Frequently Asked Questions


Ductile FAQs

The questions are divided into 3 sections:

  • Product Questions
  • Care and Adaptation
  • Day-to-day Operation

Product Questions:

1. What type of sealing arrangement is the cover supplied with?

A15 to C250 ductile iron manhole covers have a single integral, ‘dry-labyrinth’ (tongue and groove) seal, whereas D400 to F900 have no integral seals unless otherwise stated in their datasheets. Some steel manhole covers have a single integral seal, where others have no seal (at the same load category). The datasheets provide details of the existence of seals.

2. What pressures are the manhole cover seals rated to?

Standard manhole cover seals are designed to resist water/odour egress/ingress at normal atmospheric pressure only. For higher pressure requirements (up to 0.5Bar), a flood & odourcontrol plate should be used.

3. What is the coating that the covers are painted with?
The bituminous coating found on our covers is a barrier coat intended to provide some aesthetic appeal and a low level of corrosion protection. Under normal UK non-marine exposure conditions, it is not necessary to provide additional protection to ductile iron castings, as any initial surface oxidation (rust) prevents further corrosion. Rust is not thought to reduce the life of ductile irons.

4. Why are ductile iron manhole covers and gratings only available in fixed sizes?

To produce a casting, a pattern must be used which is an expensive piece of equipment. For this reason, generally, only standard chamber sizes are accommodated by castings unless quantities allow economic manufacture of covers to non-standard chamber sizes. Custom-sized covers are often accommodated by the manufacture of a bespoke steel fabrication.

5. Can Wrekin’s access covers and gratings be locked?

Most of Wrekin’s products offer screw-fastener locking as a factory-fitted option, whilst some products incorporate dedicated lock assemblies as standard. Where locking is available or as a standard fitment, this is illustrated on the respective product datasheet. In some circumstances, it may also be possible to factory lock other products on request.

6. What are the HA102/00 water evacuation rates for Wrekin’s gully gratings?

The water evacuation rates for Wrekin gully gratings vary depending on the design of the gully. Generally, for conventional designs, the larger the grating the greater the capacity for water evacuation. HA102/00 categories are available for all Wrekin gratings.

7. What are the link fasteners for between double triangular covers?
The purpose of link fasteners between adjoining cover elements is to provide a loose coupling that reduces the possibility of dropping a joined-pair of covers down a chamber during cover lifting. At the same time, they must allow the covers to perform independently under traffic loads so must remain loosely fastened in all circumstances.

Note: Link fasteners are not actually applied to help traffic security but will make some contribution to it in the event of unrestrained cover movement.

8. Are all gully grating tops reversible for opposite hinging applications? e.g. one-way streets, traffic islands, etc.

Not all of them are reversible. Usually they are reversible where provisions are made in the frame to allow it. This is usually restricted to loose-fit, flat seated designs like Highway double-triangular gully gratings. Reversibility is not possible where there are no provisions in the design. It is usually restricted to interference-fit, wedge seated designs which are manufactured to a specific orientation and component pairing.

9. What type of Wrekin product data can be provided for enquiries?

Wrekin can provide datasheets and safety information which provide the fundamental dimensions and product details. On request, additional illustrative drawings and documents may be provided at Wrekin’s discretion, including further data for BiM purposes.

10. Clear opening vs base opening vs clear area?

How access covers are measured:

Access cover dimensions are always referred to by their ‘clear opening size’ and sometimes also by their frame depth (see diagram).

Clear Opening:

The clear opening is the inside frame dimensions and these should normally be marked on the cover. If these are missing or have become illegible over time then it will be necessary to remove the cover and physically measure these dimensions. The clear opening size of the access cover frame must always be equal to or greater than the size of the chamber it is covering to ensure the frame is adequately supported.

Base Opening:

These are the most important dimensions when establishing the correct size of frame intended to fit on a given chamber or gully-top opening. i.e. The central opening at the base of a frame must never be smaller than the opening at the chamber top so that the frame flange is always completely supported by the chamber top, usually via bedding materials. This harmonisation of component dimensions is vital in achieving the most effective traffic-load transfer possible to an underlying chamber structure.

Clear Area:

The Clear Area dimensions are the two gross measurements taken perpendicular to each other and a product’s permanent frame walls or supports and which coincide with the narrowest points at that location. In view of this, it is common for one of these dimensions to be the same as the Clear Opening.

A frame product’s Clear Area often disregards the presence of cover seating or locking-feature encroachment in the corners of the frame. i.e. Such features are generally ignored as their presence is generally not considered detrimental to unrestricted chamber access unless the equipment in a given chamber requires the full use of a frame product’s Clear Area.

11. How do i know if i need 100mm or 150mm frame depth?

Minimum frame depths for NRSWA road category Types 0 to 4 shall be as Table A. Manhole covers in shared driveways that could be subject to occasional loads from heavy vehicles (e.g refuse vehicles) should see the requirements for Type 4 roads.

Table A

Road Type Traffic Capacity Road Application Minimum Frame Depth (mm)
Type 0 30-125 msa Trunk roads and dual carriageways 150
Type 1 10-30 msa All other A roads 150
Type 2 2.5-10 msa Bus services 150
Type 3 0.5-2.5 msa All other roads except residential cul-de-sacs 150
Type 4 Up to 0.5 msa Residential cul-de-sacs 100

Care and Adaptation:

12. My customer has installed a manhole cover and frame (MHC), is it possible to retro fit a flood & odour-control plate?

