Read aggregate paths.PDF text version

AGGREGATE & MINERAL PATHS and TRACKS

Contents Background Geology Slopes Traditional Construction methods Review of traditional methods Modern Aggregate Construction methods Review of Modern Aggregate paths Compaction & Consolidation Revegetation Design & Maintenance Standards ANNEX 1 Pennine Way Aggregate Surfaces ANNEX 2 Geology of Pennine Aggregates ANNEX 3 Aftercare of Hymac or Soil Inversion paths

1.0 1.1

Background The Pennine Way has 106 km of aggregate paths and tracks and another 36 km of grass tracks where natural vegetation lies over an aggregate base. Of the trail's total length of manmade surfaces, aggregates account for 65%. The vast majority of these are tracks, paths and roads which serve farms and forest, and formerly, lead or coal mines. There are both traditional and modern aggregate surfaces. They may be many hundreds of years old and include Roman roads in the Cheviots as well as Scheduled Ancient Monuments like parts of the Maiden's Way and the [so called] Roman Road off Blackstone Edge [actually an 18th Century Turnpike]. They may be recently constructed as both the two major 1980's projects in the Three Peaks [Project Officer Simon Rose] and the Peak Park [Project Officer Mollie Porter] worked extensively on rafted geotextile and aggregate paths. Since 1990 and the run down of these two high profile projects, there has been no major programme of constructing imported aggregate paths as such, though short sections of aggregate paths have been made. The use of in situ materials was pioneered on the West Highland Way in the late 1980s and then trialled in the Three Peaks area in 1989. There has been extensive use of this technique since and over 25 km of mineral surfaced Hymac or inverted subsoil paths were built on the Pennine Way between 1990 and 1996. Short sections of hand built `stalker's' type paths using in situ materials including aggregates have also been built. Other failed surfaces have been rebuilt or resurfaced. This study will cover the range of aggregate surfaces and aims to reach conclusions on current best practice through review of recent experimental works and empirical study of long established aggregate surfaces. Geology Pennine geology is mainly Carboniferous with a range of sandstone and gritstones, shales and limestones. These sedimentary rocks are dominant. Locally, igneous rocks may be important. In Upper Teesdale and along Hadrian's Wall whinstone may be used where the Whin Sill outcrops, and in the Cheviots there are outcrops of andesites and other basic and intermediate volcanics. There are not vast deposits of unsorted glacial drifts in the Pennines, as there are in other upland areas, nor are there extensive screes, alluvial or fluvioglacial deposits in river valleys, although alluvial silts and pebbles are locally important. The classic moorland soil profile is of blanket peat of depths up to six metres with a grey or buff clay below, usually about one metre thick, overlaying solid geology with some periglacially weathered sandstone gravels. Slopes Aggregate paths and tracks are more often found on reasonably level ground and slopes up to 5?. Stable aggregate surfaces can exist on slopes between 5? and 10? and, with occasional random stepped risers, even up to 12?. There is an absence of empirical work on aggregate surfaces on steep slopes despite the fact that a random, long stepped, consolidated, aggregate path may be more acceptable aesthetically than a stone pitched path in mountain areas with certain rock types, for example, granite. Constructed aggregate paths on steeper slopes should always be consolidated and not loose. There are problems with lateral spread, rainwash and gullying even on shallow slopes as low as 3? with loose aggregates. Stabilising measures are essential on all unconsolidated aggregates on slopes over 5?. These may include waterbreaks or waterbars, side and cross drainage, and edge stabilisation against lateral spread. Traditional construction methods McAdam and Telford There are lessons for path makers at the end of the 20th Century in the approach of early road builders, particularly McAdam and Telford. The outline of some of their approach is taken from W.J. Reader "MACADAM - the McAdam family and the Turnpike Roads 1798-1861" Heinemann 1980 John Loudon McAdam [1756-1836] developed the hard smooth road surface in the early 19th Century that has forever associated his name with a road type. This is a layer of clean broken

1.2

1.3

1.4

2.0 2.1

2.2

3.0 3.1

4.0 4.1

4.2

angular stone resting directly on the subsoil. The stone is what would now be termed `25mm down'. "Every piece of stone which exceeds an inch in any of its dimensions is mischievous" said McAdam in 1811. He strongly objected to any binding materials be it sand, clay or soil. The reasons for this were twofold. Binding inhibited the free draining of water from the surface and also raised clouds of dust in dry conditions - a major hazard to road users. In fact many roads were watered in summer to keep dust down. The clean stone was `consolidated' directly by the weight of traffic. 4.3 JL McAdam's instructions for repairing a road were as follows [from the Select Committee on Highways and Turnpike Roads 1810-11]. "No addition of materials is to be brought to the road, unless in any part of it is found that there is not already in it a quantity of clear stone equal to a foot thick. The stone in the road is to be loosed up to the depth of a foot and broken so as to pass through a screen or harp of an inch in opening, by which no stone above an inch in any of its dimensions can be admitted. The road is then to be laid as flat as possible, if it is not hollow in the middle it is sufficient; the less it is rounded the better; water cannot stand upon a level surface. The broken stone is then to be laid evenly on it, but if half or six inches is laid on first, and exposed a short time to the pressure of carriages, and then a second coat of six inches laid on, it has been found advantageous in consolidating the materials. Carriages, whatever are the construction of the wheels, will make ruts in a new made road, however well the materials may be prepared, or however judiciously applied, therefore a careful person must attend for some time after the road is opened for use, to take in the track that is made by the wheels..... A rake of iron with short teeth, not to exceed and inch and a half in length; the head ten inches long, is to be employed by a careful man, in raking the track crossways when the road is first used. Tracks will not occur again after the road has settled, the whole mass will become like one solid smooth surfaced stone..... Every road is to be made of broken stone without mixture of earth or any other matter; no large stone to be employed on pretence of bottomong, nor any sand, earth, or other matter on pretence of binding..... A road of stone effectually broken, will be a smooth hard even surface, it cannot be affected by water or by frost, and will therefore be equally good at all seasons of the year. Stone, in some form, is to be found in every part of this island, and therefore every road in the kingdom may be equally good". 4.4 Thomas Telford was a contemporary of McAdam but was critical of some of his methods, particularly regarding the need for a sub base. Telford recommended similar stone size to McAdam -stones laid upon the road should `weigh less six to eight ounces' [150-200 gms] and be clean gravel with rounded stones being broken up by hammer, but thereafter his prescription differed. He favoured a rolling crown - `a very gentle curve upon the cross section'; and argued for the Nineteenth Century equivalent of geotextiles - if a road were built upon `clay or other elastic substance, which would retain water' it should have a layer of `vegetable soil' between the surface and the clay. Telford considered that the small stones needed a `rough pavement' beneath them ` or otherwise, `a portion of the broken stone metalling comes in contact with the earth, sinks into it, works unequally, and can never be rendered so perfectly uniform as when the layer of broken stone is placed upon a proper pavement'. McAdam maintained that the uniformly small graded one inch down material was sufficient in his evidence to the 1819 Select Committee: What depth of solid materials would you think it right to put upon a road, in order to repair it properly? I should think that ten inches of well consolidated materials is equal to carry anything. That is provided the substratum is sound? No ; I should not care whether the substratum was soft or hard; I should rather prefer a soft one to a hard one You don't mean you would prefer a bog? If it was not such a bog as would not allow a man to walk over, I should prefer it." McAdam went on to give the road [now the A38] over the soft ground between Bridgewater and Cross, [Somerset Levels] as an example of the veracity of his approach. He restated his conviction that he never used large stones on the bottom of a road, nor would he put faggots or other layer between the ground surface and road.

