This method statement covers supply and installation of gabions along the top of cut slopes to divert and filter upland runoff where indicated on the drawings.
These will be installed ahead of major earthwork operations to minimize potential for washout of cut slopes both during the construction phase and permanently thereafter.
Such work will be in accordance with associated details from the design and drawings, and in accordance with project specifications.
This method statement describes the techniques and methods to be implemented and the resources to be utilized for the construction of Gabion Protection.
Earthworks required for the construction of temporary access roads and/ or other site facilities are dealt with in separate methods and procedures.
This Method Statement is a controlled document, and will be distributed as such to those detailed on the Internal Distribution of Documents Matrix, maintained by the Document Controller, while external distribution will be via the office secretary.
The materials for fabrication and filling of the Gabions will be according to the approved material technical submittals.
Other materials such as safety barricades, signaling etc. around the excavated areas, will be provided during the works, as per safety regulations.
All equipment to be deployed for the construction of the gabions (transportation of materials, assembly, filling etc.) will be in good working condition and well maintained throughout the duration of the gabion wall construction works.
Roles and Responsibilities
Project Manager is responsible for appoint site supervisory personnel to organize, coordinate, and oversee the execution of the work detailed in this Method Statement.
Construction Manager is responsible for the organization and coordination of the entire work detailed in this Method Statement.
The responsible Engineer is responsible for the execution of the works detailed in this Method Statement. He will be responsible for instructing the Foremen / Superintendent about the requirements of the contract, including all restrictions, safety regulations, scope of work and time schedule. In addition, he will be responsible for related logistic matters and all matters related to HSE and the quality of the works.
Superintendent/Foreman is responsible for the day-to-day implementation and supervision of the works detailed in this method statement.
Assigned HSE staff is responsible to assist the construction team to develop safe methods of working and to monitor implementation of the controls as required.
Gabion Construction Methodology / Procedure
All the activities will take place in a manner that will suit the natural ground conditions and will allow maximum deployment of equipment.
Auxiliary works such as temporary access roads, maneuvering plateaus, assembly and stockpile areas etc. will be also constructed and maintained in order to ensure easy, safe, unobstructed and well supervised execution of the works.
The dimensions of Gabions and Gabion Wall will be defined by the design.
Gabions can be filled either directly in their final position or at a suitable position and then transported to their final position.
Foundation and stockpile area preparation
Prior to commencement of dumping the filling material at a certain location, removal of obstacles will take place. The surveyor will give the line in which the gabion wall will be constructed.
The foundation on which the gabions will be placed will be cut or filled and graded to the lines and grades of the design and drawings.
Surface irregularities, loose material, vegetation, and all foreign matter will be removed from foundation surface area.
When fill is required, it will consist of materials conforming to the specified requirements.
Any unsuitable areas of the foundation will be excavated and replaced with the specified material.
Gabions will not be placed in their final position until the foundation preparation is completed and the surfaces have been inspected by the engineer or the engineer’s representative.
Inspection
Prior to assembly works, it will be ensured that the correct materials as specified are delivered to site.
Gabions that have been damaged during transportation or during storage on site will be repaired or disposed.
The foundation of the gabion structure will be inspected to be at the correct grades and levels as required by design.
A visual inspection of the gabions will be made to ensure the correct fixing of the gabion’s sides and during their installation the correct fixing between the adjacent cells.
All joints must be tight and correctly formed.
At each stage of filling, a visual inspection of the gabions will be made to ensure that no large voids are present.
Inspection will also take place in order to check that the overall line and level of the gabion wall is acceptable.
Periodic inspection and inspection after severe weather of flood occurrences will be carried out to ensure the performance of the structure and that no external damage has occurred to the mesh fabric which would affect the integrity of the structure.
Necessary work permits for construction activities will be obtained prior to commencement of the work.
Quality Control
All works will be undertaken in compliance with the quality requirements of the project quality manual.
All Gabions will be visually inspected and their position will be checked by surveyors.
