Liquefaction Test in the Brutus Tank

Delft University of Technology

Liquefaction Test in the Brutus Tank

Technical Report SE-690504

Molenkamp, F.; van Os, R.C. DOI

10.4233/uuid:4f98a07b-164b-4287-a1e8-6643b962d5c1 Publication date

1987

Document Version Final published version Citation (APA)

Molenkamp, F., & van Os, R. C. (1987). Liquefaction Test in the Brutus Tank: Technical Report SE-690504. Delft Geotechnics. https://doi.org/10.4233/uuid:4f98a07b-164b-4287-a1e8-6643b962d5c1

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GEOTECHNICS

Liquefaction Test i n the Brutus Tank

SE-690504/2

May I987

Mlk/MS/l/98/brutus

D r . i r . F. Molenkamp Lag. R.C. van Os

Foreign offices: Belgium, United Kingdom, Canada and Singapore

All tenders and contracts as well as all consequentdelivenes of services and products and execution of activities are sub-ject to the general terms of reference of the Foundation "Stichting Waterbouwkundig Laboratorium", which are registered at the office of the clerck of the county court in The Hague and the Chambers of Commerce.

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GEOTECHNICS

CONTENTS: P A G E 1. INTRODUCTION 1 2. EXPERIMENTAL SET-UP 2 3. PREPARATION OF SLOPE 5 4. MEASURING SYSTEM 1 0 5. CALIBRATIONS 1 1

6. VISUAL OBSERVATIONS DURING THE TEST 14

7. EXPERIMENTAL DATA 15 8. FINAL STATE AFTER LIQUEFACTION 17

9. SUMMARY 18 10. REFERENCES 1$

TABLES 2 0

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1. INTRODUCTION

I n order t o v e r i f y the c a p a b i l i t y o f f i n i t e element models w i t h the c o n s t i t u t i v e model MONOT (Molenkamp, I98O) t o p r e d i c t both the

i n i t i a t i o n o f l i q u e f a c t i o n o f a slope o f loose sand and the i n i t i a l flow p a t t e r n a series o f l i q u e f a c t i o n tests i n the BRUTUS-tank a t D e l f t Geotechnics has been proposed (Molenkamp, 1982).

I t was intended t o prepare numerical p r e d i c t i o n s before performing the actual t e s t s . For these predictions t o be r e a l i s t i c the boundary

conditions and the s o i l parameters i n the experiment should be known before hand. Besides the experimental set-up should be adapted i n such a way t h a t l i q u e f a c t i o n could be induced i n a c o n t r o l l e d way.

Therefore a series o f c a l i b r a t i o n tests were performed and required improvements o f the experimental set-up were defined (Greeuw,

Molenkamp, 1986).

Although the numerical predictions were not a v a i l a b l e yet the

experiments were s t a r t e d i n September 1986. During the preparation o f the sand bed several experimental problems were met. A l l problems involved the preparation o f the p r o f i l e o f the slope. The t e s t was performed successfully.

During the e l a b o r a t i o n o f the measured data i t was found t h a t the

p l o t t i n g software had to be adapted i n order t o p l o t a l l measured data at the same time scale.

I n t h i s report the t e s t i s described and the measured data are

presented i n such a way that a comparison w i t h numerical p r e d i c t i o n s i s possible.

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The BRUTUS-tank has the f o l l o w i n g i n t e r n a l sizes: height : 1 meter

width : 1 meter length : 2 meters

The side walls o f the box are made o f glass w i t h a thickness o f 4 cm. At a l e v e l o f 3 cm above the bottom o f t h i s box a f l u i d i s a t i o n system i s i n s t a l l e d , which consists o f 20 p a r a l l e l p.v.c. tubes w i t h a

diameter o f 5.0 cm (see f i g u r e 1 ) . These 20 tubes are interconnected by a s i m i l a r tube supplying them w i t h the water f o r f l u i d i s a t i o n . I n the 20 f l u i d i s a t i o n tubes sets o f three holes w i t h 1 mm diameter have been d r i l l e d a t distances o f 10 cm, through which the f l u i d i s a t i o n

water can enter the tank. I n f i g u r e 1 also the planned geometry of the slope i s i n d i c a t e d .

I n the Brutus-tank an uniform h o r i z o n t a l bed o f loose sand can be prepared under water by applying f l u i d i s a t i o n and subsequent sedimentation. Then the slope can be made under water by sucking

consecutive t h i n layers o f sand (see f i g u r e 2 ) . This i s done by moving a suction head w i t h a s p o i l e r (see f i g u r e 3a) across the sand surface. To prepare a f l a t surface the suction head i s mounted on two

interconnected carts on top o f the tank. One c a r t allows the

l o n g i t u d i n a l motion; the other the transverse motion. The l e v e l o f the suction head w i t h respect t o the carts can be varied continuously.

The suction i s supplied by an e j e c t o r (venturi)(see f i g u r e s 3b and 3_c) , which i s water operated. To prevent severe v i b r a t i o n s i n the tank and subsequent d e n s i f i c a t i o n the e j e c t o r with hoses i s hung f r e e l y on a rope above the c a r t s .

To apply f l u i d i s a t i o n , water i s pumped from a r e s e r v o i r ( v i a the by-pass, see f i g u r e 2) t o the tubes f o r f l u i d i s a t i o n . The tubes can be saturated before f l u i d i s a t i o n i s s t a r t e d . For l a t e r t e s t i n g the water from the r e s e r v o i r can be pumped v i a a valve (Samson AG, type 241-1) and a flow meter (flowmetering instruments LTD, type: D 357-001/002) i n the f l u i d i s a t i o n system; i n t h i s way the flow o f water t o the bottom o f the BRUTUS-tank can be c o n t r o l l e d . Also the d i f f e r e n t i a l pressure across the holes a t the end o f the f l u i d i s a t i o n tubes can be measured by means o f a d i f f e r e n t i a l pressure gauge; i t measures the d i f f e r e n c e i n f l u i d pressure a t the same l e v e l i n the tubes and i n the sand. To prevent mechanical damage t o the f l u i d i s a t i o n tubes a metal g r i d i s positioned on top o f the tubes.

