Method Statement of Steel Structure Foundations for Overhead Transmission Lines
The Method Statement of Steel Structure Foundations for Overhead Transmission Lines is a comprehensive guide that outlines the proper procedure for constructing strong and stable foundations for steel structures used in overhead transmission lines. It covers everything from the initial design stage to the actual installation process, providing detailed instructions on each step to ensure safety and efficiency. The method statement includes information on the materials and equipment needed, as well as quality control measures and safety precautions to be taken throughout the project. Following this method statement is crucial for the successful construction of steel structure foundations for overhead transmission lines.
1.0 Scope
This standard provides general recommendations for selecting methods and procedures that have been found practical for installing steel structure foundations with stub angles and anchor bolts for the overhead transmission line system.
Table of Contents
2.0 General Requirements for Foundations
2.1 It shall be the responsibility of the contractor to locate underground facilities such as gas, water, sewer lines, drain lines, electric power, and telephone cables, etc., which may be encountered in digging or drilling holes and to perform work in such a manner as to prevent damage to such facilities.
In case of any damage, the contractor shall immediately repair all such facilities to the satisfaction of the facility owners. If the facilities cannot be repaired immediately, the facility owner shall be advised when repairs will be completed.
The contractor shall inform the client representative as soon as possible after the repair works are completed, but no more than one week after any damage to such facilities occurs.
When the damage is completely repaired, the contractor shall secure and submit to the client representative one copy of a written release, signed by the contractor and the facility owners suffering the damages, within 30 days after completion of the repair work.
2.2 The footings for tangent structures shall be placed so that the longitudinal axis of the structure cross-arms will lie in a plane perpendicular to the centerline of the transmission line. The footings for angle structures, except the terminal structures, shall be placed so that the cross-arms will lie in a plane bisecting the interior angle formed by the two intersecting centerlines.
The footings for terminal dead-end structures shall be placed so that the transverse axis of the structure cross-arms shall be parallel to the transverse axis of the gantry structures placed in the substation.
If heavy angle structures are used as terminal dead-ends, their footings shall be placed the same way as that for the angle structures.
2.3 The center of the top of each drilled pier shall not vary from its designed center by more than four (4) percent of the pier diameter, and the pier shall be plumbed within one (1) percent of the total depth.
The centerline of the completed foundations shall be centered within 30 mm of the specified location across the line and 150 mm along the line.
2.4 The transverse axis of the structure foundations shall not deviate from the bisector of the interior angle of the structure by more than 30 minutes of the arc due to the rotation of the entire foundation.
2.5 Excavation of the structure foundations shall be to the elevations and dimensions indicated on the approved drawings or as required to permit construction of the foundations, erection of forms, and inspection of foundations.
2.6 The earth, rock, stumps, and all other materials encountered shall be excavated as required for the construction of foundations in accordance with the approved drawings. All excavated materials suitable for backfill shall be placed in a graded embank in the immediate area of the structure. Materials found to be unsuitable for backfill or grading shall be disposed of as directed by a client representative.
2.7 For excavation where the base is unstable or lies below the groundwater level, the water table shall be lowered, and a layer of crushed stones or selected backfill or borrow shall be placed to stabilize the base before the placement of the materials.
2.8 Foundations in the earth shall be excavated to clean, level surfaces of undisturbed material of adequate bearing value. The client representative shall approve the quality of the soil and adequacy of its bearing value before the placement of the foundation.
Materials unsuitable for sustaining the design loads shall be removed and disposed of as directed by the client representative.
Where loose rock or boulders of a size equal to or less than the foundation diameter extends above the design elevation of the footing base, such loose rock or boulders shall be removed to a minimum depth of 150 mm below the footing base, and the resulting depression shall be considered and treated as new footing base.
Where over-excavation occurs, the limit of such over-excavation shall be considered and treated as the new limits of the footing.
2.9 Foundations shall be placed as soon as practicable after excavation, and all excavations shall be maintained in a safe, clean, and sound condition up to the time of placement of footings.
Whenever necessary, all sand, mud, silt, and other objectionable material, which may accumulate in the excavation, shall be excavated before the footings are placed. All excavations shall be covered with strong covers or fenced as required until reinforcement and concrete have been placed. The bracing, sheeting, and shoring shall be installed as required for safety. All excavations shall be kept dry by pumping or draining as required.
