studies on the sett1ement force of snow...from the snow surface even if the snow depth reached its...
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Report of the National Researc11Center for Disaster Prevention.No.・u.N1arch1988
551,578.46,624,144.2
Studies on the Sett1ement Force of Snow
as a Gemeratiom Mechamism*
By
Hideomi Nakam皿m
∫/1切・肋・じ/1,一Wl・・αげ・∫ε”で/lCビ舳・〃必肋フ・P。θ舳〃。。
N・・!400・τ・冶舳・似τα肋∂伽,∫1伽・一∫〃,X舳鮒α一加・,996,切伽
Abstmct
Sett1ement forces of snow on beams which were buried in snowpack were ana1yzed
from the point of view(〕f the sett1ing movement of the snow.
In this study,various measurements were made of the movement of the snowpack
around horizontal beams and the settlement force on the beams.In addition,predictions
ofthe sett1ement force using the Finite E1ement Method were carried out.
For this study,two new kinds of devices were designed to investigate the behavior of
the snow.With these the displacement of the snow around a beam was measured,and a
fracture in the snow layer around a bean1and stake was detected,
The compressive viscosities of the snow,the density of which was1ess than about110
kg。一m3,were obtained from experiments using sett1ement gauges.
Furthermore,re1ationships between settlement forces on beams and snow depth,
snow1oad and the shape of the top of the beam were ana1yzed.
From the measurement of the sett1ement force it was found that the settlement force
sometimes increased during co1d nights in the melting season.This increase of the force
was assumed to be due to the growth of the crust1ayer around the beam.
App1yingtheFiniteE1ementMethodtothesettli㎎ofsnow,itwasconfimedthatthe
settlement force and the settling movement of snow cou1d be predicted with adequate
aCCuraCy.
Through computer simulations using the Finite Element Method the mechanisms of
force transmission to the beam cou1d also be identified.Those mechanisms ware as
fo1lows l(ユ)direct pressure from overburden snowpack、(2)shear tranfer from adjacent
snowpack.(3)snowpack weakening beneath and on the side of the beam,and(4)snow
densificati(〕n in layers above and to the side of the beam which develops a bridge for the
intensified transfer of force.
1.Introduction
Snowpack always sett1es due to its own weight.The sett1ement of the snowpack,
however,is impeded when some strucure is in the snowpack.Consequent1y the settle-
ment speed of snowpack above a structure differs from that of the snowpack far from the
ホDoctor thesis submitted to Hokkaido University
一361
Report of the Nationa1Research Center for Disaster Prevention,No.41,March1988
細 j」 工 , ;鱗. 一.メ鰯堕.
Photo1 The cross-section of the snow around a
structure,and snow layers around a structure form consequently a shape such as that
shown in Photo1.The difference of sett1ement speed produces a force which acts on the
structure.The stmcture a1so receives the weight of the snowpack on it.The summation
of these two forces is ca11ed the sett1ement force of snow.
Because snowpack invo1ved in the force generation is not on1y snow on the structure
but a1so snow adjacent to it,the force becomes very large.Owing to this factor,many
structures such as guard rai1s,fences and eaves(Oura1957-a)etc.,are destroyed every
winter in snowy countries as shown in Photo2.
Photo2 The guard rails bent by the sett1ement
force of snow.
In previous reports,sett1ement force has a1most a1ways been ana1yzed with reference
to the weight of snow or snow depth(e.g.Amano1943,Shidei et a1.1949,Shoda1953,
Furukawa1953,Kobayashi et a1.1976,Osada et a1.1975).In some of these papers,the
sett1ement force on a structure was predicted by equations reduced empirica11y from the
fie1d measurements of sett1ement force using horizonta1beams.In these predictions,
however,the movement of snow cover was not considered to be a factor that needed t0
be taken into accomt.In spite of the fact that the sett1ement force of snow needs to be
ana1yzed from the point of▽iew of the sett1ing movement since that force is produced as
a resu1t of the sett1ing movement,as has been mentioned above,there are few reports
which ana1yzed the sett1ement force of snow from this point of view(Yoshida1954,Oura
1955.1957-b,1958.1959)。Therefore the objective of this paper is to c1arify the generation
mechanism of the sett1ement force of snow and to estab1ish a way to predict the
settlement force from the perspective of the sett1ing movement of snowpack.
2.1Meas11rement of behavior of snowpack aroumd the stmctums
2.1Two-wired sett1ement gauge
(1)Design
A settlement gauge with two strings was designed in order to measure the disp1ace一
362
Studies on the Settlement Force of Snowas a Generation Mechanism H.Nakamura
Branch
SuPPort
\E1ectr1c
榊 ジTwo }1red
Sett1em㎝t嗜ouge
S1・g1.H1。。d p1・t・
ぐ岬 //Ground
Fig.1 The two-wired sett1ement gauge and the
single-wired settlement gauge for investi-
gation of the sett1ing mevement of the snow
aromd beam.
ment of snowpack around a horizonta1beam.This gauge was made up of a s1ender p1ate
of vinyl ch1oride,two fine metal strings wich heat up e1ectrica11y and two ba1ances,as
shown in Fig1,The size of the p1ate was0.3m×O.02m x0,002m.Two supports
comected by a3m1ong beam and15branches were set up on the ground in order to hang
thestrings.Thesupporte1evationwassetat1.8minordertoa11owadequatec1earancefrom the snow surface even if the snow depth reached its maximum.A1though a baIance
of18g was connected to the ends of each string,the strings could s1ide easi1y on the
branches in accordance with the movement ofthe plate.This type ofgauge wi11be ca11ed
a two-wired sett1ement gauge hereafter in this paper.
It was thought that the p1ate set up on the snow surface wou1d must probab1y be
covered with fresh snow fal1and wou1d move with the snow surromding the p1ate.
Therefore ifthe trace ofthe p1ate were measured,the two-dimensiona1movements of the
snowpack that made contact with the p1ate on the p1ane perpendicu1ar to the beam cou1d
be discovered.
iBC
B C Be日mo o
BranchTwoHiredSett1ementGauge
Ba1ance
一 一ABB日1日nce
AC
P1ate
’
Ground
Fig. 2 The position of the plate can be calculated
from three distances AB,AC and BC.AB
and AC are measured by graduations
marked on the strings.BC is obtained from
direct measurement.
363
Report of the Nationa1Research Center for Disaster Prevention・No・41・March1988
It was possib1e to determine the position of the p1ate from three distances AB,AC
and BC(F1g2) The1ength AB and AC,wh1ch s1ow1y changes,cou1d be measured by
graduations marked on each ofthe string.BC could be figured from direct measurement
preceding the experiment.E1ectric heating wires were used as the strings.Therefore,
these strings cou1d be extended straight by the me1ting snow which surrounded them,
even if a part of the strings was buried in the snow.