Several sensors have been i n s t a l l e d , namely:

4 pore pressure gauges a t the bottom o f the tank (= PPGB) 9 pore pressure gauges i n the slope (= PPGS) 3 gap sensors near the toe of the slope (= GS)

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GEOTECHNICS

The l o c a t i o n s o f these sensors are i n d i c a t e d i n figures 4a and 4b. The pore pressure gauges i n the slope have been connected t o nylon wires, which have been stretched between the metal g r i d near the bottom and a support near the top o f the tank. During the t e s t these wires have been detached from the upper support and have been

supported by small f l o a t s t o f a c i l i t a t e r e t r i e v a l o f the sensors (see f i g u r e 5 ) •

The gapsensors have been connected to very s t i f f bars, supported a t the top o f the tank. The r e f l e c t o r s consisted o f a piece o f perspex w i t h dimensions:

length : 160 mm width : 35 mm thickness : 3 pn

and a piece o f copper glued to i t w i t h dimensions: length : 85 mm

width : 35 nmi thickness : 0.5 mm

These r e f l e c t o r s were stuck v e r t i c a l l y about 90 mm i n t o the s o i l . The above mentioned equipment f o r the preparation of the p r o f i l e o f the slope has been developed during a process o f s o l u t i o n s o f

consecutive experimental problems.

In the f i r s t t r i a l the suction pressure i n the suction head was produced by the hydraulic head o f the water i n the BRUTUS-tank. The suction head was moved by hand. This produced a very i r r e g u l a r suction and o f t e n a complete blockage o f the suction pipe occured.

In the f o l l o w i n g t r i a l the suction was produced by an e j e c t o r

( v e n t u r i ) . This e j e c t o r produced b e t t e r suction without any blockage. However, the motion by hand of the suction head appeared to be too i r r e g u l a r t o o b t a i n the required p r o f i l e . Several times the sand bed l i q u e f i e d while using t h i s method. To obtain a c o n t r o l l e d h o r i z o n t a l motion o f the suction head on top o f the tank a guiding system was mounted. This system consisted o f two carts on top o f each other. The lower c a r t could move i n l o n g i t u d a l d i r e c t i o n o f the tank; i t s width was equal t o t h a t o f the tank. The upper c a r t could move i n the transverse d i r e c t i o n of the tank. The suction head was connected t o t h i s upper c a r t ; i n t h i s way i t could be moved by hand i n a h o r i z o n t a l plane a t any l e v e l i n the tank. I t was thought t o be possible t o suck o f f h o r i z o n t a l layers w i t h a constant thickness. However, several refinements o f the mounting o f the suction pipe appeared necessary t o obtain s u f f i c i e n t f l e x i b i l i t y t o remove a t h i n l a y e r o f sand across the required area. Near the sides o f the tank the sand bed became i r r e g u l a r ; besides both the slope and the surface of the h o r i z o n t a l sand layer could not be made f l a t and once an i r r e g u l a r i t y induced a spontaneous l i q u e f a c t i o n o f the slope during the suction process.

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As a possible remedy the suction head was adapted with spoxlers t o allow suction from the f r o n t of the suction head only. A f t e r these adaptations the required p r o f i l e could be made. However, i t was found t h a t during the preparation of the slope s i g n i f i c a n t d e n s i f i c a t i o n had occured; t h i s d e n s i f i c a t i o n was believed t o be caused mainly by the v i b r a t i o n s , which were induced by the ejector (venturi) which was mounted on the upper c a r t on top o f the tank. Despite the high density a f i r s t l i q u e f u c t i o n t e s t was performed, but because the sand was too dense no l i q u e f a c t i o n occured.

To prevent the d e n s i f i c a t i o n during the preparation f o r the next proper l i q u e f a c t i o n t e s t on loose sand the ejector was hung f r e e l y above the carts; i n t h i s way no v i b r a t i o n s were propagated t o the tank and a s u f f i c i e n t l y loose sand bed was obtained.

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PREPARATION OF SLOPE

A f t e r i n s t a l l i n g the 4 porepressure gauges a t the bottom and the 9 porepressure gauges f o r the slope on the nylon wires the tank was p a r t l y f i l l e d with water. Then the sand was placed h y d r a u l i c a l l y , t o prevent any damage t o the pore pressure gauges, which are r e l a t i v e l y sensitive (maximum ranges o f 175 and 350 mbar). The t o t a l dry weight of the sand was approximately 2600 kg.

Then the f l u i d i s a t i o n could be s t a r t e d . The water entered from the reservoir v i a the "by-pass". The flow o f water was c o n t r o l l e d i n such a way that the surface o f the f l u i d i z e d sand reached t o only 1 cm below the overlet o f the tank; the required flow was 50 - 60 L/min. To ensure a complete f l u i d i s a t i o n o f the sand bed a s t e e l s t i r r i n g p l a t e was moved through the sand on top o f the metal g r i d a t the bottom o f the tank. This f l u i d i s a t i o n was maintained u n t i l the outcoming water was clear; t h i s took about 4 hours.

A f t e r stopping the flow o f water the sand s e t t l e d during one hour. Then the surface reached a l e v e l o f about 750 mm above the top o f the metal g r i d .

Next again f l u i d i z a t i o n was induced. During t h i s phase both the

d i f f e r e n t i a l pressure and the pore pressures a t the bottom o f the tank were measured. The r e s u l t s are shown i n f i g u r e 6. I t should be noticed t h a t i n the p l o t the time bases o f the d i f f e r e n t signals have been s h i f t e d .

