Discussion on test resuIts

ISOTHERMAL BEHAVIOUR OF SOME GEOSYNTHETICS SUBJECTED TO SINGLE STAGE LOADING

I. Linear fit I

5.3 Discussion on test resuIts

instantaneously. Further, to prevent the clamped specimen from shooting back, thus disturbing the test and damaging the LVDT's, a guide mechanism was developed, Fig. 5.7. The loads and deformations in this stage were recorded in the data logger.

The Sustained-short term loading test was designed to simulate the sustained- earthquake loading in the laboratory. The earthquake accelerations are cyclic in nature having irregular frequencies. Due to the complications involved in simulation, they are represented by a uniform load applied over a short duration. This consideration of a uniform load is likely to be conservative, Khan.

The test was carried out on uniaxial geogrid B and comprised of three stages. Stagel consists of a single level of Sustained Load [P,] to simulate the sustained loads acting on a GRSS, Stage2 consists of the original Sustained Load [P,] plus various levels of Additional Short Term Loads [AP,] to simulate an earthquake loading and Stage3 consists of the original Sustained Load [P,] to simulate the sustained loads acting on a GRSS after the earthquake.

The loading scheme chosen for the tests is as shown in Figure 5.I.The Stage I Sustained Load [P,] was 25 kN/m which is the long term Design Strength of uniaxial geogrid B at 20°C according to BS8006 (1995). The Stage2 Additional Short Term Loads [AP,] were varied from 10 kN/m to 50 kN/m in increments of 10 kN/m, applied over a period of20 seconds. The maximum Total Load of75 kN/m in Stage2 was chosen as this was the strength obtained in CRS tests carried out at the University of Strathclyde at a strain rate of 25% per minute. The duration of Additional Short Term Load [AP,] was chosen on the basis of the durations of the main strokes of Kushiro Offshore Earthquake in 1993 and Northridge Earthquake in 1994, reported by Fujii et al (1996) and Frankenberger et al (1996) respectively. The time for application of the Stage2 loading was arbitrarily chosen to be 100 hours after the start of Stagel loading, in order to allow for the time gap between the end of construction of a structure and the occurrence of any earthquake. After removal of

the Stage2 Additional Short Tenn Load [liPs] after 20 seconds, the Stage3 loading was again equal to 25 kN/m. To observe the material behaviour in the Stage3, this sustained load was maintained for a further 100 hours.

Figures 5.8(a) and (b) portray the test results over the entire period of 200 hours in 'hours' and 'seconds' Time scale showing the strains in Stage2 and Stage3. It should be noted that only under Additional Short Tenn Load [liP,] of 50 kN/m in Stage2 the material strained more than 10% and rupture in 18 seconds. Therefore, no Stage3 was available for this combination of load. For other Additional Short Tenn Loads [liP,], i.e. for 10 kN/m to 40 kN/m, the Total Strain of the material was less than 10%. It should be noticed that at all Additional Short Tenn Loads [liP,] of upto 40 kN/m the material showed more or less linear extension behaviour, except 50kN/m at which it showed non-linear.

The results for each level of Stage2 Additional Short Tenn Load [liP,] are presented in Figures 5.9 to 5.12, in nonnal and in semi-log plots. The Total Strain in Stage3 may be observed to have remained almost constant at least up to 200 hours. At the lower levels of Stage2 Additional Short Tenn Loads [LiP,], i.e. 10 kN/m and 20 kN/m, the strain behaviour in Stage3 was similar to that of Creep test under a load of 25 kN/m. However, at higher levels of Stage2 Additional Short Tenn Load [liP,], e.g.

30 kN/m and 40 kN/m, the Total Strain in Stage3 was higher than that of creep test at a load level of 25 kN/m.

The explanation follows that after withdrawal of the additional Stage2 Short Tenn Load [liPs], the Recoverable Strain recovered immediately in Stage3, while the 'Locked-in' Strain was recovering with time. On the other hand, a constant Recoverable Strain and increasing 'Locked-in' Strain with time were contributed from the Stage I Sustained Load [P,], which effected in the net Total Strain in Stage3 to be constant. For this reason, the geosynthetic is likely to show the same strain behaviour as that of a creep test under a load of 25 kN/m. This behaviour was visible within 200 hours for lower levels of additional Short Tenn Load [liPs], e.g. 10 kN/m and 20 kN/m. However, this behaviour was not visible within 200hrs for higher

5.4 Interpretation of MSA (sustained-short term) test results onI:R - I:Lplot levels of additional Short Term Loads [t.P,], e.g. 30 kN/m and 40 kN/m, but it was likely to be observed in a course of time.

