Región de Antofagasta

 considerable level of corrosion in the invert and small holes appearing in the invert of the BCMS (Figure 4.9 and Figure 4.10)

 no evidence of material loss from soil behind the BCMS and no soil cavities evident

 BCMS is retaining circular shape or may have distorted during construction but is remaining stable

 BCMS is assessed as being condition state 4.

Treatment

 monitor and plan for installation of a concrete lining of the invert within the next 12 months. If corrosion extends above the level where it is practical to install a concrete invert consider relining

 after a significant rainfall event, check the BCMS for erosion of backfill material through the invert perforations

 repairs should be carried out within 1 year.

Source: TMR nd a.

Figure 4.9: Heavy corrosion in invert with small perforations to metal structure

Source: TMR nd a.

Figure 4.10: Heavy corrosion – considerable loss of metal thickness 4.6.4 Situation 4

Medium risk condition

 BCMS invert rusted completely through over a large portion of the length

 loss of backfill material below invert

 BCMS is retaining circular shape (loads are being carried by soil arch) or may have distorted during construction but is remaining stable

 BCMS has no significant flows

Treatment

 prop immediately and repair as soon as possible within two months or before the next wet season starts

 check BCMS and pavement levels fortnightly, after short rainfall events and during extended rainfall periods for any signs of ring compression failure or settlement of the soil arch over the BCMS

 put speed restrictions and hazard identification signage in place to ensure adequate stopping sight distance, should a hazard develop.

Safety

 if a dip in the pavement occurs the road is to be closed immediately.

4.6.5 Situation 5 High risk condition

 BCMS invert rusted completely through over a large portion of the length

 loss of backfill material below the invert (Figure 4.11)

 BCMS is retaining circular shape (loads are being carried by soil arch) or may have distorted during construction process but is remaining stable (see notes in Section 4.3)

 local flooding event occurs creating the risk that the BCMS and embankment could be washed out

 heavy vehicle traffic loading increases.

Treatment

 BCMS must be repaired as soon as practical (within one month)

 props should be installed immediately

 BCMS and pavement surface should be checked weekly, after short rainfall events and during extended rainfall periods for any signs of ring compression failure or dips in the pavement caused by soil arch settlement

 if replacement is determined to be the most appropriate repair method and the replacement BCMS cannot be obtained immediately from the manufacturer alternative options that may be considered are:

— temporarily replacing the BCMS with a readily available low flow reinforced concrete pipe. The low flow pipe can then be replaced with the appropriately sized pipe when it is available from the manufacturer

— completing the works under full road closure if sidetracks or diversions are an issue

— backfilling with concrete can be undertaken if necessary to decrease the time required for installation if the road is not available for extended closure

 put speed restrictions and hazard identification signage in place to ensure adequate stopping sight distance, should a hazard develop.

Safety

 if a dip in the pavement occurs the road is to be closed immediately.

Note: The helical lock seams have not distorted and BCMS remains circular with no evidence of ring compression failure.

Source: TMR nd a.

Figure 4.11: BCMS invert corroded away (loss of granular bedding material in invert)

4.6.6 Situation 6 Very high risk condition

 BCMS invert rusted through over large portion of the length

 compression ring movement is obvious (metal has buckled or is overlapping), but soil arch is mostly intact (Figure 4.12)

 voids possibly present behind BCMS lining but no dip in road

 BCMS structure has failed, embankment soil arch at high risk of imminent failure (condition state 5 – close structure)

 no visible settlement of road pavement over BCMS.

Treatment

 inform the owner immediately

 seek structural engineering advice – send photos of BCMS

 prop the BCMS immediately if safe to do so

 put speed restrictions and hazard identification signage in place to ensure adequate stopping sight distance, should a hazard develop

 undertake urgent repairs.

Safety

 BCMS is structurally unsafe and can fail under heavy traffic loading or in a flood due to embankment erosion

 if a dip in the pavement occurs the road is to be closed immediately.

4.6.7 Situation 7 Extreme risk condition

 BCMS invert rusted through over large portion of the length

 compression ring movement is obvious (metal has buckled or is overlapping) but soil arch is mostly intact (Figure 4.12)

 voids possibly present behind BCMS lining but no dip in road

 condition state 5 – close structure, BCMS structure has failed, embankment soil arch at high risk of imminent failure

 road pavement above BCMS shows obvious signs of settlement (Figure 4.13).