The d i f f e r e n t i a l pressure increases w i t h i n about 3 minutes t o about 43 mbar. The pore pressures a t the bottom (channels 11, 13 and 14) increase w i t h i n about 1.4 minutes t o an average maximum value of about 69 mbar; (* 6900 - j - ) ; i n f a c t also an overshoot o f about 5% occurs. The pore pressure a t channel 12 indicates only 54 mbar, (* 5400 -r) thus only 78$ o f the other sensors.

The expected increase i n pore pressure Au a t the bottom o f the tank during f l u i d i s a t i o n i s calculated by:

Au = (1 - n)(T - Y ) h

v s w (1)

i n which (see Greeuw, Molenkamp, 1986)

n =0.465 : assumed porosity

: u n i t weight o f mineral u n i t weight o f water

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GEOTECHNICS

For an estimated thickness o f the f l u i d i z e d bed above the metal g r i d of h = 0.75 m the increase i n pore pressure near the bottom reads:

Au = 5754 ~

and f o r an estimated thickness of a l l the sand above the bottom

h = 0.75 + 0.105 - 0.855 m

i t becomes

au - 6559 It

Comparing the measured and expected pore pressures i t can be concluded t h a t near the gauges of channels 11, 13 and 14 a l l the sand has been l i q u e f i e d while near the. gauge of channel 12 a layer w i t h the

thickness Ah may not have been f l u i d i z e d , while:

Ah * 0.855 * * 0.19 m

I n t h i s case the s t i r r i n g might have helped t o improve the homogeniety of the l i q u e f i e d mixture.

A f t e r stopping the f l u i d i s a t i o n again and a l l o w i n g settlement during 1 hour the sand reached the same top l e v e l of 0.75 m above the top o f

the metal g r i d . Then the sand was d e n s i f i e d by g i v i n g impacts t o the side of the tank. As a r e s u l t the surface s e t t l e d to a l e v e l of about 0.678 m above the top of the metal g r i d . Then a series of input flows were applied. The r e s u l t s are shown i n f i g u r e s 7A and 7B. I n t a b l e 1 the measured average excess pore pressures Au, the d i f f e r e n t i a l

pressure Ap (channel 15) and the flow q (channel 16) have oeen coXlsctsd. •

The properties o f the f l u i d i s a t i o n system can be expressed i n the form of a r e l a t i o n between the flow q and the d i f f e r e n t i a l pore pressure across the holes of the f l u i d i s a t i o n tubes.

The measured flow q and d i f f e r e n t i a l pressures Ap of t a b l e I are shown i n f i g u r e 8. The measured data have been f i t t e d by the f o l l o w i n g

expression:

q = C Apa (2)

i n which Ap i n kPa and q i n L/min.

The f i t o f f i g u r e 8 i s obtained by using the f o l l o w i n g parameters. C = 17.092 L/min

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GEOTECHNICS

I n f i g u r e 9 the flow q i s shown versus the excess pore pressure Au a t the bottom. The l i n e a r f i t gives

Au * 3 , 2 8 (mïn"5KPa)

From t h i s the permeability k can be estimated by

q l Tw _ 3.28^mIn7KPa^ . O.678 (m) 10 ,KPa, , L ,

k = A*AÜ " 2"(mry *~m~' 1 1' lmr7mïn;

= 11.1 * * 1.85 10-

5 [-5-]

60 Ksec' Ls e cJ

i n which A i s the area of the plan o f the tank.

Next a new sand bed was prepared by applying f l u i d i s a t i o n and

subsequent sedimentation during one hour. To get some i n s i g h t i n t o the speed by which a pore pressure wave propagates through the sand the pressure i n the f l u i d i s a t i o n tubes was increased i n one step as f a s t as possible. The responses o f the pore pressures a t the bottom

(PPGB 12) and h a l f way the height (PPGS 8) were measured by means o f an oscilloscope. The r e s u l t i n g photo i s shown i n f i g u r e 10. From t h i s p i c t u r e i t i s seen that at the bottom the pore pressure increases stepwise i n about 1.0 sec. The pore pressure halfway the height shows a very s i m i l a r response with hardly any delay i n time; the shapes o f both responses are p r a c t i c a l l y equal; consequently the v e l o c i t y o f propagation i s higher than can be measured by means o f the current instrumentation. This r e s u l t suggests a reasonable degree o f

saturation o f the sand (see also Greeuw, Molenkamp 1986).

Next again f l u i d i s a t i o n was applied. A f t e r stopping the f l u i d i s a t i o n and r e s t i n g during 1 hour the sand reached again the same top l e v e l o f 0.75 m above the top o f the metal g r i d .

Then the sand was densified by applying a s l i g h t tap at each corner o f the tank. The tap was given by hand w i t h a wooden block w i t h sizes o f 500 * 150 * 50 mm. The stroke o f the wooden block before impact was about 30 cm. I n f i g u r e 11 some measured pore pressures are shown f o r two taps; one a t the side o f the toe o f the slope, the other at the side o f the top o f the slope. The maximum excess pressures Au have been c o l l e c t e d i n table 2, together w i t h the estimated excess pressure to cause complete l i q u e f a c t i o n .

From these r e s u l t s i t i s l e a r n t that a f t e r each tap only complete l i q u e f a c t i o n occurs near the side were the tap has been given ( f o r l o c a t i o n o f gauges, see figures 4 a and 4 b ) . From f i g u r e 11 i t i s also l e a r n t t h a t the excess pressures dissipate i n about 30 sec.

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The taps caused a settlement o f the surface o f approximately 15 mm; thus the average l e v e l a f t e r d e n s i f i c a t i o n was about 735 mm above the top o f the metal g r i d .