Figure 5.1 shows the loading scheme used in combined sustained-short term loading tests. The Second stage load is applied after 100 hrs from the application of the sustained load to cater for someti me after construction. The strain- time plots for the tests are shown in the Figures 5.8(a) and (b) in hour and second scales, respectively.

The material was reported to rupture in 18 seconds at (25+50) kN/m load. The pair of strain components, i.e. recoverable strain I:R and locked in strain I:L for the peak strains as well as at the end of Stage I and Stage 2 are obtained and superposed on the strain envelope plot for lO % limiting strain, since this is considered to be the failure criteria for GRSSs by most of the codes/methods. The strains at the end of Stage2 loading is taken to be that at (lOOhrs + 30sec), because practically it takes at least 5sec to unload the Stage2 load. Figure 5.13 shows the €R -I:L plot for the Stage I and 2 loadings. Similarly, Figure 5.14 shows that for the Stagel, 2 and 3 loadings.

It may be noticed that at the end of Stage I, or at the start of Stage2, i.e. 100 hrs, the geosynthetic would have assumed I:R (100 Iu,) = I:R (0 hrs) and I:L (l00 hi,) due to the sustained load of 25 kN/m. Following the application of Stage2 loading, there has been an introduction of an additional pair of €R and I:L components into the material for each of the second stage loads. These I:WI:L plots for each Second stage load are denoted by different colours, Figure 5.13 and 5.14.

Figure 5.15 shows the superposition of €WI:L plots from the results of the test onto the strain envelope at 10%. It may be observed that the strain components at 50 kN/m only cross the 10% strain envelope. Naturally, since it was reported to be the rupture load for the specimen. Other strain components from all upto 40 kN/m of Stage2 additional short-term loads are well within the envelope. That means the specimen did not fail at the lower levels of Stage2 additional short-term loads upto 40kN/m but at 50 kN/m.

This behaviour of GRSSs may be attributed to the available strain that enables the GRSSs to withstand the quake forces. The available strain may be defined as the difference between the limiting strain and the strain just before an event. The available strain thus may be likely to reduce with the operational tenure of GRSSs due to gradual development ofEL, i.e. greater in the beginning and smaller towards

On the other hand, let the Stage2 load be assumed to apply after 1000 hrs from the commencement of the sustained load to the same specimen. Due to the application of the sustained load, the specimen would have assumed the same amount of ER (lOOhrs)

but greater EL (1000) and the strain components from Stage2 loads would shift to point2 instead of pointl, Figure 5. 16(a). In this case, the strain components in the specimen are derived from the Isochronous curves at 1000hr. It may be noticed here that although the Stage2 load of 40 kN/m does not cross the failure envelope, it is very close to it, meaning it is approaching the failure strain. To elaborate this point let the event occur at IOOOOhrfrom the completion of the construction of the structure. In this case the strain components at 10000hr are not computed but assumed, since the Isochronous curve at that time was not available. The plots are superposed in Figure 5.16(b). It may be appreciated from the figure that Stage2 loadings of 40kN/m in addition with 50kN/m surpass the envelope and even 30kN/m is very close to the failure strain. Likewise, it may be extended that even the Stage2 lower loads like 30, 20, or even IO KN/m would cross the failure envelope, had they been applied after long period since the commencement of the sustained load.

In the above test, the sustained load simulates the self-weight of the structure and Stage2 loads the forces due to earthquake events. Thus, if a GRSS were stricken with an earthquake after 100hrs of construction, it would fail (cross 10% strain) at additional short-term load of 50KN/m but remain safe at the additional short-term loads upto 40 KN/m due to the event. On the contrary, if the same GRSS were stricken after 10000 hrs, even the 40KN/m-quake force would cause a failure.

Similarly, towards the end of operational tenure of the structure, even small additional forces like 10 or 5 KN/m due to seismic events might cause a failure to the structure.

5.5 Summary

Multi stage action interpreted in strain envelope concept

the end of their age. In other words, the GRSSs may withstand higher quake forces during the initial period of their operational life, but smaller forces during the latter period of their operational life. The additional short-term load that a GRSS may withstand thus may be said to decrease with its age. This is portrayed qualitatively in the Figure 5.17, which is a plot of ~Pm", (additional maximum short-term load) against time of occurrence t. For example, at the end of the service life of a GRSS if the strain in the structure is already lO%, it can take no further load since there is no available strain left in it. On the other hand, if the strain in it is less than 10%, say 8%, there is an available strain of 2% and the structure can take further loads until the strain reaches 10% (the failure criteria). Further, the material ruptured after 18 sees at (25+50) kN/m load during the sustained-short term loading test. If the rupture is considered to be the failure criteria, the structure could have been designed as safe even for the Stage2 loading of 50kN/m upto the duration less than l8secs.

