Subject: GD707 Garden Paths and Drives

Transcription

1 + Oxford College of Garden Design Illustrated Hard Landscape Seminars - Student Notes Copyright 1996 John Heather Reviewed 2011 Time 1.25hrs Module No. GD707 Subject: GD707 Garden Paths and Drives This seminar looks at some design considerations and the construction of flexible and rigid paving. Rules of thumb are given for the design of the load bearing foundation (or sub-base) to pavings and the important difference between rigid and flexible paving design is explained. 1 Design considerations The width and slopes of domestic paths and drives affect convenience and access. Wet or icy ramped paths in winter can be lethal especially if the slip resistance of the paving material is not good. The use of smooth faced clay pavers may be hazardous. Clay or concrete paviors and pre-cast concrete (pcc) flags should normally have a dragged or textured finish - especially on a gradient. The suggested minimum widths are 2.4m for drives which must be increased at sharp bends and 600mm for paths. Main access paths to the front of a house should be at least 900mm. From October 1 st 2008 paving with an area greater than 5m 2 in front of the house will need Planning Permission unless it is constructed to be porous in accordance with SUDS principles, see GD1110. The slope or gradient of a path may be dictated partly by its drainage needs but if the site is steeply sloped it may be tempting to follow the existing ground levels without considering wheelchair users or those with arthritis. Ramps are defined as 1 : 20 or steeper. Drives should not exceed 1 : 20 (= 3º ) and pedestrian access ramps should never exceed 1 : 12 (Building Regs.1 : 15). Ramps should be provided with level resting places at intervals depending on the steepness of the slope. The length of ramp between resting places should be the gradient minus 10 in meters. (eg a 1 : 15 ramp should have resting places at 5m intervals) Steeper slopes may need to be replaced with steps and short ramps. A small cross fall or camber is desirable for two reasons. On drives and paths with a longitudinal gradient, a fall to the side 1 will help prevent water collecting at the bottom of the slope. On drives or paths on level ground a cross fall is the only means of removing storm water. The cross fall or camber should range between 1 : 50 maximum to 1 : 80 minimum. Drainage is considered in more detail in the Seminars on that subject. 2 Construction methods There are two main methods of laying paving materials: the first is known as flexible paving and allows a degree of ground movement which can be accommodated without making the paving surface dangerously uneven. A flexible pavement has a foundation or sub-base of consolidated granular material on to which the surface paving is bedded on a layer of sharp sand or weak mortar. (For granular sb-base material see section 13 of "An introduction to stone, GD708" The alternative method relies on a rigid paving based on a concrete slab foundation and designed to hold the cobbles, setts or other paving material solidly together. Some manufacturers recommend a mortar bed >75mm thick which approaches the strength of a concrete slab and therefore is almost equivalent to a rigid base. 3 Sub-base for flexible pavings The granular material for the sub base or foundation for flexible paving usually consists of well graded gravel, hoggin or quarry scalpings. Traditional hardcore made of broken brick, tile or concrete to pass a 75mm ring is usually too coarse to provide a top surface free of voids even

2 after compaction. It may be possible to use hardcore for sub bases 150mm or more deep and then, if necessary, blind the surface with fine material like sand or pfa (pulverized fuel ash) to fill the surface voids. The required thickness of sub-base will depend on the type of soil, its water table and whether the paving is solely a footpath or is a drive for cars and light vans. (see Table on next page). Drives for occasional commercial traffic like oil tankers may need specialist advice. The water table must be > 600mm below the excavated level at all times of the year. If necessary the water table must be lowered by the use of sub-soil drainage which is discussed in the seminar on land drainage, GD1112. All vegetable soil must be removed from the area of the paving. If this results in deeper excavation than is otherwise required, the levels should be made up with additional consolidated granular fill material. The rule of thumb thickness of sub-base for various soil types is shown below: Drives* Terraces Heavy clay or silt Silty Clay Sandy clay Sand 100 N/A Sandy gravel N/A N/A * Designed for private cars and light vans where there is no frequent commercial deliveries (eg oil delivery tankers, refuse collection lorries, etc.) Use a geotextile over the sub-grade. Similarly, if any part of the paving is on unconsolidated "fill" (eg an old rubbish pit), the soft area must either be excavated and replaced with granular material or the paving must be laid on a rigid foundation strong enough to span over the soft spot. 4 Bedding of flexible pavings Paving bricks, setts, concrete blocks and pcc small element paving slabs are bedded on a layer of sharp concreting sand. Suitable sand should allow 90% to pass a 5mm sieve and have <3% clay content. The sand should have a consistent moisture content when laid such that it will "ball" under hand pressure. The loose sand is screeded to precise levels to give the necessary falls and levels to the final surface. Allowance is made for consolidation so that the final thickness of the sand bed is 50mm. Thinner beds down to 35mm consolidated thickness may be satisfactory provided the work is carefully controlled. Plate vibrators are used to consolidate the sand once the paving blocks are in place. Workmanship should achieve levels within +6mm of design. Finally the surface is spread with fine sand conforming to Grade F of Table 5 of BS882 and vibrated again to cause the sand to fill the 2-3mm joints. This locks the individual units together by friction but still leaves the pavement capable of movement. In time these joints become water proof but initially it is important that the sand bed is allowed to drain freely. The surface can be used by traffic as soon as it has been finished. It was mentioned that concrete and clay block paviors were often cast with spacing nibs to ensure that there is a constant gap for the sand to run into the joints. Natural stone blocks are also available and can utilise moulded plastic spacers to achieve a similar gap for the sand. 5 Edge restraint for flexible paving and paving patterns. It is essential that brick, block, setts and small element paving is provided with edge restraint to prevent sideways movement of the surface and the bedding sand. This is particularly important for drives where the cars/lorries can exert a large sideways force. Restraint can be from features like walls or kerbs - or concealed as concrete haunching below a grass lawn or border. 2