Treatment

 as given for Situation 6 – very high risk.

Safety

 BCMS has structurally failed

 road is to be closed immediately.

Source: TMR (2010).

Figure 4.12: BCMS ring movement

Source: TMR (2010).

Figure 4.13: BCMS soil arch failure

5 MAINTENANCE AND REPAIR PROCEDURES

5.1 Emergency Propping

Install emergency propping for immediate temporary stabilisation of the structure when safety issues become evident. Propping of the BCMS can only slow the complete collapse of the

structure and there is still a future risk to the travelling public if a failure is allowed to occur. Since most conditions that require propping are caused by excessive corrosion of the invert, vertical propping is often not an option. However, propping at 45 degrees (Figure 5.1) can be effective.

For safety, 6 mm self-tapping screws can be inserted in the BCMS to hold timber spreader beams with heavy wire ties until the props are installed. Each sleeper should have a minimum of two props. Timber sleepers should be used to distribute the load from the props across numerous BCMS corrugations. The timber sleepers will be held in place against the wall by friction, but a suitable connection of the props to the sleepers should be considered to prevent the props from being washed out. Seek advice from a qualified structural engineer for the required safe working load and arrangement of the props.

Note: This culvert had not failed and props were installed pending a full structural assessment.

Figure 5.1: Example of emergency propping

An appropriate installation method, taking into consideration all applicable safety risks, and which ensures that no disturbance of the culvert walls occurs, would need to be developed prior to installing props.

5.2 Repair Methods

The following are a range of repair treatments that should be considered, taking into account:

 remaining life of the non-corroded portion of the culvert

 durability and expected service life of the repair

 size of embankment over culvert

 traffic volumes

 water flow

 ability to detour traffic

 cost.

The products and methods detailed in the repair methods should be sourced from reputable suppliers.

5.2.1 Repair and Maintenance Methods

The condition of the BCMS determines the appropriate rehabilitation or maintenance method required. Preventative maintenance measures are recommended either at construction or during its service life. Two common methods of preventative maintenance are concrete lining of the invert (Section 5.2.2) and/or a paint membrane that will prevent internal rusting (Section 5.2.3). Both of these methods are dependent on the inverts being of sound condition with no rust evident.

5.2.2 Concrete Lining of Invert

This is the most common repair for larger culverts with sufficient working space and high embankments where it is difficult to remove the culvert.

The concrete lining of culverts can be either reactive maintenance or planned maintenance.

Ideally, concrete lining is used as part of a structural management plan to optimise the life of a structure through careful management of the sacrificial metal thickness in the invert. Once the sacrificial thickness is used up the invert is lined to protect the remaining required structure

thickness in the invert. In a reactive situation the invert may be seriously deteriorated and in these instances invert lining can be used as a reactive approach to extend the life of a culvert.

When using this treatment it is essential that the concrete invert liner forms a structural bond with the uncorroded metal above the invert. Thin layers of concrete that are not structurally fixed to the metal culvert are not effective repairs, as shown in Figure 5.2. Voids that may be present beneath the invert or in the backfill material will need to be grouted.

The rest of the metal culvert (excluding the corroded invert) must be in a reasonable condition. The inside face should have adequate remaining galvanising metal. The thickness should be checked with a non-destructive thickness gauge or by drilling a small number of holes and checking with a thickness gauge reading to 0.2 mm or better, and holes must be plugged with a galvanised screw afterwards.

Where there are significant concerns regarding collapse due to the invert being corroded out, it is often not safe to enter the culvert to prop it. In this situation the invert lining can be conducted in a progressive manner by the following procedure:

 Starting at one end, use curved plate or reinforcing bars welded to the shell at sufficient centres to reinstate the hoop capacity of the section. Only a small length is repaired at a time.

 Once the appropriate reinforcement has been installed, the lining section under consideration is poured. It is important that the reinforcement has appropriate cover, therefore the hoop strengthening should not be considered as reinforcement.

If there is significant thickness loss due to corrosion of the external face of the culvert, a concrete invert may not be effective as a long-term repair and relining should be considered.