The sett1ement gauge with a sing1e string designed by Hirata(1941)is a1so shown
in Fig.1.Because this type of gauge can measure on1y vertical movement,it is usua11y
used when the disp1acement of snowpack has no horizonta1motion,such as the settle-
ment of snowpack on an open horizonta1p1ain.
(2)Accuracy The two types of sett1ement gauges were set at the same time on the snow surface
of an open fie1d,and the traces of those gauges were measured and compared in order to
examine the accuracy of the two-wired sett1ement gauge.Here the sing1e-wired gauge
was regarded as the standard gauge.The traces of the two p1ates are shown in Fig.3.
From this figure,it was found that the heights of the two gauges are a1most equa1when
the two-wired gauge was e1ectrifie1d.
On Feb.221975,the position of the p1ate of the two-wired gauge ca1cu1ated from the
graduations on the wire differed on1y1cm from the va1ue obtained by direct measure-
ment.Therefore,the two-wired gauge was considered to have an adequate accuracy in
order to measure the movement of snowpack around a horizonta1beam.
・1・・ 1/B「anch\、
B「anch↓ 1\ ■、 1 葛
⊥.。 。
J。・lO1975 1 J.n・101975 ・’1一一二∴一一・’1・’’・1’二一・’1’.㍗・.㍗r
l一.....†・….....一
1一一一一一..L....一
/1{皇…昔/
1…一一ヰニニニー 今一一一一一一一一一一一一1 中…一一一1一一一一一一→
・・・…一一1三芹三手一一・・・…
↓.■ ⊥I
l ’o’050’05皿 Fig.3 The examination of the accuracy of the
1汎珂 二1二il:1、:l1鷺、ごll、㌫a鴛=1e-Wi「ed
(3)Two dimensiona1displacement of snowpack During the winter of1974/75,the movement of the snowpack around a fence was
measured using three gauges with two strings,as shown in Photo3,and the resu1ts are
shown in Fig.4、
364
Studies on the Sett1ement Force of Snowas a Generation Mechanism
Photo3 The support with a beam and
branches.The strings are
separated from each other by
sma11p1astic plates.
1975J.13
J.14
J.15J.16
J.18J.19J.21J.22
H.Nakamura
F.
P1o t畠 ^
M.
Fe n〔e
Ground ;urf三■c昌
0.85
.80
.70
.60
.・・1
.40虐
.30
.20
.l O
Fig.
.60 一.50 一.40一.30一.20一.10 0
X(π)→
4 The movement of snowpack aromd a fence was measured
using the two wired settlement
gauge.Plate A moved in an arc-
1ike fashion around the top of
the fence.
From this figure,it can be seen that the p1ate c1osest to the fence(p1ate A)moved
in an arc-1ike fashion around the top of the fence,as did the snow around it.The pace
of the sett1ement speed of the snow aromd p1ate A decreased rapid1y from Jan118,at
which time the snow at plate C(farthest gauge from the fence)had a tendency to move
toward the fence.P1ate A,however,showed a1arge disp1acement from Jan.21to Jan.
22,and during that time plate C stopped moving towards the fence and moved vertica11y
downward on1y.
The strange behavior of these p1ates(i.e.snowpack around these plates)can be
understood by examining the snow1ayer on which these p1ates were set.This1ayer
might have been cut by the top of the fence at some time aromd Jan.21.The reason why
the settlement speed of p1ate A increased sudden1y,and why the motion of p1ate C
towards the fence stopPed on Jan.21and22is because the snow1ayer which hung on the
fence was cut by the fence top and fel1down rapid1y from the top of the fence.
From these experiments it was found that1)snowpack sett1ed in arc・1ike fashion
aromd the fence top and2)the snow1ayer is sometimes cut and disp1aced by horizonta1
beams a1though it has been reported that a snow1ayer,density of which is1arger than lOO
kg/m3,has never been cut by a horizonta1beam(Shoda,1953).
2.2Fracture sensor
Because it was found that1ayers of snow were cut by a fence top,a new device to
detect a fracture of this snow1ayer on a horizonta1beam was designed.
365
Report of the National Research Center for Disaster Prevention,No.41,March1988
Fig. 5 The mechanism to detect the fracture of the
snow1ayers by the fracture sensor. The
fracture is sensed from a rapid increase of
e1ectric resistance between both ends of the
SenSOr.
Fracture Sensor
1 E1・・t・1・
Carbon Powder
ユ
Photo4 The fracture sensor which was made up
of carbon powder wrapped in a soft
tissue with two e1ectric wires.
2
し
3
Ground
Extension
■ 、
←Broken
(1)Design
This detector,wich wi11be c41ed a Fracture Sensor in this paper,is made up of
carbon powder wrapped in soft tissue with two e1ectric wires connected to it,as shown
in Photo4.This sensor,as seen in Fig.5,can extend according to the movement of the
snow1ayer in which the sensor is inserted,and win not been cut if the1ayer remains
intact.Because carbon powder is a good e1ectric conductor,e1ectric resistance between
the outer ends of two e1ectric wires is minima1un1ess the sensor is cut.If,however,the
snow1ayer with the sensor fractures,the sensor wi11be a1so broken.In this case the
e1ectric resistance is thought to become infinite.Therefore,if the e1ectric resistance is
a1ways monitored,the fracture of snow1ayer wil1be detected due to the rapid change in
the e1ectric resistance1
(2)Detection of the fracture of the snow layer
In order to confirm the abi1ity of the sensor to detect a fracture in the snow1ayer,
the fo11owing two experiments were carried out.
1)Detection of a fracture of the snow layer hanging on a horizonta1beam
A box with a horizonta1beam was set up and new snow was put into the box unti1
the box was fi11ed,setting three sensors at heights of10,20and30cm from the top of the
beam,in order to determine the impact of the snow1ayer on the e1ectric resistance of the
SenSOr.
The changing of the snow in the box,observed through the c1ear wa11,is shown in
Photo5,and the change of the e1ectric resistance after the setting of the sensors is shown
一366一
Studies on the Sett1ement Force of Snowas a Generation Mechanism H.Nakamura
Photo5 A fracture of the snow1ayer occurs
aromd the beam.The e1ectrica1resis-
tances of the sensors are sti11smal1(3
days after)、
Hor1zonta1 Beam
■0◎
UpPerSensorlC)M1dd1e lB〕
Lower 〔A)
l071♂1げ04103◎2
←・.・十・‘・◎一一,.__
Hor1zonta1 Beam
E
呈104
七 Fig.6 The changing of electric resistance
ξI03 between both ends of the sensor set in a
snow1ayer over the horizontal beam.
