Study on the Mechanism of Piping and the Effect of Suspended Cutoff in Dike Foundation in Dike Foundation YAO Qiuling China Institute of Water Resources and Hydropower Research (IWHR) May 6, 2008, Toronto

1Introduction 2 Tests on Piping in the Single-stratum Foundation 3 Tests on Piping in the Two-stratum Foundation 4 Tests on Piping in the Three-stratum Foundation 5 Tests on Seepage Control Effects of Suspended Cutoffs in the Two-stratum Foundation 6 Conclusions Contents

1 Introduction Kinds of risks Piping Riverbank collapse LeakageChinkDropSloughing Wave erosion Culvert gate OtherstotalTotalrisks Percentage21.5%3.5%29.4%29.7%1.1%6.5%3.4%2.3%2.5%100% Total severe risks Percentage52.4%8.0%5.7%18.6%0.9%8.0%1.3%2.9%2.1%100% 1.1 Statistics of Risks in Yangtze River Main Dike during the ’98 Flood

Because the piping mechanisms are not well understood, fights are taken wherever piping occurred. It costs plenty of manpower and materials to check the possible piping occurrences and to fight against them. 1.2 Problems in Countermeasures of Dike Foundation Piping :

In some cases, the calculated widths of landside seepage berms according to the existing design criterion of China are too large to be adopted in dike stabilization design. Thus, designers have to determine them through their individual experiences. Suspended cutoffs are used to prevent piping in dike projects. However, it is not strictly coincident with the seepage control theories in dike projects. Further studies are needed to explain the action mechanism and to work out the design criteria of suspended cutoffs to prove their effectiveness. 1.2 Problems in Countermeasures of Dike Foundation Piping :

single-stratum dike foundationtwo-stratum dike foundationthree-stratum dike foundation 1.3 Three typical types of dike foundation

2 Tests on Piping in the Single-stratum Dike Foundation One perspex sheet is used to cover the sand sample as the dike body. Samples are made by depositing sand in water in thin layers. 2.1 Setup of Models

2.2 Test Method During the tests, the hydraulic head is increased step by step from a lower original one. When the seepage deformation stops, and the seepage discharge and piezometer head keep invariable under certain hydraulic head, the hydraulic head can be raised for the next step.

2.3 Several Stages (1) Some small holes, where several grains are taken up and fall down continuously, are observed at landside near the levee toe. No grains are taken away from the holes.

Some narrow and shallow piping channels are observed along the interface of the dike body and the foundation. They look like meandering rivers. Sand grains in the foundation are taken away along the channels by flow and brought out from the holes. Along the increase of the hydraulic head step by step, piping channels propagate toward the river side and stop intermediately. If the hydraulic head is lower than the critical one, the channels will stop finally. 2.3 Several Stages (2)

If the hydraulic head exceeds the critical value, the piping channels will propagate to riverside consistently. Plenty of grains are brought out by the muddy flow. The seepage discharge increases suddenly. Scoured by high flow, piping channels will enlarge continuously. Meanwhile, one main piping channel propagates toward riverside fast and connects to the river water finally that will cause dike failure. 2.3 Several Stages (3)

2.4 Relation Curve between Seepage Discharge and Average Horizontal Gradient stage 1 stage 2 stage 3

Piping initiates near the dike toe at landside firstly, then propagates horizontally toward riverside along the interface of the dike body and the foundation acted by the horizontal seepage force. 2.5 From the phenomenon mentioned above and the experimental results, it can be inferred that: If the hydraulic head is lower than the critical value, the piping channel length is limited, and piping will cease and the seepage deformation will stop ultimately. Otherwise, if the hydraulic head exceeds the critical value, the piping channel will propagate toward the river side and connect to the river water finally, that will cause the dike failure. For the single-stratum dike foundation, the critical average horizontal seepage gradient is about in these tests.

3 Tests on Piping in the Two-stratum Dike Foundation A perspex sheet with a length of 2.25m fully covers the sand layer, which simulates both the dike body and the surface clay layer. The joints between the perspex sheet and the flume is sealed up by glue. The outflow can only run out from the circular hole. 3.1 Set Up of Models

The first is to keep the hole open and to increase the hydraulic head step by step from a low original water level. 3.2 Two methods are used in the tests respectively The second is to plug the hole by a rubber stopper before the test and then raise the water level to a relatively high point. Then the plug is pulled out to begin the test.

The propagation process, scope and form of piping channel in the two-stratum foundation are similar to that in the single-stratum foundation. 3.3 Test Process

Piping in the two-stratum foundation only occurs on the top of the sand layer after the surface soil layer is penetrated by the upward seepage flow, and piping propagates horizontally under the seepage force. 3.4 From the tests, it can be inferred that: The observed piping channel development process in the two- stratum foundation is nearly the same as single-stratum foundation. For the two-stratum dike foundation, the critical average horizontal seepage gradient is about This value is smaller than that of the single-stratum foundation, which is caused by more intense 3D effects in the two-stratum foundation.

