NOTES TO THE READER THE COURSE OBJECTIVE - e … · NOTES TO THE READER THE COURSE OBJECTIVE ......

©HCC RHS LVL3 GPCP L3 2011 A5 1

NOTES TO THE READER

THE COURSE OBJECTIVE

To enable the college member completing the course to have a thorough understanding of general horticulture and to be successful in the Royal Horticultural Society Level 3 Certifi cate in the Principles of Garden Planning, Construction and Planting.

An HCC Certifi cate of Course Completion is issued to all who successfully complete the course. A Course Completion Certifi cate can be issued to those who decide to do part of the course or pick and mix the lessons from other courses. The HCC Certifi cate would state which lessons had been studied, the level of work achieved would also be described – this would be from ‘Satisfactory’ Course Completion through to ‘Good’, ‘Credit’ and ‘Distinction’. These are our assessments and our procedures. They are not part of a national scheme of awards.

ABOUT THE HCC

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DISCLAIMER

Every effort is made to ensure that the information in this text is complete and correct at the time of going to print but the HCC does not accept liability for any error or omission in the context or for any loss, damage or other accident arising from the use of the techniques or products outlined herein.

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CONTENTS

INTRODUCTION .........................................................................................9

1. SETTING OUT A SITE ......................................................................... 11

Site Access and Egress .......................................................................... 14How to Level a Garden .......................................................................15

2. SOIL MOVEMENT & STORAGE DURING CONSTRUCTION WORK 17

Signifi cance of Soil Layers in relation to Soil Separation ....................... 17Movement of Soil to Levels Specifi ed in a Design .................................. 19

Earth Moving .......................................................................................19Groundworks .......................................................................................20Land Forming ......................................................................................22Mounding ............................................................................................22Contouring ...........................................................................................23Terrace Building and Establishment ....................................................23Infi lling and Levelling ...........................................................................23Subsoiling ............................................................................................24Angle of Repose ..................................................................................25

Soil Storage ............................................................................................ 26Where to Store the Soil? ....................................................................26What Condition should it be in? ..........................................................26

Biosecurity Measures ............................................................................ 27

3. CONSTRUCTION OF DRAINAGE SYSTEMS ....................................29

Transferring Levels from Plans to Achieve Falls ..................................... 29French Drain .......................................................................................31Soakaways ..........................................................................................32Typical Drainage Specifi cations ........................................................32Laying Pipes - Safety First .................................................................33

4. CONSTRUCTION PROCEDURES FOR PATHS, PATIOS & DRIVEWAYS .............................................................................................34

Surface Materials .................................................................................... 36Foundation and Construction ................................................................. 37

Slab or Raft Foundations ....................................................................38Concrete Foundations .........................................................................39Foundations for a Slab or Natural Stone Patio ....................................40Constructing Permeable Paving (Porous) ...........................................42Reinforced Grass ...............................................................................43Driveways ...........................................................................................43


Construction of Loose Surface Areas .................................................... 44Edging and Kerbs ................................................................................45

5. CONSTRUCTION OF STEPS AND RAMPS .......................................48

Steps ...................................................................................................... 48Measurements ....................................................................................48Height ..................................................................................................49Number of Steps .................................................................................49Constituent Parts of Steps ..................................................................50Types of Steps ....................................................................................51

Ramps .................................................................................................... 56Methods for Creating Ramps ..............................................................57

6. CONSTRUCTION OF WALLS, FENCES AND PERGOLAS ..............58

Materials for Constructing Garden Walls ................................................ 58Brick Bonds .........................................................................................60Stretcher Bond ....................................................................................60Header Bond .......................................................................................60English Bond .......................................................................................61English Garden Wall Bond ..................................................................61Flemish Bond ......................................................................................62Flemish Stretcher Bond .......................................................................62

Footings or Strip Foundations ................................................................ 62Stone Retaining Walls .........................................................................66

Fences .................................................................................................... 67Panel Fences ......................................................................................67Post and Rail Fences ..........................................................................69Picket Fences ......................................................................................70Fixing Fence Posts ..............................................................................70

