Feasibility study Sandwhip Crossdam, Stability of clay-filled gunny bags

Stability of clay-filled gunny bags • I Î

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Report on investigations R 1331-40

---december 1985

Summary

The behavior of the gunny bags under strong currents has been investigated in a flume on a scale of 1:1 .

In the first test series a dam was built up with

gunny bags only. The bags in the dam are quite stabie up till a velocity of ca 2.60 mis. With larger velocities the current is removing the bags to such an extend, that the flow cross-section is enlarged, so the velocity

decreases again to ca 2.60 mis.

During a second series of tests, the bags were dropped on top of a willow mattress at floor level of the flume.

In that case, velocities ·up till ca 3.00 mis did no harm. However at velocities of ca 3.80 mis an avalanche effect was noticed and the flume was swept clean by the current removing all bags including the mattress.

1 Introduction 1 1. 1 Terms of reference 1 1.2 Scope of work 1 2 Conclusions 3 3 Materials 4 3.1 Bags 4 3.2 Filling material 4

3.3 Method of filling and closing of bags 4

4 Preparation of tests 5

4.1 Building of the dam 5

4. 2 Consolidation ··· 6

4.3 Willow mattress 6

5 Measurements 7

6 Results of the tests 8

6. 1 Test of dam structure 8

6.2 Test on willow mattress structure 10

List of figures

1 Test flume

2 Gunny bag samples 3 Sieve analysis 4 Dam composition 5 Dam profiles

6 Willow mattress layout

7 Current veloeities in cross-section 3

8 Relation current in sections 3 and 7.5 (dam) 9 Relation current in sections 3 and 7 (mattress)

Photographs

1 Barrage with flume entrance 2 Filling of the bags

3 Layered foundation of the dam

4 Building up the dam by dropping the bags 5 Results after dropping

6 After consolidation 7 Dam before tests

8 Preparation before test; attaching control wires 9 Situation after test no 2

10 Situation before test no 3 11 Situation after test no 3 12 Situation after test no 4

13 Example of material 1055 after test no 5 14 Situation after test no 6

15 Detail of mattress construction and clamp 16 Willow mattress upstream before test no 7 17 Willow mattress downstream before test no 7 18 Empty bags

19 Empty bags

20 Willow mattress upstream after test no 9 21 Willow mattress downstream after test no 9 22 Mattress after being ejected from the flume

1 Introduction

1.1 Terms of reference

Building of dams in estuarine channels is one of the proposed activities in the framework of the cooperation between the governments of the Netherlands and

Bangladesh.

Locally available material such as gunny bags, may be used for the closing of the channels. However, for an adequate use of the bags, the behavior of the bags under strong currents, must be known.

The bags are filled with a clay-silt mixture, which may influence the stability of the bags by a certain degree of consolidation.

Therefore tests were done on a scale of 1:1, in a flume where high currents can be created.

The flume, a disused fish ladder, forms part of the river regulation works in the river Maas, near the village of Lith in the Netherlands (see Figure 1).

The Delft Hydraulics Laboratory was commisioned by

Euroconsult to conduct these tests, by the letter, dated 26-08-1985 and with re. 222/4.68.057.

1.2 Scope of work

Two series of tests were held. In both tests gunny bags were used, filled with a material, that resembles as much as possible the characteristics of the locally available material in Bangladesh.

For the first test series, a dam was built inside the flume, consisting of gunny bags only.

This dam was to subjected to currents weIl over 3 mis.

For the second series of tests, a willow mattress was used as a foundation and support for a layer of bags. All alterations of the dam and the mattress , happening

during the tests, were to be described, photographed and if possible to be checked by leveling.

Current velocities to be calculated and/or measured, including measurement of waterlevels.

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-Tests were held on october 24 and 29, and on december 3 and 4, 1985.

