr in g th aw E S W L 21 ¢ 201:
d
LL
; 3-?
_
ox"
NPa: :«31 :5: § Es}; 3
. . 5M 7 w-t- .:\.\- d . I \. D \. '\ Z_.
u o .__
\\ O
.\ m
L.
4L)-.
E,
n
a
H. w
o. o d.
\
»
m.
I
.5th
5
1
/0 106K Jed cum aJnHDj
u
E
" 2
ti
. . r u v I r I l 0] mm
'Fig.
Rapport Nr 60 A
10 m o
1. Annual failure ratio through the seasons of the year for different safe loads in terms of ESWL and LCN (ICAO). Presumed design life is about 8 years corresponding to a failure ratio of 12% per year. The maximum safe load holding good for the whole year in this case is found to be an
ESWL of l7t (LCN 52) trace o a-b-c-d~e f).
Alternatively it is possible to choose a higher safe load for the period of the year from May through Febr. if the increased failure ratio made use of during this period is compensated by
choosing a lower safe load for the following spring
thaw period" In this case the safe load for the first period may be for instance an ESWL of 24t, which will give a permitted ESWL of lSt for they
Spring thaw period (geometrically found in the
diagram), if an annual failure ratio of 12% is to
JTI.
reduction was considered reasonable during a spring thaw
period of'l month, alternatively 2 months, referring to
the maximum reduction of 17% mentioned above. In addition to this, one calculation more was performed assuming a modulus reduction of 50% (Table 2) during spring thaw.
The results of the bearing capacity calculations are illustrated in Fig l, where the bituminous pavement
failure ratio" in per cent per year are plotted for different loads (ESWL) and interpreted in terms of LCN (ICAO).
By the calculation, the number of load repetitions (aircraft passages) was established at l 200 per year, which corresponds to an 8 year design if the
cumula-tive number of passages desired is fixed at the value
of 10 000. With a design-time of 8 years the "failure ratio" should not exeed 12% per year. The corresponding load (ESWL) giving this cumulative "failure ratio" per year was traced in the diagram. This load design line
is marked in Fig l (o~a~b c-d-e-f).
From the diagram it can be concluded that the fatigue effect on the bituminous pavement caused by the
repeated load is influenced to a high degree by the
environmental factors (temperature and water content).
Thus the decrease of the "failure ratio value is
equal to nil during the winter period, very small
during the summer and early fall periods, greater
during the late fall period due to low temperatures in
the bituminous layers and greatest for the spring
thaw period due to the co-operating effects of low temperatures in the bituminous layers and high water content in the top of the subgrade (i.e. low E modulus value). Basing the aspects of the loadécarrying capability
of flexible pavements in cold regions on diagrams likei
I
Fig 1 it would be quite logical to permit higher SafeJTI.
loads during the long summer and early fall period.
(and of course during the winter period) and
some-what lower safe loads during the rather short spring thaw period.
In the diagram, Fig 1, this idea was examined, starting
with the assumption that a reduction of the allowable
load during the spring thaw period may be established
at 10%. The corresponding design line is marked o k j-i h-g-f in the diagram. From classification
vieWpoint this means that, instead of stating one .
number of bearing capacity valid for the whole year (LCN 52 in this case), it would be an alternative possibility to state 333 numbers, the first giving the bearing capacity for the whole year, the spring thaw period excepted, and the second one giving the
value for the-spring thaw period (in this case LCN 70
and LCN 47, respectively). There is no question that the latter suggestion for classifying the pavement bearing capacity for cold region airfields is of great advantage in many cases, considering the desirability from an operational point of view to utilize as far as possible the real load-carrying capability of the pavement. The result of the examination of the pave ment bearing capacities, in relation to the other chosen spring thaw conditions and durations, is summerized in Table 2.
From this and many other analyses of the influence of environmental factors on the pavement strength
for cold region airfields it is evident that the conditions of the subgrade in spring thaw time are critical. In the analyses here referred a medium
E-modulus was used to characterize the elastic behaviour of the subgrade. During the spring thaw period, when mechanically the subgrade could be
regarded as a three layer system with quite different
\PTI.