Pergolas ................................................................................................. 72

7. POND CONSTRUCTION .....................................................................75

Constructing a Flexible Liner Pond ......................................................... 75Construction of a Concrete Pond ........................................................... 77Constructing a Pre-formed Pool ............................................................. 78

8. ROCK GARDEN CONSTRUCTION ....................................................80

Materials ................................................................................................. 80Construction ........................................................................................... 81

Larger Scale Rock Gardens ................................................................81


9. RISK ASSESSMENT ...........................................................................84

Identifi cation of Hazards ......................................................................... 86Risk Evaluation ....................................................................................... 87

Controlling Risks .................................................................................89Example Risk Assessment ..................................................................89

BIBLIOGRAPHY AND FURTHER READING ...........................................91


ILLUSTRATIONS

Figure 1 Example site plan ....................................................................... 11Figure 2 A well organised construction site ..............................................14Figure 3 Topsoil, subsoil and bedrock ......................................................18Figure 4 Re-use of excavated topsoil .......................................................20Figure 5 Laying of services e.g. oil pipe ...................................................21Figure 6 Water pipe installation during construction .................................22Figure 7 Figure 7 A trench being prepared to receive the drainage pipes 30Figure 8 A fi xed sight profi le .....................................................................30Figure 9 Rigid paving layers .....................................................................35Figure 10 Wide gravel path at RHS Harlow Carr......................................37Figure 11 Slab foundation in cross-section ...............................................38Figure 12 Path construction .....................................................................40Figure 13 Marking out levels using datum pegs .......................................41Figure 14 Paving ......................................................................................42Figure 15 Garden design using loose gravel with brick edging ................45Figure 16 Loose gravel path with stone block edging ..............................45Figure 17 Kerb/edge construction ............................................................46Figure 18 Ascertaining changes in level on a sloping site ........................49Figure 19 Parts of a step ..........................................................................50Figure 20 Step dimensions .......................................................................51Figure 21 Marking out cut-in steps ...........................................................52Figure 22 Informal steps using sleeper and gravel construction ..............53Figure 23 Log and gravel step construction .............................................53Figure 24 Brick and concrete slab step construction ................................54Figure 25 Free-standing step construction ...............................................55Figure 26 Free-standing step construction (plan view).............................55Figure 27 Figure 27 Using a spirit level to set step treads ......................56Figure 28 Figure 28 Depression of ‘frog’ in a brick ..................................59Figure 29 Parts and dimensions of a standard brick ................................59Figure 30 Stretcher or running bond.........................................................60Figure 31 Header bond.............................................................................60Figure 32 English bond.............................................................................61Figure 33 English garden wall bond .........................................................61Figure 34 Flemish bond ............................................................................62Figure 35 Flemish stretcher bond .............................................................62Figure 36 Marking out footings for a wall..................................................63Figure 37 Footing layers ...........................................................................64Figure 38 Brick wall retaining material beneath a raised terrace .............65Figure 39 Stone retaining wall construction..............................................66Figure 40 Waney-edged fence panel .......................................................68Figure 41 Close-board fence panel ..........................................................68Figure 42 Section of post and rail fencing ................................................69Figure 43 Figure 43 Post and rail fencing in a typically rural setting .......70


Figure 44 Fence post establishment (cross-section) ................................71Figure 45 Pergola at the Manor House, Bledlow, Bucks. .........................72Figure 46 Pergola section – front/entrance view ......................................73Figure 47 ‘Metpost’ fence post bracket .....................................................73Figure 48 Pergola roofi ng sections ...........................................................74Figure 49 Calculating the size of pool liner...............................................76Figure 50 An Informal Pond ......................................................................79Figure 51 Rock garden at RHS Harlow Carr ............................................80Figure 52 Rock garden construction.........................................................83


INTRODUCTION

This lesson covers the topic ‘Understanding the Setting Out and Construction of Landscaping Elements in the Garden’.