3 Typical edge restraint detail - pedestrian Typical edge restraint details - light vehicle traffic Paving patterns are important visually. Some patterns like the 45º herringbone layout help to spread the horizontal loads by interlocking adjacent blocks. Not all blocks are of the same size. For instance clay paviors may vary between 200 and 215mm long. Although blocks can be cut Typical paving patterns by bolster it is preferable to use a block splitter. Cutting is expensive. It may be possible to reduce cutting by adjusting the paved width to an exact block module. Some block manufacturers produce special shaped blocks for improving the detailing at the edges. 3

4 6 Bedding of stone and concrete paving flags The larger sizes of stone and concrete paving are more vulnerable to cracking under load. Although they can still be laid as a flexible pavement on a sand bed it is normal to lay them either on 5 mortar dabs or on a full bed of mortar rather than a vibrated sand bed. The mortar can be a 1 : 4 lime/sand or a 1 : 8 cement/sand weak mix. The mortar dabs method should be restricted to paths and patios for foot traffic only and only flags 50mm thick or over should be used for drives. The sand bed beneath the mortar dabs or bedding should be thicker than for vibrated block paving and should provide 65mm consolidated depth below the mortar. Frame bonding is laying a strip of mortar round the perimeter of the slab with 2 further mortar strips down the centre. This method provides more support than the mortar dabs method. Joints between slabs are normally 10mm and filled either with sand or dry mixed mortar brushed in. Some manufacturers of decorative slabs for the domestic market recommend that the edges of the slabs are buttered with mortar before laying. This eliminates the problem of ensuring the joints are properly filled later but is difficult to do with large heavy slabs. A separate sub-base will rarely be required for a garden path in addition to the bedding of the paving flags, blocks, bricks or setts described above. This assumes that it is for pedestrian use only, that all vegetable matter has been removed and the ground is well drained, undisturbed soil. 7 Bitumen macadam (bitmac) Macadam was originally stone chippings coated in coal tar. Bitumen is now use as the binder. When it is spread in layers and rolled it consolidates and coalesces into a smooth flexible surface. The texture of the surface depends on the size of the chippings and whether there is a proportion of finer chippings and grit to fill the surface voids. Where very smooth surfaces are required like infant school playgrounds the size of the chippings may be as small as 3mm. For footpaths and drives a 15-20mm thick surfacing is usually formed of 6mm sized chippings. Unless it is an overlay to an existing structurally sound drive, the surfacing (or wearing course) is laid on a 40mm base course of 20mm sized chippings. For domestic situations limestone aggregate is used rather than the more expensive granite. The tar weathers off the limestone leaving the surface a light grey colour. 8 Reinforced grass This is a form of flexible paving which has the minimum visual impact and can be self- draining. One form consists of precast concrete blocks typically 500 x 300 x 100 thick with approximately 35% perforations to allow grass to grow through.. Originally designed for emergency fire engine access paths, these systems are suitable for driveways in occasional use eg a week-end cottage. The concrete will prevent soil erosion and provide traction in wet, slippery conditions but still allow the grass to be mown like a lawn for most of the year. Other systems based on the egg-crate pattern are available in plastic as well as concrete for pedestrian and vehicle use. When designed to take vehicles, the blocks are laid on 20mm of sharp sand over a compacted sub-base as for any flexible paving. However the more recently introduced plastic egg-crate systems when used to control erosion under heavy foot traffic, require no base at all if the original ground is sound. The perforations are almost filled with a light friable topsoil and seeded with grass. A final 10mm of fine topsoil is spread and then kept watered till the turf is established. 4