At present, it is not possible to retro-fit a flood & odour-control plate. The best option is to exchange stock before installation or to re-install the MHC, replacing it with one incorporating a flood & odourcontrol plate.

Note: Wrekin Low Leak Unite manhole covers do have the facility for retro-fitting a new seal plate. 

13. Can I recoat my cover as the coating has eroded from traffic and it’s starting to rust?

Yes. Manhole covers and gratings can be recoated with any paint suitable for ferrous metal which does not unduly compromise a product’s skid resistance. However, due to bitumen residues from the original coating process, we recommend using a bitumen compatible paint e.g. ‘Johnstone’s Black Bituminous Coating’.

14. My manhole cover looks rusty. Is this a problem?

Bitumen coating acts as a barrier coat for transportation and short-term aesthetic purposes only and provides no electrolytic corrosion protection to the underlying iron. Where light rusting or corrosion has occurred on the upper surface of an iron access cover or grating product, this will have no effect on the function of the product and will certainly not affect its structural integrity.

Classification and Standards:

Wrekin is the first company to have all its ductile manhole covers and gully grates Kitemarked by the British Standards Institute to the new BS EN 1242:2015.

15. Should manhole covers have their load classification marked on them?

Yes. It is a requirement of the BS EN124-2:2015 that all manhole covers and gratings claiming conformance to this Standard clearly identify their load class on the top face of the cover or grating to provide guidance on where they can be installed to suit the anticipated traffic exposure.

16. If a cover is badged D400 can it be used in all categories of road?

In essence, yes, where the vehicle types employ pneumatic tyres. The difference between the ranges at the same D400 load classification is mostly related to durability and expected service life, but all should be capable of installation in a BS EN124 Group 4 (highway) location. E600 load class products are sometimes specified for certain highway applications but this is often due to inadequate D400-design products not fulfilling the Group 4 location traffic function. Indeed, installations incorporating inadequate E600-design products can be just as vulnerable to premature failure.

17. Is CE marking a requirement for Wrekin access covers?

CE marking is only a requirement for products covered by a harmonised European standard covered by the Construction Product Regulations. At present (November 2016), the controlling standard BS EN124 for access covers and gratings of up to 1m clear opening, is not harmonised, hence, there is not a current requirement to CE mark products supplied against this standard.

18. Where a product carries a Kitemark, what does this actually mean?

Where a product carries a Kitemark or other Third Party Certification (TPC) mark, it indicates that the product has been tested by an accredited independent body and has achieved all the requirements of the relevant standard. On our products, Kitemark is the TPC mark for BSi (British Standards Institute).

19. With the introduction of BS EN124-2015, are there any associated changes in installation practice for Wrekin’s products?

Not at the moment. Exceptions may come with the introduction of new products. Any variations from the recommended standard installation practice of using UniPak Mortar and packing accessories will be advised of in our literature and/or website.

20. What constitutes a seal in relation to BS EN124-2015 manhole and access covers?

Seals are features of manhole covers or access cover assemblies which provide resistance to fluid and gaseous egress or ingress. Unless otherwise described, seals are only designed to provide resistance to leakage at normal atmospheric pressure. Should an application require resistance to higher operating pressures, please clearly state this so a suitable product can be selected. Typical seal arrangements are illustrated on the final page of this document.

Day-to-day Operation:

21. How many 25Kg tubs of UniPak mortar do I need to install my manhole cover?

There is no definitive quantity of mortar, the amount needed depends on the volume or size of gap below the frame that needs filling and whether other packing materials are to be used in the bedding construction.

The general rule for calculation is based on a cured mortar density of 2,000Kg/m³. Wrekin can provide guidance on this for a given product size and known bedding depth.

22. Do Wrekin’s access covers or gully gratings require periodic maintenance?

Currently, none of the cast assemblies require periodic maintenance, although it is good practice to clean all seatings and cover/frame clearances when chamber access has been completed. Cover/ lid lifting keyways not already containing Wrekin’s anti-blockage keyway component may require cleaning out before cover removal to allow lifting key fitment.

23. What does Wrekin supply or recommend for access cover lifting purposes?

Over time, access covers can accumulate debris in crevices and clearances which can affect their static lifting weight. In view of this, and the fact that cast iron products are usually of significant mass, we recommend that mechanical lifting devices are used wherever possible to lift access covers from their frames. Where manual lifting is required or preferred, Wrekin offers a range of lifting keys suitable for all common access cover types.

24. My manhole cover is stuck in its frame and I’m struggling to open it with lifting keys. What should I do?

If there are prising slots available at the cover perimeter, use a suitable prising bar (e.g. crow bar) to ease it open, then use the lifting keys to complete the cover removal. If there are no prising slots, clear the gaps between the cover and its frame and then strike the cover in the geometric centre with a mallet or lump hammer (<5Kg) so that the material’s elasticity is used to ‘bounce’ the cover and break its stickiness in the frame.

Note: This ‘bounce’ technique should not be used on Class A15 covers or those in-filled with brittle surface materials.