3.0 Structure Pads
3.1 Prior to excavation or filling of structure pads, each structure site shall be verified based on plan and profile drawings to determine the structure centerline ground elevations. The top elevation of the concrete footings shall not be less than 450 mm above the finished grade of the structure pads.
3.2 The structure centerline and location shall be established after the original ground has been scarified and prior to filling of the sub-base so that a maximum distance of 15 meters shall be maintained from any point of the structure foundations to the top edge of the structure pad.
At locations where a minimum distance of 15 meters for the structure pad is not met, the client representative shall decide on the minimum requirements. The structure pad shall be graded with a 1.5% to 2% slope to drain water away from the structure foundations. Structure pads shall be constructed in accordance with the requirements.
4.0 Soil Pier Foundations
4.1 Soil pier holes shall be drilled with suitable types of drilling equipment to produce the required level of excavation. The methods and equipment used shall be such as to leave the sides of the excavation free from appreciable quantities of loose material, which would prevent intimate contact of concrete with firm soil and/or rock.
The bottom of the excavation shall be clean so that the end bearing of the footings will be on firm soil. The excavation shall be protected with a temporary casing of suitable thickness maintained at the height of 500 mm above the existing ground line.
4.2 Temporary casings shall be required at all locations of the high water table and layers of fine sand and where workmen are required to work at the bottom of the excavation. Casings shall also be required at locations where the soil will not stand alone without support or where sloughing of the sides may endanger the satisfactory completion of the pier.
Casings shall be of sufficient strength and rigidity to maintain the required excavation lines against the earth and water pressures that may be encountered and shall be installed carefully to ensure that soil around the casing is not disturbed.
The method of holding the casing in the pier hole shall be such that it will not allow the casing to sink into the ground. If casings are removed from the excavation, they shall be removed in a manner that shall not adversely affect the quality of concrete, disturb the surrounding soil, or reduce the amount of rebar cover.
4.3 When the casing top is at the proper elevation, and the vibrator-driving hammer is removed, the auger/digger shall be centered over the hole, and the soil in the casing shall be removed.
After the vibrator is positioned, the casing shall be checked for plumbness by using a transit on one side and at 90º to the first position check. The casing shall be driven until the top is about 500 mm above the structure pad.
If conditions permit and the steel casing is to be removed, the concrete shall be poured up to a construction joint. The steel casing shall be removed concurrently with the placement of concrete. However, in sandy areas, the casing may be left in place if this is considered in the design. In such cases, the exposed portion 500mm below the finished grade level of the structure pad shall be coated with two (2) coats of coal tar epoxy.
4.4 In uncased foundations, where temporary casing used in excavation and concreting has to be removed, the following requirements shall be completed before the removal of the casing:
4.4.1 Casing shall be fully plumbed in two position checks at 90º to each other.
4.4.2 It shall be ensured that concrete inside the temporary casing is sufficient to fill level up to the construction joint and the required concrete cover is maintained before removal of casing and tremie used in concreting.
4.5 If the pier does not require a steel casing or liner, the digger operator can set up the center of the cutting blades of the auger/digger so it is centered over the nail or tack of the foundation center hub and after checking that it is in plumb position both ways on the Kelly bar, digging can begin.
If the soil is satisfactory and no caving occurs, the hole can be completed to the required depth, and the other holes can be completed in a similar manner.
The excavated soil shall be kept away from around the holes to permit working and shoring and keep the soil from slipping back into the hole.
After completion of the hole, the steel-reinforcing cage shall be placed carefully. If a construction joint is to be provided, the concrete shall be poured and stopped at the level indicated in the approved drawings. The construction joint key shall be installed as called for in the approved drawings.
5.0 Rock Pier Foundations
5.1 The procedures and methods for installing rock pier foundations are the same as for soil pier foundations, except that they are partly embedded in the soft rock. They can be of the uncased or cased type above the rock strata. The approved drawings and the structure list indicate the type, size, and depth.
5.2 When soft rock with Rock Quality Designation (RQD) of 26 to 50 and ultimate core strength of 1.4kPa to 2.8kPa is encountered at a depth of three (3) meters or less, the foundations shall be soil pier type. Rock pier-type foundations shall be of bell-bottom or straight shaft piers type and shall be designed and constructed to resist the required design loads.
