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Anchor Trench -
Design
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Anchorage is designed to prevent wind and water from moving under the geomembrane; it is not designed to allow geomembranes to be tensioned. The anchor trench design should allow pullout of the geomembrane before tension failure. This is directly reflected in the anchorage ratio:
| AR | Anchorage Ratio |
| TGM allow | Allowable geomembrane tension from ASTM D3886 |
| TAT allow | Allowable anchor trench tension from analytic model |
Following situations can occur:
| Anchorage Ratio > 1 | Geomembrane pull-out mode controls |
| Anchorage Ratio = 1 | Balanced Design |
| Anchorage Ratio < 1 | Geomembrane tension rupture mode controls |
Figure 1- Cross section of anchor trench section and related stresses and
forces involved
Note that the factor of safety is placed on the geomembrane force T, which is used as an allowable value.
| σallow | The allowable geomembrane stress |
| t | The geomembrane thickness |
| σult | The ultimate geomembrane stress, e.g., yield or break |
| TATallow | Allowable anchor trench tension |
| γAT | Soil unit weight |
| dAT | Depth of the anchor trench |
| d | Thickness of the cover soil |
| L | Embedment length |
| δL | FML / soil friction angle (below geomembrane) |
| δU | Cover soil / geomembrane friction angle (above geomembrane) |
| Φ | Soil internal friction angle |
| β | Side slope angle |
| Kp | Coefficient of passive earth pressure = tan2(45o+Φ/2) |
| Ka | Coefficient of active earth pressure = tan2(45o-Φ/2) |
| FS | The factor of safety for geomembrane against tension response |
| FUσ | Shear force above geomembrane due to cover soil (for thin cover soils tensile cracking will occur and this value will then be negligible) |
| FLσ | Shear force below the geomembrane due to cover soil |
| FLT | Shear force below geomembrane due to vertical component of Tallow |
| σn | Applied normal stress from the cover soil |
"Designing with Geosynthetics". R.M. Koerner, Prentice Hall Publishing Co., Englewood Cliffs, NJ, 1998.
"Geosynthetic Design Guidance for Hazardous Waste Landfill Cells and Surface Impoundments", G. N. Richardson and R. M. Koerner, 1987
Copyright 2001 Advanced Geotech Systems. All rights reserved.