The main learning outcomes for this lesson are to:

Understand the practical procedures for setting out ‘site to scale’ plans and drawings.

Understand the reasons for correct soil movement and storage during construction works.

Understand the factors which determine the type of drainage system required in various situations.

Know materials and construction procedures for paths, patios and driveways for parking and light use.

Know materials and construction procedures for steps and ramps.

Know materials and construction procedures for low garden walls, retaining walls, fences and pergolas.

Know material and construction procedures for a water feature.

Know materials and construction procedures for a rock garden.

Understand risk assessments.


1. SETTING OUT A SITE

By the end of this section, you will be able to understand the practical procedures for setting out ‘site to scale’ plans and drawings.

Describe how scale drawings are interpreted to set out the major features of a design on the ground.

Describe how to set out the required levels on site.

Describe the sequence of work involved in the realisation of a design.

Introduction

The fi rst step when preparing to set out the major features of a design on to the ground is to interpret and understand the scale plans.

Figure 1 Example site plan

A B

C

House

Patio

Mixed border Lawn

Driveway

N

Scale = 1:100 (this diagram has been reduced in size by 70%)


Here is a step by step guide to interpreting a site plan:

1. Start by understanding the scale of the plan. The scale assists in locating and siting features on the ground. In the previous illustrated plan (Figure 1) the scale is 1:100, which means that every centimetre on the plan represents one metre on the ground.

Check that the scale works and that the measurements are accurate. Choose a given feature or line, for example, the left corner of the house and any other feature of one’s choosing. (NOTE: choosing a long baseline is best for this task).

For the purposes of initial practice, place the scale rule zero mark precisely on the plan’s given point for the left corner of the house. Measure to the feature marked A, which is the site for the planting of an ornamental tree.

Now repeat this exercise, reading from A to B and from B to C and note the distances between these points.

This exercise is known as ‘scaling-off’ from the plan and is an invaluable and essential technique when marking out a site, prior to ‘on-the-ground’ positioning of given features from the plan.

It is best to check all the main house and boundary measurements. If these are accurate then it is reasonably safe to assume that the rest will correspond. If there are discrepancies then, unfortunately, all the measurements relating to the inaccurate line will have to be re-measured.

Use an encased spooled surveyor’s tape measure to check the measurements.

2. Now observe the north orientation indicated on the plan, noting its presence and gaining a general idea and evaluation of the orientation or aspect of the site.

For now, simply note that the sun rises in the East and sets in the West and that the orientation of the garden landscape will determine its aspect to the sun, thus indicating the general disposition of light and shade across the landscape once the garden becomes established and the various features are in place. This will also indicate something of the designer’s intentions.


3. The third and fi nal essential feature (or convention) to look for in the plan or design drawing at this stage is the key, or the interpretation of symbols. This is usually in a box or panel that should be readily located on the drawing.

The purpose of the key is to interpret the symbols used on the design plan so that they can be ‘read’ or identifi ed, and so one may further interpret the designer’s intention. The role of the key is to render otherwise complex information simple and accessible.

Details that are not made available in the key are usually written onto the plan, often as feature headings. Measurements and dimensions are always written or noted on the main body of the plan.

The review of these three principal points covers the salient features to interpret a plan and to gain an understanding of the intentions of a design.

The designer will have surveyed and measured the area in question. It simply remains for the landscaper to both interpret accurately the design and its intentions, and to check over the accuracy of certain primary details during the early site assessment.

This is not a presumptuous procedure - mistakes (and omissions too) can be made at the design plan stage. There can also be ‘grey areas’ of presentation and/or ambiguities requiring clarifi cation, preferably at a relatively early stage in the proceedings.

Landscape construction commences from the moment of entering a new site area for the purposes of visual and physical inspection.

It is the landscaper’s responsibility and business to check over the site thoroughly, to become acquainted and familiar with it and to develop an organised understanding of the terrain and general disposition of the site. Getting ‘a feel for the land’, or ‘one’s eye in’ are both colloquial ways of expressing this process.

