Method Statement for Construction of Manhole Chambers and Bases

What is the Method Statement for Construction of Manhole?

This Method Statement for Construction of Manhole Chambers, Bases, and Appurtenances is the systematic procedure for safely performing the detailed installation of precast manholes and appurtenances ensuring that quality control objectives are maintained.

This method statement also covers, in brief, the installation procedure with respect to approved shop drawings, design drawings, and manufacturer recommendations, and in accordance with project specifications.

Execution is not limited to the following scope of works and varies as the construction of the manhole advances to the finishing stage which includes:

• Survey
• Trench Excavation
• Installation of Permeable Geo-textile
• Installation of Underground HDPE Perforated Pipes
• Construction of Manhole Chambers and Bases
• Installation of Manhole
• Benching
• Backfilling

The method statement provides guidance for assessing the activities i.e. transportation, handling storage, site preparation, installation, and inspection. This guidance is based on the requirements set forth in the HSE Plan and the Quality Assurance Program (QAP).

This method statement will be implemented in conjunction with the related manhole construction methodology. Wherein example of these is the typical section for drainage and manhole construction as per the approved shop drawings.

The Sequence of Works for the Method Statement for the Construction of Manhole

Prior to installation, it must be ensured that:

All relevant documentation such as Approved Shop drawings i.e Underground Manhole Construction Details, Method Statements, ITPs, and Checklist.

Method Statement for Construction of Manhole Chambers, Bases, Appurtenances, etc., and the material applicable to a particular section of work will be checked by the Site engineer prior to the commencement of work ensuring that these are of the correct type as reviewed by the Engineer.

Approval of the manhole production schedule will be submitted and obtained.

All necessary documents, such as a work permit and other relevant QC inspections to be carried out, shall be obtained prior to the commencement of the concrete manhole works.

Before the commencement of installation of the soak-away network, a Material inspection of the soakaway system will be carried out prior to being withdrawn from the storage area i.e. quantity, physical damage, and brand/make.

Before the beginning of the installation, it will be ensured that the approved valid Permit To Work (PTW) is available on site.

The engineer in charge will ensure that approval and standard tools and tackles are available to carry out the work.

Safe access to work areas will be provided and will be approved by the competent safety person prior to the commencement of work if applicable.

Check the work area tidy and remove flammable materials if any and a safe environment will be ensured.

Where the Risk Assessment identifies high-risk works a permit to work will be obtained from the safety department.

The HSE Manager and the Safety Supervisor shall carry out safety site inspections to ensure that the safety specifications set out in the Safety Management Plan are complied with during the execution of precast concrete manhole work.

Persons engaged in precast concrete manhole work shall be supplied with and used with the requisite PPE and as a standard, each person shall wear safety helmets and safety footwear, etc. The requirement for personal protective equipment shall be approved by the client’s representative

Competent manpower and builders will be employed to carry out the installation work.

Necessary civil clearance will be obtained (if applicable) prior to the start of installation.

For loading and unloading operations, the proposed crane location is to be ready with a compacted floor for the safe positioning of crane and boom trucks.

The boom truck loading capacity and certificates will be ensured prior to the start of the loading.

Material Transport and Handling

All materials involved in the soakaway network will be handled and transported and unloaded at the site strictly as per the manufacturer’s instruction.

After delivery, equipment will be inspected for any transport both internal and external precast manhole components casing damage, in compliance with project specification & materials submittal.

All the accessories will be tagged, crated, and protected for shipping as per the manufacturer’s recommendation.

Approved shop drawings will be posted at the workplace for site reference used and installation works.

Storage

HDPE Pipes need to be stored in a shaded area or protected with a tarpaulin covering.

Waterproofing membranes will be stored vertically in well-covered and ventilated places not subject to direct sunlight.

Any discrepancies, damage, etc, found in the materials will be notified and reported to QA/QC Engineer.

Materials found not suitable for site use will be removed immediately.

Replace materials that are damaged or unfit for use.

No materials will be permitted to enter the work location without being inspected and approved by the Engineer.