4.5

4.6

"Q:

Q

Q

4.7

4.8

McAdam was an empiricist, as was Telford, but McAdam's specification was more minimalist than Telford's in terms of load bearing capacity. Both men contributed significantly to highway engineering and many aggregate path specifications have been and are still, in many cases, merely watered down road specifications based upon their pioneering work almost two hundred years ago. Other Traditional Surfaces Most vernacular aggregate surfaces in the Pennines still in existence were not built by road builders of the reputation or methods of McAdam or Telford. They were ancient drove roads, some on the lines of old Roman Roads; some Nineteenth century access tracks built to give farmers access to newly enclosed allotments; and some were miners' tracks to coal and lead workings in the North Pennines and Yorkshire Dales. Many of these are in good condition and are in equilibrium. That is, they are relatively free from erosion damage and, unless damaged by high levels of vehicular traffic, relatively low maintenance. The tracks were built for sheep and cattle driving and horse and cart traffic. They now represent a well integrated part of the landscape. The evidence of the construction methods is entirely contained in the surfaces themselves. They are all well consolidated with compacted stone in the range 15 -75 mm in a matrix of soil or other mineral material [cf. McAdam]. Most tracks are wheel rutted, but some surfaces showing less usage still have evidence of a slight rolling crown to shed water into side drains. Frequently cut in, some tracks do not have side drainage and some consolidated surfaces are allowed to carry run off along their length. There is evidence of side drains that have become silted up over time but others show no sign of ever having had any drainage. Where water crosses these traditional aggregate paths there is usually a culverted cross drain. There is evidence of edge revetting on some well built surfaces above mean ground level, others have a grassed verge. There is no evidence of `vegetable soil' or `faggots' being laid as a sub base on soft ground; rather there seems to have been more reliance on a `fill to refusal' approach. In many cases there does not seem to have been any excavation with stone simply laid on the topsoil and rammed in, though it is difficult to confirm this on site, given the well revegetated nature of a lot of these surfaces. [The majority of grass track surfaces have an aggregate surface underneath]. Review of the Characteristics of Traditional Aggregate Surfaces All surfaces were built for people, sheep and cattle droving or horse and cart traffic [not for modern vehicular traffic], hence they were built to empirically determined loading standards; They all used in situ materials or that derived from nearby delphs, borrow pits or mine waste; Some wearing courses are very long established and are stable; The most stable surfaces have a continuous visible wearing course with little or no matrix material showing; There are no obvious problems with aggregates consolidated directly into clay subsoils; Most aggregates are unscreened, i.e. have a random mix of fragment sizes; Fragment sizes commonly vary from 6mm to 75mm [0.25in to 3in]; The best consolidation is evident in angular or subangular materials of 12 -50mm; Coarser particled rocks [e.g.sands/grits] consolidate better than fine grained [e.g.limestones]; The worst consolidation is evident in rounded pebbles of fine grained material; Most are now rutted but there is evidence of slight rolling crowns; Older aggregate paths tend to have fewer waterbreaks or even none at all; Traditional and natural aggregate paths perform well on slopes up to the maximum for aggregate surfaces with random risers or `long steps' usually every 2-3 metres run.

4.9

4.10

4.11

5.0 1.

2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16.

There is little evidence of pure `Macadamming' with clean stone; There is little or no evidence of graded construction; Over substantially boggy ground many paths show `turnpike' construction or have shallow revetments to build up above the zone of saturation.

Note that despite the original type and volume of traffic, many consolidated aggregate tracks have coped reasonably well with tractors and 4WD vehicles. However, when the surface wearing course is broken through exposing mineral or clay substrate, damage accelerates very rapidly.