25. Is there any way I can stop the lifting keyways in my manhole covers from becoming blocked?

Yes, if buying new from Wrekin, ask if the product being offered has the option of the Anti-Blockage Insert. Alternatively, if it’s existing products that require the anti-blockage feature, determine the approximate internal dimensions of the keyways as it may be possible to fit our standard items due to their universal-fit characteristics.

26. Can I interchange manhole cover lids between frames of identical product types?

We do not recommend this as cast components are usually paired at the manufacturing foundry, so that the likelihood of combining components not originally assembled together [at the=”” foundry=””][/at], may not result in stable cover operation. If this process is unavoidable or preferred, please consult Wrekin Technical office for advice.

27. What does Wrekin’s Friction™ anti-skid coating provide in terms of traffic skid resistance?

The factory application of Wrekin’s Friction™ antiskid coating enhances the surface skid resistance of conventional cast iron manhole covers to a PSRV value in excess of that required by Highways England HA104/09 requirement for high skid-risk areas. Friction™ is manufactured from materials typically used in road anti-skid surface treatment applications. This means it benefits from the significant advantage of having its wear characteristics closely follow those of the surrounding road surface, which is a fundamental requirement in reducing the likelihood of induced vehicle-skid (from surfaces having differential skid resistance).

Steel FAQs

Frequently Asked Questions:

1. How do I know what size manhole cover I need?

Manhole cover dimensions are determined by the internal size of the access chamber, referred to as the clear opening, this is the hole through which a person or equipment can pass. To obtain an accurate clear opening, any existing cover or coverings must first be removed. The clear opening is critical as in many cases parts of the cover / frame hang within this void.

2. I can only take measurements of the cover & frame at surface level, can you work from these sizes?

We can produce a cover and frame to match these dimensions but internal frame opening may not align with the chamber. This is generally not an issue with side lid units but can cause problems with twin and multiple covers where support beams are incorporated, the beams must fit accurately between the chamber walls.

3. I have a chamber with a clear opening that is different from any shown on your web site what do I do?

Non-standard sizes are available on request, contact our sales department for a quotation.

4. Can I set two manhole covers side by side to cover a larger clear opening?

Not without installing a central support, all four sides of the cover and frame need to be supported to achieve the required loading. We can supply a single piece frame with multiple covers and a removable centre support bar for these situations.

5. I am installing a recessed cover & frame in a swimming pool area, are there any special requirements?

Due to the corrosive nature of swimming pool environments we would only recommend the use of fully stainless steel grade 316 products.

6. Can you supply a recessed cover and frame with a grade 316 stainless visible edge?

If the environment requires the use of grade 316 stainless steel for corrosion protection an edged cover & frame is not suitable. Stainless steel edges on products are for aesthetic purposes only, the base material (galvanised steel) is still susceptible to atmospheric conditions.

7. What is the definition of a slow moving wheel load (SMWL)?

Slow moving wheel loads are deemed to be speeds no greater than 20 mph in low intensity trafficked areas.

Geotextile FAQs

1. What is a Geotextile?

A geotextile is a man-made synthetic textile that can be used in a wide variety of Civil Engineering applications. In order to be  classed as a geotextile it must be permeable and it must separate the soil layers either side of it.

2. Why use a Geotextile?

Fundamentally the use of a geotextile will save money. By correctly selecting a quality product, manufactured in controlled traceable conditions, this will be achieved by a product which delivers consistent performance, that can be rapidly installed with a reduction in the use of raw materials. Furthermore, inclusion of a geotextile will be cost effective against soil or other construction materials with a carbon footprint that is much lower than traditional solutions.

3. How are Geotextiles manufactured and does this make a difference?

Geotextiles fall into two broad groups being either woven or non woven. Within these 2 groups there are also more subtle variances such as whether a non woven geotextile is thermally or mechanically bonded (or varying combinations of both). The method of manufacture will affect the properties of the finished product and how it performs.

For example, in the majority of cases a basic woven (such as FasTrack 609) or non woven (MultiTrack NW8) is suitable to use as a separator. However, woven geotextiles are generally better at providing additional reinforcement to the soil where as non woven geotextiles provide greater permeability and superior filtration.

4. What functions can a Geotextile perform on my project?

Depending on the type and grade of product used a geotextile can perform the functions of separation, filtration,  reinforcement, and protection. To a lesser extent a geotextile can also provide erosion protection or drainage (where fluid/ gas travels in the plane of the product rather than directly through it although this is often better provided by other geosynthetics)

5. What ranges of Geotexile do Wrekin offer, how do they differ?

Wrekin offer extensive ranges of both woven and non woven geotextiles so, unlike many suppliers, can offer the best product for a particular application rather than the ‘next best’ based on what they have to offer. Our woven geotextile range is called FasTrack and comes in Standard Grade (SG) and High Flow (HF) Our non woven geotextile range is called MultiTrack™ and comes as non woven (NW), Superior Needle Punched (SNW), and ‘coloured’ Needle Punched (VNW). The various ranges utilise thermal and mechanical bonding to different extents to produce products best suited to their expected application. Irrespective of the method of manufacture or extensive ranges enable us to offer the best product for any given application.

6. How are Geotextiles specified?

Geotextiles can be specified either by a number of performance properties (e.g. tensile strength, CBR puncture resistance), function (e.g. separation or filtration), or trade name and grade (e.g. Terram 1000).