I◎2 The rapid increase in the electric resis-
O1234567 tancesuggeststhatthefracturejustoc・ set Days curred at that time.
in Fig.6.From this figure it was discovered that the resistance of sensor A,which was
set at a height of10cm from the top of the beam,increased rapid1y4days after the
setting.This rapid increase of the resistance suggests that,at that time,sensor A was
cut,and the snow1ayer where the sensor had been inserted was a1so fractured.Three
days after that,the situation of the sensors was observed by shoveling the1ayers of
snowpack in the box.At that time the electric resistance from the other two detectors
sti11remained sma11er than103ohms.It was ascertained that sensor A had been cut as
shown in Photo6,though the others were intact though stretched.From the change of
e1ectric resistance and the situation of sensor A,it can be seen that the fracture of the
snow1ayer in which sensor A had been inserted must have occurred4days after the
setting of the sensor.This experiment proved that this type of sensor is very useful for
detecting a fracture of the snow1ayer hanging on a horizonta1beam.
Photo6 Only the fracture sensor which was set
atthelowesthei・ht(10←m・b…f・・m
the top of beam)was broken,a1though
the others were a11intact.
367
Report ofthe National Research Center for Disaster Pre、・ention,No.41,March1988
2)Detection of the fracture of the snow layer on a stake
A Fracture Sensor was also set up on the snow surface above a stake driven into the
ground,and the change of e1ectric resistance was measured in order to examine the
abi1ity to detect the fracture of the snow1ayer using a stake. The resu1ts of the
experiment are shown in Fig.7,
In this experiment,because the e1ectric resistance became very1arge on the13th day
after the setting of the sensor,the snow1ayer on the stake was considered to have been
cut by the top of the stake.When the layers of snow around the stake were removed it
was confirmed that the1ayer in which the sensor had been inserted had been cut by the
Stake
ξl03 Fig.7 The electric resistance of the fracture sen-
sor set in a snow layer over a stake.The
sno,v layer is considered to have been cut
0 2 4 6 8 10 12 set Day, on the14th day after the setting,
stake top.Therefore it was confirmed that this sensor was a1so usefu1in detecting the
fracture of the snow layer on top of a stake.
2.3Measuring the compressive viscosity of snow using sett1ement gauges
Later in this paper,the settlement force of snow on a beam wi11be predicted using
the Finite E1ement Method.In order to carry out the prediction,it is necessary for the
compressive viscosity of snow to be known.
The viscosity of snow has been measured by severa1researchers(Shinojima,1962;
Kojima,1955.1956.1957.1974;Saito et a1.1971),but there are few papers on the viscosity
of snow with a density of1ess than100kg/m3.It was therefore necessary to investigate
the compressive viscosity of snow especia11y in those regions with densities of1ess than
100kg/m3.For this reason,the viscosity of1ight snow was measured using two sett1e-
ment gauges,as shown in Fig.8.The gauges used in this experiment were deve1oped to
measure disp1acement of the p1ate automatica11y by potentiometers.
A settlement gauge plate was set on top of the new1y fa11en snow and the p1ate of
another gauge was inserted underneath in order to measure the thickness of the snow
1ayer.The initia1interval between the two p1ates was measured by a sca1e,and its
change after the initial setting of the p1ates was ca1cu1ated from the displacements
measured by each potentiometer.The1oad which acts on the snow layer was assumed
to be the summation ofha1fweight ofthe snow1ayer p1us the weight ofsnow on the upper
p1ate,if there was any snow.The measurement of the1oad was carried out about every
two hours after setting the p1ate.Compressive viscosityη、is expressed as fo11ows:
η、=σ/;, (1)
btaヒe
1◎61051◎4103102
’ 1 1 1
368
Studies on the Settlement Force of Snowas a Generation Mechanism H.Nakamura
。ll。、。、、
Potentio爬ter
’’
1・
Sing1e
;1
H1redG日uge
11
lllI
Su叩ort
er
Ii3亡;
She1ter
一 一 . 一
P1・t・ W
B柵;。H 一 一 ■ ■ 一
一 ■ 一
.・Sn㎝1 Pate
〈 ~〔..。一 /Ground
Fig.8 The measuring system of the thickness
variation of the snow 1ayer using two
single-wired settlement gauges.
whereσis the normal stress in the snow1ayer and;is the strain rate of the1ayer.
When the thic㎞ess H and the1oad W change from H1to H2and from W1to W2in
time interva1△T,the viscosity can be represented in the fo11owing equation whereσand
;are regarded as(W1+W2)/2and2(Hl-H2)/△T(H1+H2)
_△T(Hエ十H2)(WI+W2) η、一 (2) 4(H1-H2)
Compressive viscosities of snow obtained from these measurements are shown in
Fig.9by b1ack dots.Because the snow temperature was about-2℃in the region where
the density was sma1ler than l1Okg/m3,the re1ationship between compressive viscosity
and snow density when the snow temperature was equal to-2℃was derived as fo11ows:
η、=2.12x105Exp(5.24x10■2ρ), (3)
whereη、is in N・s/m2andρis the density of the snow in kg/m3.Assuming that the
re1ationship between the viscosity and temperature is the same as that derived by
Shinojima(1962),the previous equation(3)was modified as fo11ows:
ηc=1.75×105Exp{10-2(5.24ρ十9,581T,1)}, (4)
where T,is the snow temperature iパC.
Since the va1ues calcu1ated from equation(4)matched with the resu1ts achieved by
Shinojima(1962)at a snow density of11Okg/m3,in a simu1ation of sett1ement force using
Finite E1ement Method,η、for snow of densities1arger than110kg/m3was ca1culated
using Shinojima’s equation,and for snow sma11er than this density,after equation(4).
3.Meas11rement of the settlement force of snow
In Japan,the settlement force on stakes or beams has been measured by many
researchers,main1y in Niigata prefecture in the Hokuriku district,which is a rather
warm area among the snowy regions of Japan.In the colder areas such as Shinjo,
however,few systematic measumerents of sett1ement force have been carried out.
Furthermore,sett1ement force is supPosed to depend on the snow properties,and
369
Report of the Nationa1Research Center for Disaster Prevention,No.41,March1988
E
~、
E
91◎
I♂
1♂
ぞ・・1・44X1・6・・pl10-2(2・・3ア・9・581・。1)}
・1.7
一2,2’
一24t \
’2・へ ■
c
Sh1mJ1・・11963〕lT。・’2、
!
一0.4.C
“
、
■
■
■
(T。・イ・)
.・、・.. \
一0.2C
/伽・1・…1・5…l1O-2(…ザ・・。・・1・、1)1 Fig.9
5◎ 1◎O 150DENS1TY・戸(kg/・3)
2◎O
Photo7
The compressive viscosities
obtained from the measurement
using two sing1e-wired sett1e-
ment gaugeS.
The two types of beam at the Hijiori
experimental site.The settlement force
on the middle part of each beam is
measured by the electric force gauge.
although the Japanese is1ands are sma11,snow properties vary significant1y from region
tO regiOn.
For these two main reasons,the sett1ement force of snow cover onto horizonta1
beams has been measured since the winter of1977/78in the test fie1d of the Shinjo Branch
of the Nationa1Research Center for Disaster Prevention,and since the winter1978/79at
the Hijiori experimenta1site(Photo7)1The maximum snow depths on the ground at
Shinjo and Hijiori experimenta1sites are,on the average,1.5m and4.5m,respective1y.