6.0 6.1

Modern Aggregate Path Construction Methods This account and review will not cover existing published material. Readers with a specific interest in modern aggregate construction are recommended to refer to the reports of the Three Peaks Project and Pennine Way Management Project for detailed accounts of the experimental work done at that time. Most of these surfaces have since been replaced and are no longer available for inspection, yet the technical background is essential reading. There are three basic methods of constructing aggregate paths with variations providing a range of individual specifications. Variations are usually of two types:Type of Aggregate

6.2

6.3 a]

Types 1 and 2 DoT sub-base. These are standard road specifications used as surfacing on paths, sometimes with a blinding layer of 4mm to dust to `seal off' the top. They can be of different source materials - the specification refers to the percentage passing through different screens. Type 1, the more commonly used specification, is basically a 75mm down sorted specification. Screened specifications These are customised aggregate specifications. For example, the whinstone specification used on the High Force path breaks down as follows:40-60mm 25mm 12- 18mm 6-12mm 4mm to dust [fines] 12.5% 12.5% 12.5% 12.5% 50%

The key is in balancing the proportion of fines to larger material so that the material binds. This will vary with the geological type of stone used. Generally, path builders use between 40-60% fines with other material screened up to 25mm - this, at least, is in line with McAdam's 19th Century specification. For the High Force specification the larger material [40 mm +] was included to provide variety in the size range and hence texture of the wearing course. Random aggregates are visually better as an upland path surface. Quarry Bottoms This refers to spilled materials scraped up from below the screens and other unsorted aggregates. There is no standard specification but path makers have usually chosen quarry bottoms on inspection. It is cheaper than a screened aggregate. Often there is a high percentage of fines. b] Grids Woven geotextiles Type of Geofabric Tensar Lotrak Extruded plastic grids of various mesh sizes Woven plastic fabrics of various weights and weave densities Compressed felt type fabric [needled] of different

Non-woven fabric

Terram

weights On many paths geotextiles have been used to separate imported aggregates from the substrate. Both Three Peaks and Peak Park Pennine Way Projects did extensive tests in the late 1980s on various combinations. The detailed reports of these projects should be referred to for the experimental conclusions. The rationale for current recommended use of geofabrics for the Pennine Way is as follows [See 10.3 for best practice notes]; R The installation of geofabrics adds another complication and cost into the management equation. In the past their use has often been unnecessary and their inclusion in a specification has been part of the whole process of over-engineering paths. Alternatives are fill to refusal - as is the usual practice for Hymac paths, and use of a mulch layer of cut heather , brash etc. [The West Highland Railway was floated on brash over Rannoch Moor]; Geofabrics have caused problems where they have come to the surface and they are very difficult to remove from rebuilds of failed aggregate paths. Plastic materials exposed or buried in the environment are generally not an indication of good management practices; In practice geofabrics have become exposed through 4" [100mm] of compacted material, or where washouts occur on unconsolidated aggregates over geofabrics. The loading of paths is much lower than surfaces designed for vehicular traffic and the upwards migration of clays is rarely a problem on paths. It is often better to have path materials bound into the substrate. Where clays might pump up, it is easier to top up with aggregate than remove exposed geofabric and start again. Aggregates filled over geotextiles [not geogrid] can slide off downslope for paths not properly benched on cross slopes and graded on long slopes. In these circumstances it is more important to ensure the aggregate is consolidated into the subbase. There is tendency for geofabrics to become exposed, particularly at path edges. Geotextiles near the surface can actually hinder adequate revegetation establishment by providing a barrier to root growth. It is often better to work with grids than fabrics. Aggregate Road specification Path specifications are frequently derived from contemporary road specifications using either McAdam or Telford as a `role' model. There are two types; Single layer of graded material [McAdam type] Multiple layers of different grades [Telford type] 6.5 The conventional engineering approach would be to test the sub-soil for California Bearing Ratio [CBR], which is a measurement for load bearing calculations, and then decide on the type of construction. In practice aggregate road construction [with or without a geotextile] has often been aimed at the railroad method of construction. This means over engineering the path to take dumper trucks or equivalent to carry the aggregate along the path to the point of construction. It also means a wider path than is necessary for foot traffic [usually c.1.8 metres, but may be up to 3 metres], and problems with lateral spread. The surface is then finished by working backwards raking out and rolling in with a vibrating roller to compact the surface. A blinding layer of 4 mm down may be laid. Edges may be stabilised to hold back lateral spread and washouts across the path.

R

R R

R

R

6.4

6.6

Geofabric and Aggregate A layer of geotextile or geogrid is laid directly on the ground and overfilled with aggregate [usually Type 1]. There are several types of specification depending on whether the geofabric is laid directly or a trench [tray] dug to take the geofabric before backfilling with aggregate. The path is then rolled in and compacted, though vibrating rollers are ineffective on mobile subsoils like peat because the vibrating action works from the bottom up and the interface between geotextile and subsoil will slurry, or collapse, if it is peat. Peat has thixotropic qualities. That is, peat is a solid that liquefies under pressure variations, such as trampling. Edge stabilisation is usually essential, partly to ensure that geofabrics do not show and partly to