Quality Assurance
Measurement Accuracy – Instrument Calibration
Measuring devices will be calibrated regularly in accordance of the established calibration and maintenance program, as described in the relevant data sheets and/or applicable method statement.
Health, Safety and Environment
The significant safety hazards have been identified and analyzed in accordance with the applicable HSE legal and contractual requirements as well as with the project hse plan.
Health and Safety Hazards
As required by the Project HSE Plan, the significant health and safety hazards of this work have been analyzed in accordance with the general procedure Hazard Analysis and Environmental Impact Assessment and are listed below.
Tripping hazards slip, trips and falls
Trapping injuries
Falling materials
Traffic accident
Defective plant and equipment
Collapse of loose materials at slopes
Personnel struck by equipment or vehicle
Equipment tipping
Equipment fall from embankment edges Controls will be implemented to minimize these hazards as set out in the form HSE Risk Assessment.
Protective and Safety Equipment
The Protective & Safety Equipment shall be in accordance with the specifications and the requirements of the contract. All personnel will be handed out these items during their site induction.
Safety Helmets
Safety glasses
Safety shoes
Coveralls
Work gloves
High visibility vests
Emergency Procedures
In case of emergency situations, everybody should follow the project emergency plan available in hse office.
Assembly, Installation and Filling of Gabions
Assembly and Forming the Structure
Each gabion mesh will be unfolded on a hard, flat surface. All kinks will be stretched and stamped out. It will be made sure that all creases are in the correct positions for forming the box, one at the edge of each end panel and each diaphragm.
The side and end panels will be folded into the upright position to form a rectangular box. The top corners of the box will be joined together with the thick selvedge wire which is sticking out from the corners of each panel. This will make sure that the tops of all four sides of the box are level.
Wiring up will begin by securing the binding wire at the top corners of the panels to be joined, by looping it through and twisting it together.
Then the wire will be laced around the selvedges with single loops and double loops in turn at 100 mm intervals. Finally, the wire will be fastened securely at the bottom corner, and the loose end will be poked inside the gabion. Next, the diaphragms will be lifted into the vertical position and will be wired up to the side panels in exactly the same way.
When possible, small groups of gabions will be wired together off the site, and then will be carried to the structure as complete sections for joining to other gabions already in position. The ground where the gabion structure is to be built will be leveled as much as possible before placing the empty gabions in position.
The gabions will be wired together, using exactly the same method as for assembling single gabions. The gabions will be placed front to front and back to back so that pairs of facing lids can later be wired down in one go.
The empty gabion or group of gabions will be placed in position. The end from which work is to start will be secured, through the two corners, into the ground and will be tied firmly to a good anchorage.
The remaining empty gabions will be wired one to another as work proceeds. The opposite side or end of the gabion will be stretched.
Top and bottom are then will be kept stretched until the gabion has been filled, by tying them to a fixed point. While the gabion is being stretched, it will be checked that the wiring together has been properly carried out and is not pulling apart. If it is, the section will be rewired. When on the structure, the gabions will be wired securely along the sides and ends to the adjoining gabions and to the already filled gabions below.
Filling The Gabions
The filling material to be used will not be larger than 200 mm and not smaller than the size of the mesh.
Gabions will be filled by hand or by machine, making sure that the stone is tightly packed and has a minimum of voids.
When possible, the last gabion will left empty, as it is easier to join the next one to it.
All the outside compartments of 1 m high gabions will be filled only up to one third full.
Then horizontal bracing wires will be fixed in the gabions directly above the level of the stone in those compartments and the bracing wires will be tied to keep the face even and free from bulging.
Next, the gabion will be filled up to two thirds and the bracing will be repeated.
Finally it will be filled to the top. 500 mm gabions will be filled in 2 ‘lifts’ with bracing at half full. In order to allow for settlement the fill will be leveled off 25 mm to 50 mm above the top of the mesh.
The lids will be tightly stretched over the filling and will be wired down.
Corners will be temporarily secured first, to ensure that there is enough mesh to cover the whole area.
Some filling may have to be removed from the top of the gabion to prevent the lid from being overstretched.