There is,
- modulus values as mentioned above , the assumption
of substituting medium modulus is an approximation.
however, no problems to theoretically ana-lysing the strains of the different layers of a
pavement resting on a three layers subgrade, provided
that the values of modulus and Poisson ratio are
stated.
Table 2. Calculated bearing capacity in terms of LCN at different assumptions as to the conditions of the subgrade during Spring thaw.
Spring Subgrade E-modulus of Bearing capacity thaw treated subgrade in Alt I r Alt II_
duration as per cent of y
summer value For the For the For the whole spring rest of
year thaw the year
period Month Z LCN LCN LCN Group a 7 1 1 layer 30 59 53 7O 2 (semi- 3O 52 47 70 l infi- 50 67 60 80 2 nite) 50 61 55 82 Group b 1/2 + 3 layers 35 » 60 54 71 + 1/2 (compare 20 Fig 4) Rapport Nr 60 A
TI.
10.
4. PAVEMENT STRENGTH CALCULATION USING A THREE-LAYER SYSTEM AS SUBGRADE DURING THAW TIME
In order to obtain a conception of the elaStic condi-'
tions of a frost susceptible subgrade during the spring thaw period, two small test sections were
arranged in the Northern part of Sweden. The sections were constructed in a very simple way, consisting of a base gravel course spread-and compacted on the
levelled ground which consisted of silty soils. The levels of the upper and lower surfaces of the frost zone were registered during the winter and the Spring of 1973. During the spring thaw period, bearing tests were performed at four occasions. A comparative
hearing test was performed in July well after the frost had left the ground, Fig 2. Two methods were used for the performance of the bearing tests, the
falling weight deflectometer method (6) and the wave
prOpagation method (7), both of them giving dynamic modulus values as final result of the data treatment.
In this study, only results from the falling weight test method were used. Assuming that the E-value of the gravel layer was about 2,5 times the value of the underlying, softed top layer of the subbase, which is a reasonable approach (3) (7), it was possible to
evaluate the E-modulus of the latter layer. Thus the
construction tested consisted of the following layers in order, (a) a gravel layer with known thickness and a modulus which was as mentioned related to the
modulus of the underlying layer, (b) a top subgrade layer softened by high water content during the Spring » thaw period and with a known thickness, (c) a
layer-of frozen subgrade soil with known thickness and a very high modulus and (d) a layer of unfrozen subgrade with semieinfinite thickness and a modulus normal
for the soil and environmental conditions in question.
11.
Test sec ons A
1 73 _ _
P=18 t
Rebo un d de f le ct io ' ' ' ' Gravel ,. 0 ' 0 I o q . n O . . o O I u G C I . a . . . base _ Sub grade
MAR APR MAY ' JUN JUL
30 Inm 20 10
Re bo un d de fl ec ti on +-0 . .. . . ,.A '. '
. . . ...,jGrc1vel
. ..
.. . .. . .
.. .
.
.
.
; . . . .. base
Sub-grade Km cm_MAR APR MAX JUN JUL
Fig 2. Frost penetratidn diagrams and recorded rebound.
de lections at a maximum test load of l 800 hp
during and well after spring thaw of 1973.
Falling weight deflectometer used.VTI.
12.
The reSults of the evaluation are given in the diagram
Fig 3 as the variation in time of the E-modulus of the
subgrade top layer. From the diagram it can beconcluded that the minimum value of the modulus in May was about 20% of the maximum value in July for
'the two test.sections A and B.
These results were applied to the typical airfield pavement construction previously used for bearing capacity calculations in this study. The chosen input data valid for the critical spring thaw period are shown in Table 3. The total duration of the Spring
thaw time was assumed to be 1 month, divided into
two periods of equal length. The modulus valid for the first period following the Spring thaw start in the subgrade top layer is averaged at 35% of the V .maximum summer value based on the results given in
Fig 3. For the_second period the corresponding value
is estimated at 20% in the.same way.
Finally a strain analysis of the pavement
construc-tion given in Table 3 was performed, using different. ESWL values and the same design periods throughout the year as shown in Table 1 (Alt a). The "failure ratios" were calculated and the design line giving
an allowable cumulative number of 10 000 wheelpassages was traced in the same way as-indicated in Fig l.