Fテ8淵干H‘oπ L0■「、、H為甘嘉.一 一 一
PE~ε朋0METER‘STεEL COR~aハLM’MuM PL^丁目
甘合0.’ 0.’ 0.’
STEELSuPPORTELεCTRO~’CFORCE6^6E
.’
d
≡妻1婁萎111111嚢1≡…≡≡…≡11l1l1……1111111;1襲…11111l1l 11l1111 6κOU~O SURFム6ε 0.55→一一.0.55‘FRONT VlEW, lS10E VlEW,
Fig.
‘FRONT VlEW, lS10E VlEW,
37・
10The horizonta1beam with force measurement devices,and the cross-sections of
different types of beam.
一370一
Studies on the Settlement Force of Snowas a Generation Mechanism H.Nakamura
3.1ApparatusThe cross-sections of beams used in this research were rectangu1ar,square,equi1atera1
triangu1ar and triangu1ar with a37.vertica1ang1e,as shown in Fig.10.The height of a11
beams was0.8m above the gromd,and the horizonta1width of a11beams was O.1m.
Eachbeamconsistedofthreehorizota1parts,havinga1engthof1.Om(2.0matHijiori).The sett1ement force on the midd1e part ofthe beam on1y was measured.A penetrometer
consisting of a steel com and an a1uminum p1ate was a1so inserted between the horizonta1
bar of the beam and another support in case the e1ectric gauge shou1d fail.
3.2Seasonal variation of the settlement force
(1)The re1ationship between the maximum settlement force and the maximum snow
depth during each year
A variation in the sett1ement forces obtained from measurements taken during the
winter of1980/81at Shinjo is shown in Fig11,In this case,forces were created when the
snow depth exceeded the beam height(0.8m)and then increased with increasing snow
depth.The forces began to decrease two weeks1ater after the snow depth reached its
maximum as reported by Shoda(1953).It is thought that the difference in the maximum
sett1ement force(1.3kN/m)between the rectangu1ar beam and the triangu1ar beam
(6ぴ)was caused by the difference of shape of the beams tops.The forces vanished when
the snow depth was1ess than the beam height again in spring.
As a particu1ar case,sudden sharp increases of force were observed at the end of the
wintξrs1980/81and1983/84at Hijiori.Because they appeared during the me1ting season
only,they might have been due to crust formation,as pointed out1ater.
In Fig.12,the increment of settlement force onto a beam due to the increase of the
snow depth is shown in comparison with the maximum force under maximum snow
depth. From the measured va1ues shown in Fig.12,the re1ationships between the
ユ980一’81 WINTER
2 SNOW DEPTH(m) SHI㎜JO
ユ
O1l・π・・旺1・1・・舳11 π・…
RECT^“6uL^R BE^m
^ ■ \
3 廿 14days \
2 1 TR1州6u山R(60・〕BE州
1 1
0ユ0 20 30 9 ユ9 29 8 ユ8 28 ユ0 20 30 9 ユ9 29
DEc. JAN. FEヨ. 門^R. ^PR,
Fig.11The daily change of the settle-
ment force on two different
types of beam in comparison
with the snow depth(Shinjo).
371
RePort of the National Research Center for Disaster Prevention,No・41,March1988
二35EZよ301」」
Oo=25◎
L←z20]婁山」15トト山
ω102⊃
; 5支く
…O
一一□一REC.81SQu.8EAM一・士・・TR1ANGULAR BEAM ‘VERT1CAL ANGLE1370,
口
.“
ミ “
メ
“φ q □ }
12kN〕 手 1 くφ官/1 ▲OSADA ot o[.一1975〕 一 ▲ ▲
REC.BEAM>仁・□1二;.;急丁河
!づ、百伽血…
▲
▲
Fig.
O O,81 2
MAXl MuM 3 4
SNOWDEPTH lm〕
12The increase of the sett1ement
force on a beam due to increase
of snow depth.
maximum sett1ement force onto a rectangu1ar beam(FRm、、)and a triangu1ar beam of37。
(FT37m、、)and the maximum snow depth in each winter
(beam height=0.8m)
FRma、=4.20(Sma、一0.8)L43,
Sm。。 were obtained as fo11ows
(5)
FT37m、、=2.93(Sm、、一0.8)o.73, (6)
where FRm、、and FT37m。、are in kN/m,and Sm、、is in m.
0sada and others(1975),ana1yzing sett1ement force data measured at Niigata
prefecture,proposed an equationto estimate maximum sett1ement force on a rectangular
horizonta1beam from the maximum snow depth on1y.Their re1ationship between the
force and the maximum snow depth is shown in Fig.12by a dashed1ine.The magnitude
ofmaximum sett1ement force on the rectangu1ar and the square-section beams,measured
during each winter,coincided with the va1ues ca1cu1ated from Osada’s equation when
snow1ies1essthanabout3monthegromd. It is thought that the reason why the maximum sett1ement force onto a rectangu1ar
beam increases exponentia11y with the maximum snow depth,as shown in Fig.12,is that
the average snow density throughout the depth of snow cover becomes1arger in genera1
with the increase in snow depth before the me1ting season(Shoda1953).
Photo8 The snow1ayers on the triangu1ar beam(37。)were cut by the top of the beam
1ike a b1ade,though on the rectangu1ar beam,the snow layers are intact though
stretched.
一372一
Studies on the Sett1ement Force of Snowas a Generation Mechanism H.Nakamura
On the other hand,it is thought that the reason why the maximum settlement force
on a triangu1ar beam(37。)doesn’t show such a tendency,but rather shows a saturation
curve,is that a part of the snow on the beam was perhaps cut by the beam top,acting
1ike a b1ade mder high pressure.Therefore it is thought that the amount of snow
hanging on the beam became less than before the cutting,and that the forces didn’t
become as1arge as that in the case of the rectangu1ar beam.It has often been observed
that a part of the snow1ayers on the triangu1ar beam(37。)had been cut by the top of the
beam,as shown in Photo8、
(2)Sensitivity of the sett1ement force to the increase of the snow1oad hanging on the
beam It is wel1known empirica11y that the sett1ement force on a beam increases with the
1oad of snow on the beam.Hence the sensitivities of the force to the snow load were
investigated.
The sensitivity was defined as ratio ofthe sett1ement force increment to the addition
of the snow load.Fig.13shows the re1ationships between the sett1ement force and the
Sett1emEnt Forc2 InCrement
^dd1t1onofSnow阯eight 。
椛3/砕 。/9冴0/8/ 1 o’’
ノ
、。/t一伽■。孕
二二0 2 4 6
Sn㎝…eight,
8
阯(kN/・2〕
10
Table1
12
Fig.