Then the sand was l e f t f o r another 24 hours; during t h i s period no f u r t h e r settlement occured. Then the density o f the sand was measured by means o f a perspex c y l i n d e r w i t h wings (see f i g u r e 12). This c y l i n d e r had been submerged i n the l i q u e f i e d sand during f l u i d i s a t i o n and f i x e d a t a s t a b l e p o s i t i o n near the end o f the tank a t the side o f the toe o f the slope by means o f 2 s t e e l rods connected t o the top o f the tank. A f t e r settlement o f the sand t h i s c y l i n d e r was s t i c k i n g out of the sand about 40 - 50 mm. Then a t h i n f l a t l a y e r (about 30 - 40 mm) o f sand was sucked o f f by means o f a s u c t i o n pipe w i t h a diameter of about 20 mm. The s u c t i o n pressure was supplied by the h y d r a u l i c head o f the water i n the Brutus tank. Then the s u c t i o n pipe was adjusted i n such a way that again a f l a t layer w i t h a thickness o f exactly 100 mm could be sucked; t h i s sand was c o l l e c t e d , d r i e d and weighted. I n t h i s way both the i n - s i t u volume and weight were known and the i n - s i t y density could be c a l c u l a t e d ; a p o r o s i t y o f n = 0.453 was obtained.

Then the slope was prepared. As described i n chapter 2 t h i s aspect was found t o be unexpectedly d i f f i c u l t . Many problems had t o be overcome before a s u i t a b l e slope could be made. Here, only the f i n a l procedure i s described.

F i r s t the shape o f the slope was i n d i c a t e d by s t i c k i n g tape on both glass side w a l l s .

Then f o r each h o r i z o n t a l l a y e r t o be sucked o f f a small groove was dug by means o f a small spade near the end o f the tank, i n which the

suction head could be lowered. Next the t h i n l a y e r o f sand (about 2 cm) was sucked o f f across the required area. The operator had t o move the upper c a r t by hand while standing up and l o o k i n g a t the s u c t i o n head through the water surface, which gives a d i s t o r t e d view.

To improve h i s observation also a video camera and screen were

i n s t a l l e d ; the camera was p o s i t i o n e d near the glass w a l l . I n t h i s way i t was found t o be possible t o guide the s u c t i o n head c a r e f u l l y past the nylon wires supporting the PPGS's and t o obtained a f l a t

h o r i z o n t a l surface.

The slope was formed by sucking successive h o r i z o n t a l layers w i t h d i f f e r e n t lengths; these lengths were extended so f a r t h a t a f l a t n a t u r a l slope occured w i t h a steepless o f about 30-9°, which i s very s i m i l a r t o the planned steepness.

F i n a l l y the gapsensors were i n s t a l l e d . The r e f l e c t o r s o f the gapsensors were stuck i n the sand very g e n t l y and the gapsensors were mounted on the r i g i d bars and p o s i t i o n e d near the r e f l e c t o r s . I n a d d i t i o n 3 h o r i z o n t a l rows o f small p l a s t i c nobs were put on top o f the sand t o improve the l a t e r v i s u a l observations o f the motion o f the surface. A photo o f t h i s s i t u a t i o n i s shown i n f i g u r e

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I t i s seen t h a t the water i s very clear. During previous stages sometimes the water had become very cloudy by growing algae. Several methods were t r i e d t o improve the clearness o f the water; e.g. by adding poison and chloride and reducing the amount of l i g h t . The best method was .found t o be a refreshing of the water above the slope.

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4. MEASURING SYSTEM

The measuring system i s only described s u p e r f i c i a l y . I t i s i n d i c a t e d schematically i n f i g u r e 14.

Each sensor i s indicated by a typename, namely: PPGS - (Pore pressure gauge o f slope), PPGB (pore pressure gauge o f bottom), DIFFPR

( d i f f e r e n t i a l pressure across holes i n f l u i d i s a t i o n tubes), FLOW -(flow meter), GS - (gap sensors). I f required also a sequence number i s given (see figures 4A, 4B and 14).

Each sensor i s connected t o an a m p l i f i e r v i a a channel; each channel has a sequence number as indicated i n f i g u r e 14. Because a series o f pore pressure gauges were already i n f a u l t before the t e s t was

s t a r t e d , they are not shown i n f i g u r e 14; a t o t a l o f 16 channels were s u f f i c i e n t . The signals produced by the a m p l i f i e r s are e l e c t r i c

analog. From those 16 channels 12 have been connected to 2

penrecorders w i t h each 6 channels. Besides a l l 16 channels have been connected t o a datalogger (HP 3497A) w i t h f o r each channel an

i n t e g r a t i o n time o f about 30 usee and a sample i n t e r v a l o f about 550 Usee. The d i g i t a l data can be p r i n t e d and p l o t t e d . For the p l o t s per channel the actual sampling times have been determined. Because a l l channels are sampled, one a f t e r the other, w i t h i n each sample i n t e r v a l of 550 usee f o r each channel the actual sample time d i f f e r s from those of the others. On average the time i n t e r v a l between successive

channels was found t o be about 35 usee.

The valve o f the f l u i d i s a t i o n system has been c o n t r o l l e d by means o f a PID-controller (proportional i n t e g r a l d i f f e r e n t i a l c o n t r o l l e r ; VDO Mess- und Regeltechnik GMBH, type 24/81-14). The input signal has been generated by a f u n c t i o n generator; the coupling has been obtained by means o f the s i g n a l from the pore pressure gauge PPGB1 (see f i g u r e s 4A, 4B and 14). The PID-controller produced the input s i g n a l f o r the I/P (current/pressure) converter, which produces an a i r pressure t o c o n t r o l the valve o f the f l u i d i s a t i o n system.

page our r e f . date 11 -SE-690504/2 May 1987 5. CALIBRATIONS

F i r s t the flowmeter has been c a l i b r a t e d . I n t h i s c a l i b r a t i o n an output s i g n a l i n Volts has been used. The c a l i b r a t i o n has been performed i n the range: 0 S q i 50 L/min.