Pipe and Manhole Trench Excavation

a. Survey/Setting out

Setting out of the proposed manhole position will be provided as per the latest approved shop drawings and design drawings. This activity is applicable for various sizes and depths of manholes such as for example:
-1500mm diameter manhole opening on 200-500mm diameter surface waterlines up to 6m depth
-2000mm diameter manhole on 600-900mm diameter surface water lines of depth more than 6m

Marking shall be done according to the excavation site area with coordinates by permanent marker or will use steel pins (12 mm dia. steel bars with safety cap) to secure points given on the cross-section and along the longitudinal direction.

The contractor is to provide the as-built survey record drawings including the existing underground utilities and existing manhole in the vicinity of work once it is completed.

Surveyor will recheck to ensure that finish levels are acceptable. Any discrepancy in the levels, an action notice is issued and work is rectified as per requirement.

During pipe laying, if the alignment deviated within the permissible limit, the manhole will be adjusted so that all the lines should be at the center of the manhole as well as the invert level according to the approved profile shop drawing.

The contractor will provide proper access for other traffic (like a pedestrian) where the pipeline works in construction access roads and road crossing within the manhole construction area.

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b. Utilities Scanning

Buried/existing utilities are to be scanned, identified, and marked on the area using markers.

If any unidentified utilities have been detected during scanning and excavation, then machinery work will be stopped and will be reported to the site in charge.

Based on permission, only an authorized person shall work near utility lines.

Besides any utility line, the excavation shall be cleaned by manual excavation. If possible the use of a manhole box for shoring during the excavation should be implemented.

If any damage occurs to an unidentified utility line during excavation, then it will be reported to the emergency number immediately for temporary mitigation and permanent mitigation to be done after reporting to the authorized department then take necessary approval.

As-built has to be recorded and updated by the contractor if any changes as per site condition.

c. Trench Excavation

Excavation of the trench shall be done as per the approved methodology for trench excavation.

The excavation levels will be carefully checked/verified and recorded by the surveyor prior to start-up to ensure achievement of the formation level of the manhole and manhole chamber which are approximately 130 mm below the slab (allowing for 75mm thick blinding and 50 mm concrete screed under the base and waterproofing membrane or up to the required depth.

d. Permeable Geotextile Installation

Approved material shall be used.

The application shall be in accordance with the manufacturer’s recommendation and as indicated in the approved shop drawing.

Prepare and cut the geotextile to the proper width prior to the installation of the trench drain.

Place the prepared geotextile along the excavated trench.

Anchor the edge of the geotextile with heavy objects or pin the geotextiles on the ground to prevent movement of the geotextile during the installation process.

After the geotextile surround installation, the trench will be filled with gabbro aggregate up to the required bedding level of the perforated HDPE pipe.

To prevent slippage of the overlapping fabric, the areas of overlap shall be stabilized, as approved by the engineer, with pins, anchor blocks, or piles of aggregate.

e. Installation of Underground HDPE Perforated Pipes

Related installation of underground HDPE perforated pipes shall be done as per the approved methodology.

Bedding and Surround:
Bedding material shall be in accordance with the project specification and approved shop drawing.
20 mm granular material shall be used as per the approved material submittal.
Completion of pipe surround: Gabbro shall extend from not less than 150 mm beneath the pipe and 300 mm above the crown of the pipes in accordance with the latest approved shop drawings.

Relative to laid bedding, a plain concrete cradle will be constructed to support the pipes.

Pipe Laying with Gravel Surround

Drawings shall be reviewed by the Engineer and shall be communicated to the site in charge of the pipe laying. The pipes will be laid out in accordance with the instructions of the manufacturer and as per project specifications.

Prior to this, the written inspection requests will be submitted to the Engineer/Inspector for approval of the various stages of the work.

The bedding material with a thickness of 150 mm as directed by the Engineer shall be spread and leveled to the gradient of the pipe at a depth of 2.0 cm above the inverted level.

The pipes shall be laid in a granular bed and surrounded by aggregates of 10 mm maximum in size and then contained within the approved geotextile trench liner material.

For the safe setting out of the pipelines, the stalked pipes will be lowered and laid down from the downstream side to the upstream direction.