Sensitivity(D)
13The re1ationship between the
settlement force and the snow
weight.The inclinations of the
curves represent the sensitivities
of the sett1ement force to the
increase of the snow weight.
Year Place Researcher Beam MaximumHeight Snow Depth D
1977/78 Shinjo Nakamura et a1. 80CM 147CM 1,30
1980/81 〃 〃 〃 183 1.53
1981/82 〃 〃 〃 138 0.58
ユ983/84 〃 〃 〃 171 1.30
1984/85 〃 〃 〃 128 O,86
1978/79 Hijiori 〃 〃 252 1,08
ユ980/81 〃 〃 〃 402 1.43
1981/82 〃 〃 〃 398 1,17
1983/84 〃 〃 〃 425 1.00
1951/52 Shiozawa Shoda 77 170 1.86
1952/53 〃 〃 80 240 1.41
1954/55 SapPoro Oura 60 112 0,42
1967 Sekiyama Kobayashi et al. 68 210 1.58
1967/68 Koide Osada et al. 90 332 1.71
1968/69 〃 〃 〃 217 1.00
373
Report of the Nationa1Research Center for Disaster Prevention,No.41,March1988
O SヨPPoro ^H}o■
◎Shrnjo ■N1rgata
’ ◎ Fig.14The sensitivity of the sett1ement force to /6 ・ . ム the snow weight(D)versus the difference 冶 o
∠ o ム between the maximum snow depth and 1 ■ / 1 beam height(SB).The magnitude of D is / /
〆 th…htt・d・…d・…th・d・・・…ff・・一 mation of the hard1ayer on the top of the
o beam. 0 1 2 3 4
SB(而〕
snow1oad for a rectangu1ar beam in three cases.
The average sensitivities,(D),over each winter were regarded as the mean inc1ina-
tion of each somewhat irregu1ar curve,as shown in Fig.13.Va1ues of D obtained from
the experiments and resu1ts by other researchers(Shoda1953,Oura1955,Kobayashi et
a1.1976,Osada et a1.1975)are1isted in Tab1e l and the same resu1ts are p1otted in Fig,
14,In this figure the X-axis ischosen as the difference between the maximum snow depth
and the beam height,(SB),because beam heights are not a1ways just equa1to0.8m.
It was found from this figure that in the region where SB is1arger than1m,va1ues
of D are a1most constant,though the deviation is a1itt1e1arge,On the other hand,in the
region where SB is1ess than1m,D decreases rapid1y according to the decrease of SB.
Because it has been fomd from the simu1ation of the sett1ement force using the
Finite Element Method that the force becomes1arger if a hard snow1ayer is formed on
the beam,the reason why D is sma11in the region where SB is1ess than1m was a1so
considered to have been a resu1t of the fact that the hard1ayer hadn’t been formed on the
beam adequate1y in such cases.In other words,there was not sufficient snow in the
winters for such a hard1ayer to be formed on the beam.
(3)Re1ationship between the sett1ement force and the shape of top of the beam
Comparison of the sett1ement forces on beams,the tops of which are f1at,and on
triangu1ar beams with ang1es of37但and60むare shown in Fig.15and16.It was found that
in the region where the sett1ement forces on f1at beam were sma11er than2kN/m,the
sett1ement force on triangu1ar beam was a1most equa1to the force on the flat beam.The
HIJIOR工
τR一^“OuL^R.ヨ7.I
日E^M
10
(1.5m)
十1982-83
1978-79
1989-81
1979-88
1981-82
SOuARE 目EAM
5SETTLEmENT
1臼 15
FORCE 〔kN■m〕
2θ
Fig.15
A comparison of the settlement force on
the triangu1ar beam(37。)with that on the
square beam.In the region where snow
depth is smal1er than1.5m,both are almost
equa1.
374
Studies on the Settlement Force of Snowas a Generation Mechanism H.Nakamura
SHINJ0
E 4\
z1山
OαOL 2←
zLuΣL]
」←←L] Ooo
1980-81
6 2 4 6
SETTLEmENT FORCE 〔kN!m〕
Fig. 16 A comparison of the settlement force on
the triangu1ar beam(60。)with that on the
rectangu1ar beam.The resu1ts are simi1ar
to those found in the case of the triangular
beam(37℃)、
……
f l O
\峯:
島
止05
1 5
MAX1MuM SNOW DEPTH lml
Fig. 17The ratio FT37M^x/FRMAx is larger than1.0
when the maximum snow depth is less than
1.5m.It is thought that the cause is due to
the difference between the thickness of
each beam.
snow depth when the force on the flat beam equaned to2kN/m was about1.5m.
On the other hand,the re1ationship between the ratio of the amua1maximum
sett1ement force on the triangu1ar beam(37。)to the amua1maximum force on the
rectangu1ar beam,and the amua1maximum snow depth are shown in Fig.17.Though
the ratio was0.3-0.5when maximum snow depthwas1arger than about1.5m,it became
1arger than1.O when the maximum snow depth was1ess than1.5m.It is thought that
the reason why the ratio was sma11in the region where the maximum snow depth was
large is because the snow1ayer on the triangu1ar beam was easy to be cut by the top of
the beam due to the snow load,and because the amount of snow hanging on the triangu・
1ar beam decreased as the result.
The reason why the ratio is1arger than1.O when the snow depth is1ess than about
1.5m,is considered to be due to the thickness of the beams.Because the triangu1ar beam
(37。)was three times thick than the rectangu1ar beam as shown in Fig.17,the amount
ofthe snow hanging on the triangu1ar beam was1arger than that on the rectangu1ar beam
except when the snow layer on the each beam was cut by the beam top.The sett1ement
force in this region was not so1arge that the cutting of snow1ayer cou1d not occur.Due
to these two factors,the maximum sett1ement force on the triangu1ar beam was becomes
greater than that on the rectangu1ar beam in the region where the maximum snow depth
was sma11.
一375一
Report of the National Research Center for Disaster Prevention,No.41,March1988
1.OO.5 0 10 0-lO
Artificia1 Cutting㌣淵㌻帷鮒一/
SHlNJO
AlR TEMP.ドC〕
1210
18 24MARCトl1983 6 1211
18 24 6 1212
18 24■ ∈1;
IO MARC1-l1983 11 12 1ヨ
Fig.18
The changes in the settlement force and air
temperature at the Shinjo experimental
site.The sudden decrease of the sett1ement
force was due to the artificiaI cutting of the
hard snow1ayer,as wm be mentioned1ater.
3.3Increase of sett1ement force due to hardening of snow
It was found that on a co1d night during the melting season the sett1ement force
increased even if there was no addition of snow,
One examp1e of such a phenomenon was observed on Mar.11.1983at the Shinjo
experimenta1site and it is shown in Fig.18.Variations in the sett1ement forces and air
temperature are shown in the figure.The variations occurred during a period in which
there was no snowfa11though this is not indicated in the figure.A sudden decrease in the
sett1ement force during the night of Mar.11to Mar.12was caused artificia11y,as wil1be
mentioned1ater.