Figure 5.2: A thin concrete invert lining which has separated from the culvert and washed away in flood VicRoads specifies the following for concrete linings (VicRoads 2009):

 The minimum thickness of concrete lining shall be 130 mm above the crest of corrugations.

 The minimum height of lining shall be normal water level plus 300 mm or one-third height of the structure, whichever is greater.

 Top edges of concrete lining shall slope towards the centreline of the structure to prevent ponding of water against the wall of the structure.

 At both ends of the structure the concrete invert lining shall terminate with a 900 mm deep reinforced concrete cut-off wall. The cut-off wall depth shall be measured below the finished invert level, and the wall shall be detailed to connect to the reinforced concrete headwall if this is present.

 Concrete for the lining shall be special class performance concrete having a grade not less than VR 330/32 as specified in VicRoads (1997).

 The concrete lining shall be reinforced with a steel fabric having a minimum steel area of 500 mm²/m in both directions and mesh dimensions not greater than 200 mm and bar size not less than 8 mm.

 Cover to the mesh at the edges of the concrete lining shall be not less than 50 mm and not more than 100 mm. Minimum cover shall be 50 mm to all other faces, including to the crest of the BCMS corrugations.

 For steel BCMS, reinforcement in the concrete lining shall be lapped and welded for

electrical conductivity and supported by steel bars welded or bolted to the structure at 1.0 m maximum spacing in both directions.

The following steps are recommended for preparation of an existing BCMS for concrete lining. This information has been extracted from the Bridge Technical Note 2005/009, (VicRoads 2009):

 temporarily divert water flow

 remove sediment in culvert

 abrasive sweep blast area to be lined to equivalent to class 1 finish to AS 1627.9 (2002)

 for steel BCMS, paint penetrating primer 50 microns DFT over area to be lined using Xymax MonoLock PP, Wasser MC-Prebond, Zinga or other approved equivalent

 for aluminium BCMS, paint with an approved bitumastic coating.

Advantages

 concrete can be applied in situ

 cement mortar linings have been found to dramatically reduce internal corrosion

 abrasion forces on the BCMS are reduced

 the afflux can be controlled by using more/larger rock baffles

 depressions can be made in the concrete layer to allow rest areas for fish and marine animals

 increased service life of the BCMS

 safe progressive methods can be used

 it is a low-cost repair method.

Disadvantages

 BCMS under 900 mm in diameter have restricted access for personnel to lay the concrete

 approach and departure aprons will need to be raised or constructed to maintain inflows and outflows

 the diameter of the BCMS will be reduced.

5.2.3 Painting the Invert

This repair method can be used where the culvert is not extensively corroded and significant metal thickness remains. The type of paint system used will depend on the abrasive conditions the culvert is subject to. If a significant volume of debris flows through the culvert regularly, high abrasion resistant systems can be used. Alternatively, paint systems with lower abrasion

resistance can be used in culverts which do not flow regularly or if the flow does not contain debris.

Painting of the invert of a BCMS was traditionally done using a coal tar epoxy coating. This method is no longer used due to carcinogenic and environmental concerns. The preferred method would be to obtain coated BCMS before installation. If repairing or prolonging the life of an existing structure, a number of paint options are available including:

 bituminous paint

 polymer coatings

 metal oxide based paints.

All metal surfaces need to be in sound condition and free of dust, dirt and moisture. Application of the selected product can be done with a sprayer unit, brush or roller. This method could be used with a concrete liner as mentioned above, to add better protection to the invert of the BCMS.

Disadvantages

 impossible to get complete coverage on moist inverts

 diversion of stream waters required to allow BCMS to dry

 breathing apparatus may be required due to confined space and lack of ventilation.

5.2.4 Joint Repairs

Separated joints can be repaired using a chemical grout such as polyurethane foams. This will stop joints from leaking and prevent the erosion of backfill material behind the culvert. Where backfill material has been lost, the voids can be grouted using a low pressure cement based grout.

5.2.5 Replacement of the Culvert

Removing and replacing a culvert should be considered when culverts:

 have large distortion from original shape (greater than 5%)

 experience significant reduction in waterway area due to the culvert shape not being circular (they can be oval etc. in shape) or not having a consistent diameter along the length

 have significant voids in the embankment material and grouting is not practical

 are considered to have structurally failed.

