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Application of Geosynthetic Clay Liner in Hydraulic Engineering

 1.The Introduction of Geosynthetic Clay Liner

The development of hydraulic engineering can not only meet the needs of people’s lives, industrial and agricultural production and development, but also serve more for the protection and improvement of the environment. Impermeability is the key of hydraulic engineering, and the selection of natural eco-friendly impermeable material is the consensus. Geosynthetic clay liner (hereinafter referred to as GCL) is a kind of new environment-friendly impervious material, consists of two layers of geotextile through a special needle punching technology to wrap together the graded bentonite particles, which is a new generation of GCL developed on the basis of compacted clay liner. GCL was first applied to the liner system of a refuse landfill in the United States in 1986, and around the same period, Germany also studied the application of GCL products and it was successfully applied to canal liner systems, liner systems of refuse landfill and so on. In the 1990s, GCL began to be applied in many domestic projects and achieved a good impermeable effect. At present, GCL is widely used in water conservancy projects, underground engineering, artificial lakes, garbage disposal sites, tunnels, roof gardens and so on.

a)The Waterproof Principle

GCL belongs to the montmorillonite clay minerals, sodium bentonite has a high expansibility that a small particles in the laboratory conditions can be expanded more than24 times when meeting water. When GCL swells with water, the sodium bentonite expands. Due to the squeezing force that from the tensile force of the basement, the temporary protective layer, the backfill and the fiber inside GCL and so on, interspersed between the molecules arranged within the narrowing of the internal gap to form a dense layer of gel structure, the water molecules can not be through, which play a waterproof effect. The greater the squeezing force from both sides, the higher the degree of densification, the better the waterproofing effect.
b)Main Specifications
According to the industry standard that is Sodium Bentonite Geosynthetic Clay Liner (JG / T193-2006 ), the main physical properties of geosynthetic clay liner are listed below:
Items Specifications
Mass Content Per Unit Area of Geosynthetic Clay Liner(g/㎡) ≥4000
Swell Index of Bentonite(mL/2g) ≥24
Blue Absorption Power(g/100g) ≥30
Tensile Strength(N/100mm) ≥600
Elongation Under the Maximum Load(%) ≥10
Peel Strength(N/100mm) ≥40
Permeablity Coefficient(m/s) ≤5.0 × 10﹣¹¹
Hydrostatic Pressure(1h,No leakage MPa) 0.4
Filtrate Loss(mL) ≥18
Bentonite Durability(mL/2g) ≥20

The Main Specifications of GCL
c)Main Characteristic of Product
i.Excellent impermeability: GCL can automatically expand when meeting water with high anti-hydrostatic pressure.
ii.Simple construction: GCL has low requirements for the soil subgrade , with the natural overlapping not limited by the temperature of the construction environment, and can working in the negative temperature, it has the shortest construction period of the existing waterproof materials.
iii.Special self-healing: using the expansion property of the sodium bentonite when meeting water to realize the automatic healing of  waterproof layer defects or damage.
iv.Strong anti-deformation ability: using the natural overlapping to adapt the uneven settlement.
v.Durability and environmental protection: the sodium bentonite is inorganic material with aging resistance, good durability and long service life.
vi.Unique breathability: it conducive to hydrosphere exchange.
vii.Good penetration: the plant roots can penetrate the GCL without destroying it.
viii.Frost resisting: GCL can avoid the destruction of waterproof layer caused by frost heaving, which can ensure the perfect waterproof effect as ever.