Because there had been positive air temperature since the daytime of Mar.10,the
snow cover was wet and of a granu1ar type.Under these conditions the sett1ement forces
increasedduringthenightofMar.11.Detai1softhevariations inthesett1ement forces,air
temperatureandthicknessofthecrustlayerfromMar.11to Mar.12are shownin Fig.19and a cross-section of snow cover adjacent to the beam on Mar.11is shown in Fig.20.
A1thoughitwaswarminthedaytimeonMar.11(maximumairtemperature:7.7℃),
it became co1der in the night and the air temperature dropped to-7.6℃in the ear1y
momingofMar.12.Thesnowcoversurfacetumedto ahardcrust asthe airtemperaturefe11,the thickness ofthe crust reaching15cm at7.35a.m.The sett1ement force increased
with the growth of the crust.
This increase of the force was assumed to be due to hardening of the snow,because,
a1though the sett1ing speed of the snow was a1most the same as before the hardening,the
viscosity ofthe snowpack of the surface became very1arge,and the resu1tant shear stress
(=viscosity x strain rate)a1so became1arge.Because the shear force transferred from
the adjacent snowpack to the beam and therein intensified the sett1ement force,this
mechanism was assumed. This hypothesis was confirmed in two ways.One method was to cut the hardened
snow layer a1ong the beam and the other was to perform calcu1ations using the Finite
Element Method. When the crust was cut off a1ong a section of the triangular-topped beam(Fig20),
it was found that the sett1ement force decreased sudden1y by200-300N/m(Fig,19).On
theotherhand,thesimu1ationofthesett1ement force using FiniteE1ementMethodwhich
took into account the hardening of the snow(i1e.using the viscosity of snowpack of
negative temperature)showed that the sett1ement force increases if the snow becomes
hard,as is shown in Fig.19by a dotted line.Therefore,it is believed that the growth of
the crust1ayer on the beam caused the gradua1increase of the sett1ement force.
376
Studies on the Settlement Force of Snowas a Generation Mechanism H.Nakamura
l O
⊆o○卜 mo』)O E』 oO O」L」O〇
一一 」
⊆ o塞;睾旨O・5岩.睾
8ξ
ω
ωr⊃Φ 』 』
⊆ o oy=1= >,
O dlp←一 15芦o
O o 』」 コ
・F ・1一
くε 9 -8 ;
←
Art1f1c1a1.O kNlm Cutt1ng
▲: Observed
◎: Ca1cu1atedby F.E,M.
Artif1cia1 ’’
’’
Cutting 7.5
!1’
!’’
O ■11,l1ll
!5
Cml l一
6 モ
O12 18 6 12 18 24
o Mn■11 1◎q司 M【F I9Mor1l,1983 Mor.12
Fig.19The details of the variation of the air temperature,settlement force and thickness
of the crust1ayer.The sett1ement force increased with the growth of the crust.
C∪TT1N6SEC1.・◎NS SuRFACE Or \SN◎WCOVER…。.11
1鞭主撚蟻睡螂・
1喜ギー1・1l、二二⊥・・^・・
1L鵬㍑、、L,19明
o,lm“ BEAM
Fig.20A cross-section of the snow cover adjacent to the triangular beam,and the growth
of the hard crust layer adjucent to the beam.The downward arrows indicate the
growth direction of the crust1ayer.
A simi1ar process of bridging was derived through a computer ana1ysis of snow
sett1ement,i.e,the increase of the sett1ement force from the snow densification of a snow
1ayer within the snowpack(Lang and others1984).
4.Simulation of the settlement force by computer mode1ing
4.1Settlement force simu1ation assuming the density distribution of the snow aromd a
beam The sett1ement force of snow is produced due to a1ack of uniformity of the sett1e-
ment speed of snow around a beam.Therefore it is necessary to ana1yze the force taking
377
Report ofthe Nationa1Research Center for Disaster Prevention,No.41,March1988
into accomt the movement of the snow as mentioned before.
However,simp1e estimates with many attendant approximations do not general1y
yie1d accurate predictions of force.The reasons for this are that snowpack is a1ayered
’continuum of viscoe1astic materia1which in the vicinity of a structure has a comp1ex
pattem of f1ow and force transmission.To incorporate these physica1effects into an
ana1ysis of sett1ement requires the use of a numerica1method such as the Finite E1ement
Method.Sett1ement forces measured during the winter of1980/81at the Shinjo observa-
tion site were simulated using the Finite E1ement Method.
(1)Experimenta1Data Experimenta1data were taken on the settlement force on the beam,on the new snow
deposition and on the density.These were measured in the area once each day after the
intia1buria1of the beam.The distribution ofthe density in the snowpack was a1so known
from the pit data taken at1O-day interva1s at a1ocation in the vicinity of the beam test
site (but not at the test site itse1f).
(2)Computer program
A computer program that uses the Finite E1ement Method in ana1ysis of p1anar
e1asticity prob1ems,inc1uding materia1orthotropy(Lang et a1.1984)was used to analyze
the beam sett1ement prob1em.Whi1e ordinari1y used for e1astic materia1prob1ems,the
computer code can also be app1ied to secondary or steady creep prob1ems by simp1e
redefinition of the materia1constants.The program does not incorporate materia1
densification,which o㏄urs in snow sett1ement,but with the density distribution known
at1O day interva1s,stepwise1iner interpo1ation on a dai1y basis can be made.The
approach is to use the density and force data as reference information and to investigate
the conditions by which a finite e1ement representation of the beam-snow confiquration
fits the data.
Severa1considerations dictate the finite e1ement mode1that is se1ected to ana1yze the
snow-beam sett1ement problem.Knowing that the material f1ow shou1d vary most
rapid1y in the1oca1region surrounding the beam,the finite e1ement array shou1d account
for this gradation.A1so,at some horizontal distance away from the beam,the snow
sett1ement should approach the infinite s1ab condition of the simp1e vertica1deflection.
Advantage is a1so taken of the symmetry about the vertica1axis through the center of the
beam.In assimi1ating these factors,a grid of finite e1ements was se1ected with115
gridpoints and107e1ements.The boundary condition with no horizonta1disp1acement
was imposed at the gridpoints on the vertica1axis throubh the beam and at the vertica1
bomdary1.Omtothe1eftofthebeam.Atthegromdsurfaceandontopofthebeamno vertica1and horizonta1disp1acement was permitted.
(3)Materia1description
Se▽era1assumptions are made to reduce the comp1exity of the materia1description
and make a so1ution tractab1e.One assumption is that of p1ane strain,that a section of
the beam cross-section and snow-s1ab is!’typica1”in the sence of constancy in the third
coordinate direction.And another assumption is that transient material response is
neg1igib1e.So that on1y sencondary(steady)creep of the materia1needs to be defined,
一378一
Studies on the Sett1ement Force of Snowas a Generation Mechanism H.Nakamura
2500
2000
E1500
竃1000
500
。邊丁;ROSS-SECT1㎝
OF THE 8E^M
Fig.21
ソ、E炸E,ENT^L The experimental and computed settlement
。。=㏄岬UTE0 force on the beam.