The following personel was involved in the actual testing: T. van der Meulen DHL

R. Bruinsma DHL

K.

van der Hoek Public Works Department F. Rijnberg _{Public Works Department}

T. van Vliet Public Works Department

2 Conclusions

The critical velocity for the gunny bags on the top of the dam in a situation simular as in the flume is

ca 2.60 mis.

This velocity is only slightly higher than the calculated critical velocity determined for a stone with the same mass and volume as a gunny bag. The expected positive effect on the stability due to a certain degree of

consolidation caysed by the clay in the filling material of the bag is therefore smalle

The supporting effect of the willow bundles of the

mattress on the stability of the gunny bags could not be determined in the test.

Due to a loss of filling material of the bags and a too abrupt increase of the velocity in the last test, the entire structure became instable and was discharged from the flume after reaching a velocity of ca 3.80 mis.

Under the current conditions during the test there is a considerable loss of filling material from the bags. Without supplementary measures (larger grain diametres of the filling material and/or finer meshes of the fabric ) the bags can not be used as ballast for a willow mattress at current velocities as large as adjusted in the flume .

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- 4 -3 Materials 3.1 Gunny bags

In Figure 2 pictures of some gunny samples are shown. It was decided that sample A was the most suitable, because it was the heaviest material and had the thickest thread. An amount of 1000 bags was ordered to be made from this material, at a price of approx. Dfl 1.60 each, at the firm. Wed. van Ommeren BV, Arnhem.

3.2 Filling material

In the Netherlands the material th at resembled the material from Bangladesh closest, was found in the southern part of the Netherlands. This material is locally known as "18ss". The grainsize distribution of both, the material from the Netherlands and from

Bangladesh, is shown in Figure 3.

3.3 Method of filling and closing of the bags

To get some preconsolidation of the filling material, the bags were filled by lifting, dropping and refilling in three steps. The bags were then closed by sewing

mechanically the loose end, without folding double the end first.

Although, during the test many bags lost their contents partially, none of them was found to be opened.

4 Preparations for the test

4.1

---Building of the dam

The flume was prepared by filling it up with sand, topped with a geo-textile, till a level of +1.48 m. After that,

a layer of bags was laid down carefully, from section 3 to the downstream regulating gate.

The base of the dam was built up further, by another three layers of bags, till a level of +2,30 m.

Activities sofar were taking place in a dry flume. After letting water into the flume, the remaining part of the dam was built by dropping bags into the water.

Figure 4 shows the composition of the dam.

After filling, the average weight of the bags was

determined by weighing some sample bags.. The weight was between 42 and 50 kg. The upper part of the dam was

built underwater by dropping the bags from a height of ca 2.00 m above the waterlevel.

After the theoretically needed amount of 235 bags were dropped and the water had been let out of the flume, it appeared that still an additional amount of 75 bags was needed to build up the dam to it's specifications.

This was probably caused by the fact that the slopes of the dam had become less steep than the required 1:2.

It is however not certain how this came about. Whether it is just the effect of the movement of the bags during their descend to the bottom, or the dropping program was not correctly made up or executed, was not clear.

A levelling in three sections was executed after

completion of the dam, the result of which is presented in Figure 5. The average level of the dam was calculated to·be +2.74m. Taking a level of +1.48 as the reference, then the volume of the dam is ca 25 cubic metres, while ca 600 bags were used for the equivalent part of the dam. This leads to a volume of ca 42 cubic decimetre per bag,

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-4.2 Consolidation

For three times, water was let into and out of the flume to check if any consolidation had taken place.

The mean level of the dam, after another levelling, was found to be a few centimetres higher than before, so consolidation certainly had not taken place. Due to the irregular surface of the dam and insufficient measuring points, any actual change is hardly distinguishable from the approximation of the levelling, so not too much

value can be given to the levelling results.

The last levelling, executed just before the dam was taken away completely, by the current, gave a mean level of about the same value as before the test.

During the tests it looked as if the empty spaces between the bags had decreased, but levelling lateron did not confirm this.