The most common specification method in the UK is by trade name for which Wrekin have a range of equivalent, often superior, products against the established trade names.

7. Can Wrekin assist in choosing the correct geotextile for a particular scenario?

Yes. Regardless of which of the above specification methods has been used, Wrekin can provide technical assistance to select the correct product.

8. What are the key properties to assess to correctly specify a Geotextile?

The key properties will depend on what application the product is to be used in which will dictate which function or functions the geotextile needs to perform. In turn this will govern the relative importance of mechanical properties such as tensile strength, elongation, puncture resistance and cone drop, or hydraulic properties such as permeability and pore size.

9. What is tensile strength?

Tensile strength is measured by clamping a sample of product and pulling it in opposing directions. The strength, normally at break, is then recorded in kN/m. Certain applications may require strengths to be reported prior to the product breaking.

10. What is elongation?

Elongation is a measure of how much a sample of product has stretched from its original length when it is loaded. This is recorded as a % increase. Elongation values will vary widely between different geotextiles and can be an important factor in selecting the correct product.

11. What is CBR puncture resistance?

This is a measure of a product’s resistance to pushing a plunger through a clamped sample. The load required is recorded in Newtons. The sample’s vertical displacement from its original position can also be shown as the CBR displacement.

12. What is cone drop?

Cone drop measures the product’s resistance to installation damage and sharp stones in the ground. A weighted metal spike is dropped onto a clamped sample and the diameter of the hole is recorded in mm.

13. What is permeability and how does it differ between geotextiles/ types?

Permeability is the measure of the rate at which water will flow through a product. It is measured in litres/m2/sec or m/s. For example, the permeability of MultiTrack™ NW8 is 110 litres/m2/s, this is the same as 110 x 10-3m/s With the exception of specific High Flow woven geotextiles, non woven geotextiles will offer superior permeability.

14. What is the Geotextiles ‘Pore’ or ‘Opening’ Size ?

The pore size defines the size of the largest particle that can pass through the geotextile and defines its ability to trap particles and prevent their passage. Smaller and more consistent openings will be found in NON WOVEN geotextiles which is why they generally provide superior filtration performance.

15. Why do woven geotextiles have a higher tensile strength than non wovens?

This is only partly true and may also not be relevant to the application. The reason why woven geotextiles appear to have a higher strength is that the manufacturing process uses straight yarns of polymer. The product therefore takes up load more quickly when stressed than a non woven product where the fibres in the finished product are not straight. This difference in the manufacturing process also accounts for the difference in elongation performance between a woven and a non woven geotextile and also the fact that a non woven geotextile exhibits a more isotropic behaviour.

16. Are all geotextile tests standardised?

The correct test methods for geotextiles in the UK are defined by EN ISO. However, different countries have historically developed their own tests and even if you think you may be comparing like-for-like results this may not necessarily be the case. For example, the EN ISO test for recording tensile strength tests a particular width of sample at a defined loading rate. This may be different, and hence give different results, from the American (ASTM) testing or the German (DIN) testing. European tests are now standardised but you may still see specifications referring to old/historic tests such as BS or DIN. Similarly if a specification has been copied from an overseas project it may refer to an incorrect test method.

17. Is the Geotextile’s weight important?

No, weight is an identification property and, unlike mechanical or hydraulic properties does not affect how the geotextile will perform in-situ. Despite this, weight is frequently incorrectly included in the specification for a geotextile. This is very misleading as different types of geotextile of the same weight will have very different mechanical and hydraulic properties.

18. Should I use a geotextile manufactured from recycled fibre?

In the vast majority of applications only geotextiles manufactured from virgin polymer will offer a suitable product in terms of performance, cost effectiveness and design life. However in certain applications, only a short design life (e.g. <5yrs) may be required, and geotextiles manufactured from recycled fibre may provide a suitable solution. Wrekin offer their MultiTrack™ VNW range for this purpose.

19. Is the Geotextiles thickness important?

Thickness is fundamentally an identification property and as such does not directly contribute to the geotextiles performance. If specified on its own, thickness is meaningless. Even if specified in conjunction with another property, such as puncture resistance or weight, it can be misleading and prevent the approval of a suitable product or mean that an unsuitable product is approved. For example, products manufactured from virgin fibres will tend to achieve a puncture resistance using approximately half the weight of fibre required to achieve the same puncture resistance when recycled fibre is used. Generally the recycled fibre cannot be needled to the same degree as virgin fibre so will result in a significantly thicker product.

20. Which type and grade of Geotextile should I use for my project?

Fastrack™ SG609 – SG40/40: Standard separation and filtration geotextile with grade dependant on ground conditions and size of granular fill.

Fastrack™ HF180 – HF1300: Geotextile where high water flow rates are required

MultiTrack™ NW6 – NW45: Separation and filtration geotextile with high permeability and superior filtration properties. Required grade is dependant on ground conditions and size of granular fill.

MultiTrack™ SNW14 – SNW140: Separation and filtration geotextile with high puncture resistance to provide protection.

MultiTrack™ VNW200 – VNW2000 : Medium performance geotextiles manufactured from coloured fibres. Suitable where a thick cushioning layer and a high elongation are key design requirements.