6.0 Rock Anchor Foundations
6.1 Where sound hard rock is encountered at a depth of three (3) meters or less, the hard rock shall be drilled, and the reinforcing bars (or tendons) shall be grouted into the rock. All holes shall be protected as soon as drilled with plugs at the collars of the holes to prevent the entrance of sand, silt, and other objectionable materials. The bottom of the drilled hole shall be at least 100 mm lower than the tip of the tendons. The minimum center-to-center distance between rock anchor holes shall be 300 mm.
Prior to grouting, each grout hole, including the area immediately around the collar of the hole, shall be thoroughly cleaned by air or other satisfactory means to provide clean contact surfaces against which to place the grout. Water entering the excavation shall be removed by pumping from a sump provided at the bottom to keep the excavation reasonably dry during the grouting operations.
The grout hole shall be completely filled with grout prior to inserting the rebars. The grout forced out from the hole by inserting the reinforcing dowel shall be left around the dowel (tendon) to form a raised crown above the adjacent rock surface.
6.2 If entry of groundwater prevents the complete dewatering of a grout hole, the grout shall be pumped through a small diameter pipe lowered to the bottom of the hole. Grout shall be pumped slowly into the hole until all water is displaced and clean grout overflows the hole.
6.3 After insertion, the reinforcing dowel (tendon) shall be subjected to vibration to ensure intimate contact with the grout. After the dowel (tendon) has been properly positioned in the freshly placed grout, special care shall be taken to prevent it from being moved out of position or otherwise disturbed by subsequent operations. A centering device shall be used to ensure the rod (tendon) is centered in the hole and completely surrounded by the grout.
7.0 Spread Foundations
7.1 Spread foundations have large rectangular or square plan dimensions. These are placed in shallow excavations and are then backfilled. One foundation is commonly used for each structure leg. Spread foundations typically consist of a buried rectangular or square pad with a leg stub or column connecting the foundation to the tower body.
The typical foundation depth–to-width ratio is between 1 and 3, with the maximum depth often limited to 4-6 m because of construction equipment limitations. The foundation usually is set horizontally, with a leg stub battered to the same slope as the tower legs. Steel, concrete, or a combination of both are usually used for the foundation.
7.2 The tower foundation construction survey shall establish the foundation center hub, reference hubs, elevations, and the required depth of excavation. Before excavation, the tower foundations shall be marked (staked), and the depth of excavation shall be computed.
Ground staking shall include establishing a reference point (RP) hub to the pit center (PC) for each foundation. The elevation of the RP hub shall be established, and the depth of cut from this hub shall be computed. This hub shall be used during excavation to control the depth of excavation. During the staking process, a PC stake and depth of cut at the PC shall be established. The four corners of large excavations shall also be staked according to the foundation excavation dimensions.
7.3 The equipment and techniques to be used for excavation shall depend on the type of material encountered at the excavation site. When soil, loose or fractured rock, boulders, or any combinations thereof are encountered, the excavation shall be done with a track-mounted or rubber-tired backhoe. When the terrain is steep, and the backhoe equipment can not be used, other specialized digging machines shall be used.
Drilling and blasting may be required whenever machine digging alone cannot proceed because of the excavated material’s hardness. Rock-drilling machines shall be used to drill holes for blasting. If rock is encountered during hand excavation on steep hillsides, small portable compressors, and jackhammers shall be used to loosen the rock material. Excavated material shall be removed from hand–dug excavations using a bucket and rope.
7.4 High water table shall require dewatering of the excavations. Depending on the site-specific conditions, open pumping, cutoff trenches, or drainage with wells shall be required to remove the water. If water continues to run into or seep in from the walls or bottom of the excavation after the initial dewatering, a sump hole shall be dug at one or more corners of the foundation bottom, and small portable suction pumps shall be used in these pumping points to keep the excavation dry during foundation installation.
7.5 After excavation, the stability of the foundation bottom shall be checked to ensure adequate bearing capacity. If soil conditions exist that lead to inadequate load-bearing capacity caused by water or poor soil properties, additional excavation below the foundation, to a depth of 230-1200mm (depending on the subgrade soil), shall be required to remove the soils.
