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Page Flip: Enabled. Under field conditions, the geocomposite drain is permanently stressed by a combined action of pressure and shear forces. This creep deformation has a strong effect on the drainage capacity and the drain core stability in the long run. There are two types of behavior [ 7 , 14 ]. First, in case of a very flexible drain core, the creep will lead to a continuous thickness reduction of the drain core structure and thereby to a continuous reduction in drainage capacity. Second, in case of a stiff and rigid drain core, the reduction in thickness due to creep will in fact be small.
However, a stability failure may occur in the long run even though the stresses, which are induced by the field conditions, are much lower than the failure stresses, which were observed in a short-term compression experiment. Therefore, the drain core may collapse in the field in the long run, in which case it will lose most of its water flow capacity. The same reasoning about the effect of creep applies to the shear strength [ 15 ]. If the in-plane deformation due to creep becomes larger than a certain critical deformation, shear failure might occur in the field at a stress level which is significantly smaller than the one which was measured with a shear box equipment direct shear test.
In the following, we will show in which manner these long-term effects can be characterized by combining the results of creep experiments, long-term rupture tests under normal and shear stress, measurements of water flow capacity at high pressure as well as shear box tests. In the long run, aging will significantly influence failure by inducing brittle rupture. Aging effects are very important.
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However, the phenomena exemplified in this review are solely due to creep and ductile creep rupture of pristine samples. Information about aging effects may be obtained elsewhere [ 17 ]. Installation of a geocomposite drain top. Geocomposite drains are in most cases used on more or less steep slopes of landfill capping systems bottom.
Drainage capacity or water flow capacity q p within the plane is defined as the volume of water, which can flow through the cross section of a geocomposite drain per unit width and per unit of time. It depends on the properties of the geocomposite drain, especially its thickness t , and on the hydraulic gradient i enforcing the flow in the plane of the geocomposite drain as well as on the so-called bedding, i.
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The residual thicknesses t 1 y , t 2 y , etc are extrapolated for the relevant design life e. Thereby, the thickness as function of pressure is likewise obtained. Then, the pressures p 1 , p 2 , etc, which correspond to the respective residual thicknesses, are read from the pressure versus thickness curve. These long-term values may be quite different from the experimental values of the declaration of performance associated with the CE-marking of the products or of other technical data sheets. The difference will depend strongly on the type of product.
With the method described, the introduction of an arbitrarily chosen general reduction factor for creep in the construction design is avoided.
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However, there is an important problem. One measures a creep curve for, say, about one or two years, and one extrapolates to years. During the 99 or 98 years, over which one extrapolates, things can happen, which could render the extrapolation senseless. This leads to the question of long-term shear strength and drain core stability.
Schematic illustration of the method to determine the long-term water flow capacity of geocomposite drains from short-term experimental values. The components of the geocomposite drain can be solely connected to achieve easy handling during transportation and installation, e. In many cases, the components are permanently bonded, e. A large adhesion stress a is additionally observed in the direct shear test. However, bonds between the components are susceptible to creep.
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The creep deformation might cause shear failure at a combination of shear stress and normal stress in the field well below the stress level, which was derived from the failure envelope of the direct shear test. In other words, the stresses at shear failure, which are observed in direct shear testing with its more or less high deformation velocity, are usually significantly higher than those actually relevant to the conditions in the field, where the deformation velocities is smaller by orders of magnitude. Therefore, direct shear test results are not applicable to a proper design.
However, a conservative estimate of the relevant stress at shear failure under field conditions might be obtained as follows. In the first case, yielding of the nonwoven geotextile at the welding points took place at a critical deformation s k. This was the relevant failure mode, even though shear stress continued to increase slightly to a maximum value at a larger deformation. In the second case, a sudden rupture occurred at the maximum level of shear stress and at a critical deformation s k.
It is quite independent from the applied normal stress and a characteristic feature of the product. We may now argue that as long as the creep deformation is below this critical deformation, there will be no shear failure in the long run. In both cases, the connection between the components of the GCL may be considered as quite strong and only weakly affected by creep for the given loading conditions. Arrows mark the critical displacement s k at which shear failure starts. Collapse of the drain core and shear rupture are independent phenomena.
One may have a collapse of the drain core without any effect on the shear strength and one may have shear failure even though the drain core remains completely intact. In the long-term creep rupture test at high pressures, the failure behavior is usually dominated by drain core collapse. The line of reasoning as described in section 2. After a certain failure time, the drain core collapsed. Decreasing normal stress the failure time increased significantly.
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Superposing a shear stress in addition to the normal stress will decrease the failure time. Therefore, the tests were performed at a given ratio of shear stress to normal stress and the failure times are measured for varying normal stresses. With enough data available, the critical normal stress related to an envisaged service life, above which collapse might be imminent, can be extrapolated by a best-fit straight line. The critical normal stress is the abscissa of the intersection of the best-fit straight line with the horizontal line indicating the envisaged lifetime. The polymeric drain core recovers from collapse after releasing the stress.
Gray circles: ratio of shear stress to normal stress Black circles: pure normal stress. The use of geocomposite drains in capping systems of landfills is a typical example were loading conditions are severe and envisaged lifetime is very long. Design life has to be many decades, actually at least years according to the German regulations [ 19 ]. To design the drainage layer reliably, it is very important to know the long-term water flow capacity as function of the loading conditions and the critical normal stress and the critical ratio of shear stress to normal stress for a geocomposite drain product.
In sections 2. However, various other impacts may affect the performance of the geocomposite drain. For an appropriate design the relevant design values of the water flow capacity have to be calculated from the characteristic values of the long-term water flow capacity see section 2. They may be considered as the inverse of partial factors of safety with respect to the various impacts. They have been the result of a survey of the opinions of the experts of this group.
The values are still under discussion.
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However, they are in the lower part of the ranges, which are given by other sources [ 1 ]. It is recommended by the working group to use an additional partial factor of safety for the uncertainty in assessing the inflow of rain water to the drainage system. The overall global factor of safety obtained by multiplying the inverse of the reduction factors is about 2 with respect to the assumed water inflow and experimentally determined long term water flow capacity.
Currently, there is a controversy in Germany, whether this factor is still too small for landfill capping systems, because the effects of root penetration might be underestimated in case of a reclamation layer, which is typically 1 m thick. Reduction factors RF. Long-term water flow capacity see section 2. Therefore, the critical normal stress should be either multiplied by a factor of safety and the ratio of shear stress to normal stress taken as it is, or the ratio should be multiplied by a factor of safety and the critical normal stress may be applied as taken from the diagram.
The design of landfill drainage layers is described in more detail in the above-mentioned recommendation E of the German Geotechnical Society. In the following, the pull-out behavior of an idealized geogrid with limited junction strength is considered and some conclusions are drawn with respect to the material properties of geogrids, which are actually relevant for a safe design of the anchoring [ 9 ].
The soil—geogrid interaction is due to friction between the soil particles and the surface of the geogrid [ 22 ]. If one assumes that the LEs are totally stiff and that the junctions have a strength, which is always much larger than the induced stresses at the junctions, one may derive a simple formula for the pull-out resistance: The pull-out force per unit with of a geogrid is proportional to the anchorage length and to the normal stress on the geogrid in the anchorage [ 22 ]. It follows that the pull-out resistance can be made arbitrarily large by increasing anchorage length or normal stress.
Therefore, the tensile strength of the LE is usually considered as the only relevant material property of a geogrid [ 23 ]. However, the assumptions may apply to metal geogrids. They do not apply to polymeric geogrids. Geogrids are used to prevent sliding on long and steep slopes during installation and use of a landfill capping system [ 58 ].
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