21. What is the life expectancy of a Geotextile?

Provided that the geotextile is installed in accordance with installation instructions and it is not adversely exposed to degradation mechanisms such as heat or chemicals we are confident that our geotextiles will have a service life in excess of 60 years. This is in accordance with other manufacturers. If you come across a product with a design life in excess of 60 years then please contact us as there may well be caveats to this apparent extra longevity.

22. How soon should I cover the Geotextile on site?

To avoid the detrimental effect of UV degradation we recommend that our geotextiles are covered as soon as is practicably possibly and certainly within 7 days. If it is not possible to cover the geotextile within 7 days then please contact us to see if another product with higher UV protection may be more suitable.

23. How can Geotextiles be joined?

There are several ways in which geotextiles can be joined such as overlapping, sewing, gluing / taping, or leistering. Of these, the most commonly used method in the UK is overlapping. As a rule we would suggest an overlap of 300-500mm to ensure a full overlap is maintained once any irregularities in the sub grade and post construction settlement have been accounted for. In certain applications, such as underwater, the overlap may need to be increased.

24. Can Geotextiles be installed underwater?

Yes. However, when installed in water the majority of geotextiles will float hence specific consideration needs to given to the installation method as this may, for example, require divers, methods to sink the geotextile, and bespoke lengths of geotextile to avoid jointing/cutting the product underwater.

25. Can Geotextiles get damaged and is one type more susceptible than another?

All geotextiles can get damaged during the installation process especially if the installation guidelines are not adhered to. Installation stresses can be one of the most severe a product will be subjected to hence if instructions are not followed then the effect on a product’s properties can be detrimental.

It is often believed that woven geotextiles are more susceptible to installation damage than non woven geotextiles. Although this is a widely debated point there is little or no actual evidence to support this. As Wrekin offer both woven and non woven geotextile ranges, unlike manufacturers who only have one range, we have a balanced view with no ‘axe to grind’.

26. Can I drive on the Geotextile before covering it with stone?

No. This will cause significant damage to the product.

27. What types of Polymer are used to manufacture Geotextiles?

The main polymers used in the manufacture of geotextiles are polypropylene, polyester, polyethylene, polyvinylidene and polyamide. Of these the most commonly used is polypropylene.

Different polymers will have different properties such as melting point, specific gravity, resistance to acid and alkali, creep resistance and flammability and these may be relevant in specifying the correct product for an application.

28. How can holes in the Geotextile be repaired?

If the Installation Instructions are followed then this should not be an issue. However if required the best remedial measure would be to overlay the affected area with a ‘patch’ of the same product ensuring an overlap of at least 300mm around the hole/s.

29. What grade of Geotextile should I use with which grade of stone fill?

Should you require we can offer more advice but as a ‘rule of thumb’ the larger the stone size the more robust the grade of geotextile that is required. For example, for a stone with a maximum size of 200-250mm we would recommend our MultiTrack™ NW26 whereas for stone with a maximum size up to approximately 50mm either our MultiTrack™ NW8 or FasTrack™ 609 should be adequate.

30. Can a Geotextile be used for weed suppression?

Any geotextile will offer a degree of weed suppression however for the best results we have our Weedstoppa product. It should be borne in mind that after installation new seeds will be brought in and dropped above the geotextile, for example, by wind or birds.

Geogrid FAQs

1. What are Geogrids?

Geogrids are geosynthetic material made from polymers such as polypropylene, polyethylene or polyester and are used widely in Civil Engineering applications to provide tensile reinforcement of soil. They are in the form of open grids so that soil can strike through the apertures and the two materials interlock together to give composite behaviour. They are used in the construction of retaining walls, steep slopes, roadway bases and foundations.

2. What functions can a Geogrid perform?

The primary function a Geogrid will perform is reinforcement. Depending on the application under consideration either a uniaxial (strength in one direction) or biaxial (strength in all directions) Geogrid will be required.

3. Why use a Geogrid?

The correct use of a Geogrid can offer many benefits to a scheme such as increasing the speed of construction, and/or reducing the quantity of soil that needs to be exported from/imported to a site. Common uses of Geogrids include increasing the amount of usable land on a site by enabling construction of steep green slopes or walls, enabling construction of a road over poor ground conditions or decreasing the thickness of fill required to construct a road.

4. How are Geogrids manufactured and does this make a difference?

Currently there are three categories of geogrids available:

The first, and ‘original’, geogrids are commonly referred to as ‘punched and drawn’ geogrids.

A sheet of either HDPE or Polypropylene has holes punched into it in a regular pattern and the sheet is then ‘drawn’ or ‘stretched’ into the finished product. The drawing is done under controlled conditions of temperature and strain rates to avoid fracture whilst allowing ductile flow of the molecular chains. This operation aligns the molecular chains in the direction of drawing to convert low-strength polymer into high-strength grids.

The second category of Geogrids is ‘coated yarn’ types. They are, in fact, technical textiles in the form of grids and use bundles of fibres (most commonly Polyester) as the reinforcing component that are then coated to provide protection during installation and in service. The grid structure is formed by knitting or intertwining the transverse and longitudinal bundles of fibres.

The third category of Geogrids is made by laser or ultrasonically welding together polyester or polypropylene rods or straps (as used in packaging/shipping) in a gridlike pattern.