The excavation shall be backfilled with select soil or rock materials to improve the bearing capacity of the foundation bottom. After compaction of such material and subgrade preparation, the excavation shall be made ready for foundation placement.
Care shall be taken to avoid saturation of the foundation bottom during periods of heavy rains. The excavation shall be kept free of water whenever the water table is at or above foundation bottom elevations. The recommendations of the Soil Investigation Agency shall be followed for poor soil conditions.
7.6 After all the foundations have been excavated and their subgrade elevations have been established to be within allowable tolerances, the excavation with the highest elevation shall be set first. The remaining foundations shall be set using the first foundation as a reference for elevation and angular placement. For the setting of foundations, the position of the tower center hub for the alignment and the line angle, if any, shall be checked. Templates shall be used to ensure correct setting dimensions.
7.7 The following major items shall be considered for the placement of foundations. Generally, these items shall be considered in order:
- Batter
- Working Point Elevation
- Back to Back (Transverse) Distance
- Diagonal Distance
- Twist
In case the distance between the center of the tower and the center of the foundation at the bottom of the excavation is established, the foundation shall be set to this distance first, and then the above items shall be considered in order.
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8.0 Reinforcing Steel Cage
8.1 The steel reinforcing bar cage shall either be assembled at the storage yard and hauled to the job site, or the proper steel bars be hauled and assembled at the job site. Both of these methods shall be acceptable and satisfactory. As the approved construction drawings show, the vertical reinforcing bars and ties shall be spaced and assembled.
8.2 The reinforcing bar cage shall be properly located and held in position using concrete blocks of proper dimensions and wired into place at about 75 mm to 100 mm intervals outside the cage. Concrete blocks shall have the same compressive strength as concrete to be used for the foundation.
They shall be placed in several locations around the cage to position the cage in the center of the hole. The reinforcing steel shall be bonded to the stub angle/anchor bolts by using a suitable size of steel wire in accordance with the procedures.
8.3 Steel reinforcement, before being positioned, shall be thoroughly cleaned of mill scales and all coatings that may destroy or reduce the bonding. Loosemil scale, dirt, grease, and heavy flaky rust shall be removed and cleaned.
Reinforcement bars (rebars) shall be carefully formed to the dimensions indicated on the approved construction drawings.
All rebars shall be bent cold before being placed in the forms. Rebars with kinks and bends shall not be used. No heating of rebars shall be permitted.
Rebars shall be accurately positioned and secured against displacement by using annealed iron wires of suitable size or clips at all intersections and shall be supported by concrete or metal chairs or spacers or by metal hangers. Vertical column bars shall be rigidly tied to binders at every intersection.
8.4 Pier cages shall be made, as the approved construction drawings describe. No visible deformation of the cage shall be allowed. Splicing of vertical bars shall not be permissible unless shown in the approved drawings.
The cages shall be accurately positioned and secured against displacement during the placement of concrete.
The method of securing pier cages shall be such that it will not allow the reinforcement to sink into the ground.
The tops of the reinforcement cages in piers shall neither extend more than 12 mm above the specified elevation nor more than 25mm below this elevation.
The center of the cage at the top shall not deviate horizontally by more than 12 mm from the specified center of the pier.
Fabrication and bending, where required, shall be in accordance with the approved drawings and within the tolerances specified in the latest edition of ACI-315, “Details and Detailing of Concrete Reinforcement.”
9.0 Stub Angles
9.1 Stub angles shall be placed in the tower footings as shown on the approved drawings and shall be supported in proper position by means of a rigid frame (setting template) or an equivalent device.
The stub angles shall be held rigidly in a manner to prevent displacement during the placing of the concrete. All stub angles shall be set accurately to the grade and alignment indicated on the approved drawings and within the following limits:
9.1.1 The center (punch mark) of the four stub angles of a tower shall be within 30mm of its theoretical position across the line and within 150 mm along the line.
9.1.2 The faces of each stub angle shall not deviate from the corresponding faces of the tower by more than one part in 300 horizontally, and the batter of each face of the stub angle shall be within 5mm per meter of the specified batter in part exposed.
9.1.3 The relative elevations between the punch marks on the heel of the stub angles for each set of the tower footings shall not differ by more than 6 mm.