prevent lateral spread. 6.7 In practice, the use of fabric geotextiles was abandoned in the major projects around 1989-90. This is because geogrids were more effective in the trials then undertaken [Three Peaks Project and Peak Park Pennine Way Management Project]. Geotextile reinforcement techniques have been trialled by Bradford M.B. over a number of years. These are now only used over mineral subsoil and do not involve new aggregate. The roll of terracoir [a biodegradable woven fabric] is laid on top of the existing surface, pinned down and seeded. The matting acts as a wearing course and has a life of two to five years, depending on local conditions. The main variables are volume of foot traffic, altitude, aspect, slope and soil acidity. The matting has two immediate effects. Firstly, it traps any loose mineral materials and prevents washouts. Secondly, it stabilises the path edges and provides a significantly improved chance of strong edge vegetation recovery. The slow degrading of the fibres into the path surface tends to improve consolidation of mineral materials. Unlike Geojute, the fibres are short. Long strings of half rotted geofibre are not a characteristic of the rotting down process. Hymac or Inverted subsoil This method was originated on the West Highland Way in the late 1980s on peat soils. It uses a Hymac or equivalent single hydraulic armed plant to dig through peat and bring mineral subsoil to the surface. This is basically a linear borrow pit. The mineral substrate is then laid directly on the peat and forms a walking surface. Ideally the mineral materials are laid directly on surface vegetation. Vegetation mats stripped off to allow excavation of mineral materials are then replaced on the haunches of the path to stabilise the edges. There are a number of methods of construction, dependent on the site, depth of peat and driver preference. 6.10 Most Hymac paths are dug through peat up to 2 metres deep but the technique has been applied to peat of depths up to 4-5 metres. In the Pennines the frequency of grey clay [gley] as the subsoil means that coring along the proposed line of the Hymac path to ascertain the nature of the substrate, and peat depth, is an essential precursor to commissioning a Hymac path. The best substrates found in Scotland are fluvioglacial materials and drift. These tend to have a lower clay fraction [though drift includes boulder clays, generically] and a range of mineral fragments including sands and gravels. As the products of local glacial and periglacial action rather than ice sheets they tend to be less rounded, though the largest fragments can be cobble or boulder sized. They tend to be free-draining when excavated. There may be some pockets of clay/silt grade particles. In the north of England, the object is to get down to sandstones and weathered sandstones which form good angular aggregates and can consolidate well. However, these can be of a type that are too free-draining. On shales the materials tend to be very saturated and clayey. They take a long time to dry out and may need a clean stone layer of matched sandstone imported and rammed in as a wearing course. The problem with inverted subsoils is their unpredictability. There is a need for aftercare to both consolidate and revegetate such surfaces. This has often been neglected. It has also proven difficult to revegetate many Hymac paths. The problems seem to be associated with inverting materials from below the water table which were permanently saturated. The mineral substrate is thus from a reducing environment with anaerobic soil bacteria. These soil bacteria have been observed to severely inhibit vegetation colonisation and regrowth where substrates have a high proportion of clays and are deep excavated. These take considerably longer to dry out and hence for oxidising bacteria to `colonise'. It is not possible to put a time limit on the continued presence of inhibiting bacteria but one site built in 1990 was still showing unvegetated patches on deep excavated material five years later. One possible solution is for the machine driver to break up the substrate as construction is taking place. Another is for the path surface to be rotovated after the path has dried out. There is potential here for further research and development. Review of Modern Aggregate paths and tracks Many modern surfaces are built to `reduced' civil engineering aggregate road specifications, to uniform widths determined by the need to bring material in aggregate paths by construction plant. Loadings are `calculated' for vehicular access for large plant and not small equipment

6.8

6.9

6.11

6.12

6.13

7.0 1.

such as powered wheelbarrows and ATVs, or even for the end use of foot traffic. Many aggregate paths are still therefore very heavily over engineered for their eventual use; 2. The use of helicopters to spread aggregate directly, as pioneered by the Three Peaks Project, can avoid the over-specification for aggregate paths by removing the rationale for vehicular access and allowing narrower paths to be built; There is no apparent difference in performance between paths built with graded aggregate layers [Telford type] and those of a single grade throughout [McAdam type]. The conventional wisdom is still that over 100-150 mm depth a sub-base of 150 mm clean stone ought to be used, but there is no empirical evidence to support this view; Modern aggregates have an estimated average life of 20-25 years but virtually all the experimental geotextile and aggregate engineered surfaces built in the late 1980s have already been replaced; Most early sections used imported aggregates with very little attempt to match to the local geology. In the Peak Park and South Pennines basalt chippings were used to provide a colour match with the peat but these merely look like tarmacadam; white limestone was used in the Yorkshire Dales. In recent years a change in thinking has resulted in considerable efforts to select matching aggregates, with the result that some newer aggregate paths are increasingly difficult to distinguish as manmade surfaces using imported materials; There is a continuing problem of exposure of geotextiles, both at path edges and within the wearing course. Geogrids have performed best; Revegetating imported aggregates and some inverted subsoil paths has proven very difficult [see review]; Hymac paths require further work often over a number of years to create a consolidated vegetated surface. The presence of large and often loose stone of 200mm diameter plus has made some Hymac surfaces difficult to walk on and the breaking down of surface stone of this size [according to McAdam's specification] has improved this situation locally [See Annex 3] ; There have been major problems of lateral spread with most imported aggregates. Though these surfaces have been compacted, they have rarely consolidated. They are still prone to washouts and kicking off of loose surface material; There are significant differences in performance between compacted and consolidated aggregates. Compacted aggregates are high maintenance surfaces, consolidated aggregates can require minimal attention.

3.

4.

5.

6.

7.

8.

9.

10.

8.0 8.1

Compaction & Consolidation There are critical differences between compaction of loose aggregates and consolidation. The behaviour of an aggregate when compacted varies with its range of particle sizes, and their chemical make up. Inert minerals such as quartz tend not to absorb water or react chemically even as fines. In sandstones, iron minerals are often the cementing agent. Clay minerals, chemically a range of complex hydrated metal silicates, do absorb water and may bond chemically. The mineralogy of the parent material of the aggregate is thus a key variable in determining whether the whole aggregate consolidates or merely compacts. Compaction involves the reduction of pore spaces within an aggregate by rolling or otherwise compressing the surface. Though the interstitial pore spaces are reduced, the material does not bind together as a cohesive unit. Compacted aggregates are: prone to washouts and lateral spread; prone to dessication with dried out surfaces capable of `wind blow'; uncomfortable for users where loose surface material acts as `ball bearings' underfoot; difficult for use by wheeled access such as wheel or push chairs; very slow to revegetate, damage to early root growth by moving gravel preventing colonisation. Consolidation involves both compaction and the binding together of the entire range of particles into a cohesive whole. This can take place through progressive compaction, but the