5. What ranges of Geogrids do Wrekin offer?

Wrekin currently offer two ranges of Geogrid. The first range is our E’Grid™ Geogrids which are ‘punched and drawn’ Geogrids, with a design life of 100 years. The second range is our SX Grid™ Geogrids which are a cost effective, punched and drawn Geogrid with a 50 year design life. Please contact us for advice on the most suitable type of Geogrid for your application.

6. Does the method of manufacture make a difference?

Yes. The different methods of manufacture create products that look and feel quite different. It therefore follows that the different forms of Geogrid will work with the soil to perform the reinforcing function in differing ways.

There have been numerous studies into the performance of Geogrids. These have concluded that whilst there are a number of mechanisms which enable a geogrid to function, the principal and most effective mechanism is lateral restraint or confinement of the compacted fill that is interlocked within the grid.

The best type of Geogrid for mobilising this mechanism is a ‘punched and drawn’ geogrid such as E’Grid™ and SX Grid™ geogrid.

7. How are Geogrids specified?

Geogrids will either be uniaxial or biaxial and can be specified either by a number of performance properties (e.g. tensile strength, junction effciency), or trade name and grade (e.g. E’Grid 3030™).

The most common method of specification in the UK is by trade name for which Wrekin have a range of equivalent, often superior, products against other trade names. Wrekin are the only supplier in the UK that can offer true alternatives to the geogrids manufactured by the market leader, Tensar.

8. Can Wrekin assist in choosing the correct Geogrid for a particular scenario?

Yes. Wrekin can provide technical assistance to select the correct product and number of layers of geogrid required for a particular scheme.

9. What are the key properties to ensure you have a quality geogrid that is ‘fit for purpose’?

The key attributes to consider in selecting a Geogrid are the height and thickness of the rib, the aperture area, the tensile loads at 2% and 5% strain, the junction efficiency, the open area percentage, the carbon black content, the quality of the raw material used, whether the product has a CE Mark with full traceability, and whether it has been manufactured under a certified ISO 9001 system.

10. Does the aperture shape make a difference?

No. For 25 years Geogrids were constructed with square or rectangular apertures, however, since 2007 geogrids with triangular apertures have been available. The introduction of a new aperture shape was purely a commercial decision by the manufacturer concerned. No data has been published to show superior performance of the triangular grid.

However, to evaluate the new product Wrekin and it’s manufacturer undertook comparative testing which concluded that E’Grid™ geogrid is at least as good as the triangular aperture TriAx geogrid.

11. Does the aperture size make a difference?

Yes. The geogrid needs to be designed to optimise the key physical attributes listed previously – essentially to get the best possible mix of the key attributes from a finite sheet of polymer. One of these key attributes is aperture size.

To ensure the best possible mechanical interlock with the soil particles Wrekin offer two different aperture sizes in their E’Grid™ geogrid range – standard and large aperture. Standard aperture E’Grid™ geogrid is used where the average particle size is a maximum of approximately 50-60mm. Beyond this the large aperture E’Grid™ products should be used.

12. Does the rib height make a difference?

Yes. The prime mechanism for reinforcing the soil is mechanical interlock which is achieved by the soil particles pushing against the vertical face of the ribs. A higher rib is therefore better as it provides a larger surface to effectively transfer the stresses from the soil to the geogrid.

E’Grid™ geogrids have been designed to optimise the rib height. Lesser quality geogrids will have shallower ribs and may not state this parameter on their data sheets to conceal this inferiority.

13. Does the rib width make a difference?

Yes. The width of a geogrid rib is a key factor in the ability of the geogrid to adequately survive the not insignificant stresses when the granular material is spread over the geogrid and then mechanically compacted. A narrower rib will clearly be more susceptible to installation damage thereby rendering the geogrid less able to perform its function during the service life.

The ribs which make the TriAx geogrid are narrower than those of E’Grid™ geogrids, and testing has shown that, as would be expected, they are far more susceptible to installation damage and potentially lose around 25% of their original strength.

14. Is the junction efficiency important?

Yes. Junction efficiency is a measure of the strength of the node compared to the strength of the rib (expressed as a %). Both the rib strength and the junction strength are important since the soil particles will impart load against the transverse ribs which will then be transmitted to the longitudinal ribs through the junctions and vice versa.

E’Grid™ geogrids have been designed to have excellent junction efficiency >=95%.

15. What is tensile strength?

Tensile strength is measured by clamping a sample of product and pulling it in opposing directions. The strength is then recorded in kN/m either at break (Ultimate Tensile Strength) or at varying elongations. Certain applications may require strengths to be reported prior to the product breaking. In particular, for the low strain conditions in road construction, the load measured at 2% strain is a good indicator of performance potential.

16. What is elongation?

Elongation is a measure of how much a sample of product has stretched from its original length when it is loaded. This is recorded as a % increase (strain).

Elongation values will vary widely between different geogrids and can be an important factor in selecting the correct product.

17. Are all Geogrid tests standardised?

The principal test methods for comparing geogrids are standardised. These include tensile strength, creep tests, junction efficiency and physical dimensions. With the exception of the Junction Efficiency test, which is a USA ASTM test, the test methodologies for products used in the UK market should be EN ISO tests. However, the triangular aperture grid introduced in 2005 cannot be tested using conventional and established methods for tensile strength to give meaningful results. Its performance can therefore not be measured against other products. To address this issue, Wrekin and its geogrid manufacturer have developed a test that can measure the omni-axial performance in the X-Y plane of any grid, irrespective of its aperture shape. Additionally the test better represents the plate loading effect of a wheel on a carriageway in a real scenario.