9.1.4 The actual elevation of any set of the stub angles shall be within 60mm of the specified elevations.
9.1.5 The actual horizontal distance between adjacent stub angles of a tower shall be within 6mm of the specified distance (back to back), and the distance between the diagonally opposite stub angles shall be within 12mm of the specified distance (diagonal distance).
9.1.6 The transverse axis of each angle tower shall not deviate from the bisector of the interior line angle at that tower by more than 30 minutes of arc.
9.2 The stub angles and the setting template shall be assembled together into place to obtain the proper setting location for each stub angle.
The stub angle setting template shall be furnished as part of the steel furnished by the steel supplier or shall be fabricated separately for the intended purpose. The templates shall be adjustable for different spacing and batter as required for the tower type under construction.
9.3 The concrete shall be poured after all the adjustments and checks have been made as required. The top surface shall be crowned and given a trowel finish. After three days (72 hours) of curing, the templates and the forms may be removed.
Waterproof elastomeric coatings and coal tar epoxy coatings shall be applied as shown in the approved drawings and/or Scope of Work &Technical Specifications.
10.0 Anchor Bolts
10.1 Anchor bolts shall be placed in the steel pole footings as shown on the approved drawings and supported in proper position by a rigid template.
The anchor bolts and the template shall be held rigidly in a manner to prevent displacement during concrete pouring. If required, anchor bolts may be tack-welded with the foundation reinforcing bars to avoid their displacement.
The vertical reinforcing bars and the anchor bolts shall be bonded electrically. Anchor bolts shall be set accurately to the grade and alignment indicated on the drawings and within the following limits:
10.1.1 The center of the circle of the anchor bolts shall be within 25mm of its theoretical position.
10.1.2 The angular variations in the specified location of the anchor bolt group shall not exceed one (1) degree for a single pole.
10.1.3 The actual elevation of any set of anchor bolts shall be within 25mm of the specified elevation.
10.2 The anchor bolts and the setting template shall be assembled together into place to obtain the proper setting location for each anchor bolt. The anchor bolt-setting template shall be furnished as part of the steel poles furnished by the manufacturer or fabricated separately for the purpose.
The location of each anchor bolt shall be accurately set by measuring from the top of the bolt to the required length of the anchor bolt to be exposed at the top of the concrete.
10.3 The concrete shall be poured after all the adjustments and checks have been made as required. The top surface shall be crowned and given a trowel finish.
After three days (72 hours) of curing, the templates and the forms may be removed. The approved drawings show that waterproof elastomeric coatings and coal tar epoxy coatings shall be applied.
11.0 Concrete
11.1 General
The measuring, mixing, and placing of concrete shall conform to the latest requirements ACI 318 and ACI 304.
Concrete shall have a minimum 28-day compressive strength based on the degree of exposure required.
11.2 Preparation for Concrete Placement
11.2.1 Prior to placement of concrete, excavations and formed areas for the footings shall be cleaned. All water at the bottom of excavations shall be removed or absorbed.
Hoppers and elephant trunks shall be used to direct concrete flow down the center. For initial pours, sacked cement shall be used at the bottom of the pumped-out excavations. Concrete shall not be placed until the client representative inspects the excavations and/or forms and embedded items.
The holes shall be kept in acceptable condition until the placement of concrete is completed. The placing of concrete shall be accomplished in such a manner as to prevent segregation of the aggregates.
Concrete shall be consolidated by using vibrators, with just sufficient vibration being done to ensure the compaction of the concrete into a dense homogeneous mass without honeycombs.
Where concrete for the pier is deposited against formed surfaces or earth, the contact surfaces shall be wetted prior to the placement of the concrete unless the surface treatment of the forms or moisture in the earth is sufficient to make this requirement unnecessary.
11.2.2 If the amount of groundwater present will result in the concrete of unacceptable quality, tremie or other pumping methods shall be used for placing the concrete per ACI 304R, “Guide for Measuring, Mixing, Transporting and Placing Concrete.”
11.3 Placement of Concrete
11.3.1 Except as shown in the approved drawings, construction joints in foundations shall be avoided. The use of such joints shall require approval from the client representative. In cases where such joints are permitted, the first lift to be placed as soon as practicable after excavation is completed is to be made to an elevation approximately 150 mm below the bottom of the stub angle.
The second lift shall then be placed to meet the requirements indicated on the drawings.