The measured data are shown i n f i g u r e

15-I n the range 0 < q < 50 L/min a reasonable l i n e a r f i t i s obtained w i t h :

q = F * V (3) i n which:

F * 23 7 ( 5 2 *

és)

The data o f the c a l i b r a t i o n s of a l l sensors have been c o l l e c t e d i n tables 3 t o 19.

For each type o f sensor the f o l l o w i n g maximum absolute errors were found:

pore pressure gauges i n slope (PPGS8) pore pressure gagues a t bottom (PPGB1) gap sensors (GS1)

d i f f e r e n t i a l pressure gauge

Next the r e g u l a t i o n o f the PID-control o f the valve o f the

f l u i d i s a t i o n system i s discussed. This r e g u l a t i o n has been performed a f t e r performing the f i r s t f l u i d i s a t i o n and subsequent sedimentation as discussed i n chapter 3- Some t y p i c a l data o f t h i s r e g u l a t i o n are shown i n f i g u r e s 16, 17 and 18. The aim o f the r e g u l a t i o n i s t o obtain an instantaneous response o f the flow when changing the input s i g n a l instantaneously by means o f the f u n c t i o n generator. However, i n

r e a l i t y always some delay and overshoot w i l l occur and besides o s c i l l a t i o n s may be induced. Therefore a kind o f optimum adjustment must be chosen. The method t o f i n d t h i s optimum i s described by e.g. Cool, S c h i j f f , Viersma (1979, i n Dutch).

The c o n t r o l l i n g process i s i l l u s t r a t e d i n f i g u r e 19. The input s i g n a l R(s) represents the required values o f the flow, as generated by a f u n c t i o n generator as a function o f the frequency s.

The output s i g n a l C(s) i s the actual input flow i n the tank; i n f a c t i n the current t e s t s the r e s u l t i n g excess porepressure a t the bottom of the tank are considered.

The frequency response function H(s) o f the f l u i d i s a t i o n system causes delays o f the output s i g n a l C(s) compared t o the input s i g n a l R(s).

0.038 kPa 0.040 mm 0.021 kPa

page our r e f . date 12 -SE-690504/2 May 1987

GEOTECHNICS

The c o n t r o l l e r w i t h frequency response G(s) must be adjusted i n such a way t h a t the delay i n the output C(s) becomes minimum. The properties of the c o n t r o l l e r are usually approximated by the f u n c t i o n :

G(s) = K (1 + --- + T, s) (4) v ' p s T . s d 1 i n which: K - a m p l i f i c a t i o n f a c t o r P T. - time constant of i n t e g r a t i o n ₁ T ., - time constant of d i f f e r e n t i a t i o n _d

The magnitudes of these parameters have to be chosen. To t h i s end the constants T. and T , of the c o n t r o l l e r are put at zero and the ₁

d

a m p l i f i c a t i o n f a c t o r K i s increased stepwise u n t i l l o s c i l l a t i o n P

occurs.

The current value at o s c i l l a t i o n of the a m p l i f i c a t i o n f a c t o r K a n d the r e s u l t i n g period ^Q S C of o s c i l l a t i o n are noted.

Then the optimum values o f a l l constants K , and can be

estimated using the rules by Ziegler and Nichols as i n d i c a t e d i n table 20. Depending o f the values of K , T. and x d i f f e r e n t types of

JP 1 Q.

c o n t r o l can be applied.

I n the t e s t f i r s t p r o p o r t i o n a l type of c o n t r o l (P) has been applied w i t h Kp = 6 (see f i g u r e 16). O s c i l l a t i o n occured.

Then a value of K = 4 has been applied. S t i l l o s c i l l a t i o n occured. P

Next also values of K =2.5 and K =1.6 have been used. P P

I n both cases o s c i l l a t i o n s occured only i n the r i s i n g p a r t o f the response. The period of o s c i l l a t i o n was about TQ g c = 30 sec.

A p p l i c a t i o n o f the c o n t r o l r u l e of table 20 gives:

Type of K

P

T .

1 Td

action sec sec

P 2 0 0

PI 1.8 25.5 0

page our r e f . date 13 -SE-690504/2 May 1987

GEOTECHNICS

When t r y i n g the Pl-type o f a c t i o n w i t h = 1.6 and x^ = 30 sec too much damping has been measured. Reduction o f x.^ t o x. = 0.3 sec

resulted i n too much overshoot (see f i g u r e 1 7 ) .

F i n a l l y only p r o p o r t i o n a l type o f c o n t r o l has been chosen w i t h K = 1 (see f i g u r e 18). I n t h i s case the flow could increase from zero t o 3I.5 L/min w i t h i n 1.5 sec (see f i g u r e 18). An acceptable overshoot o f only k% occured.

page our r e f . date 14 -SE-690504/2 May 1987

m 0 •: li'Ii'l!'

• GEOTECHNICS

6. VISUAL OBSERVATIONS DURING THE TEST

As shown i n photo 1 ( f i g u r e 1,0) a t time 11 hours - 6 minutes - 53-2 sec. (11-6-53.2) a t t h i s stage the slope i s s t a b l e . At about 11-6-54 the pump o f the hydraulic system i s switched on; t h i s can be heart from the sound track o f the video recording.

At 11-6-55 no v i s u a l changes have occured y e t (see f i g u r e 20, photo At H-6-57.2 the slope has deformed, causing h o r i z o n t a l displacements of a l l r e f l e c t o r s o f about 1 t o 2 cm (see f i g u r e 21, photo 3 ) . A t t h i s stage the lower gap sensors (GS) has raised by about 1 t o 2 cm, t h e middle GS has s e t t l e d by about 1 cm and the higher GS has s e t t l e d by about 3 cm. The lower p a r t o f the slope i s bulging while the higher p a r t i s s e t t l i n g .

At H-6-59.4 s i g n i f i c a n t flow has occured (see photo 4, f i g u r e 21) while a t I I - 7 - I . 6 (see f i g u r e 22 photo 5) the surface changes towards a h o r i z o n t a l surface.