The above interpretation is based on the assumption that the same amount of strain components, due to the same earthquake event, would be introduced into the geosynthetic irrespective of the time of occurrence of the event. That is a GRSS may acquire the same amount of strain due to the same earthquake, had it been hit at different stages of its service life. If the earthquake induces an equal strain at any time of the service life of the GRSS, the above extension that the capacity to withstand an additional short-term load decreases with time might be true. However, it should be verified with tests as how far it conforms to the assumption.

•

The occurrence of an event like earthquake has a great influence on the load carrying capacity of a GRSS. This capacity of a GRSS to withstand an additional earthquake load depends on the available strain in it, which reduces with the service life of the GRSS. The available strain is considered to be the difference in the limiting strain and that just before an earthquake event.

5.6 Suggested approach of designing GRSS for' MSA

On the bases of understandings of the geosynthetics made so far, it may be noted that the ability of a GRSS to withstand additional short-term load due to an earthquake decreases with its service life, Fig 5.17. This may be attributed to the reduction in ,Available Strain' in the geosynthetics, which is defined as the difference between the limiting strain and the strain just before an event. Naturally, the available strain decreases with the age of the structure, because there is a continuous development of 'Locked- in Strain' in the geosynthetics due to the sustained load. For this reason, a GRSS is likely to withstand greater shocks in the initial stage of its service life and smaller shocks at the latter stages.

Therefore, while designing GRSSs for combined sustained plus short-term loading, the approach should be such that there is still some amount of 'Available Strain' at the end of design life (EDL) of the GRSS. This is to ensure that the GRSS is able to withstand the shocks due to probable earthquakes even at EDL.

In order to design a GRSS for sustained plus short-term loading, the forces induced from an earthquake event may be estimated in advance, say L\P. In Fig 5. I 8, it may be noticed that there is a pair of strain components namely ER and EL,contributed from the short-term load I1P. The major contribution is in 'Recoverable Strain' ER part since the material does not get much time to develop significant amount of 'Locked-in Strain' EL.For design purposes, this ELpart may be ignored and the total strain due to the earthquake load I1P may be considered to be wholly Recoverable, viz.ER. The additional 'Recoverable Strain' I1ERdue to this additional load I1P may be obtained from the Load-Recoverable Strain curves from Unloading test, Fig 5.19, knowing the slope K ofthe curve which is more or less a straight line. Let the value of I1ERbe equal to 4%,

Thus the Available Strain which should be equal to t'.ERor 4%, should be left in the geosynthetic in order to take the additional short-term load of L\P. In Fig 5.20, after

Further, a small Partial Factor (PF) like 1.1 may be applied to this Design Strength PMult! stage to cater for neglecting the 'Locked-in Strain' ^!:L component. However, the exact value of this PF should be established on the basis of further MSL testing.

deducting this strain from the limiting strain of 10%, the strain level of 6% may be considered to be the failure criteria and the sustained load PMult! stage corresponding to that to be the Design Strength for the combined sustained plus seismic load. In other words, PMult! stage may be taken as the sustained load for which the GRSS is designed and is capable of taking an additional short-term load of L'>Pat the EDL.

FIGURE 5.1 Loading schemes used for combined sustained- short term loading tests, after Khan (1999).

(kN/m)

Stagel

25 k\'fm

100 hrs.'

Stage2 loads

.7

5.'.kNi;ii~:

"

'"^,

,.. '•. ;i

65.kNim

55 k:'\/m

45 k.'l/m

J5k\flll

Stage3

25 I,]'\/m

I ^{20 sec.} I ^{100 hrs.} I

•• • -.---. TIme

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Whittier, 7October 1987

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Northridge, 17January 1994

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Lorna Prieta, 18October 1989

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5.