Joints not indicated on the drawings shall be designed and located least to impair the strength and appearance of the structure and shall be approved by the client’s representative.
The surface of the previously placed concrete shall be thoroughly cleaned of all laitance and the foreign matter before placing the next lift of concrete.
Prior to placing the next lift of concrete, the surface of the poured joint shall be covered with a brushed coat of grout.
The grout shall be introduced into the holes or formed area in the space between the reinforcement cage and the sides of the excavation or forms, and the grout brushed over the concrete surfaces to ensure thorough coverage, particularly between the bars of the reinforcement cage and the sides of excavation or forms.
The whole of a showing face between prescribed construction joints shall be cast in one continuous operation.
Concrete that is to have an exposed surface, whether any particular finish is called for or not, shall be placed and worked as may be necessary to secure, at the face, a uniform distribution of the aggregates, freedom from void spaces, and uniform texture.
For exposed surfaces, the forms shall be smooth and watertight. The top of the foundation shall be sloped to prevent the accumulation of water.
The slope shall start at the heel of the stub angles and shall have a minimum rise of 6 mm in 300mm to form a level plane at the edge of the foundation.
At the periphery, an edging tool shall be used to form a bevel at the edge, approximately 12mm.
Exposed uniform surfaces shall be wood float finished.
11.3.2 During hot weather or under conditions contributing to the rapid setting of concrete, the mixing and delivery time shall not exceed the following limits:
a. Air temperature below 29ºC 90 minutes
b. Air temperature above 29ºC but less than 32ºC 75 minutesc. Air temperature above 32ºC 60 minutes.
All concrete that has not been placed within 90 minutes after all the ingredients have been introduced into the mixer shall be rejected. The temperature of the mixed concrete shall not exceed 32ºC.
11.4 Curing and Testing
11.4.1 All exposed surfaces shall be cured for a period of at least seven (7)days.
Curing shall be accomplished by continuously applying water of similar quality to that used for mixing through ponding, sprays, or saturated cover materials such as burlap or cotton mats. If saturated cover materials are used, they shall not be allowed to dry out.
11.4.2 Concrete cylinders shall be made under the supervision of a client representative, and testing shall be made by an approved Independent Testing Laboratory in accordance with ASTM C39, ‘Standard Test Method for Compressive Strength of CylindricalConcrete Specimens”.
Test cylinders shall be kept in a similar environment to the foundation concrete until testing.
11.4.3 Sixteen (16) concrete cylinders shall be made (four per tower leg) for every strain tower. Four (4) concrete cylinders shall be made for every tangent tower foundation for the first thirty (30) tangent towers.
One additional cylinder for each tower shall be taken for the chloride permeability test.
If the cylinders pass the following evaluation and acceptance criteria, the requirement may be reduced to four (4) cylinders for every fourth tangent tower.
Whenever the sample fails to meet the evaluation and acceptance criteria, the cycle shall begin again with four (4) cylinders taken for each of the next thirty (30)tangent towers.
11.4.4 One set of four (4) cylinders shall be made for every angle pole and one set of four (4) cylinders for every tangent steel mono-pole foundation for the first thirty (30) tangent monopole structures. One additional cylinder for each tower shall be taken for the chloride permeability test.
If the cylinders pass the evaluation and acceptance criteria specified in clause 11.4.6, the requirement may be reduced to four (4) cylinders for every fourth tangent monopole.
Whenever the sample fails to meet the evaluation and acceptance criteria, the cycle shall begin again with four (4) cylinders taken for each of the next thirty (30) tangent steel monopoles.
11.4.5 Client representative may require additional cylinders to be taken whenever there is a just cause, such as a change in batching procedures, change in concrete materials, breaking of batching, etc.
11.4.6 Concrete shall be considered acceptable if the following criteria are met:
a. The average of any three (3) consecutive strength tests shall be equal to or greater than the specified compressive strength.
b. No individual strength test (average of two cylinders) shall be less than the specified compressive strength by more than 3.50MPa.