4 Tests on Piping in the Three-stratum Dike Foundation Tests also adopted the same method as the second one for the two-stratum foundation. The conditions of different thickness of sand layer and gravel layer are considered. 4.1 Set Up of Models

Conditions and results of different test schemes in the three-stratum foundation No. Thickness distribution (cm) Critical average horizontal gradient Pattern of piping development Sand layer Gravel layer Percentage of sand layer thickness %0.078 deep-seated piping % %0.165 shallow-seated piping % Test Schemes

Several stages (1) 4.3 Scheme No. 1 and No.2 As soon as the stopper is pulled out, some grains will be brought out from the hole along with the seepage flow, and a large sand ring will be formed around the hole. In the hole, sand boils appear. Beyond the sand ring, no deformation is observed.

When the hydraulic head is high, both sand boils and shallow channels appear. They propagate toward the river side together. If the hydraulic head is lower than the critical value, this process can stop. Several stages (2)

Finally, if the hydraulic head exceeds the critical value, the channel will develop in an accelerated way to the river side until it connects to the water tank. In this case, the development of piping is mainly driven by the vertical hydraulic gradient, and the depth of piping channels is larger than that of the former two types of dike foundations. This piping phenomenon is suggested to be called “deep-seated piping”. In contrast, piping developing in surface areas in horizontal direction as in the former two types of dike foundation is suggested to be called “shallow- seated piping”. Several stages (3)

4.4 Scheme No. 3 If the sand layer in the three-stratum dike foundation is thick enough as in the scheme No. 3, the mechanism of piping will be similar to that in the single-stratum and the two- stratum foundations and can be classified as “shallow-seated piping”.

It is indicated that the average critical gradient of piping in the three-stratum dike foundation will decrease when there exists a gravel layer. What’s more, the piping development mechanism is changed in this case. These effects will be enhanced when the sand layer is getting thinner. 4.5

5 Tests on seepage control effect of suspended cutoff in the two-stratum dike foundation 5.1 Setup of Models

No.LocationX/LPenetrationt/T Critical average horizontal gradient Percent of increment Vertical efficiency 1 no cutoff --0/ %-- 1(a) riverside 35/1406/ %4.4 1(b)35/14012/ %10.5 2(a) landside 105/1406/ %10.5 2(b)105/14012/ %11.8 2(c)105/14018/ %12.2 2(d)125/1406/ % Conditions and results of tests with suspended cutoffs

Effect of the Depth of Cutoffs Effect of the Location of Cutoffs Effect of the Location of Cutoffs The critical average horizontal gradient of piping failure increases along with the increase of the cutoff depth penetrated into the sand layer at a definite location. The critical average horizontal gradient of piping failure is higher when the cutoff is placed at landside than that placed at riverside with a definite penetration. 5.3 Effects of Depth and Location of Cutoffs

piping hole The seepage vector at the front and back sides of the cutoff is changed from horizontal to vertical directions. As a result, the anti- permeability strength of the dike foundation can be significantly improved. 5.4 The Action Mechanism of Cutoffs The cutoff can increase the seepage path after piping occurrence. The cutoff can prevent the piping channel from propagation toward riverside. When the piping channel propagates to landside of the cutoff, the effective seepage path will be composed of two parts, namely the horizontal distance from the river water to the cutoff, as well as the product of the vertical efficiency and the cutoff depth. It can be lengthened if the cutoff is placed at landside in contrast to riverside. Therefore, it can be concluded that cutoffs at landside will be more effective than at riverside.

6 Conclusions If the hydraulic head is larger than the critical value, piping will propagate persistently toward riverside and it will eventually cause dike failure for all the three types of dike foundations. However, piping can only extend to a limited scope and it will stop. In this case, piping will not cause dike failure. Therefore it is suggested that it is not necessary to fight against piping wherever it appears. For the single-stratum and two-stratum foundations, as well as the three stratum foundation when the sand layer is thick enough, piping propagates horizontally along the surface of the sand layer. However, for the three-stratum foundation with a very thin sand layer, “deep-seated piping” will occurs, and the damaged zone will be large in both width and depth.

Critical average horizontal seepage gradients are obtained for different types of dike foundations in experimental conditions. These can be referenced as a basis for the establishment of the horizontal control criteria of dike-foundation piping. It can be used in the dike project design and emergency management such as the determination of landside berm width as well as the reasonable scope in which whether it is necessary or not to fight against piping. Suspended cutoffs can effectively increase foundation resistance to piping, and they are more efficient when placed at landside than at riverside. Therefore it is suggested that suspended cutoffs can be used for seepage control in future design. In addition, it is recommended that suspended cutoffs placed near landside dike toe or the end of landside berm will be taken as an alternative in the selection of possible seepage control measures. 6 Conclusions

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