Consecutive photo's a t 11-7-3-7 ( f i g u r e 22 photo 6) 11-7-6 ( f i g u r e 23 photo 7) and H-7-8 ( f i g u r e 23, photo 8) show t h a t the surface becomes p r a c t i c a l l y h o r i z o n t a l ; thus the sand behaves l i k e a t h i c k f l u i d .

page our r e f . date 15 -SE-690504/2 May 1987

D E L F T

GEOTECHNICS

7. EXPERIMENTAL DATA

The measured data during the f i r s t 15 sec. o f the t e s t as d i g i t i z e d by means of the datalogger have been p l o t t e d as a f u n c t i o n of time i n f i g u r e s 24 up to 31. Because the time i n t e r v a l o f 0.55 sec between consecutive samples of each measured q u a n t i t y i s r e l a t i v e l y large compared t o the step-response time of about 1.0 sec. (see f i g u r e 10) of the system the above mentioned p l o t s w i l l be rather rough

approximations of the actual measured data. Therefore also the

measured analog data as p l o t t e d by means of the penrecorders have been added i n figures 32 and 33. I n those figures the curves have been s h i f t e d as a f u n c t i o n o f time due t o the mechanical properties of the penrecorders. I n these f i g u r e s also the measured data during the switching o f f of the flow are shown.

-As shown i n f i g u r e 24 the flow q has been increased from zero t o a value of q ~ 8.6 L/min w i t h i n a r i s i n g time of about 2.5 sec.

According t o f i g u r e 24 the flow s t a r t e d t o increase a t a time of about 1.125 sec. of the time scale. From the data of the penrecorders (see f i g u r e 32, channel 16) i t was understood t h a t f i r s t during about 0.3 sec the flow increased r e l a t i v e l y slowly a t a r a t e o f about 2

L/min/sec. Then i t increased f a s t e r a t a r a t e of about 1 L/min/sec during about 0.8 sec. F i n a l l y the rate decreased t o zero during about 1.5 sec when the flow reached the maximum value o f q ~ 8.6 L/min. The measured data of the excess porepressure a t the bottom (PPGB's, channels 11, 12, 13 and 14) are shown i n f i g u r e 25. The data o f

channels 12, 13 and 14 are very s i m i l a r . They increase almost l i n e a r l y w i t h i n about 3•75 sec t o a value o f about Au = 4.5 KPa.

The data of channel 11 are s i m i l a r t o the above mentioned data d u r i n g about 2.7 sec when reaching an excess porepressure of about 3-25 KPa but then f o r channel 11 a much slower response i s measured; the value of Au = 4.5 KPa i s only reached a t about 9-4 sec a f t e r the s t a r t of the flow. The same conclusions can be drawn on the basis of f i g u r e 32 i n which analog data of channels 11 and 12 are shown.

The measured data of the excess pore pressure i n the slope (PPGS) are shown i n figures 26, 27 and 28. I n f i g u r e 26 the data of channels 1, 3 and 4 i n a cross-section of the slope (see f i g u r e 4b) have been

c o l l e c t e d ; f o r channels 1 and 4 also the analog data according t o

f i g u r e 33 have been i n d i c a t e d . I t i s found that f o r these channels the porepressure increase i s very s i m i l a r u n t i l l about 1.35 sec a f t e r t h e s t a r t o f the flow. A f t e r t h a t channel 4 shows an increasing

porepressure while channel 1 shows a sharp decrease and even tension during about 0.9 sec. Channel 3 shows an intermediate behaviour. I n the next phase, thus a f t e r about 2.25 sec a f t e r the s t a r t o f the flow, the excess porepressures increase f u r t h e r to reach a maximum o f about 3 t o 4 KPa at.about 10 sec a f t e r the s t a r t o f flow.

I n f i g u r e 27 the measured excess porepressures o f channels 2 and 5 are shown together w i t h the data o f the d i f f e r e n t i a l pressure (channel 15). Also the analog data of channels 5 and 15 are i n d i c a t e d on the

page our r e f . date 16 -SE-690504/2 May 1987

basis of the data i n figures 32 and 33- I t i s found t h a t the response of channel 2 i s very s i m i l a r to t h a t of channel 1 as shown i n f i g u r e 26, but unexpectedly the data of channel 5 are not s i m i l a r t o those of channel 4; namely channel 5 only s t a r t s to increase a f t e r about 2.1 sec a f t e r the s t a r t of the flow.

I n f i g u r e 28 the excess porepressure at channels 4, 5 and 6 i n the slope p a r a l l e l to the toe of the slope are shown. Also the analog data of channel 6 according to f i g u r e 33 are indicated. I t i s found t h a t the data of channels 4 and 6 are s i m i l a r thus channel 5 remains the exception.

The d i g i t i z e d data of the gapsensors (GS) have been p l o t t e d i n f i g u r e 29. Besides also the analog data of channels 7 and 9 according t o

f i g u r e 32 are drawn. I t i s found t h a t the sensors of channels 1 and 9 become large (10 mm) a f t e r about 1.7 sec a f t e r the s t a r t of the flow, while f o r channel 8 t h i s occurs about 2.2 sec a f t e r t h i s s t a r t .

Besides i t i s l e a r n t t h a t channel 8 becomes again measurable a f t e r about 8.1 sec.

From the v i s u a l observations i t i s l e a r n t t h a t the l a t t e r phenomenon i s caused by the r e f l e c t o r of channel 9 which approaches the sensor of channel 8.

Comparision of the v i s u a l observations as described i n chapter 6 w i t h the current measured data i n d i c a t e t h a t zero time of the measurements coincides roughly w i t h about 11 hours 6 minutes and 54 seconds of the v i s u a l observations.

To f a c i l i t a t e comparisons a l l d i g i t i z e d measured data as shown i n f i g u r e s 24 up t o 29 have been c o l l e c t e d i n f i g u r e 30.