5 9 Rigid paving Areas <1m² of cobbles (or duckstones) and mortar jointed brickwork can be laid on a flexible sub-base. Larger areas will need a solid concrete slab to prevent cracking. The surface finish is held together by the concrete slab so that edge restrain is not essential. Information on the specification of concrete has been given in the seminar on concrete. Rigid paving may need movement joints to divide areas into no more than 12-16m² bays, depending on the foundation thickness. A typical specification for rigid brick paving laid on a concrete slab would be 65mm brick paviors laid basket weave pattern on a 25mm bed of 1: 4 mortar with plasticiser. The joints will be 10mm wide slightly recessed and rounded using the same mortar. A variation on this method is used to lay tapered setts eg those produced by Blanc de Bierges. Blanc de Bierges recommend that their 40mm or 80mm tapered sided setts are pressed into a thick bed of 1 : 5 mortar so that the mortar fills 75% of the joint. This leaves the top 25% to be filled either by the laborious business of using a moist pointing mortar 1 : 3 mix. or with 1 : 2-3 semi-dry mix brushed in diagonally. The rough surface of the setts tends to hold the dry mix and the next rain will hydrate the cement and tend to stain the surface. Pointing with mortar requires careful workmanship for the same reason that the damp mortar must be confined to the 10mm wide joint and not smeared on the surface of the setts. Note that rigid paving with setts does not require the edge restraint that flexibly bedded blocks must have. (see seminar on concrete and the laying of slabs.) As with brickwork, the colour and profile of the pointing mortar can have a dramatic effect on the finished appearance of the paving. 10 Paving near trees It may not be wise to make even shallow excavations within the root zone of existing mature trees which are to be retained. Where Planning Permission is needed Local Authorities may make special construction techniques mandatory for any paving and drainage, etc. The objective is to avoid damaging roots, to prevent compaction of the soil and to allow water and oxygen to permeate into the soil. Details are shown in the current edition of BS 5837 e.g. using a geogrid fabric, Type 3 sub-base and a porous block pavers or a no-fines gravel topping. The area should be restricted to 20% of the root zone 11 Surface sealants Maintenance of block paving and other concrete slabs may be difficult if they become stained with engine oil or perhaps contaminated with the gummy dropping from lime trees. Attempts to power hose off the surface may blast out the new sand jointing and loosen the blocks. Use of proprietary surface sealants may be worthwhile. They may add over 50p/m² to the cost but they will make the surface less absorbent and will bind the sand jointing without destroying the flexibility of the pavement as a whole. Sealants will cause colours to become brighter as if rain had recently fallen. 11 Bibliography and further reading GD707 and 708 BS6677 Pt 2 Clay and calcium silicate pavers for flexible pavements: the design of lightly trafficked pavements. BS 5837 Trees in relation to Construction Arboricultural Practice Note APN Driveways Close to Trees AAIS The Tree Advice Trust Build it with Concrete. Published by British Cement Association 1988 Tel: (01344) Interpave Data Sheets. Pub. free on their web site The Precast Concrete Paving and Kerb Assoc. Tel: (0116) Design, construction (with illustrations) and specification clauses. Paving manufacturers catalogues usually contain laying recommendations. eg Blanc de Bierges, Marshall s,. BRE Digest 420 Selecting natural building stones. Building Research Establishment. Department of Transport. Specification for Highway Works HMSO - not essential reading!! Type 1 spec. Chilterns Conservation Board: Building Design Guide, supplementary technical note Chiltern Flint pub office@chilternsaonb.org See also various web sites 5

6 GD707 Design of garden paths and private drives GD708 Introduction to stone, paving materials and aggregates Combined Self Assessment Questionnaire 1. Name a well known sandstone used for paving? 2. What type of stone is quarried at Portland, Dorset? 3. What type of stone is quartzite? 4. What is the normal width of Yorkstone paving slabs? 5. What would you pay per tonne for Cotswold stone rubble at the quarry? 6. What is small element paving? 7. What is the effect on concrete block paving of lime bloom and is it permanent?. 8. Why do blocks and small element paving slabs often have chamfered edges? 9. What thickness of paving block is usual for the domestic market? 10. What proportion of the thickness of a dry stone wall are "bonding" stones? 11. What mix of mortar would you specify for a knapped flint wall? 12. Explain the term "flexible paving" 13 What is the minimum recommended width of a domestic drive? 14. What is the maximum recommended gradient for a drive? 15. What is DOT Type 1 and Type 2 material? 16 What is reinforced grass? 17 In what circumstances would you use 5 mortar dabs to lay flag stones? 18 Describe granular material suitable for use as a sub-base. 19 Why is the water table level important in relation to paving? 20 Do you need more or less thickness of sub-base on heavy clay soils? 21 What are the basic reqirements for paving in tree protection zones (TPZ)? Model answers to Questionnaire 1 Yorkstone. 2 Limestone 3 Quartzitic sandstone 4 600mm 5 About 60 per ton 6 small pressed paving flags 300 up to 450mm square 7 Apparent fading; No, about 12 months. (Brick paviours don t fade but do get dirty) 8 To avoid tripping hazard from small differences in level mm 10 Two thirds or more 11 1 : 2 : 9 cement : lime : sand 12 Paving which is bedded on sand and granular material to allow small ground movements m 14 1 : Department of Transport specifications for a stone graded aggegates for use as a paving sub-base. 16 Flexible paving of concrete or plastic egg crate section blocks which allows grass to grow through the perforations. 17 Footpaths made of large size flags 18 Well graded gravel, hoggin, Type 1 or Type 2 material or scalpings. 19 The load bearing capacity of the ground is greatly reduced by a high water table. 20 More; also probably a geotextile separating layer. 21 Less than 20% of root zone; avoid root damage, compaction and allow oxygen+ water. Copyright John Heather 1996 Revised 20/11/06 6 C:\Data\Cons\Seminars\Ston-sno.wp