5 6 7 8 9 10 ■1 12 13 14 15 16 17 18 19 20 21 2223 2425
0^Y一」^“u^RY 1981
and this in tum is1imited to1inear viscoe1astic representation.From these assumptions
the fo1lowing constitutive equation for steady creep was derived by T.E.Lang and T.
Nakamura(1984):
ぽ/一(1一・リ)(1・リ)[1デ・1ひ112レ/ぽ/(7)
Thematrix inthisequationcorrespondstothe[Bコmatrix inthe FiniteE1ement Method・
The Poissonic coefficient of snow,ひ,in compression is set at0.27.
(4) Computation resu1t
Computation of the settlement force by the Finite E1ement Method was carried out
assuming the density distribution around the beam based on the pit observations carried
out at a1ocation in the vicinity of the beam test site.As has been described,the
experimenta11y measured beam sett1ement force was used as a means of finding the
intemal arrangement of finite e1ements in order to predict the force correct1y.The
resu1ting comparison between the experimental and computer predicted sett1ement force
from5January through14January is shown in Fig.21.
Despite the fact that this ca1cu1ation was carried out assuming the density distribu-
tion around the beam,from these simu1ations,the mechanisms of the force transmission
to the beam cou1d be identified.They are1)direct pressure from overburden snowpack,
2)shear transfer from adjacent snowpack,3)snowpack weakening beneath and to the
side of the beam,and4)snow densification in1ayers above and to the side of the beam
to deve1op a bridge for an intensified transfer of force. It is thought that due to the
bridging mechanism of step4)the force intensification on the beam became as1arge as
has been measured empirica1ly.Resu1ts of this computer study show the bridging region
to extend horizonta11y from the beam by a factor of2to3times the width of the beam.
Photo g The experimental box with a horizontal
beam(2m×0.6m×0.85m).
379
Report of the Nationa1Research Center for Disaster Prevention,No.41,March1988
0,9
0.8
(A)
」く
←
Z
0BsERvEDO.9
0.8
(B)
0.61。.、
=
N O.2
O.O
・^ ・o
・0 ・E
・C
Bea
CALcuLATED
O.6/。.、
=
N O.2
一ユ.O
O.8
O.6
一〇.8 一〇、6 -O.4
X(M)→
O.O
一〇.2 0.O -1.O
ω>くρ
NH
/。.。
=
N O.2
O.O
O.8
一0.8 一〇.6 -O.匂
X(M)→
一〇.2 0.O
O,61。.、
=
N O.2
ω>く口
\
旧
一1.O
O.8
O.61。.、
=
N O.2
O.O
一〇.8 一〇.6 -O.4
X(M)→
O.0
一012 0.O 一ユ10
一〇.6 -O.匂
X(M)一一“
O.8
O.6/。.與
Σ
N
一〇.8 一〇.6 -O.4
X(M)一一“
一0.2 0.O
O.2
一1.O 一〇.8
O.O
一〇.2 0.O -1.O 一〇、8 一〇.6 -O.4
X(M)一→
一〇.2 0.O
Fig. 22The disp1acement ofthe snow in the box.(A)observed(B)calcu1ated by F.E.M..
The shape calcu1ated is quite simi1ar to the measured va1ue.
一380一
Studies on the Settlement Force of Snowas a Generation Mechanism H.Nakamura
Because this ca1cu1ation was carried out assuming the density distribution as
mentioned above,in the next stage,the sett1ement force was ca1cu1ated based on the
density distributions that had been actua11y measured.
4-2Sett1ement force simulations based on measured density distribution of the snow
(1)ApParatus
The measurement of the sett1ement force and the density distribution was done using
a box(1.Om x0.9m x2.0m)as shown in Photo9.Because the front wal1of the box
was made of clear acry1ic board,the densification of snow in the box was measured
through the board without disturbing the snow.The densification pattem was meamred
using straws as marks of snow disp1acement.
Newly fa11en snow which had accumu1ated natura11y on the ground was put into the
box gent1y.The box,the top of which was she1tered,was covered with snow in order to
insu1ate it from the outer air.The measurements of the densification were carried out
after the snow which had covered the front side of the box had been removed.
(2)Experimenta1resu1t
The resu1ts of the movement of the snow in the box are shown in Fig.22a1ong with
theresu1tscalcu1ated usingthe Finite E1ement Method.The shape ofthe snowca1cu1ated
by the Finite E1ement Method is quite simi1ar to the measured va1ue.It bears an
accuracy of within1cm.The movement of the marks showed a simi1arity to the
disp1acements which had been fomd in the obsemations using severa1two-wired sett1e-
ment gauges around the fence.Therefore the resu1ts achieved from this experiment are
thought to be reasonable.
Sett1ement forces measured and ca1cu1ated are shown in Fig.23.Both forces are
a1most equa1in the initia1stage of the experiment a1though there is a difference between
the two forces of about50N/m in the1ater stage.
The density distributions are shown in Fig.24.From these figures it was found that
a high density zone was formed on the beam,and a1ow density zone beneath the beam.
From Fig.22and Fig.24it was found that the snow f1ows to the lower density zone be1ow
the beam.
Distributions of shear stress on the X-Z p1ane were shown in Fig.25.Snow in the
region where the sign of the shear stress is positive intensifies the sett1ement force,whi1e
←
Σ
←
0 ユ 23 4 5 5 7Fig.23The settlement force measured and cal-
cu1ated usingthe Finite E1ement Method.
DAYs
381
Report of the National Research Center for Disaster Prevention,No,41,March1988
」く
←
Z
O.9
0.8
O,61。.。
Nedsured 98伽/π3
129
一
11ア
」く
←
Z
O,9-
0.8
O,6
ω>く
口
NH
O.2一
O.O
一ユ.C
O.8
O.6f。.。
O.2
O.O
一〇.8 -0.6 -0.^ 一〇.2 0,O
X(”〕→
160
> く ρ
N
H
/。.。
一ユ.o
O.8
一〇.8-0,6 -0.與一〇.2 0.0
X(H)一一“
1與0
180
=
O.2
O.O
一ユI0
/。.勾
0.6
姜1。.。
口 ( ;
N くパ O.2
oo > く 口100 N
旧
O.O
一〇.8 -O.6 -0.與 一〇.2 0.0
X(”)一一}
Fig.
O.O
一ユ.O 一〇.8 -O.5 -0.^ 一〇.2 0,O
X(M)一→
24The variation of density distri-
bution.It was found that a high
density zone was formed on the
beam.
Fig.