4.3 Willow mattress

The willow mattress was built up in a grid in such a way that pockets with an area of approx. 1 square metre and a depth of approx. 0.35 m, were formed.

This mattress was laid down at floor level as shown in figure 6. On this mattress, while submerged in the water bags were dropped from a level of approx. 2 m above the waterlevel, in the same way as the upper part of the dam was built. The mattress was completely covered by

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5 Measurements

During the tests, currents were measured with an

impellor-type currentmeter in section 3 ,in three verticals, in three points in the vertical. The measuring points and the results are shown if Figure 7.

During the test of the dam the current in section 7.5 was measured in the centre of the flume at halfway the waterdepth.

The relations between the measured current velocities in section 3 and 7.5 are shown in Figure 8.

During the test of the willow mattress the current in section 7 was measured at various depths, as indicated in the appropiate test description.

The relation between the measured current veloeities in section 3 and 7 is shown in Figure 9.

Waterlevels were measured also, but the reference level used in the form of the perimeter of the flume, appeared to be so irregular, that the results were considerd to be unreliable.

To be able to observe any movement of the bags, some were attached to a thin wire, suspended over a pulley and provided with a small counterweight.

During test 9 the maximum possible veloeities were reached, in the actual flume configuration. The get even higher velocities , part of the downstream gate had to be dismantled first, before the tests could be continued.

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-6 Results of the tests

6.1 Test of the dam structure

---Control wires were connected to bags in sections 6, 7.5 , 9.5 and 10.5 .

Each test condition was kept stabie during at least 30 minutes.

Test no 1

The current velocity over the dam was 1.50 mis.

No detection of any movement of bags.

The flume was not emptied and test no 2 was conducted shortly after test no 1.

Test no 2

The current velocitiy over the dam was 1.91 mis.

Immediately after reaching this velocity, movement of the control bag in section 6 was noticed.

During the rest of the time no further movement could be observed. After this test the flume was emptied for the first time and the control bag in section 6 could be seen in section 8.

Some more bags had apparently moved in a bid to find some shelter for the current.

Test no 3

The current velocity over the dam was 2.22 mis.

No movement of control bags was noticed during the test.

After setting the flume dry again, some more bags appeared to have moved again, but this still did not cause much change in the overall shape of the dam.

It seems that any movement of bags sofar, served only to equalise the current over the dam by moving some bags to less vulnerable places.

Test no 4

The current velocity over the dam

=

2.40 mis.

After the test, it could be noticed that the upper layer of the dam, forming the crest, started to move with the current. But still the bags settie down on the

downstream slope.

This is the velocity during which single bags cannot withstand the current on their own any longer.

Test no 5

The current velocity over the dam reaches a maximum value of 2.80 mis, but decreases after a short while to

2.60 mis.

After the test it could be noticed that again more bags in the upper layer had moved, but they still settie on the downward slope, and sofar no bags have been lost. Now clouds of filling material can be seen, passing by

in the water. Some bags showed loss of contents after the test which confirms the observation during the test.

It seems that the stability limit is reached now and that each individual bag cannot rely any longer on more

support from it's neighbours. Test no 6

Each time the current velocity was increased beyond 3

mis, it settled down again after a short while to a velocity of 2.60 mis.

After a maximum initial velocity of 3.40 mis, and another failure to hold on the velocity for a longer period of time, above the critical velocity of 2.60 mis,

the test was terminated.

When the flume was emptied afterwards, it could be

observed that the bags that made up the dam were in the mean time spread out evenly in one layer on the bottom of the flume.

Sometimes large clouds of material were visible in the water. It could be confirmed afterwards th at this always coincided with movements of the bags. Some bags were found to be filled only for a quarter or less of their original size. Since not a single opened bag was

found, it may savely be assumed, that the current was able to extract material from the bags through the gunny fabric.

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-6.2 Test on the willow mattress structure

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---Control wires were connected to bags in the first and last row. Each test situation was kept stabIe for at least 30 minutes.