Always have a number of copies of a site plan made and a master copy or ‘original’ kept out of general use - plan copies used either in the fi eld or in the offi ce are subject to considerable wear and tear.

It may be helpful to gain a general, but close, idea of the slope or ‘fall’ of given areas of ground as the site stands at point of entry. A fall is normally included as an aspect of otherwise fl at, fi rm surfaces that are not free-draining (e.g. patios, lawns on heavy soils) to allow surface water to run off so that puddling does not occur and surface water does not gather.


A well stretched and pinned builder’s line laid across relevant surface sections will provide a general guide as regards the extent of slopes and falls for the purposes of a visual impression, whilst the application of the spirit-level to the line will provide for a more exact assessment.

Site Access and Egress

Adequate and clear physical points of entry to, and exit from a site are important for the smooth and unhindered progress of work.

Storage areas for site materials should be organised so that open stores of loose construction materials (e.g. topsoil, sand, gravel, loose chippings) and all other materials and equipment are placed so as not to impede the free movement of machinery, nor the general access to, from and across the site.

A well-organised and productive site is tidily arranged and work planned in logical, progressive steps. The even fl ow of deliveries inwards and the removal of arisings outwards need to be straightforwardly catered for in keeping with boundary restrictions and those imposed by areas of work.

Note the site’s general readiness for working and access - e.g. is there a great deal of refuse or rubbish around, or other unwanted features that require attention? Do existing trees require felling or pruning? In other words, are there elements of site reclamation or improvement to be attended to initially? Are there landscape features or particular plants that require physical removal?

Figure 2 A well organised construction site


How to Level a Garden

Levelling is most important when preparing to construct features such as patios and lawns. The fi rst step is to assess the area which needs levelling as a considerable variation may render a more realistic approach of working with the contours, constructing walls or terraces. The method which minimises the amount of soil being moved is the best one.

Choose your level and knock a master marker peg. Using a straight plank, say 2m long and 2cm thick, knock in more pegs level with the fi rst, by placing the board on the master peg and the new peg and aligning them with a spirit level (which can be placed on top of the board). The site area should be worked across in this fashion (be careful not to trip over the pegs!). Soil can then be removed or added until the appropriate levels are achieved, as gauged by the spirit level.

If the ground is very uneven use ‘boning rods’ (‘T’ shaped markers). They are usually used in threes, in conjunction with the marker pegs. The master peg is placed in at the highest part of the plot. The next peg is placed about half a metre away and checked with the spirit level, as above. Then a boning rod is placed upright on the fi rst marker peg and another rod on the second peg. ‘Sight’ across the ‘T’ pieces of these two to a boning rod placed in the distance and knock in a marker peg to the correct level. This is useful because it can be carried out over particularly undulating ground, over plants and rubble to give initial levelling.

There is another method to fi nd the difference in level between two points – using a length of hosepipe, funnel and some water - a more fun method but not so accurate! One person must have the end of the hose with a funnel sticking in the end. It must be stood at the highest point in the area (like the master peg was in the other method). The height of the funnel, from the ground, is noted and should remain constant (so fastening it onto a stake in the ground can help). The other end of the hose is placed wherever the difference in levels needs to be measured. Water is poured into the hose, slowly, via the funnel. The ‘free’ end of the hose is raised until water no longer seeps out. Measure how high off the ground the hose is being raised. Deduct the measurement, made earlier, of the height of the funnel. This gives the difference in levels and so soil can again be added until level.

Remember that just because a plot looks level, it absolutely does not mean that it is! If an effort is made to ensure the groundwork is done properly, then any work carried out later, on say foundations, will go much easier and more accurately. It saves time and money in the long run if work is carried out thoroughly.


Also note that where soil levels require altering, it is preferable to remove the topsoil and save it for later use, rather than to bury it and end up with poorer soil on the top. If hollows are to be fi lled in, consolidate (tread down) regularly to prevent subsequent sinking.