11.4.7 In the event that the concrete fails to meet the strength requirements,in-place testing for concrete strength shall be conducted under the supervision of a client representative. In-place testing shall be conducted by one or a combination of the following methods:
a. ASTM C42, “Method of Obtaining and Testing Drilled Cores and Sawed Beams of Concrete”
b. ASTM C803, Test Method for Penetration Resistance of Hardened Concrete”
c. ASTM C805, Test Method for Rebound Number of Hardened Concrete”
11.5 Protection of Concrete
11.5.1 On above-grade surfaces, a single component, liquid applied, moisture cure, polyurethane waterproofing membrane with the following properties shall be applied in accordance with the Scope of Work & Technical Specifications:
a. Chemically resistant to alkalies, acids, chlorides, and sulfates and can withstand service temperatures up to 90 °C.
b. Priming Coat and Subsequent Coats (except Top Coats), Aromatic formulation, 700 microns DFT (Dry Film Thickness):
- Color Black
- Modulus at break 3 MPa
- Tear resistance 25 kN/m
- Ultimate elongation 400%
- Tensile set recovery 95%
- Adhesion to mortar 3000 N/m
- Shore “A” hardness 45
- Permeability to water vapor 0.4 metric perms
- Re-coatability Excellent
c. Topcoats, aliphatic formulation, 300 microns DFT:
- Color White
- Ultimate elongation 130%
- Tensile strength 22 Mpa
- Shore “A” hardness 85 ± 5
- Tear resistance 17.5 kN/m
- Total solids 60%(by weight & volume)
11.5.2 On below-grade surfaces, a two-component, high-build, high-solids (at least 90%) coal tar and epoxy resin with the following properties shall be applied in accordance with the Scope of Work & Technical Specifications:
- Bond strength to concrete 3 Mpa
- Bond strength to steel (sandblast prepared) 7.5 Mpa
- Heat deflection temperature 60% or above(as per ASTM D468)
- Potential health hazard None (during application and in-service)
12.0 Backfill
12.1 Excavated material that is suitable for backfill around the foundations shall be stockpiled separately for use as a backfill. The stockpiles of backfill material shall be sloped to drain water and shall be protected from other elements, which may render the material unsuitable for backfill. A client representative shall approve the quality of backfill material for each foundation.
12.2 Backfill shall be placed in layers not greater than 200 mm before compaction. Each layer shall be thoroughly compacted before the next lift is placed. Steel-wheeled roller compactors shall be used on cohesive materials; vibratory compactors shall be used on non-cohesive materials.
Each layer of non-cohesive material shall be thoroughly compacted to 85% of the relative density at optimum moisture content. Relative density for non-cohesive compacted material shall be determined in accordance with ASTM D4253. Each layer of marl or cohesive materials shall be compacted to 95% modified proctor density.
The maximum density for marl and other cohesive compacted materials shall be determined in accordance with ASTM D1557 modified proctor densities. The terms “maximum density” and “optimum moisture content” shall be defined in ASTM D1557.
When the backfill material is too dry for proper compaction, water shall be applied to obtain a relatively uniform moisture content throughout the backfill. Stone and rock fragments may be used in the backfill, provided they do not interfere with proper compaction.
Rock particles larger than 100 mm shall not be in contact with the concrete.
12.3 All locations that settle below the surface of the surrounding ground shall be refilled. The final grade after backfill shall be sloped and ditched to direct drainage away from the foundations.
13.0 Sand Stabilization
13.1 At structure locations where, as determined by client representatives, a potential for shifting sand exists, the sand shall be stabilized with crude oil. The crude oil shall be applied with metered spray nozzle.
An average of 4.5 liters of crude oil shall be applied to stabilize every square meter of sand. The amount of crude oil per square meter shall be increased if the minimum oil penetration is less than 13mm. The composition of crude oil shall be as approved by the client representative.
13.2 The area of sand to be stabilized shall be determined on a structure-by-structure basis by the client representative, but in all cases, the minimum amount of area to be stabilized shall consist of an area within a minimum distance of three (3) meters or more from the edge of the structure pad.
Based on actual site conditions, the client representative shall decide whether a minimum distance of three (3) meters is sufficient for sand stabilization or needs to be increased. Sand stabilization shall also be done on opposite sides of the access roads as necessary.
14.0 Surface Excavation
14.1 For erosion control, grading shall be performed to produce a more uniform or level surface at structure sites and other locations. Excavation in earth and rock shall be made at approved areas on the site, and the resulting materials wasted or placed in embankments at designated locations.