8.2 R R R R R 8.3

R R R 9.0 9.1

cementing together of particles is more a product of the nature of the aggregate matrix. Angular fragments bind better than rounded smooth material. Silts and clays bind mechanically and chemically, partly through the presence of water. This is comparable to the process by which Portland cement works but with clays and silts the bond is weaker and the overall aggregate layer has elasticity, unlike concrete which is rigid. [The use of Portland cement is totally unacceptable in upland path construction]. Consolidated aggregates have: a surface that limits the impact of rain splash and runoff; wearing courses of stone bedded into a mineral matrix with fines between the stones still capable of acting as a rooting medium; wearing courses that prevent dessication and protect root systems under the stone layers. Revegetation The revegetation of aggregate paths and tracks varies considerably. It has proved to be a very inexact science with both superb examples of successful revegetation like the grassing up of the Fountain's Fell aggregate path and yet other persistent bare surfaces like the High Force contract path. The reasons for success and failure are complex. The key variables which affect the chance of successful revegetation are as follows:Mineralogical Composition & Biochemistry Nutrient availability; depends on rock and matrix type, see Annex 2, both base and top dressing with fertiliser such as Enmag may be required or admixture with mineral soil. Water retention capacity; varies with source stone and matrix type. Free-draining aggregates like whinstone will be very prone to dessication and revegetation will fail. Mixing in a mineral soil with the wearing course stone is the best method to enhance water retention. Bacterial associations; inverted subsoil, particularly gley, from below the water table has a different bacterial association as it is a reducing environment. Experimentation to establish the impact of this has yet to be undertaken but soil bacteria are very important in re-establishing a vegetation cover. Soil oxygen; deficits are also caused by waterlogged subsoils such as clays at the surface and compacting down. Breaking up of clay pans etc., by forking over or rotovating should ameliorate this condition. It may be necessary to add Calcium Sulphate in clays or otherwise introduce and admix organic/fibrous or coarser mineral matter to improve soil oxygen via increased pore space. Note that there may be a trade off in lower water retention. Experimentation should yield a balance. Physical Nature of Aggregate Range & balance of fragment sizes; Each soil inversion path will have its own characteristics which may be highly variable over the length of the path; causes problems 2 and 4 above. Drainage characteristics [pore space]; Essential for aeration and drainage, often balance between the two. Free-draining soils are well aerated and highly compacted material is poorly aerated and prone to water-logging. Larger consolidated surface stones limit dessication in longer periods of dry weather by retaining moisture underneath; Degree of mobility of surface layer; an unconsolidated aggregate is a mobile surface. Root systems of germinating vegetation cannot survive or establish on a surface being constantly disturbed; Protection of root systems from trampling; Larger embedded surface stones, optimum 50-75 mm but any upwards of 25 mm, can stand slightly proud of the overall surface and protect seedlings growing between stones from trampling. Aggregates on the Pennine Way - Design and Maintenance Standards The Pennine Way Strategy, Management and Maintenance Handbooks all include information on agreed current best practice. This information is reproduced here. General Design Standards Immediate and progressive aftercare of all work on hard surfaces should attempt to achieve as high a vegetation cover as possible on and adjacent to the surface; New hard surfacing should be designed to combine functionality with aesthetic and landscape

9.2 1.

2.

3.

4.

9.3 1.

2.

3.

4.

10.0

10.1 1. 2.

3. 4.

5. 6. 7. 8.

integration. Traditional and vernacular surfaces/materials are preferable to modern engineered surfaces/materials. Hard surfaces should be constructed or maintained at the minimum width feasible for the passage of legitimate users. For example, a moorland path should have a maximum width of 1.2 m and preferably be 0.9 m or less [min 0.6 m]; Surfaced paths should not have straight line edges or be of uniform width; Hard surfaces should cover the minimum length necessary and natural surfaces be managed between extended lengths of hard surfacing in open country; Path design on slopes should recognise the different walking patterns necessary for ascent and descent; Unstepped [pitching or comparable random risers] path surfaces should not be constructed on slopes of over 10-12 degrees without zigzags to relieve the gradient; Drainage Drainage is only required on `natural' or aggregate surfaces where water flow may cause sheetwash or gullying of unconsolidated surfaces; to prevent excess standing water and/or maintain natural drainage patterns; All drains should be adequately sized for storm conditions. Unconsolidated path surfaces must be adequately drained on slopes over 3 degrees and should not be built without waterbreaks/cutoffs or cross ditches on slopes over 5 degrees; Open ditched drainage must have battered sides for both side and cross drains with sumps, chocks and splash stones to prevent accelerated water erosion [as required]; Drainage across paths should be step across open drains wherever possible with stone landings or stone lined [random, pitched or flagged]; Where covered drains are unavoidable, the preference is for stone lined box drains, with wing walls as necessary. Where plastic section drains are unavoidable visible pipework is unacceptable. Cross drain ends must be dry stone or turf faced. There should be no visible cement work present. No use of concrete section drains is acceptable [acidic runoff rots concrete]. Waterbreaks/cutoffs must lead water away from the path line and be angled correctly across the path at 45 degrees [+/- 15 degrees dependent on long slope angle]; The use of timber, clay or concrete products for cutoffs/waterbreaks is unacceptable; The height of waterbreaks/cutoffs above path level should not constitute an obstruction or present a hazard to legitimate users. Choice of Materials - Aggregates Aggregates for surfacing should be locally derived; Aggregates should not be texturally uniform. That is, they should be unsorted/ungraded where possible; Aggregates should be consolidated to provide a stone wearing course with a binding matrix of mineral fines [which may include a clay fraction]; Borrow pits must be restored and landscaped after use; Imported aggregates should match locally derived material [surface geology, drift or alluvial deposits] in weathered colour, texture, mixture of fragment sizes and shape and in geologic origin; Imported aggregates must have no potentially adverse effects on the Ph or nutrient balance of adjacent ground: Aggregates with known difficulties for revegetation [some limestones, whinstone, slate] should be mixed with a mineral matrix that will assist vegetation establishment; The use of aggregates of regular grading profile giving a uniform surface texture should be avoided; A proportion of larger stones should be consolidated into aggregate surfaces to assist progressive revegetation by providing protection from trampling and dessication; On older unconsolidated aggregate paths edge management should ensure lateral spill does not occur Use of Geofabrics The alternative to geotextiles of "fill to refusal" should be considered as an alternative wherever practicable; Geotextiles and geogrids should be used only where layer separation is essential or where the substrate load bearing capacity means a path will not be adequately supported without them e.g. bare amorphous peat; It is preferable to use biodegradable materials as geotextiles and grids wherever practicable; Geofabrics require a minimum covering layer of 15cm compacted material to limit the risk of them becoming exposed at the surface;

10.2 1.