There follows a logical sequence of events for the realisation of a design.

1. Ensure copies of the plan have been obtained, checked and verifi ed.

2. Prepare a work schedule (time for delays must be built-in to form a realistic timetable).

3. Research the market and order construction materials (and plants). Delivery dates must be appropriate to the schedule and the season.

4. Clear the garden of unwanted materials, weeds and plants.

5. Carry out any drainage improvements.

6. Install underground services (service lines, pipes, drains).

7. Carry out major ground contouring (if appropriate).

8. Lay foundations and bases and erect boundaries.

9. Construct vertical elements (pergolas, trellising, walls, steps etc.).

10. Construct horizontal elements (paths and patios).

11. Finalise soil contouring to bring all adjacent features level (e.g. make sure paths and patios meet other groundwork).

12. Cultivate areas to be planted (bring soil up to the required standard).

13. Implement the planting plan.

14. Finely cultivate areas to be grassed, then turf or seed.

15. Make fi nal checks to see if any adjustments need making.

16. Work out an annual maintenance schedule.

17. Relax in a comfortable seat and enjoy the maturing garden!

Some tasks will require contractors. Check they are members recognised by a professional trading body. Do not forget to obtain estimates before getting into any contracts.


2. SOIL MOVEMENT & STORAGE DURING CONSTRUCTION WORK

By the end of this section, you will be able to understand the reasons for correct soil movement and storage during construction works.

Describe the correct handling, storage and reinstatement of soil during site construction; to include separation, angle of repose and maintenance of soil quality during storage.

Describe the procedures required to reinstate the soil to the levels specifi ed in the design.

Describe how bio-security measures are used to prevent the distribution of pests and diseases through soil handling/storage and reinstatement.

Soil is a complex entity. Generally speaking, the less it is disturbed the better; it should not be radically intervened upon or overly disturbed in its placement and structure, unless to clearly diagnosed purposes, whilst employing known and reliable methods. With this principle in mind, one is able to approach the topic and the practice of earth-moving in a sensible fashion.

Signifi cance of Soil Layers in relation to Soil Separation

Soil can be perceived as comprising two horizons (or layers). Firstly, there is the topsoil, which is the surface layer of any soil profi le. The topsoil is the actual growing level or layer in the profi le; it is the level in which plants are able to put down their main root systems and from which they mostly derive water.

The topsoil is the fertile layer, alive with micro-organisms and decomposing, naturally-occurring organic matter, such as decaying vegetative matter. It is such decompositions of matter across extensive periods of time that actually form topsoil and give it its relatively dark and loamy appearance. Humus-rich loams are particularly favourable to plant growth as this humus is especially retentive of soil water and plant nutrients which do not readily leach or drain away.


Secondly, the subsoil is the layer situated beneath the topsoil. It is visually distinguishable from the topsoil, being generally lighter in colour, more solid and compacted and less varied in physical nature than the topsoil. Subsoil often contains the colouration of the dominant stones and bedrock of a given region. Thus, in a predominantly red granite area, the subsoil has generally got a deep reddish hue. In limestone areas the subsoil will be lighter and creamy in visual appearance and in texture. In sandstone and gritstone areas, it is yellowish to brown, and so on. It is simply necessary, for landscape construction purposes, to be aware that the soil demarcates in this way so as to know where the topsoil commences and a subsoil ceases (and vice versa).

Figure 3 Topsoil, subsoil and bedrock

Altering site levels, in particular, can have a profound effect on soil. Most plant roots will spread in the top metre of soil, so clearly removal of this layer, even in part, will have serious consequences for normal woody plant growth and development. It can destroy a plant’s ability to extract water and nutrients from the soil and the condition of stag-headedness can occur rapidly.

It is, therefore, most desirable to ensure that the topsoil layers and the subsoil layers remain separate. They have different qualities, nutrient statuses and tasks to perform, in relation to growing plants in particular. Poor drainage characteristics are also likely to result from mixing the layers. So when performing any of the following construction works, ensure that a separate


pile is made for each layer. Then make sure that the subsoil goes back in fi rst and that the topsoil remains on top.