14.2 Areas to be filled shall be stripped of topsoil and organic material before forming embankments thereon. Stripped materials shall not be incorporated in embankments except when permitted at topsoil. Material not permitted shall be wasted. Fill materials shall be placed in 200mm layers and compacted.
15.0 Erosion Control
15.1 The slopes adjacent to structures, which may be washed or otherwise eroded, shall be protected. Where erosion control is required, no slope or excavation shall be steeper than one and a half (1 1/2)) horizontal to one (1) vertical without erosion protection.
Structure site slopes shall have contour trenches for each 2-meter change in elevation, constructed by hand 0.4 meters deep. The trenches shall drain to the sides of the cleared area, i.e., the area shall be high in the middle and slope down each side at about 2% from the high point. Where structure sites are so located that trenching is inadequate to carry water from the cleared area, drains shall be installed to carry water away from the area. A client representative shall approve such drains and their locations.
15.2 All foundations located in wadi areas shall be designed to resist a two (2) meter flood running at a velocity of 20km per hour.
15.3 In wadi areas, where a water level of 300mm or more above the existing bed may be expected, concrete barriers shall be installed to protect the structure pads against erosion. A client representative shall approve the design of the concrete barriers. Erosion control shall be completed at each structure site as soon as practicable after installation of the structure. If required, the concrete barriers shall be made of concrete slabs.
15.4 When transmission line structures are located on sloping terrain in hilly areas, gravel blanket/retaining walls may be required for the erosion protection of foundations/structure pads against water currents. Gravel blankets shall be placed in such a way that they do not flow away with water current. The client representative shall approve the placement of a gravel blanket or the construction of a retaining wall.
The gravel shall contain stones not larger than 610 mm, be reasonably clean and free from other foreign matter, and be distributed and graded evenly over the required areas. No compaction shall be required.
15.5 Riprap shall be placed on the slopes of the structure pads and in such places along the right-of-way or access roads, which, in the client representative’s opinion, is required to control erosion.
15.6 Riprap shall be durable stones or broken concrete without projecting reinforcement bars. Riprap shall be well graded and shall be individually placed in a manner that larger pieces are uniformly distributed, and the smaller pieces are filled in the spaces between the larger pieces to produce a compact uniform layer of Riprap at least 300mm thick.
15.7 Where, in the opinion of the client representative, gabions are required for the protection of foundations against erosion, these shall be installed as per the following specifications:
15.7.1 The box gabion shall be mesh type 8 x 10 with 27 mm diameter wire diaphragms. The sizes of boxes shall be 1.5m x 1.0m x 1.0m,2.0m x 1.0m x 1.0m, and 4.0m x 1.0m x 1.0m as required.
15.7.2 The box gabion shall be a rectangular basket fabricated from a double twist, a hexagonal mesh of soft, annealed, heavily galvanized wire; It shall be filled with rounded river or quarried stone of suitable size.
15.7.3 All the edges of the main base and end panels shall be reinforced with galvanized wire of greater diameter. The selvages wires and strengthening basket facilitate its assembly and assist in keeping it square.
15.7.4 Where there is more than one course of gabions, the ones in the upper course shall be securely laced to those below.
15.7.5 The stone must be hard to withstand abrasion, non-friable, and resistant to weather. Its packing inside the compartment shall be as tight as practicable.15.7.6 After filling the gabions slightly over-full, the lid shall be laced down with binding wire to the tops of all sides and diaphragm panels to allow for subsequent settlement.
15.7.7 The steel wire shall be in accordance with the Specifications Soft and shall be hot-dip galvanized.
16.0 Quality Assurance
The client representative shall inspect both materials and workmanship at each stage of the work, focusing attention on such items as:
Establishment of the correct location for the structure (span length, radial distance, elevation, etc.)
Type of structure, height of basic body and leg/body extensions, etc.
Suitability of location for the proposed type of foundation
Suitability of excavated material for backfill
Excavation depth
The setting of templates and placement of stub angles/anchor bolts
Distance between adjacent and diagonally opposite stub angles
Stub angle slope/batter
Quality of concrete
Sand stabilization
Typical Proforma for keeping the record of monitoring, inspection, and installation of steel structure foundations.
This Proforma shall be signed by the contractor as well as by the client representative.
.tag: Method Statement of Steel Structure Foundations for Overhead Transmission Lines