2. 3. 4. 5. 6. 7.

8. 9. 10.

10.3 1. 2. 3. 4. 5.

6. 7. 8. 9. 10.

10.4 1. 2.

3. 4.

5. 6.

7.

Geotextiles or grids should not normally be used on any mineral substrate. On clay and similar substrates stone should be consolidated into the surface; Geofabrics should not be used on slopes where the material can act as a shear surface allowing aggregate fill above to slide off laterally or longitudinally - use a geogrid in this circumstance if unavoidable; Geofabrics and geogrids must not become exposed at path edges. They must be dug in at the sides of the path and covered to sufficient depth [min 1.5 x max. aggregate size] or with turfs and boulders to prevent them showing at or rising to the surface.

10.5

Maintenance Schedules for Aggregate Surfaces The cross referenced schedules for drainage and fertilising are included at the end of this section.

A1

Wash out of fines from walking surface, exposing rough and protruding stone base, and/or loosening previously bedded in stone TO DO Ram exposed stone to relevel surface, pack in stone fragments. Prevent washouts by drainage works if appropriate FOLLOW UP Incorporate matching soil or fines and consolidate between exposed stones as necessary. Revegetate by seeding and fertilising between exposed stones or by finger transplants. In severe cases on shallow soils consider importing & consolidating soil/mineral mix over rough stone base, but try to break up stones smaller and ram in first. A2 TO DO Slurrying of walking surface As A1 plus ram aggregate into slurried surface,[if clay/silt] to provide consolidated surface FOLLOW UP Ensure all stone used matches local stone. Use only angular and sub angular stone unless on riverbank when use broken river stone if appropriate. Consider turnpiking to restore rolling crown. A3 Sheetwash/gullying of unconsolidated aggregate paths TO DO Insert cutoffs/waterbreaks [See D4] , Repack aggregate FOLLOW UP Ensure flow on to path diverted into side drains. Topdress and aggregate if necessary. [Ensure sufficient fines in rammed in top dressed material]. A4 TO DO Spill and lateral movement of unconsolidated material Screef off and remove, all loose and unconsolidated material migrated to path edges [especially imported material] FOLLOW UP Reprofile surface if necessary. For unconsolidated surfaces mix matching soil or fines with unconsolidated stone and repack. If repeated sheetwash, consider drainage options. A5 TO DO Rutting and compaction [especially through vehicular use] Spike or otherwise scarify top surface, rake out aggregate, refill and reprofile, consolidate. FOLLOW UP Revegetate [seed & fertilise (as N1 schedule)] where appropriate. Review management actions and solutions to prevent recurrence [dependent on PROW status]. Consolidate loose aggregate surfaces by ramming in with mineral matrix. A6 Geotextile/geogrid exposed within main surface TO DO Cut min. 0.5 sq m. around exposed geotextile/grid, cut out, refill and consolidate. FOLLOW UP It is necessary to excavate to check whether the overall depth of aggregate over geotextile is adequate or pumping of clay subsoils into aggregate is taking place. Extensive problems will require path rebuilding as consolidated aggregate surface. A7 TO DO Loss of established vegetation Spike, rake out or otherwise scarify top surface [25 - 50mm], fertilise, reseed, transplant or turf as necessary. FOLLOW UP Ensure seed mix matches existing plant association. Apply lime and/or gypsum as necessary, if vegetation loss is attributable to other causes, remedy as required. Patch seed small areas.

D4 Check structure of cutoffs/waterbreaks TO DO Reform cutoff. Ensure correct upstand. FOLLOW UP Extend cutoffs to path edges if required. Ensure free flow off path line. [Place splash stones if backcutting]. Replace timber cutoffs with stone as necessary. Ensure cutoffs at correct angle to path line. Repack or line back of cutoff if scouring. D9 Standing water and pooling on path surfaces TO DO Reprofile path, cut catch drains and/or exit ditches as necessary FOLLOW UP Stabilise path edges to prevent wash off with stone or turves as appropriate. Reprofile surface with additional matching aggregate to rolling crown surface. Ram in matched in situ or mineral material on persistently bare areas and reseed. N1 TO DO Retention of vegetation Partial loss of established vegetation Spike, rake out or otherwise scarify top surface [25 - 50mm], fertilise [see N6], reseed, transplant or turf as necessary. Patch seed where necessary FOLLOW UP Ensure seed mix matches existing plant association. Apply lime and/or gypsum as necessary, if vegetation loss is attributable to other causes, remedy as required. Ensure compaction is relieved by regular spiking. Frequency of maintenance regime dependent on use levels, seek to show expected improvement over medium term [3-5 years]. Control areas are required for various fertiliser specs.. Monitoring essential. N2 TO DO FOLLOW Wash out and gullying of walking surface. Insert stone cutoffs/waterbreaks. Ram exposed stone to relevel surface, pack in in situ mineral fragments. Alternatively form turfed breaks UP Consider realigning on to zigzags for slopes over 10? Incorporate matching soil or fines and consolidate between exposed stones as necessary. Revegetate by seeding and fertilising between exposed stones or by finger transplants.