2.Construction Technology and Node Processing

a)Construction Process
The construction of GCL is convenient, and the specific construction as follows: smooth and clean the base surface→construction preparation and GCL cutting→GCL laying→protective layer construction
i.Smooth and clean the base surface: the base surface must be smooth, no foreign body, no water, no holes and cracks, and the degree of compaction must be more than 85%.
ii.Construction preparation: preparing the bentonite sealing powder, bentonite sealing paste, cement nails and other construction materials and the related equipment for construction.
iii.GCL laying: according to the order of laying the elevation at first and laying the bottom surface at last with triangular distribution, and it must be avoided the cross-shaped gap. When laying the GCL, the non-woven fabrics should face the upstream face, and the GCL should be clung to the soil subgrade naturally with no fold and no dangling, and should check the GCL surface has no damage, holes and other defects during the laying process at any time.

b)Construction of Protective Layer 

Protective layer plays an important role in ensuring the waterproof effect of GCL, and it must meet the following requirements: 
i.After completion of laying construction, the protective layer must be completed on the same day with making sure not to be wet with water;
ii.If the bottom protective layer is concrete, the thickness should be greater than 200mm (include 200mm);
iii.The bottom surface can be backfilled directly with soil, and all backfill should be the sieved soil that shall not contain stones and other debris more than 10mm particle size. The backfill must be compacted (or vibrated, etc.), the density should be greater than 85% and the thickness should be greater than 300mm (include 300mm) after compacting. Do not to damage the GCL during the process of compacting, if there is damaged, should be promptly repaired;
iv.The bank protection can not be directly backfilled with soil, and avoid the erosion of mobile water destructs the protective layer;
v.If the bottom protective layer is composite of mortar and pebbles or other materials, the total thickness should be greater than 200mm (include 200mm). To prevent the mortar cracking due to the expansion of GCL, the mortar thickness should be greater than 120mm (include 120mm).

c)Node Processing

i.Fixation method
Generally require the use of cement nails and iron gasket of diameter greater than 3cm to fix the GCL, the nail holes should be coated with the bentonite sealant for processing, and the pitch of nails between the elevation and the slope should controlled at about 50cm, the plane can only be fixed at the the overlap joint. The gasket is required to press on the surface of GCL and not damage the cladding material. When fixed with a nail gun, due to the different strength of the grass-roots may sometimes result in penetrating the fixed parts of GCL, Therefore, according to the specific circumstances to use the adjustable impact size nail gun or artificial nails to fix the GCL, especially the concrete, cast-in-place concrete and other different substrates.
ii.Through-wall pipe treatment
About through-wall pipe and upright column, first of all, evenly spread the bentonite sealing powder or bentonite sealant around the through-wall pipe or upright column. Then cut a square GCL with the side length of through-wall pipe or upright column diameter plus 500mm, and cut a hole with diameter equal to the outer diameter of through-wall pipe or upright column at the center of GCL, repairing the edge to make the GCL  fit snugly around the through-wall pipe or upright column. At the joint between the through-wall pipe or upright column and the GCL should evenly spread the bentonite sealing powder or bentonite sealant.
As shown in Figure 1.
Figure 1: Through-wall pipe treatment
The reinforced waterproof treatment of structures of other shapes with penetrating the GCL should cut the GCL according to the shape of the structure for the convenient construction. It must make a temporary protection upper the GCL to prevent swelling in advance when laying the GCL.
iii.Treatment on the nightside and sunny side
The nightside and sunny side of the revetment elevation should be made of arc-shaped or obtuse angle, and cut a GCL with width of 500mm on the nightside and sunny side to strengthen processing, and then with a large area of laying. When laying on the revetment elevation, in order to avoid loosening, it can be fixed by the steel nails and shims with diameter of 25mm, or it can be directly pre-pressed by masonry.
As shown in Figure 2.
Figure 2: Treatment on the nightside and sunny side
iv.Treatment on the lapping zone
The GCL should be avoided overlapping between the elevation and slope. If it is necessary, the construction should in accordance with the the order of that the GCL on the high place should on the outside and the GCL on the low place should on the inside for construction, which is in order to prevent the foreign body into the lapped joint when backfilling the soil. When lapping the GCL on the elevation, the joint should be processed by using the bentonite sealant, as shown in Figure 3a.
The lap width of GCL should greater than 200mm (include 200mm), and the bentonite sealing powder should be sprinkled at the 75 diameters of the edge of GCL on the bottom of the lap joint, the width is 50mm and weight is 0.5kg/m. In case of windy weather, it should be used the bentonite sealant for sealing treatment, as shown in Figure 3b.
Figure 3: Treatment on the lapping zone
v.Treatment on the damaged part
In case of the damaged or holes and other defects, we should promptly use the bentonite sealing powder or bentonite sealant to repair, and put a GCL with diameter of greater than 200mm around the damaged area with using the partial coverage repair.
As shown in Figure 4.
Figure 4: Treatment on the damaged part
vi.Treatment on the revetment
If the elevation (revetment) protective layer is using the masonry, the thickness should be greater than 200mm. And the cement mortar must be used to fill the gap between the masonry and the GCL While bricklaying.
As shown in Figure 5.
Figure 5: Treatment on the revetment
d)Construction Notes
i.Storage and transportation should be waterproof, moisture-proof and strong sun-proofing. The ground should be adapted the overhead method when stored, and the GCL transported to the site should be finished on the same day.
ii.The construction should be carried out in a rain-free and snow-free weather. In case of rain or snow during construction, the GCL shall be covered in time to prevent the water-swelling in advance.
iii.During the construction of the next working procedure or the adjacent works, the completed procedures shall be properly protected and no person shall damage the GCL.
iv.Should try to avoid wearing spiked shoes, high-heeled shoes to stamp on the GCL, and the vehicles and other machinery shall not cross the GCL.
v.Laying, lapping and closing should meet the design requirements and construction practices, and be solid and stable, no hollowing, no damage,no slipping, no tilting and no wrinkles and so on. The lap length should be greater than 100mm (include 100mm).
vi.After the pavement is completed and passed the acceptance test, soil backfill or concrete slab laying should be carried out as soon as possible to avoid trampling damage. The backfill can not contain the gravel with diameter greater than 10mm to prevent the damage of GCL when backfilling, and the thickness of the backfill should be greater than 300mm (include 300mm) after compacted. The backfill should be graded to backfill compaction, and the compaction should be greater than 85%.