Test no 7

Current above the willow mattress was measured at 0.27 d from the local waterlevel.

No movement of bags could be noticed.

The flume was not emptied and the next test was conducted immediately after th~s one.

Test no 8 .

Current above the mattress was measured at 0.16 d from the surface. A velocity of 3.15 mis was to be maintained at this measuring position.

After measurements were carried out at section 3 and returning to the previous point in section 7, it appeared that the current had stabilized itself at a velocity of 2.96 mis.

The same effect of enlarging the cross-section, as described earlier, had obviously repeated itself. This was in agreement with the observation that the marker of the control bag in the most upstream row had disappeared and of the downstream row had shifted

it's position.

After disposal of the water from the flume , the above mentioned process could be confirmed visually.

The top layer had moved in the downstream direction for about half the length of the mattress, while as an

average, lost approx. 50 % of it's content in filling material.

Still no bags we re lost and all of them were still laying on the mattress.

Test no 9

The gates were completely opened during this test. The velocity above the mattress was measured at 0.37 d from the surface. The maximum velocity reached was 3.23 mis.

After the measurement in section 3 and returning to the same position in section 7, the velocity had

decreased to 2.98 mis.

Although some bags had moved, including the control bag downstream, at the mattress position there was

not much difference.

More downstream near the gate , a patch of 5 bags had disappeared.

Test 10

The stationary part of the downstream gate was lowered by dismantling.

The downstream layer of bags, starting from the mattress was secured by topping it with steelwire webbing.

After reaching a velocity of more than 3.80 mis,

everything started to move and it was not longer possible to maintain a stabilized velocity.

All material in the flume, including the complete willow mattress was discharged from it by the strong current .

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-6 Discussion on the results

---The dam built from gunny bags appeared to be stable up

till a velocity of ca 2.60 mis.

At higher velocities the bags are removed until a flow

cross-section is created, large enough to permit a

decrease of the velocity to ca. 2.60 mis again.

In the end, at high starting velocities, the dam was

swept away completely.

Each test had a duration of approx. 30 minutes.

During that period, the bags started to lose their

filling material. Longer periods were not tested

but it may be assumed that the loss of material will

be a main factor to take into account.

The stability of the gunny bags may be approximated

by regarding the bag as a stone with a mass of 45 Kg

and a volume determined from this mass and the volume

density. The volume density is estimated to be

1600 Kg/cubic metre.

The nominal diameter becomes :

3 3

Dn

= V

Volume

=

V'45/1600

=

0.308 m

and with :

A

=

(Rho - Rho

)

1

Rho

=

0.6

bag water water

The determining parameter

A

Dn becomes

On

=

0.185 m

According to Isbash the critical velocity can be

determined by :

Vcr

=

1.7

V

A On g

=

2.30 mis broad crested dam

The critical velocity found in the flume does not

differ appreciately from the calculated one and therefore

the effect of consolidation due to the clay part of the

filling material is only small and has no practical

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, The critical velocity for the bags on the top of the willow mattress was found to be around 3.00 mis for short periods. During longer periods, the 1055 of material from the bags will become eventually the deciding factor. Due to the 1055 of material the load on the mattress will also decrease.

At a certain moment parts of the mattress are becoming free from the underground and start vibrating, caused by the turbulence of the current. This increases the turbulence even more. Then after all the bags start moving , the mattress itself is taken along by the current. In fact all the bags, including the mattress as a whole were ejected from the flume.

The larger critical velocites found for the situation with the mattress may be an indication of some

support by the willow bundIes. However from the photo-graphs it can be seen that on this stage of the test the upper layer of bags is still present and a maximum

support by the bundles is not yet available.

The support from the bundIes is however not the deter-mining stability factor. More important for the stability

is the loss of filling material from the bags under conditions with large current veloeities. This 1055 causes an important restriction in the use of the bags in particular as ballast for a willow mattress. The

loss on the spot can be even more due to a finer filling material for the bags in Bangladesh (see Figure 3).