N6 Routine fertilising TO DO Apply fertiliser by hand or from ATV spreader. FOLLOW UP Ensure correct application rate by calibrating machinery. Ensure burn off is prevented. Do not apply in dry bright weather. Must be damp conditions, [heavy dews may be adequate]. Apply at rates up to 50gm per Sq.m. Use a low nitrogen fertiliser approx 5.15.10 NPK.[ Enmag is recommended]. Do not use slow release fertiliser which is temperature dependent. Spring [April/May] and Autumn [August/Sept] applications only. No fertilising after Sept. - earlier at altitude. For edge vegetation use a higher P to encourage heading up and seeding. N7 Loose or dislodged rammed stone in naturally consolidated [or Hymac] mineral surfaces TO DO Scarify sub-base, ram in matching stone. Ensure surface is sloped off [rolling crown or other] FOLLOW UP Ensure replacement stone matches in size, origin and texture if in situ stone is not available in quantities required. Reseed, brash or fertilise surface to encourage revegetation where appropriate.

ANNEX 1

KEY SITES - Pennine Way Aggregate Surfaces SITE HISTORY MODERN Early 1980s on paraweb, rebuilt as a series of trials late 1980s. Current surface 1990 Grough path 1990/1 TYPE of SURFACE Rafted geotextile, Aggregate subbase + basic slag surfacing. Devil's Dike first attempt at following grough lines using in situ mineral materials - known as `designer groughs'. Some topping added in 1991 Rafted geotextile & unconsolidated aggregate of uniform width and basalt chip surface [Tutti aggregati] Three sections Hymac path plus basalt chippings. 1994 works in Kirklees no imported topping, revegetated, stone clapper crossings. Geotextile & aggregate [Bacup sandstone] plus Hymac path Redmires to Blackstone Edge using borrow pits and railroading on dumpers. Edge revegetation works evident. Peat up to 6m deep. machine built path on long and cross slope, mineral soil. Good revegetation using incremental approach with several seed and fertiliser applications. Heavy cattle poaching. a good area for observing variations on the basic types of construction. Buckley Lane has been lined with terracoir to stabilise traditional consolidated aggregate Heavily used access to Cove. Conventional limestone aggregate path of uniform width [2.3m]. Compacted but unconsolidated. Shows periodic wash outs from stream flooding. Hymac path with exceptional revegetation. Crosses very mixed geology with limestone, sandstone and shale sections. Additional hand built drainage various aggregate & geotextile, Bacup sandstone direct laid aggregate plus Hymac paths. From south, trad track to Dalehead, black limestone and geotex to Brackenbottom link with Bacup sandstone path [no

LOCATION Snake Summit and Devil's Dike

Standedge to Haigh Gutter

MODERN Materials airlifted1990. Construction 1990-91 MODERN Trial Hymac 1990/91 with basalt chippings overlaid. Later section 1994. White Hill completed 1998 MODERN Windy Hill to M62 early 1990s. M62 to B. Edge 1992-4

Haigh Gutter to Windy Hill

Windy Hill to Blackstone Edge

Heptonstall Moor

MODERN 500m JCB built path early 1990s using excavated material from side drain

Top Withins to Keighley Moor

TRADITIONAL MODERN Buckley Lane machine built rebuild.

Pinhaw Beacon Malham Cove

TRADITIONAL from road to Pinhaw Beacon MODERN originally 1980s

Fountain's Fell

MODERN Hymac 1993-4

Pen Y Ghent Area

MODERN/TRADITIONAL Original works late 1980s 3 Peaks on N. side, rebuilt 1995/6 as Hymac. Dale head side includes traditional access tracks plus various aggregates late 1980s up to 1994, Hymac

path to summit 1995/6 replaced boardwalk

geotex.] Summit area Hymac. White limestone Hunt Pot path rebuilt as Hymac, trad. Track down to Horton. Very side range of types in this area old drove road and consolidated aggregate tracks traditional consolidated aggregate tracks miners tracks and relict `Roman Road' various aggregates and geotextile combinations with whinstone and river stone topping, some short sections of river cobbles. Hymac path, several sections, over varied geology from limestone to 4m deep peat, and `designer grough' section. tradtional and semi natural paths and tracks Ancient drove road, machine built sections. Some sections grass tracks. Sunken lane section JCB opened side drains and use spoil to build central turnpike traditional miner's track, some severe damage. Failed attempt to install cutoffs in tarmacadam at bottom end consolidated path in whinstone semi consolidated aggregate path in whinstone Hymac path, poor revegetation Short section of well revegetated machine built path

Cam High Road to Ten End Tracks to south and north of Shunner Fell Track to south of Tan Hill Low Force to High Force

TRADITIONAL TRADITIONAL TRADITIONAL MODERN High Force aggregate path 1989. Other short sections at Low Force early 1990s MODERN 1990/91

Dufton Fell

Dufton to High Cup Hurning Lane

TRADITIONAL TRADITIONAL Lane reopened by JCB early 1990s

Corpse Road

TRADITIONAL

East of Walltown Car Park Above Crag Lough Padon Hill North of Black Hag

MODERN MSC scheme late 1980s MODERN Early 1990s MODERN 1995/7 MODERN Early 1990s

Please note that there are many other examples of aggregate paths on the Pennine Way