3.Project case

a)Dragon-shaped Water System Waterproof Engineering of Beijing Olympic Forest Park
Dragon-shaped water system is not only one of the most important landscapes of the Olympic Park, but also an artificial river with special functions. The dragon-shaped water system has a total length of 2.7km, a width of water surface of 25-150m and a total surface area of 183,000 m2, of which 70,000 m2 is built above the underground space. Therefore, the selection of waterproof and impermeable material of the dragon-shaped water system is very crucial. It is necessary to consider the self-purification and respiration function of the water system itself, to create a symbiotic ecosystem for the plants and animals in the water, and also to take into account the communication function between the water system and the shore plants. After comparison and demonstration, the GCL is chosen as  an impermeable material. After laying the GCL on the bottom of dragon shaped water system, covered with the plain soil of 300mm and compacted as protective layer, the top is the planted soil of 400mm.
Dragon-shaped water system waterproof engineering was completed in October 2007, with water storage operation, after years of testing, it has been proved that have a good anti-seepage effect.
b)South-to-North Water Transfer Project: Tuancheng Lake Regulation Pool Project
Tuancheng Lake Regulation Pool is located in Yuquan Village, Sijiqing Town, Haidian District, Beijing. It is a key project for supplying water from the South-North Water Diversion Project to Beijing. It connects the two major water sources, namely the South-North Water Diversion Project and the Miyun Reservoir in Beijing. The storage volume of the regulation pool is 1.27 million m3, the surface area is 330,000 m2. Due to the strong permeability of the strata in the project site, the GCL is used for the regulation pool with anti-seepage treatment.
Tuancheng Lake Regulation Pool Project has passed the acceptance of storage on September 26, 2014, with the storage conditions after the water operation of Middle Line of South-to-North Water Transfer.

4.Conclusion 

Geosynthetic clay liner is a new type of geosynthetic material made of natural bentonite as raw material, with environmental protection, aging resistance, long service life, adapting to different seepage-proof shapes and uneven settlement, good self-healing, good anti-seepage performance, simple laying construction, low cost and other advantages, so that it has a broad application prospects in water conservancy projects.

 

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