1. Introduction
2. Theoretical background and design diagram
3. General shear behaviour of GCLs
4. Examples of interface shear values
5. Long-term laboratory shear behaviour
6. Long-term field study
Summary
References

1. Introduction

Bentofix^® Geosynthetic Clay Liners (GCLs) are industrially manufactured composite materials combining high swelling bentonite clay and geosynthetics for sealing applications. The low hydrated midplane friction angle of the bentonite alone (peak approx. 9°, residual about 4 to 5°) is overcome by the needle-punching of all components (Fig. 1) creating a uniform shear strength transmitting GCL. Bentofix^® GCLs have been employed world-wide by the industry for well over a decade now. They are generally used to replace or augment compacted clay liners. The hydraulic conductivity is in the range of < 5 x 10^-11 m/s. One main advantage of needle-punched GCLs is that they can be installed in steep slope applications (e.g.: Bentofix^® on a 45 m long slope in a landfill cap application on a 2:1 (26,6°) slope). For such applications it is important to evaluate the interface shear stress of the GCL and prove that the internal shear stress of the GCL is sufficient to meet design criteria.


2. Theoretical background and design diagram

From theory, approximately 2.5 million fibres per m² reinforce the bentonite clay layer as they are needle-punched from the cover geotextile to the carrier geotextile.

Fig. 1: Schematic cross-section of a needle-punched Bentofix^® GCL

The needle-punched fibres have a minimum tensile strength of 40 cN/tex so that the reinforcement can create a short-term shear stress of approx. 1,000 kN/m² . Assuming the fibre reinforcement interlocks completely, a safety factor for polymer creep should be taken into consideration. Using a safety factor of 4, a theoretical long-term shear stress of 250 kN/m² is obtained.


Fig. 2: Bentofix^® peel correlation for design issues


In several hundred shear tests, the internal shear stress of Bentofix® GCLs was evaluated after a 24-hour-prehydration without confining stress to simulate worst case installation conditions or an underwater installation. The Bentofix® GCLs also varied in the quality control peel value so that the shear stress results could all be plotted against the peel value by converting shear stress into a confining stress (cover soil with a density of 20 kN/m2) and slope inclinations (Fig. 2). A cohesion intercept was not taken into consideration so that the obtained values are on the conservative side.

It can be seen from Fig. 2 that a relationship exists between the peel value and the confining stress. The shear plane is outside of the Bentofix^® GCL if the given peel strength value is above the chosen slope inclination for the selected cover soil depth. The value A in Fig. 2 indicates that no internal failure occurs for Bentofix^® on 1.5:1 (33.7°) slopes with a confining stress of 4 m cover soil (80 kN/m²) but could occur with 5 m cover soil.

It can be shown that the achieved manufacturing quality control (MQC) peel values for the Bentofix® GCLs satisfy the design needs for most low confining stress applications.


Fig. 3: Automatic tilt table and shear box


3. General shear behaviour of GCLs

To examine the general shear behaviour of GCLs in a hydrated condition, tests on different GCL types were conducted after 24 hours horizontal prehydration with no confining stress in the laboratories of NAUE GmbH & Co. KG GmbH & Co. KG, Germany. On an automatic tilt-table (1 m x 1 m), the GCLs were sheared between textured geomembranes (GM). The set-up (Fig. 3) was then loaded with a 30 cm thick gravel layer (approx. 6 kN/m²). The box was constructed in such a way that the shear plane could only occur between one of the geomembranes and the GCL or in the GCL itself. After a short-term consolidation time of 0.5 hours, the tilt-table was inclined at a rate of 1°/min. Some representative results from the conducted tests are shown in table 1.

Structure	shear plane	shear angle
5 kg bentonite between two geotextiles (no reinforcement)	internal	8°
5 kg bentonite between two geotextiles, fixed with water-soluble glue	internal	8°
4.5 kg bentonite between two geotextiles, needle-punched with 8 N/10 cm peel strength	internal	18°
4.5 kg bentonite between a 200 g/m2 nonwoven and a 100 g/m2 woven, needle-punched with a peel strength of 65 N/10 cm	external (woven)	22°
4.5 kg bentonite, stitch-bonded between two nonwovens (200 g/m2)	external	29°
4.5 kg bentonite between two nonwovens 300 g/m2, needle-punched with a peel strength of 30 N/10 cm	external	33°

Table 1: Results of the tilt-table tests for determination of the general shear behaviour of GCL types

The test results highlight two significant factors in the behaviour of GCLs:

a) The peel strength of needle-punched GCLs has a decisive influence on the shear behaviour.

b) At a sufficient internal shear strength transfer, the selection of the adjacent geosynthetics is significant for interface shear transfer. Light mechanically bonded nonwovens (~ 200 g/m²) and wovens show lower shear angles than thicker mechanically bonded nonwovens (~ 300 g/m²).