Another problem is compaction. Little room for water and air exchange in the soil will result in a build-up of carbon dioxide. This sort of problem can occur if planting is done before hard landscaping elements (such as paving) are constructed. The plant’s root systems will have established and then construction work will compact and ruin the soil structure in the area. Trees will show symptoms of yellowish, smaller-than-usual leaves, marginal browning of leaves and twig dieback. Attempts at correcting this sort of problem have been made using a machine which uses compressed air to alleviate the compaction – with some success.

This is why hard landscaping should be done before planting. When digging out areas of ground and piling it, do not push or squash it all together as the structure will be ruined before it is even reinstated. Also, think about where equipment and machinery is being stored or used. Do not place anything on top of the soil which has been dug out or is going to be used.

It is indeed a logistical problem that, throughout construction, soil will need to be moved without ruining its structure and stability. How do landscapers manage the soil?

Movement of Soil to Levels Specifi ed in a Design

Earth Moving

This is necessary on a landscape construction site for a number of reasons.

1. Typically, excavations occur for the purposes of laying foundations for hard landscape surfaces.

2. For structures such as seats or benches, for instance.3. In trenching-out footings for walls.

The arisings of soil from such excavations are often referred to as ‘spoil’. Where excavations are limited in their extent, or where there are holes or depressions on the site to level up, the topsoil is normally distributed about the site to even up the surface.

It is important in such a process to take care not to mix large volumes of subsoil into the topsoil zone since, as indicated, subsoil is usually structurally and nutritionally inferior to topsoil for most horticultural purposes. Excess excavated subsoil is usually removed from a landscaping site and hauled to a landfi ll site and so disposed of appropriately, harmlessly and relatively naturally.


Sometimes it is necessary to remove and replace especially defi cient, unworkable, polluted, toxic or simply excessively thin topsoils, substituting them with a more appropriate grade of soil from another excavation site or similar. This is not an especially common occurrence, however.

Normally any additions of topsoil to a site are simply by way of adding topsoil depth to areas to be soft landscaped (perhaps after the raising of paths or road levels, in the course of new property or new hard landscape construction, or where there is necessary levelling-up, grading, contouring, or mounding to be performed in keeping with a new design plan for the landscape). Topsoiling is also a time-consuming and costly operation and so is a practice best exercised with discretion and undertaken only where clearly necessary.

It may sometimes be necessary to move topsoil from one part of a site to another (e.g. for levelling, mound contouring and terrace establishment). This should not be carried out in wet weather. In the process of new topsoiling, further improvements can be made by evenly mixing in composts. Grading and de-stoning can be undertaken, whilst any identifi able residual perennial weed roots can be removed from the topsoil, to prevent re-emergence.

Figure 4 Re-use of excavated topsoil

Groundworks are common to many landscape construction projects and the following is a general overview of the signifi cance of such practices.

Groundworks

This is something of a catch-all term to cover a number of relatively independent aspects of land-working that are linked literally at ground level and in their being basic to the preparation of ground for more visible, more featured work.In the fi rst instance, one of the prime considerations when assessing the nature


and potential of a site is the condition and nature of the soil. Purposeful soil cultivations constitute one aspect of groundworks.

At some variance to this, groundworks are also carried out in preparation for the construction of hard landscaping features. Excavations, trenching, foundations, footings, and the preparation of rammed sub-bases of broken stone as a part of this process all constitute groundworking. (More on foundations will follow later in this lesson).

Similarly, the laying of ‘services’ to a garden or other planned landscape, all constitute groundwork.

Figure 5 Laying of services e.g. oil pipe

Such aspects include:

1. The provision of garden, fi eld or land drainage systems.

2. The provision of soakaways (in specifi c parts of land where drainage water fl ow might be intensive and otherwise problematical).

3. The provision of storm ditches and peripheral storm drainage in general - as may be justifi ed in especially rainy parts of the country where and when low subsoil porosity is also a factor.