The pipes will be checked for line and level jointly by the contractor’s and consultant’s surveyors respectively.

Each pipe joint shall be enclosed with approved PVC wrapping tape. Side bedding is carefully placed by means of a crane and side discharge skip, discharging the material centrally.

Subsequently, the pipe surround material will be carefully placed in two layers at a level of 30 cm. above the crown of the pipe and compacted. The overlapping of the geotextile sheets will be 30 cm.

Selected materials previously excavated from the trench will be used for the trench backfilling in 150 mm layers, carefully compacted by hand rammers, up to a level of 600 mm above the crown of the pipe.

The water test shall be carried out in accordance with the project specifications.

When pipe laying is not in progress, the pipe ends will be closed by wooden plugs.

Backfilling will continue in layers, compacted in the lower parts by mechanical plate compactors and in the upper parts by hand-operated drum rollers.

The deflection test will be carried out immediately after the completion of the backfilling activity.

f. General Procedure for the Construction of Manhole Chambers and Bases

Precast manhole units as per standard details approved with the internal coating will be used.  

Safety procedures will be followed as outlined in the safety manual and risk assessment submitted and will generally comply with the requirements.

Both precast manholes shall have a circular base unit over which a circular shaft shall be built. The base is going to be cast in one part.

Upon receipt of the Engineer’s approval for the formation, the concrete blinding shall be poured to a depth of 50 mm and the trowel shall be finished to a height of 55 mm below the manhole invert. Prior approval will be sought from the Manhole Production Schedule.

The ‘torch-applied waterproofing membrane (5mm) will be mounted in compliance with the manufacturer’s instructions.

The membrane will be covered by a 50 mm screen.

The precast base pre-painted is set to line & level followed by shafts, adjustment rings cover slabs

The fixing process shall be maintained by fixing 2 no. Gasket strips with a width of 20 mm between each joint.

The 250 mm thick, removable concrete class b cover slab with a circular opening of 600 Ø is excentric.

Its D.I. cover & frame as defined will be fixed to line & level as per the approved details. 

g. General Method for Manhole Construction

Upon completion of the screed, the steel reinforcement for the base unit shall be cut, bent, and installed at the steelyard as per approved shop drawings as well as the approved bar bending schedules.

After fixing the mold for the manhole steel cage, the casting will be finished.

Proper curing of the precast element will be observed.

It will then be taken to the site and raised above the screen with the aid of a crane.

Frame and cover fixing shall be as specification and our D.I. cover & frame Fixing Method Statement.

h. Installation of Precast Manholes

1. Blinding concrete Grade SRC 25

For setting out, the site engineer will be ensured that a specific area/location is handed over to proceed with concrete operations at the site. The survey will be carried out for marking of coordinates & levels.

Using GPS based on the Total Stations instrument, mark each manhole construction coordinates of the concrete element or structure using permanent markers as shown in the approved drawing layout.

For blinding concrete, a blinding concrete of (SRC C20/25) as per the approved design mix and thickness and as indicated on approved drawings will be placed over the final excavated level as per approved drawing locations. Blinding concrete will be protected with approved waterproofing primer and membrane as per approved material. The application will be according to the manufacturer’s recommendation.

Blinding is then placed, spread with shovels, and rakes leveled/compacted by tamping with aluminum straight edge bars, the top of the blinding concrete will be steel float finished to provide a suitable surface for torch-applied membrane. Curing is carried out by water ponding or curing agents, covered with hessian cloth.

2. Waterproofing

Prior to waterproofing activities, the contractor will ensure that the concrete element has completed the curing period as stipulated under the method statement of cast in-situ concrete.

The contractor will ensure that the delivered waterproofing material follows the approved material submittal, and material inspection request (MIR) has been raised and approved by Supervision Engineer.

For the installation of waterproofing at the manhole, the waterproofing application shall be implemented as per the approved method statement for waterproofing.

i. Erection of Precast Manholes

The survey team should have established the precast concrete manhole’s true coordinates and elevation prior to the placement.

Precast installation shall be checked and verified to its dimension and elevation prior to placement of the precast concrete manhole, erection must follow manufacturer instructions unless otherwise indicated in the approved shop drawing.