一ユ.o
08
1。.。
一〇、8 -O.6 -O.O -O.2 0.0
X(…)一一,
U.δ 1日1 1田1一 一 〇
0.6一一30! 100
O.^一
.’’’㌧O! Z’
O.2一
一ユO’ 50 -10 1夕13・仙於、。
20
010
100
一1.O -0.8 -O.5 -O.與 一〇.2 0.0
X(H)一一“
25The distribution of the shear
stress. It was found that the
area of snow which intensified
the settlement force on the beam
became narrow fast.
snow in a negative zone doesn’t.The zero-1ine is the boundary between the two zones.
Va1ues of the sett1ement force calcu1ated using the Finite E1ement Method became
rapid1y sma11er than the measured values after setting the snow,due to the fast approach
of the zero-1ine towards the beam.In other words,the area of snow which was concemed
with the intensification of the sett1ement force became narrow fast.
5.Coc1uding remarks
Measurements of the movement of the snowpack around the horizonta1beams and
the settlement force ofthe snowpack onto the beams,and the prediction of the sett1ement
force on the beams using the Finite E1ement Method were carried out in order to make
c1ear the mechanism of the sett1ement force generation.The resu1ts can be summarized
as fo1lows:
(!)Snowpack around the top of a horizonta1beam sett1es in an arc-1ike fashion.
382
Studies on the Sett1ement Force of Snowas a Genera仁ion Mechanism H,Nakamura
(2)The compressive viscosity of snow,the density of which is1ess than11Okg/m3,is
represented in the fo11owing equation:
η。=1.75×105Exp{10■2(5.24ρ十9,581Ts l)},
whereη。is compressive viscosity in N・s/m2,ρis snow density in kg/m3and T,is snow
temperature iパC.Then the re1ation of the snow temperature to the viscosity is assumed
to be the same as the resu1t of Shinojima’s experiments.
(3)Amua1maximum sett1ement forces on the rectangu1ar beam(FRm、、)and the tri・
angu1ar beam of37。(FT37m、、),the e1evation for both of which is0.8m,are represented in
the fo11owing equations as a fmction of maximum snow depth(Sm、、)in each winter:
FRm、、 =4.20(Sm、、一0.8)1.43,
FT37m、、=2.93(Sma、一〇.8)o’73,
where FRm、、,FT3.m、、are in kN/m and Sm、、is in m.
(4)The ratio of the increase of the sett1ement force versus the addition of the1oad of
snow1ayer hanging on a beam is a1most a constant in the region where the difference
between maximum snow depth and beam height is larger than lm.However in the region
where that difference was1ess than1m,the ratio became very sma11.
It is thought that the va1ue of the ratio depends upon the formation of a hard1ayer
on the beam top.
(5)In genera1,the force on a beam of triangular shape is sma11er than the force on a
square-type beam,but,in the region where the maximum snow depth is smaI1er than1.5
m,the forces on both types are a1most equa1.
(6)The settlement force increased due to the hardening of the surface layer of the snow
cover,even if there was no snowfa1L
(7)It was found by computer simu1ations using the Finite E1ement Method that mecha-
nisms of force transmission to the beam can be identified as fo11ows;1)direct pressure
from overburden snowpack,2)shear transfer from adjacent snowpack,3)snowpack
weakening beneath and the side of the beam,and4)snow densification in1ayers above
and to the side of the beam to develop a bridge for intensified transfer of force.
(8)Sett1ement movements and sett1ement force of snowpack can be ca1culated with
adequate accuracy using the Finite E1ement Method、
Ackllow1edgment
The author is deep1y indebted to Profs.S.Kinoshita and G.Wakahama and Dr.E.
Akitaya of the Institute of Low Temperature Science(ILTS)of Hokkaido University
for their helpfu1discussions and encouragement.The author is great1y indebted to the
staff of ILTS for giving va1uab1e comments.The author a1so wishes to express his
sincere gratitude to Dr,T.Nakamura of Nationa1Research Center for Disaster Preven.
tion(NRCDP)for his many usefu1suggentions,to Mr.O.Abe of NRCDP for his he1pfu1
assistance andtothestaffoftheShinjo Branch ofNRCDP.The auther a1so sincere1y
thanks Prof.T.E.Lang for his guidance and he1p with the computer simu1ations using
the Finite EIement Method,and Dr.H.Takahashi,Director-General of NRCDP,for his
kind encouragement in preparation for this paper.
383一
Report of the Nationa1Research Center for Disaster Prevention,No.41,March1988
I己eferences
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(Mamscript Received December24.1988)
積雪の沈降力の発生機構に関する研究(和文要旨)
中村秀臣*
国立防災科学技術センター新庄支所
要 旨
積雪が流動するという観点から,積雪の沈降力の解析を行った。
このため,水平桁の周りでの雪の振舞いおよび積雪の沈降力の測定を行うとともに,有限要
素法を積雪の沈降現象へ適用して,積雪の沈降力や沈降量の推定を行った。
まず,積雪の動きを知るために,2線式の沈降計および積雪層切断検知器を考察した。これ
らを自然積雪中にセットし,雪の動きを測定した結果,積雪が桁の周りをゆるやかに円弧を描
きながら沈降する事などの知見を得た。また,沈降計を用いて,徒来測定データが極めて少な
かった低密度の雪の圧縮粘性係数の測定を行った。このデータは後述の有限要素法を用いて
行うシミュレーションには欠く事のできないものである。
次に,高さ80cmの水平桁を用いて積雪の沈降力の実測を行った。用いた桁はその頂部の形
状がそれぞれ四角形,三角形(頂角60口)および三角形(頂角3τ)をした3種類の桁である。
深さが4mまでの雪について行った測定から,最大積雪深さと最大沈降力との関係を求める
と次のようになった。
四角桁 :F。。。、=4.20(S。、、一〇.8)L43 kN/m
三角桁(37■):F・、、・、、=2.93(S・、、一〇.8ザ3 kN/m
ここでF。㎜”FT。。m肌はそれぞれ最大沈降力,Sm、、は最大積雪深(m)である。
一般に積雪量が増加すると沈降力も増加するが,融雪期には積雪量が増加しなくても沈降
力が増える事がある事を見出し,その原因が積雪表面の硬化によるものと結論づけた。
最後に,桁のまわリの雪の流動現象に対し,有限要素法を適用し,積雪の沈降力および沈降
量を計算のみで求める試みを行った。
水平桁を組込んだ2m×O.6m×(〕.85mの箱の中に雪を入れ,その沈降力および沈降量を有限
要素法を用いてシミュレーションし,そσ)計算結果と実測値とを比較した。その結果,沈降力
に関しては最大40%の誤差が生じたが,沈降量については1cmの精度で一致した。
有限要素法を用いたシミュレーションを通して,沈降力の発生機構として,①桁上の積雪の
重量,②桁の側方からの勇断力の伝達,③桁の下方の雪の弱化および④桁頂部での圧密層の形
成,の4つの機構がある事が判明した。
一385一