ANNEX 2 Geology of Pennine Aggregates Local aggregates in the Pennines [and some imported equivalents] have a number of characteristics and may be described as follows:GRITSTONE Aggregate Weathered products are coarse grained [0.5mm+] sub rounded sand and grit fragments often with rounded quartz pebbles with a colour range from white/grey [quartz and quartzite pebbles] to buffs to browns. Fresh surfaces may be a prominent reddish orange. Matrix The matrix is sandy, well weathered fragments may be friable, though capable of compaction. It requires a finer silt/clay matrix for full consolidation. More frequent in Peak and South Pennines. Revegetation Often poor due to the inert qualities of quartz fragments i.e. low nutrient availabilty. The sandy matrix means it is very free draining and hence prone to dessication. SANDSTONE Aggregate Weathered products vary from angular to sub rounded. May be `flaggy' or shaly if stone is well or thinly bedded. Sandy fragments dominate with some coarser gritty sandstones locally, and some ragstones [silty sandstones or sandy siltstones], especially from Yoredale Series. Some quartzites [pure white sandstones]. Colours range from white/grey quartzites to subdued greys and browns with bright unweathered sandy yellows and oranges. Matrix Matrix sandy but with a clay/silt fraction, especially where source stone is finer grained and includes micas and iron minerals [rather than quartz]. Bacup sandstone or similar most frequently used as an imported match. Revegetation Good where matrix includes nutrient rich silt and clay minerals [e.g. micas]. In this case dessication in dry spells is less. Quartzites and purer sandstones are more difficult to revegetate [cf. grits] as quartz is inert and does not provide nutrients. SHALES & MUDSTONES Aggregate Weathered products are not really suitable unless mixed or matched with sandstones etc., but are common in Yoredale Series and should be used if locally appropriate. Often needs special attention to drainage. Small flaggy, frequently brittle, platy fragments may include ironstone nodules. Shales and mudstones are not ideal as a wearing course. Colour usually grey to black. Matrix Matrix often grey clay/silt fractions, frequently micaceous. Generally shales have too high a clay/silt fraction and tend to `puddle up'. Where mixed with sandy partings can be ideal. Revegetation Can be first class for revegetation, nutrient rich matrix and holds soil moisture well. Shale exposures frequently have spring lines adjacent which usually assists revegetation. LIMESTONE Aggregate Locally important in Yorkshire Dales, Durham and parts of Cumbria. Rock fragments angular to sub rounded. Fine textured. Colours from mid grey to blues and black. Limestones often take a fine polish and old established surfaces may be slippery. Matrix Matrix very variable, often insufficient to provide binding layer. No longer used as imported sub base or surfacing. Can alter pH adjacent to path line where used on acid peats. Revegetation Often poor revegetation, except at edges, where can be exceptional regrowth and colonisation.. Very free draining and prone to dessication. WHINSTONE Aggregate Locally important in Teesdale, Cumbria and Wall country in Northumberland. Fragments angular, very hard fine/medium grained rock, almost inert. Very slow to weather from bright blues and greys, quarry bottom material may partially bind fragments. Matrix Has tendency to be gravelly and difficult to consolidate. One local quarried source in Northumberland yields a rich brown weathered aggregate which blends and will consolidate with a combined soil/mineral matrix.

Revegetation Very poor for revegetation given very low nutrient availability from weathered products and difficulties of consolidation [except where mixed with soil as matrix]. Very free draining and prone to dessication. VOLCANICS Aggregate Important in Cheviots. Frequently random angular and sub angular fragments, weathered products. Small quantities can be sourced from the borrow pits within forestry plantations used to supply stone for forest roads. Subdued weathered colours, and even acceptable when `fresh' as mostly blue/greys and red/browns. Geochemically mostly intermediates such as fine grained andesites. Matrix Has tendency to be gravelly and difficult to consolidate [cf. granites]. Weathers out to brown coloured silty grade. Revegetation Acceptable: nutrient availability from weathered products is better than most volcanics, tends to very thin soils, but dries out reasonably, without being too free draining. Some interesting local plant associations can develop, and forest tracks show capacity for revegetation.

ANNEX 3

AFTERCARE OF HYMAC or SOIL INVERSION PATHS

Hymac or soil inversion paths are relatively cheap and can be very durable if immediate aftercare is put into place. The actual machine contract should be viewed as only the first stage of the establishment of an in situ mineral/aggregate path. The surface after the machinery contract can be very variable depending on the nature of the mineral subsoil. The full consolidation of a new Hymac path may take several years and will involve several elements. These will probably involve a varying degree of hand working over the new surface. Locally, areas where there is too much unconsolidated random stone usually match up with areas where there is too much clay and insufficient wearing course materials. Local redistribution and consolidation is the solution. AFTERCARE ELEMENTS 1. Loose unconsolidated surface stone Frequently occurs when insufficient clay/silt fraction to bind material together. After a suitable settling in time materials need to be rammed into matrix or moved by powered wheelbarrow or equivalent into adjacent areas with insufficient stone. 2. Large surface stones These occur in areas where the substrate includes large flaggy stones or glacially dumped boulders. The solution is either to move the materials to assist in stabilising the haunches of the path or break them down into material small enough to be consolidated. 3. Path haunches Usually the path haunches will have been turfed up by the driver as part of the path construction. Where gaps exist then there may need to be additional stabilisation as in 5 below. 4. Gley/clay patches The alternation of sandstones and shales in the Carboniferous frequently gives rise to gley/clay patches with alternating sandy areas in inverted surfaces. Transfer of broken stone may solve the problem, failing that introduction of a 50mm wearing course of matching stone consolidated into the clay. Often the clay areas have the fines completely washed out through the first winter. This may solve the problem but may expose large surface stones. A combination of washout of fines and incorporation of wearing course stone aggregate is the most frequent combination of remedial measures. 5. Restoration of adjacent bare areas In a surprising number of cases removal of foot traffic allows full regeneration of adjacent areas to take place. Fertiliser applications may speed up this process. There may need to be stabilisation of bare ground through mulches or a material such as Terracoir. The introduction of spade sized transplants or a nurse grass may be required. 6. Piped cross drains Piped cross drains should have silt traps constructed at the inflow and splash stones placed at the outflow. All pipework should be covered with a depth of material in excess of 150mm, hidden at the ends as far as is possible and turf and/or stone wing walls built where necessary. 7. Seeding up path surfaces This should take place as soon as the path is built. Seed will become trapped in the surface and germinate when conditions allow. After the surface has consolidated through rain and foot traffic it is much more difficult to get seed to lay on the path surface without raking/scarifying. Refer to Maintenance Schedules Para 10.4 for other details

Information

aggregate paths.PDF

18 pages

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

283024


You might also be interested in

BETA
aggregate paths.PDF