18. Is the Geogrid weight important?

Whilst the weight of a geogrid will have an impact on the various mechanical properties of a particular geogrid, it does not itself provide values that can be used in a design or for checking quality. Generally, lower quality geogrids will have higher weight for a given strength than higher quality products.

19. Should I use a Geogrid manufactured from recycled material?

No. A product made (or part made) from recycled material will not provide the optimum parameters for a given application and will therefore be less efficient. The variable quality of post-consumer re-cycle material prevents the manufacture of high strength materials with uniform molecular alignment.

20. Which type and grade of Geogrid should I use for my project?

Wrekin can offer technical guidance on which grade and type of geogrid to use. If the geogrid is to be used in a uniaxial application then factors such as height of slope, surcharge loads, and soil parameters will be relevant to establish the correct product. Alternatively, if the geogrid is to be used in a biaxial application then axle loadings, the CBR of the subgrade and the size of granular fill to be used will be relevant.

21. How can Geogrids be joined?

Depending on the application geogrids may need to be joined. This is particularly the case when using uniaxial geogrids and a common way of doing this is by means of a ‘bodkin’ joint.

When biaxial geogrids are used, by far the most common procedure is to have a simple overlap on adjoining edges of 300-500mm. In certain applications a larger overlap (or bond length) may be required by the design.

UniPak FAQs

Frequently Asked Questions:

Assumptions made in the following advice:

  • The chamber top construction on which the UniPak system will sit is in a robust condition and is suitably prepared (dry and clean with a textured surface for good bonding).
  • Installation operatives are familiar with and trained in the conventional method of highway ironwork installation. E.g. Frame tamping.
  • Installation operatives are competent in using mechanical mixing techniques with mortar materials.
1. How many 25Kg tubs of UniPak mortar do I need to install my manhole cover?

There is no definitive quantity of mortar, the amount needed depends on the volume or size of gap below the frame that needs filling and whether other packing materials are to be used in the bedding construction. The general rule for calculation is based on a cured mortar density of 2,000Kg/m³.

Wrekin can provide guidance on this for a given product size and known bedding depth.

2. Are wet conditions problematic for Polyester Resin based mortars, like UniPak?

Ideally, chamber top slabs should be in a dry condition when receiving a bed of UniPak mortar in order maximise the likelihood of bonding with the overlaying bedding mortar. However, whilst this should always be attempted (e.g. by force drying), it is sometimes impractical to expect UK climatic conditions to allow this scenario. But with over 20 years history and experience of Polyester Resin’s successful service in highway civils situations, there has been no definitive evidence of a particular sensitivity to moist environments. Indeed, cured Polyester Resins as known to be resistant to all types water (e.g Sea, Distilled, Fresh, etc…), so this would suggest that simply protecting the unmixed ingredients from direct, excessive moisture (e.g. heavy rain) will afford all the protection the material needs to fulfil its expected requirement.

Note: Good practice should always be to prevent constant water immersion of bedding materials as the water could contain salts that are detrimental to such bedding.    

3. The instructions describe compaction of the as-laid mortar bed(s).  Is this important?

Yes, it’s vital.  In order to achieve the optimum bond strength between resin-coated granular particles, those particles need to maximise the contact area between them. i.e. be in intimate contact. This is effectively achieved through compaction and can result from conventional Frame Tamping operations and/or manual compaction.

4. The instructions describe the need for a clean chamber top/slab surface for the UniPak mortar bed. Is this important?

Yes, this is an important preparation step as the UniPak mortar has high adhesion properties, which means that it will stick to the first solid material it comes into contact with. If that is dirt, dust or loose debris, that’s what the UniPak mortar will stick to… not the chamber top/slab.

Note: Grease and Oil chamber top contaminants are particularly detrimental to mortar adhesion as these used in the moulding industry to deliberately prevent the materials they’re using from sticking to the moulds.

5. The instructions describe that it is advantageous to have a roughened or textured chamber top on which to apply the UniPak bedding mortar, why is this?

Again, this is a particularly useful preparatory step in optimising the bedding construction as roughened (but clean) chamber top surfaces increase the contact area with the bedding mortar, thereby increasing the likelihood of adhesion between the two. Additionally, a textured the surface also allows mechanical interlock between the mortar and chamber top so that lateral movement between the two is prevented, even in the event of loss of adhesion.

6. Can I mix less than the recommended 1:1 (tin-to-bag) ratio?

No, as any variation from this ratio will; affect the mortar’s expected mechanical and physical properties, extend the curing time or even adversely affect its ability to fully cure.

The blend for the 1:1 mix ratio has been carefully selected to achieve the optimum balance of properties.

7. Can I dispose of unmixed UniPak mortar ingredients in landfill?

No. Only mixed (in a 1:1 resin-to-powder ratio) and cured UniPak mortar may be sent to landfill as in this condition, it forms an inert, non-hazardous material. Ideally, any excess ingredients should  be mixed and used in the ironwork installation.

Note: UniPak mortar’s packaging containers may be recycled where suitable facilities exist (contact Wrekin for details of materials employed in same).

8. Polyester Resin mortar is known to have a characteristic odour so do I need to take any particular precautions in using it?

