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A geomembrane is one of the standard solutions for lining toxic ponds. Plastic liners are however prone to physical damage, e.g. by vandals or animals, by fire, by ultraviolet degradation in tropical climes and by frost in cold regions and they must be protected.
Following the success of Hyson Cells protection to a geomembrane liner at Injaka, Mpululanga, we have applied our minds to improvements (and added some more patents to the Hyson Cells portfolio). While some details of patent applications are not divulged at this stage, the essentials of the system are discussed for your interest.
Design objectives of the Hyson Cells liner
Summary of the system
Reliability of waterproofing is achieved by using a conventional impervious layer such as a geomembrane liner or a layer formed from a material such as bitumen sprayed on the underlying cell layer.
The impervious layer is sandwiched between two layers of Hyson Cells with ready-mixed concrete infill. The resulting 3 layers form a composite with high mechanical strength
Sandwiching the impervious layer between Hyson Cells interlocking "cell slabs" and "soil nailing" the first cell slab layer to the underlying material allows construction of steeper walls with far less need for compaction than is normally the case.
The Hyson Cells cast in-situ system is itself waterproof to the extent that the system is used for casting water carrying canals. However an impervious intermediate layer such as a conventional high-density polyethylene (HDPE) is incorporated to lend superior security
Economy may be achieved by using a geomembrane of lighter grade than would otherwise be the case.
The Hyson Cells system derives its mechanical strength from a 3-dimensional interlock of the concrete blocks, which are cast in situ. The flexible cell walls are deformed during construction to give smooth, rounded depressions. In the event of e.g. subsidence of the underlying material the blocks are able to rotate slightly to achieve equilibrium and the normal danger of shear is prevented. There is no need for reinforcing steel as the "pavement" is therefore slightly flexible.
Why have two layers of Hyson Cells ?
The first (bottom) layer gives basic strength to the structure, especially load bearing. Walls may slope as much as 60° , which would not otherwise be the case as they would not be sufficiently stable due to inadequate compaction.
The main purpose of the top layer is to serve as armour against potential abrasion. However its characteristics under load are the same as the bottom layer and the three layer "sandwich" will therefore behave as a composite. The intermediate membrane will therefore be protected against puncture and tearing.
Some details of construction
Product code 300/210 is generally recommended although code 400/280 could also be used. Each cell layer will be a minimum of 75 mm deep giving a minimum thickness of the composite of 150 mm.
Infill will be ready-mix concrete with minimum strength of a 10 MPa base with 20 MPa walls. Slump of concrete will be at least 100 mm and preferably higher. Air entrainment and the use of additives is advised to improve flow and to help control workability. A trial section should be constructed to ascertain ideal composition of the concrete and to gain the understanding and co-operation of the concrete supplier. Conventional concrete mixes are completely unsuitable.
The bottom cell layer
A special geocell is optionally but preferably used. This geocell has flaps attached at regular intervals.
It should be arranged with the geomembrane installer that he in turn weld flaps onto the liner. The flaps of the geomembrane can then be stitched onto the flaps protruding from the bottom Hyson Cells layer at 1 to 2m intervals. This will secure the geomembrane to the slope and give continuous support over the entire slope thus relieving the membrane of much stress.
Rigging the Hyson Cells on top of the liner
It is not permitted to puncture the geomembrane or impervious layer with the Y10 anchoring pegs. The terminating trenches for the anchor beams are therefore excavated outside the area covered by the geomembrane or impervious layer. (See diagram)
(a) Before the Hyson-Cells are laid, Y10 anchor pegs are driven into the perimeter anchoring trench at approximate 1 metre intervals. The pegs must be very secure as they will be carrying a load during the installation process. (an alternative is to secure the cells to a pipe buried in a trench to form a dead man anchor).
(b)"Additional rigging strings" are used as well as the integral internal rigging supplied ex factory.These "additional rigging strings" are secured to the perimeter anchor pegs in (a) above and led down the sloping wall of the embankment on top of the geomembrane liner but resting on it. These "additional rigging strings" therefore lie beneath the Hyson Cell mattress. They must bear tightly against the liner.
The rigging is secured to pegs in the anchor trench and to a sacrificial pipe at the foot of the slope. This "footer anchor pipe" will be buried in the concrete armouring layer.
The Hyson-Cells are expanded across the slope (not down it) and theintegralrigging strings are tied to theAdditional rigging stringsalready placed in (b) above. (If a geogrid net were used instead of the "additional rigging" then the integral strings would be tied to this net). This draws the Hyson Cell mat to bear tightly against the sloping geomembrane walls.
Hyson Cell mats will normally be manufactured to be of length to cover the entire fall of the slope in a single drop. Hyson Cells pack sizes may be as large as 600 m² and yet still be light enough to handle on site. A single pack could therefore cover 60 metres across a slope with a fall of 10 metres before a second pack has to be joined.
Filling the cells
Concrete for filling the cells is preferably delivered at the top of the embankment. The concrete is transported down the slope to the cells being filled by means of long plastic chutes (made from glass fibre sheeting) or grout pumps may be used.
After filling the floor filling of the cells on the walls takes place from the foot of the slope in the sequence shown in the diagram.
Because of the weight that must be supported by the rigging while the concrete is wet, the concrete must first be laid in a strip about 1 metre wide across the foot of the slope. This "footer slab" will be placed on top of the sacrificial "footer anchor pipe. This cell "footer slab" serves to support fill, which is subsequently placed higher up the slope. It also provides a platform for the ladders and the workers to stand on.
Once the initial 1 metre "footer slab" has been placed it is preferred to fill strips in the sequence indicated on the diagram, working progressively across the slope in vertical strips. There should be no construction joints down the fall of the slope, the entire fall being cast in one day. The construction joint between a day’s work and the "footer slab" should not be noticeable.
If work is not continuous then there will be construction joints between one day’s work and the next. It is important that the contractor uses a small plastic hand trowel to completely empty any cells at the end of the pour that may have only been partially filled. In this way the "boundary cells" for the following day’s pour will be completely empty and there need be no weak joint.
When the concrete has set the upper layer can be installed working in a similar manner
Special note on "look alike" geocells
Hyson Cells has a proven track record in the field. The Hyson Cell is our primary product, not a minor secondary one. We place priority on research, especially with concrete fill. It must be emphasised that our research data does NOT apply to products that are apparently similar but in fact have very different performance characteristics. Potential users are particularly warned that cells manufactured from woven or non-woven geotextiles or with rigid or near rigid walls do NOT function in the same manner as our smooth, thin-film walls do. Phone or email our office. We will gladly discuss the technical details and why apparent short cuts can lead to problems such as uncontrolled cracking of the concrete slab resulting in a torn membrane.