.2.Effective Durations for soil (A) and rock (0) sites as a function of distance

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Effectlvo Durations versus Moment Magnitude at distances of less than 10km from Epicentre

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Ell A= 1.3mm Ell B=3.5mm

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³

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Sustained load

i Short term load

FIGUREE 5.5 Scheme of test apparatus used in combined sustained-short term loading tests

.'----;

Specimen

Short term load

~~

~ Sustained load Specimen

I

^{Short term}

load

~ Sustained

•••

load

\0

(a) Stage I (b) Stage2 (c) Stage3

FIGURE 5.6 Various stages in combined sustained-short term loading tests

.., ••

.Si

== _••

oS a .~ _•• =

~

a

:E ••

=

I.:) c(

- _. _'!.

,HL'>:1

c(

.%_--

---

200 150 175

75 125

25 50 ₁₀₀

Time (hours)

Figure 5.8(a) Results of combined sustained-short term loading tests (Time in hours scale), after Khan (1999)

'"

Slagle Stage SustaiDed Load -25kNlm

Slage 1&3 load: 25kN/m

~ aod Stage210ads

-- 25kNim +50kN/m - 25kN/m +40kNim - 25kN/m +30kN/m -- 25kN/m +20kN/m

Rupture x - 25kN/m +IOkN/m

,

,-

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40 50

20 30

Time (seconds)

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Single Stage Sustained Load

~ - 25kNim

Stagel &3 load: 2SkNIm and Stage2 loads:

- 25kNim+50kNIm - 25kNim+40kNim -- 25kNfm+30kNfm - 25kNim+20kNim

_~xRupture -- 25kNim+IOkNlm

.~~ ~ r l

~ Stage2 Stage3

••

•••

•• ---

••

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_•

.

_•

18

16

14

--

12

~~

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¹⁰

I'l 8

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6

4

2

0

Figure S.8(b) Results of combined sustained-short term loading tests (Time in seconds scale), after Khan (1999)

Figure 5.9 Results of combined sustained-short term loading testsfor Stage2 loading of 10kN/m, after Khan (1999)

200

1000 175

100 150 100 125

TIme (boul'$)

10 Time (bours) 75

50

CreepWll

..--

25kNim

Combined

- --

Stagel: 25kNim

Slage2: 25kNim +lOkNlm Slagc3: 25kNim

,

.

. . .. .

Creepiest

-

••

_25kNim

••

^Combined

- --

-

Slagel: 25kNim

Stage2: 25kNim+10kN1m

-

Stage3: 25kNim

-

••

^,

~

l-

. . . . .

o

0.1 1

(b) Slrain-Iog.time plot 2

(ft)Strain-time plot 4

o o

²⁵

6 18 16 14

20 18 16 14 '0'~ 12 '-'

.~

10

J::

'11 8

6

4 2

Figure 5.10 Results of combined sustained-short term loading tests for Stage2 loading of 20kN/m, after Khan (1999)

200

1000 150 175

Creep test 2SkN/m Combined StBgel: 2SkN/01

Slage2: 2SkN/m +20kN/m Stage 25kN/m

100 Creep te,t 2SkN/m Combined Stagel: 2SkN/m

StBge2:2SkN/m +20kN/m Stage2SkN/m

125

L

•

100

10

Time (hours) Time (hours)

75 18

16 14

~ _{12 •.•}

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en

6 ~ 41' 2 ~

0

. .

0 25 50

(a) Strain-time plot

20 18 16 14 '0'~ ₁₂

.~

'-' 10 en

::

8 6 4 2 0

0.1

(b) Strain-Iog.time plot

Figure 5.11 Results of combined sustained.short term loading tests for Stage2 loading of30 kN/m, after Khan (1999)

200

1000 175

100 150

10

Time (hours)

100 125

Time (hours)

75

I

25 50

(a) StraIn-time plot

Creepiest

--

I- 25kN/m

I- CombIned

- --

I- ^Stagel:_Stagel: ^25kNim_25kN/m +30kN/m

~ Stage3: 25kNim

- - ^I

~ ~

_.

~

~

_. _.

Creep test

-

25kN/m

Combined

- --

Stage I : 25kNim

Stage2: 25kN/m +30kN/m StageJ: 25kN/m

l-

I L ______________

•

. . . . .

•

o

0.1 6 4

(b) Strain-Iog.time plot 2

o o

16 14 18 20

'0' 12

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] 10

00 8

18 16 14

'0' 12

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6 4 2

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25kN/m 16 I- _Combined 14 I- ^{Stage I:25kNim}

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^{Stage2: 25kN/m +40kN/m}

"'

^{12 I- Stage3: 25kN/m}

'-'

.)

^{10 l-}

v.>

8 I

I

6

L_-

•

4 2

0

. _.