Firstly, always wear appropriate PPE and read the Material Safety Data Sheet (MSDS) before using a particular material, working in accordance with its safety guidelines. UniPak’s MSDS is available at the bottom of this page and further directions are located on its packaging/mixing tubs.   Additional to this, apply logical practical actions. E.g. stand on the upwind side of mixing tubs, close opened resin containers where possible, avoid wiping uncured mortar on clothing, limit dust generation of the powder component in the mixing tub by slowly adding it to the resin or using the resin as a seal on top of the powder, etc…

Note: Mixed UniPak mortar forms an inert, non-hazardous material, so any excess ingredients should always be mixed (in a 1:1 resin-to-powder ratio) for use or render it safe for disposal.

9. When would I use UniPak Wedges?

As with any chemical reaction (curing in the case of bedding mortar), the speed of it is affected by temperature. Generally, a higher temperature will result in a faster reaction (i.e. a shorter curing/working time in the case of mortar).

Therefore, when working with UniPak mortar in elevated atmospheric temperatures, the mortar’s working time can be reduced to minutes.   In these circumstances, it may not be possible to mix & lay a full bed of mortar and have time to Tamp ironwork frames into it before the mortar has ‘gone-off’.   In such conditions, UniPak wedges can be used to align an ironwork frame with the surrounding road surface (in lieu of frame Tamping), thereby allowing mixable volumes of UniPak to be mixed and applied sequentially under and over the wedge-supported ironwork frame.

Note: UniPak Wedges may be left in-situ as a set of 4nr are themselves capable of supporting a 400kN (c.40-tonne) load.

10. I am planning to use UniPak Wedges in the installation construction. Will these need to be removed prior to trafficking?

Not necessarily, particularly if they’ve been fitted around the outside flange of the frame, rather than from the chamber-opening side of the frame.   Leaving the wedges in-situ is permissible because they (like UniPak mortar) are also manufactured from polyester resin and hence, form a coherent structure in cured polyester resin bedding structure. If wedge removal is preferred, this can be achieved once partial cure of the installed UniPak mortar has occurred by simply pulling the wedges out from under the frame or cutting off any unwanted protruding parts. Where complete removal of the wedges has been carried out, the resulting bedding void should be filled with a fresh mix of UniPak mortar.

Note: UniPak Wedges may be left in-situ as a set of 4nr are themselves capable of supporting a 400kN (c.40-tonne) load.

11. I am planning to use UniPak packing plates in the installation construction. Are there any characteristics of these accessories that I need to be mindful of?

Yes. When laying the packing plates into the mortar bed, they should NOT be tamped into it, only firmly pushed into the bedding using a side-to-side motion. This motion allows the packing plate to compact the underlying bedding and ensures that it also effectively interlocks with it.

Note: Tamping of packing plates is not recommended as they can flex under this form of impact load and thus create a void in the bedding.

12. When I’ve mixed a batch of UniPak mortar, my mixing paddle is heavily coated with a residual mixture. How can I remove this for further reuse of the paddle?

UniPak mortar incorporates deliberately-high adhesion properties which are indiscriminate in what they stick to, including mixing paddles.   The remedy is; immerse the contaminated mixing paddle in a bucket or fine, sharp grit/gravel at the earliest opportunity following mixing of the UniPak mortar, then simply replicate the mixing technique in the gravel until the UniPak is eroded off the paddle.

13. After applying the UniPak mortar on top of an ironwork Frame Flange, should I leave the surface float-finished?

Ideally no. Whilst a float-finished surface looks neat, it is difficult for the intended overlying reinstatement materials to bond to it. Therefore, by all means, float the surface to ensure mortar compaction, but then lightly abrade it using for example; a stiff bristle brush. This roughened bedding surface provides an increased contact area to bond with the reinstatement materials and also allows their particulates (e.g. aggregates) to ‘key’ or mechanically interlock with the roughened surface. Thus there are two major benefits in not leaving the bedding float-finished.

Note: Where the overlying reinstatement material is Bitumen-based (e.g. Asphalt), such materials are themselves a derivative of Oil, which is also a lubricant.  Therefore, to expect a lubricant-based reinstatement material to adhere well to a smooth, float-finished bedding surface is somewhat unreasonable. Hence, to improve the situation, a final float finish to bedding should be avoided.

14. Can I use the UniPak mortar as a trafficable surface.  i.e. In lieu of asphalt, concrete, etc?

Whilst it is possible to use a cured mortar as a trafficable surface, Wrekin don’t recommend using UniPak in this scenario as its polymer compound is not formulated to resist exposure to highly-abrasive environments, like trafficked surfaces.

15. My UniPak mortar seems to go off too fast for me to tamp my ironwork frame, what should I do?  

See FAQ 9 for the installation technique used in rapid cure situations.

16. Does the UniPak Mortar need to be laid in a particular bed thickness?  

Yes, the minimum bed thickness should be 10mm and may be as much as 75mm thick where site conditions dictate.   However, where a thick layer of bedding is required, it may not be possible to lay it in one pass and, indeed, many codes of practice dictate that multiple bedding layers should be employed.   Further, where a thick layer of bedding is required, it is worthwhile considering employing either a further course of Engineering Bricks or UniPak Packing Plates as bulk-fillers between separate beds of UniPak Mortar.