Removing and replacing a culvert is the most practical option when:

 culverts are smaller than 1.5 m diameter making it difficult to work inside

 embankment height is low and traffic can be easily diverted

 larger culverts have failed in ring compression, are badly corroded or have distorted significantly, so they cannot be easily relined.

All of the issues described in Section 2 on appropriate selection of BCMS need to be considered when selecting the replacement material and structural system.

Advantages

 culvert can be redesigned to perform adequately over the appropriate design

 significant site-specific experience can be gained from the performance of the old culvert which can be used in the design of the new culvert.

Disadvantages

 road will need to be closed for the replacement, possibly requiring staged construction

 costs may be higher.

5.2.6 Shotcrete Lining

Shotcrete lining is performed by pneumatically applying cement plaster or concrete to the area where relining is required. A gun operated by compressed air is used to apply the cement mixture.

Water is controlled and added to the dry material as it passes the nozzle of the gun. Shotcrete lining is considered to be stronger that the hand-placed mortar of the same aggregate-cement proportion because it permits placement with a lower water-to-cement ratio.

It is recommended that the shotcrete thickness should be between 50 mm to 100 mm. A minimum area of steel of 0.4% of the area of lining in both directions is recommended.

Advantages

 culvert can be retrofitted in place with minimal interruption to the traffic above.

Disadvantages

 skilled labour is required to successfully implement

 cost may be high; this is a trade-off with traffic delays of other options.

5.2.7 Slip Lining

Where significant metal thickness has been lost from the entire circumference of the culvert, a repair option is to reline the culvert with a new culvert of smaller diameter and grout the void between the new and old culvert.

Slip lining involves inserting a sleeve inside the existing pipe, in situ. Before the designer makes a final choice of material type it is recommended that tests are conducted on both the ground water and flowing water in order to determine their chemical content and, hence, the potential for

corrosion. Where the ground water contains concentrations of pH levels lower than 5 (acidic), steel may not be suitable and other materials such as concrete. High density polyethylene (HDPE) or aluminium may be more appropriate subject to consideration of cost and time. It is also

recommended that the chemical composition of the backfill material is specified in order to reduce the risk of corrosion (VicRoads 2008). In short, all the issues relevant to selecting a new BCMS are relevant to the selection of a suitable liner.

Materials

Slip lining can be undertaken using the following materials:

 HDPE

 aluminium

 steel

 stainless steel

 concrete.

Techniques

A HDPE lining is a push or pull technique where a new lining is inserted into the existing BCMS (Figure 5.3). Once the new lining is in place an annulus grout is poured between the existing BCMS and the new lining. Systems which rely on the host pipe for some measure of structural support are sometimes known as ‘interactive lining’ techniques.

Figure 5.3: HDPE lining being installed

The culvert must be closest to its theoretical shape for efficient relining. If there is significant distortion, then a much smaller pipe liner would be required for relining which can cause several problems such as:

 The void between the new liner and the old culvert or in the backfill material can be very large and expensive to grout.

 The smaller diameter culvert may not carry the design flood flow, resulting in the road embankment overtopping and washing away.

Relining needs careful consideration and relevant experts should be consulted. Figure 5.4 illustrates the relining process.

Figure 5.4: Relining process

In order to line the culvert, the largest possible liner size (outside diameter) needs to be determined. This can be achieved by the following method:

 At either end of the culvert, establish the horizontal and vertical centre point with a level and straight edge of suitable length to fit inside the culvert.

 Put self-tapping screws through the culvert and establish horizontal and vertical stringlines at each end with a laser at one end and aim it at the centre of the other end.

 At 2.0 m centres, measure the horizontal and vertical diameters and the laser centreline

 The dimensions can be plotted, and the largest straight internal tube can be measured.

 Plot all horizontal and vertical diameters and laser intercepts.

 Measure the largest included circle (Figure 5.5).

 Choose a liner based on suitable construction clearances and grout thickness requirements.

Figure 5.5: Estimating largest liner diameter Advantages

 slip lining allows for the repair of the BCMS to be conducted with minimal disruption to traffic

 slip lining allows for the repair of the BCMS to be conducted with minimal disruption to traffic

In document IBEROARSEN Distribución del arsénico en las regiones Ibérica e Iberoamericana (página 172-176)