I n f i g u r e 31 again a l l d i g i t i z e d measured data are shown during a period of 100 sec. During t h i s period the l i q u e f a c t i o n induced flow occurs completely and even the porepressures d i s s i p a t e .

page our r e f . date

-

17

-SE-690504/2 May

1987

D E L F T

GEOTECHNICS

8. FINAL STATE AFTER LIQUEFACTION

To obtain some i n s i g h t i n t o the i n t e r n a l deformation o f the sand, a t the end o f the t e s t a f t e r switching o f f the flow also the f i n a l s t a t e has been determined. The mean height o f sand a t that stage was about 50.5 cm above the top o f the metal g r i d .

The positions o f the f l o a t s supporting the nylon wires by which the pore pressure gauges i n the slope had been placed were measured again. These values have been collected i n table 21 together w i t h the data as measured before the t e s t was performed.

A f t e r switching o f f the flow the penrecorders showed that f o r the pore pressure gauges i n the slope pore pressures remained. I n f i g u r e 28 the recorded data w i t h the flow are shown. Only the i n i t i a l p a r t o f the t e s t and the p a r t w i t h the switching o f f o f the flow are shown

(channel 16). I n f i g u r e 29 the recorded data o f PPGS 1, 4, 5 and 6 during t h i s period are given. The remaining pressures a f t e r switching o f f have been c o l l e c t e d i n table 22. The r e l a t e d v e r t i c a l

displacements have also been c o l l e c t e d i n table 21. Assuming that the nylon wires between the top of the metal g r i d and the pore pressure gauges remain stretched at constant length the h o r i z o n t a l displacement can be estimated when the settlement o f the gauges are known. These calculated h o r i z o n t a l displacements also have been c o l l e c t e d i n table 21.

To v i s u a l i z e these r e s u l t s i n f i g u r e 30 the estimated new positions o f the wires are sketched roughly. From these r e s u l t s i t can be

understood that a l l gauges have moved t o the l e f t , especially the upper layer has moved s i g n i f i c a n t l y .

A f t e r draining the sand from the tank the f i n a l density o f the sand was measured again by means of a metal r i n g o f approximately 30 cm diameter which was pushed approximately 15 cm i n the sand. The measured p o r o s i t y read: n =

0.439-page our r e f . date 18 -SE-690504/2 May

1987

9. SUMMARY

By f l u i d a t i o n and subrequest'sedimentation i n the Brutustank an

uniform sand bed has been prepared. By sucking o f f t h i n layers o f sand underwater an uniform underwater slope has been made. This slope has been loaded i n a c o n t r o l l e d way by water flowing from the f l u i d i s a t i o n

tubes a t the bottom o f the sand bed; i n t h i s way l i q u e f a c t i o n has been induced.

The measured data o f displacement and porepressure allow the v e r i f i c a t i o n o f the c a p a b i l i t i e s o f the MONOT model t o p r e d i c t l i q u e f a c t i o n .

page our r e f . date 19 -SE-690504/2 May 1987

GEOTECHNICS

10. REFERENCES Molenkamp,' F. , (1980)

E l a s t o - p l a s t i c double hardening model MONOT

D e l f t S o i l Mechanics Laboratory, CO-218595, October Molenkamp, F., (1982)

Plan f o r l i q u e f a c t i o n t e s t i n BRUTUS-tank and i t s numerical v e r i f i c a t i o n s

D e l f t S o i l Mechanics Laboratory, CO-218596/9. February Greeuw, G., Molenkamp, F., (1986)

Calibrations f o r preparation o f a l i q u e f a c t i o n t e s t i n the BRUTUS-tank D e l f t S o i l Mechanics Laboratory, SE-690500/2, March

Cool, J.C., S c h i j f f , F.J., Viersma, T.J. (1979) Regeltechniek ( i n Dutch)

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^ DELFT GEOTECHNICS

Dt: 87-02-19 LIQUEFACTIONTEST BRUTUSTANK F l o w v e r s u s e x c e s s p o r e p r e s s u r e a t b o t t o m Chann./Scans : 11-12-13-14 CO - 287130 LIQUEFACTIONTEST BRUTUSTANK F l o w v e r s u s e x c e s s p o r e p r e s s u r e a t b o t t o m Chann./Scans : 11-12-13-14 Figure : 9

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DELFT GEOTECHNICS

Dt: 87-02-19 LIQUEFACTIONTEST BRUTUSTANK R e s p o n s e o f t h e p o r e p r e s s u r e g a u g e s CO - 287130 LIQUEFACTIONTEST BRUTUSTANK R e s p o n s e o f t h e p o r e p r e s s u r e g a u g e s Figure : 10

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CO-287130 B O - 6 9 0 5 0 4

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OF SAND

LIQUEFACTIONTEST BRUTUSBAK FIG. 12

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DELFT GEOTECHNICS

Dt: 87-02-19 LIQUEFACTIONTEST BRUTUSTANK

S l o p e b e f o r e t h e t e s t

CO - 287130 Figure : 13

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LIQUEFACTIONTEST BRUTUSTANK S c h e m a t i c e l e c t r o n i c p a r t CO - 287130

V

LIQUEFACTIONTEST BRUTUSTANK S c h e m a t i c e l e c t r o n i c p a r t Figure : 14

45 DELFT GEOTECHNICS

Dt: 87-02-19 4. LIQUEFACTIONTEST BRUTUSTANK F l o w ( 1 / m i n ) v e r s u s o u t p u t f l o w m e t e r (V) Chann./Scans : 16 CO - 287130 w LIQUEFACTIONTEST BRUTUSTANK F l o w ( 1 / m i n ) v e r s u s o u t p u t f l o w m e t e r (V) Chann./Scans : 16 Figure : 15