4. Examples of interface shear values

Upon selecting a GCL, not only is the reinforcing internal shear stress relevant but the interface shear behaviour is just as important. For a first assumption, the relationship (tan j' / tan y') for interface friction angles of geotextiles can be assumed according to Grett (j' = interface friction angle of soil vs. geotextile, y' = soil friction angle), but cannot replace shear tests with site soils:

	needle-punched nonwoven	woven
clay	~ 0.92	~ 0.84
fine sand	~ 0.92	~ 0.80
coarse sand	~ 0.95	~ 0.83

Table 2: Assumed interface friction relationship of geotextile and soil

Direct shear test with on-site-soils should follow as close as possible on-site conditions, including for example confining stress and hydration of GCL. Table 3 summarises the range of shear angles which have been achieved in various interface shear tests against geosynthetics and soils, and clearly show that the woven component of the GCL in general achieves lower friction angles than needle-punched nonwoven components of a GCL.

adjacent geosynthetic or soil	range of friction angle woven	range of friction angle nonwoven
smooth geomembrane	8° to 12°	8° to 12°
textured geomembrane	10° to 25°	18° to 35°
top soil	18° to 28°	21° to 32°
sand	21° to 28°	24° to 32°
sandy gravel	23° to 28°	25° to 34°

Table 3: Evaluated interface shear angles of GCL geotextile components against geosynthetics or soils


5. Long-term laboratory shear behaviour

To prove the long-term durability of the needle-punching, NAUE GmbH & Co. KG designed long-term shear boxes (Fig. 4) and examined the long-term shear behaviour of Bentofix^® (3,500 g/m² natural sodium bentonite, needle-punched between 300 g/m² needle-punched nonwovens) on a slope inclination of 2.1:1 (25°) with a confining stress of 30 cm thick crushed gravel (6 kN/m²) and steel plate loading (25 kN/m²). The GCL is hydrated daily with 10 litres of water and only the bottom carrier geotextile was fixed at the upper edge of the long-term shear box to ensure that the shear force is actually transferred by the fibre reinforcement. The first set-up was installed on October 3, 1993, and in the first 10 days, a displacement of less than 2 cm occurred. Since that time no further displacement has been encountered, not even after more than 46,000 hours (March 3, 1999).


Fig. 4: Long-term study of shear behaviour with Bentofix^® GCLs

A second test (Bentofix®, standard peel value 60 N/10 cm), installed in April 1994, was dismantled in November 1998. The results from the direct shear strength at a normal load of 25 kN/m² showed a shear stress of 64.7 kN/m² after 48-hour-prehydration under no confining stress. Neither creep nor a deterioration after a period of four years was evident.

6. Long-term field study

This long-term shear performance of needle-punched Bentofix^® GCLs was also confirmed on the Cincinnati EPA slope stability study (Fig. 5), where, amongst other GCLs, Bentofix^® GCLs were installed on 2:1 (26°), 30 m long slopes. The Bentofix^® GCLs have not failed internally or interfacially. One non-reinforced GCL failed internally in the bentonite layer due to hydration of the bentonite. In two other slides, the cover soil, the drainage net, and the geomembrane overlaying the GCL slid at the weaker interface between the woven side of the GCL and the textured geomembrane. In direct shear tests it was determined that this interface only appeared to have a friction angle of approx. 20 to 24°, too low for a 2:1 slope. In slopes 3:1 or steeper, it is therefore recommended to use needle-punched GCLs with needle-punched nonwovens on both sides.


Fig. 5: EPA field study on 2:1 slopes

Summary

Bentofix^® needle-punched geosynthetic clay liners show many technical advantages. Besides the low hydraulic conductivity, the self-sealing capability and the elongation properties, the peel value and the shear strength are important criteria for the long-term efficiency of GCLs. The requirement for a minimum peel strength is necessary for every slope application. It is important that the proof of long-term stability is conducted. In order to achieve necessary interface friction angle against the adjacent interfaces (e. g. textured geomembrane or soil), mechanically bonded nonwovens are especially suited. With a mass per unit area of ~ 300 g/m² good interface shear performance is achieved.

The existing examinations on the long-term performance of Bentofix^® show that needle-punched Bentofix^® geosynthetic clay liners are predictable sealing elements and provide long-term stability.


References:

1. Von Maubeuge, K. P.; Eberle, M. A.; "Can geosynthetic clay liners be used on slopes to achieve long-term stability?" 3rd International Congress on Environmental Geotechnics, Lisbon, Portugal, Sept. 1998, pages 375 – 380.
2. Heerten, G.; Saathoff, F.; Scheu, C.; von Maubeuge, K. P.; "On the long-term shear behavior of geosynthetic clay liners (GCLs) in capping sealing systems"; Proceedings of the International Symposium "Geosynthetic clay liners"; Nuremberg, April 1995, pages 141 – 150.
3. Grett, H. D.; "Das Reibungsverhalten von Geotextilien in bindigem und nichtbindigem Boden", Heft 59, Mitteilungen des Franzius-Instituts für Wasserbau und Küsteningenieurwesen, Hanover, 1984.
4. Koerner, R. M.; Carson, D. A.; Daniel, D. E.; Bonaparte, R.; "Update of the Cincinnati test plots" Geo-Bento '98, Paris, February 1998.
5. Bentofix Installation Guidelines, NAUE GmbH & Co. KG GmbH & Co. KG, Luebbecke, Germany, September 1996.