0.1 I 10 100 1000

(b) Strain.log.time plot Time (hours)

Figure 5.12 Results of combined sustained-short term loading tests for Stage2 loading of 40kN/m, after Khan (1999)

175 200 150

156 I

I

L _

.

•

75 100 125

Time (hours)

.

50

.

25

(a) Strain-time plot

o o

18 ^{Creep test} 25kN/m 16 • _Combined 14 I- Stagel: 25kNim

~

Stage2: 25kN/m +40kN/m

"'

^121- Stage3: 25kNim

'-'

.~

^{10 I-}

~ 8 I-

6 4

••..

2

8 7

6 5

Rupture at (100 hrs +18 sec)

••••••••

(100 hrs +20 sec)

(100 hrs +20 sec)

-+"'++1"III lllill (lOObrs+20sec)

(100 hrs

+

20 sec) end ofstage 1

-_._---_._.-.-._._.

3 100hrs 4 stage 1

--+- (25+50)kN/m

-+- (25+40)kN/m

~-(25+30)kN/rn

--'0- (25+20)kN/m

-+- (25+ I0)kN/m

o

2 7

6

5

Locked In strain ^SI,(%)

Figure 5.13 ^BR-S" plot from combined sustained plus short-term loading test for SR80 stage 1+2

2

o

8 6 7

3 100hrs 4 5 2

1

Locked in strain

"L

(%)

-+- (25+50)kN/m --(25+40)kN/m -+-(25+30)kN/m ----(25+20)kN/m -- (25+10)kN/m

Figure 5.14

"R-"L

plot from combined sustained plus short-term loading test for SR80, stage (1+2+3)

o

7

o

6

Rupture at (100 hrs + 18 sec)

5

'0'~

'-' ~-+"'JIIJlllIll'l

••

4

] ••

JI

••

1

³

J

2 stage 1

-_

stage 3^..

^_

^{.•.-.•-..}

_.-._

^..

(100 hrs + 30 sec)

I

Rupture at

(100 hrs + 18 sec)

(l00 hrs+20 sec) (100 hrs+20 sec) (l00 hrs+20 sec)

(.. (100 hrs+20 sec)

1

.~._._._._._._._._._._._._._.-

2 End of Stage 1

100hrs Locked in strain &L(%)

--+--(25+50)kN/m --+--(25+40)kN/m --+--(25+30) kN/m -- (25+20) kN/m --+-(25+10)kN/m

FigureS.lS Superposition ofMSA test results on the strain envelope at 10% limiting strain (time of event after 100 hrs of construction)

8

Rupture

7 5 6

End of Stage I

I I I I I I I I

I

1 _

3 4

1000 hrs Locked in strain EL(%) (1000 hrs + 20 sec)

(1000 hrs + 20 sec)

(1000 hrs + 20 sec) (1000 hrs + 20 sec)

I 2

-.-Strain envelop 10%

--+--(25+50)kN/rn -+- (25+40) kN/rn ...•...(25+30)kN/m -- (25+20) kN/m -+-(25+10)kN/m

o

Figure 5.16(a) Superposition ofMSA test results on the strain envelope at 10% limiting strain (time of event after 1000 hrs of construction)

7

6

4 5

2 3

I

o

7 Rupture

5 6

10000 hrs

._---_.-._---

End of Stage I

I I I I I I I I

4

May rupture ~ ..•.^{+ --} to;:~

3

(10000 brs + 20 sec)

(10000 brs + 20 sec)

1 2

(10000 brs + 20 sec)

Locked in strain 8L(%) 2

-a-Strain envelop 10%

---+- (25+50)kN/m

--(25+40) kN/m

---+- (25+30)kN/m

-- (25+20)kN/m -+-(25+10) kN/m

o o

2 3 4 6 7

Figure 5.16(b) Superposition ofMSA test results on the strain envelope at 10% limiting strain (time of event after 10000 hrs of construction)

EDL (end of design life)

B

Strain envelop at limiting strain

=

10%

FIGURE 5.18

"R • ilL

plot for combined sustained-earthquake load

A

Time of occurrence 't'

FIGURE 5.17 Additional short-term load Vs time of occurrence of the event

.:lIlR(Eq) ilL (!=tl)

IlR(~l ⁱ

to

tl

FIGURE 5.19 Load- Recoverable strain curve

Strain

ELimJting=lOo/Cl

Available strain t.t

^R

=

^4%

Design curve for sustained loading

FIGURE 5.20 Determination of design strength for a combination of sustained short term actions

P^Sustained

Load/m

PMulti stage