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G ( s ) H ( s ) R : S e t - v a l u e / d e s i r e d v a l u e C : C o n t r o l l e d v a l u e M : Measured v a l u e E : E r r o r s i g n a l / o f f s e t G ( s ) : F r e q u e n c y - r e s p o n s e f u n c t i o n o f c o n t r o l l e r H ( s ) : F r e q u e n c y - r e s p o n s e f u n c t i o n o f system

DELFT GEOTECHNICS

Dt: 87-02-19 LIQUEFACTIONTEST BRUTUSTANK P r i n c i p l e o f c o n t r o l l i n g p r o c e s s CO - 287130

V

LIQUEFACTIONTEST BRUTUSTANK P r i n c i p l e o f c o n t r o l l i n g p r o c e s s Figure : 19

Photo 2

.DELFT GEOTECHNICS

LIQUEFACTIONTEST BRUTUSTANK L i q u e f a c t i o n o f t h e s l o p e Dt: 87-02-19 CO - 287130 Figure : 20

Photo 4 4 5 DELFT GEOTECHNICS _{Dt: 87-02-19} LIQUEFACTIONTEST BRUTUSTANK L i q u e f a c t i o n o f t h e s l o p e CO - 287130 LIQUEFACTIONTEST BRUTUSTANK L i q u e f a c t i o n o f t h e s l o p e Figure : 21

Photo 6 4 5 DELFT GEOTECHNICS Dt: 87-02-19 LIQUEFACTIONTEST BRUTUSTANK L i q u e f a c t i o n o f t h e s l o p e CO - 2B7130 LIQUEFACTIONTEST BRUTUSTANK L i q u e f a c t i o n o f t h e s l o p e F i g u r e : 22

Photo 7 Photo 8 ^ DELFT GEOTECHNICS _{Dt: 87-02-19} LIQUEFACTIONTEST BRUTUSTANK L i q u e f a c t i o n o f t h e s l o p e CO - 287130 LIQUEFACTIONTEST BRUTUSTANK L i q u e f a c t i o n o f t h e s l o p e F i g u r e : 23

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CD A in CM Bd>| ) a j n s s a j d a j Q c j <-<Q D E L F T G E O T E C H N I C S _Dt:87-02-19 LIQUEFACTIONTEST BRUTUSTANK P o r e p r e s s u r e ( bottom ) - f u n c . ( t ) Chann./Scans : 11-12-13-14 / #95-#122 CO - 287130 T LIQUEFACTIONTEST BRUTUSTANK P o r e p r e s s u r e ( bottom ) - f u n c . ( t ) Chann./Scans : 11-12-13-14 / #95-#122 Figure : 25

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LIQUEFACTIONTEST BRUTUSTANK

P o r e p r e s s u r e ( s l o p e .) - f u n c . ( t )

Chann./Scans : 1-3-4 / #95 - #122 CO - 287130

LIQUEFACTIONTEST BRUTUSTANK

P o r e p r e s s u r e ( s l o p e .) - f u n c . ( t )

Chann./Scans : 1-3-4 / #95 - #122 Figure : 26

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DELFT GEOTECHNICS

D t : 8 7 - 0 2 - 1 9

LIQUEFACTIONTEST BRUTUSTANK

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LIQUEFACTIONTEST BRUTUSTANK

P o r e p r e s s u r e ( s l o p e ) = f u n c . ( t )

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DELFT GEOTECHNICS

D t : 8 7 - 0 2 - 1 9

LIQUEFACTIONTEST BRUTUSTANK

PPGS-PPGB-DIFF-GS-FLOW - func . ( t )

C h a n n . / S c a n s : 1 t o 9 - 11 t o 16 / # 9 5 - # 1 2 2 CO - 2 8 7 1 3 0 '•-id

LIQUEFACTIONTEST BRUTUSTANK

PPGS-PPGB-DIFF-GS-FLOW - func . ( t )

C h a n n . / S c a n s : 1 t o 9 - 11 t o 16 / # 9 5 - # 1 2 2 F i g u r e 30

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-DELFT GEOTECHNICS

D t : 8 7 - 0 2 - 1 9

LIQUEFACTIONTEST BRUTUSTANK

PPGS-PPGB-DIFF-GS-FLOW - f u n c . ( t )

C h a n n . / S c a n s : 1 t o 9 - 11 t o 16 / # 9 5 - # 2 7 6 CO - 2 8 7 1 3 0 F i g u r e : 31

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ESTIMATED POSITION OF PORE PRESSURE GAUGES BEFORE AND AFTER TEST

CO-287130 B O - 6 9 0 5 0 4 gez. FIG. - 3 4 form. A 4

excess pore pressure flow channel 16 L/min d i f f . pressure channel 15 kPa bottom PPGS 1 PPGS 2 PPGS 9 channels channel 1 channel 2 channel 6

, 1 2 , 1 3 , 14

kPa kPa kPa

2 . 0 1.25 1.20 3-1 1.95 1.85 zero value 4 . 1 2 . 6 0 2 . 5 0 not 5-1 3 - 3 0 3 - 1 0 on paper 6 . 0 3 . 8 5 3 . 6 0 6 . 3 • . 4 . 2 5 4 . 0 0 f l u i d i s a t i o n 6 . 4 1 0 . 0 1 3 . 3 1 6 . 7 2 0 . 0 0 . 2 6 0 . 4 7 0 . 6 9 0 . 9 5 1.24 2 8 . 5 2.1

Table 1. Measured excess pore pressures f o r a s e r i e s of applied flows.

Excess pore pressure kPa

Gauge Channel Tap near toe tap near top required complete for l i q u i f a c t i o n PPGS 1 PPGS 9 PPGB 1 PPGB 4 1 6 11 14 4 . 3 5 1 . 8 0 5 . 5 0 3 . 7 0 2. 1 3 6 • 70 • 25 .80 .40 3 - 8 3 5 3 . 4 5 2 6 . 5 5 9 6 . 5 5 9

Table 2 . Maximum excess pore pressure a f t e r tapping and the required excess pore pressure f o r l i q u i f a c t i o n