Crane shall lift the precast elements from the trailer directly to its final location and coordinates already established by a competent surveyor.

Fitting of manhole chamber ring: Make sure that the joints are clean and free from foreign objects before fitting the next manhole chamber ring (shaft) unit. Two strips of 20 x 25 bitumen-based flexible strip sealant shall in placed prior to mounting. Repeat with further ring units until the chamber has been constructed to the required height.

Once completed, verify the alignment and correctness of the elevation of each newly installed/placed precast concrete manhole, in layman’s terms, use a string line can be used to align the manhole. Proper alignment shall be confirmed before releasing the manhole base from the crane.

After the completion placement is done and has been verified, checked, and confirmed, all lifting devices shall be recessed. Lifting devices shall be removed and patched to match the interior face of the manhole after installation is complete. Lifting hooks shall be filled with non-shrink mortar.

After the execution of shaft installation, a pre-cast cover slab shall be installed.

Place the external precast manhole (reducer slab) cover slab on top of the last pre-cast manhole wall segment.

If the pre-cast shaft is equipped with the backdrop, all HDPE pipe fittings shall be installed and shall offer for inspection and taking approval. The concrete box will be constructed to complete the encasement.

j. Manhole Benching

Manhole floor benching is made after pipes installation with manholes walls, where site-made plywood shuttering (where needed) will be placed to the dimensions shown in approved shop drawings, Grade SRC C20/25 concrete of an approved mix will be poured by means of a concrete pump or manually, top of surface will be working with a good finish. Shuttering will be removed 24 hrs after casting and curing similarly. The benching is carried out after the pipe installation.

k. Wall Surface Finish and Painting Touch Up

Manhole chambers’ internal concrete surface shall be repaired against any uneven surface. Applicable method statements shall be submitted separately.

l. Ductile Iron Manhole Covers

The ductile iron cover frames are placed on pre-cast concrete seating rings/blocks that are positioned above the edge of the opening of cover slabs. After properly mounting the cover frame and mortar is placed all around the precast rings and frame to the dimensions shown in approved shop drawings, Finally, a manhole cover will be installed.

Elevation of the manhole cover, frame, and basic manhole chambers shall be as per the approved shop drawing.

m. Installation of Access Ladder

Installation of GRP ladder will be approved material and will be supplied by the approved experienced manufacturer and will be purposely made to suit the depth of each installation as shown in the approved shop drawing furthermore, the installation shall be in line with the manufacturer’s guide for installation.

n. Backfilling

Backfilling will be carried out according to the approved method statement.

The chamber/base slab is not to be backfilled until all required inspections are performed and until the engineer has verified the works have been done as per approved shop drawings and in accordance with project specifications.

After approval from the supervising consultant, backfilling shall start after the erection of the manhole shaft. A small equipment shaft is used for compaction to avoid damage to the shaft tanking membrane, like tamping rammer, as required.

A compaction test (FDT) will be carried out for the backfilling to obtain a minimum of 95% of the maximum density for the materials. The test shall be conducted in each layer.

Marking will be done on manholes to control the thickness of compacted layers.

Backfilling shall continue uniformly around the shaft to ensure stability during backfilling over the effective length of the trench.

Inspection Related To Method Statement for Construction of Manhole

Mock-up and subsequent inspection shall be offered to supervising consultant for approval.

For surface water drainage systems, storm manholes, HDPE pipes, and fittings shall be inspected in accordance with the Inspection and Testing Plan and Inspection checklist.

The inspection may be made on isolated portions of piping from manhole to manhole as well, to facilitate general progress in construction.

All open-end shall be sealed off after inspection is carried out successfully. The inspection shall be recorded and witnessed by supervising consultant with the corresponding Work inspection request (WIR).

Quality Controls

The Inspection and Test Plan (ITP) for this Method Statement for Construction of Manhole summarizes various characteristics to be checked. The concerned Site Engineer or Site Supervisor will be responsible to ensure compliance for this manhole construction and the site QA/QC Engineer will carry out quality control checks and report the inspection results.

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