The bearing.capacity values derived in terms of LCN are noted in Table 2 (Group b) as one value valid' for the whole year as Well as two differentiated
values valid for the spring thaw period and the
rest of the year, respectively. It can be concluded
from Table 2 that the result of the bearing capacitycalculation with the assumption of a 3-layered
l3.
E2
kp/cr'n2
sm E-modulus
Test section A E1 " / Grove! 250m
400 __,V_ I
'
'
-
' Thowmg
l // [E] . subgr ode
/ . ~ ._... .
/ >>E -__ __ _______ Frozen. ~_._: __. .
300 ' ,/ Test section B 2 __-_-_:___:__' subgrode
I ,.. - o- . I ,/ V Y El. ' ' ' ' ' '.Unfrozen 203- V II I _/ ' 1 '_ Z ', ,' subgrode
\
/, /
°°
°Q_///
0'
M
i
A
I
M
I
J
I
J j
Fig 3. Seasonal variations of the dynamic
'
E-modulus of the thawing top subgrade
layer (E2), calculated from recorded
rebound deflections, assuming that
the loaded test section consists of a 4~layered system.
14.
subgrade differs very little from that of a spring
thaw modulus of 30% of the summer value (Group a,
line l).-This indicates that the conditions in
the tOp layer of the subbase has a considerably greater effect on the bearing capacity than the underlaying layers of the ground.
Table 3.
Design data for an airfield pavement
on frost susceptible subgrade
(3wlayered).
Spring thaw design period (month): 1/2 + 1/2
I Dynamic Ewmodulus (kp/cmz)
Poissons Ratio v
Bituminous concrete +
100 000
"
100 000
cm+ Bitum. bound gravel '0.35 0.35
i§ §§é Base course 1 800 1 000
AEXQY f 15 cm . ' .' r .w;gng' Gravel material 0.35 0.33
Subbase
'
900
500
75 cm Sandy material I 0.35 0.35 MEI/LEI ; ' . Frostususceptible ' a a 10 .a a 20 Thawing a cm 350 200 ___}___ Subgrade _ ______Q._5_(_)_ _______ __0_._5__0_____
b a 20
b a 10
Frozen . , - b cm 50 000 50 000 0.35 v 0.35 lvn- 500 500 frozen 0.35 0.35 VTI. Rapport Nr 60 A.15.
5. CONCLUSIONS
From this study, carried through as a theoretical
calculation based on the properties of frost susceptible subbases in spring thaw conditions, evaluated from field investigations in terms of
elasticity, the following conclusions may be drawn.
- The condition of the subgrade during the springthaw is of determining signification for the bearing capacity and classification of the flexible pavement
- For the chosen type of flexible pavement a
variation of the Spring thaw period within 2
months seems to be relatively insignificant- The theoretical bearing capacity is influenced
much more by the condition of the subgrade tOp
layer than by the underlying layers- By restricting the load limits during the Spring thaw period by a few per cent it is
possible to increase the safe load substantially during the whole remaining part of the year, which seems to be a good policy from an opera-tional point of View.
16. References.
(l)
(2)
(3)
(4)
(5)
(6)
(7)
Scrivner, Peohl, Moore and Phillips, "Detecting Seasonal Changes in Load-Carrying Capabilities of Flexible Pavements", N C'H R P Report 76, 1969.
Witczak, "Design Analysis - Full-Depth Asphalt' Pavement for Dallas Fort Worth Regional Airport", The Asphalt Institute, 1970.
Edwards and Valkering, "Structural Design of
Asphalt Pavements for Road Vehicles-the Influence of High Temperatures", Shell International
Petroleum Company Ltd, 1974..
"A Guide to the Structural Design of Flexible
and Rigid Pavements in Canada", Canadian Good Roads Association, 1965.
Edwards and Valkering, Structural Design of
Asphalt Pavements for Heavy Aircraft", Shell
Construction Service, 1970.
Bohn, Ullidtz, Stubstad and "Danish Experiments with the French Falling Weight
sorensen,
DeflectOmeter", Proceedings - Third International Conference - Structural Design of Asphalt
Pave-ments, London, 1972.
Heukelom and Klomp, "Dynamic Testing as a Means
of Controlling Pavements during and after
Construction", Shell Bitumen Reprint No. 12,