Whether the weather is changing or not, the recent frequency of very heavy local downpours has been something to contend with. Consideration of suffi cient provision and enough to cope with, say, 50mm in 2 hours should be in the plan. For 100mm in 4 hours (which might occur once in 50 years), if this volume of water would destroy everything, then a review of the plan is justifi ed. Around the world (and maybe we hear more about it because of better reporting), much heavier and more prolonged rainfall has occurred.


Near built-up areas and major roads, the rain water drains very quickly into the nearby rivers. As a result, these may rise quickly. Forethought and contingency planning is a sensible procedure. - but what is reasonable – once in 50 years – once in 100 years?

Groundworks relating to soft landscaping includes the full range of available cultivation tasks (and possibly also of drainage tasks). It also includes the principles and practices of soil improvements. Topsoiling has been mentioned, beyond which is the improvement of the soil by mulching practices and by the application of fertiliser. Vegetative groundworks may include the clearance and/or renovation of neglected land and the general reclamation or retrieval of post-industrial land prior to re-landscaping.

At this shallow depth the water supply will not be reliable in very cold weather. Plastic pipes burst very rarely, but they could freeze. At 300mm deep, the water is most unlikely to freeze.

Land Forming

Land forming can be something of a specialist technical discipline as used in, for example, civil construction engineering, but it can, to some extent, be practised as a feature of some landscaping work and some relatively simple and straightforward descriptions of practices follow.

Land forming is made up of a series of practices and techniques in so far as amenity landscapes are concerned and are briefl y described below.

Mounding

This is the practice of creating artifi cial or ornamental mounds of earth. These can be laid with grass, planted or hard feature landscaped. The technique is perhaps most commonly used to add interest and variation to an otherwise fl at landscape, in accordance with the design plans.

Figure 6 Water pipe installation during construction


The key to success with such an undertaking is to make it look totally natural (so that a viewer would not question its existence) and therefore, effective. This goal can be achieved through precise planning, positioning and by placing it with the utmost care. Mounding should be able to be performed even in relatively small grounds or gardens, where applied with discretion.

Contouring

This is the next step up from mounding. Principally, it is a larger scale device for the creation of entirely artifi cial but natural-looking hills. Again, the goal is to produce a naturally contoured shape. This is a highly expensive, large-scale operation, typically undertaken at the re-landscaping conclusion stage of the infi ll of open cast mines. It is also practised in the transformation of large industrial waste spoil heaps from rubbish heap to hillside. A contemporary example of this is at the Drax power station site in North Yorkshire. It is therefore not very commonly done in domestic gardens.

Terrace Building and Establishment

This is a highly formal and often elegant device for gardens. The purpose of terracing is to produce a number of different, fl at levels to a garden. It helps utilise space effectively on steep slopes or to add visual interest to a reasonably fl at site. It is also possible to create a better ‘aspect to the sun’ and to permit variation in planting areas in an organised way. It can be achieved in large or small gardens, albeit in different proportions. Terraces are often accompanied by steps (the construction of which is discussed later). They may be adjacent to, or even contain, a rock garden area. Terraces can be extensive, elaborate and expensive. They may provide a situation for seating or for container or patio-style miniature shrub plantings.

Infi lling and Levelling

These techniques are perhaps the simplest but most required - supportive to many kinds of scheme. Infi lling and levelling are frequently carried out to smooth out hollows and variations in the lie (or slope) of land that is required to be either truly level, or at least relatively even. It may be exercised on naturally-occurring uneven land, where bumps are removed and hollows fi lled in, or where the soil has been disturbed or moved around to an unacceptable degree.


Infi lling and levelling are possibly the most routine and also the most widely practised. There is a great risk with such levelling operations that there will be an uneven layer of topsoil. In the classic scenario for ‘cut and fi ll’, the hillocks are graded-off to fi ll the hollows. As a result, the former hillocks have no topsoil and the hollows may now be areas of deep topsoil.

In such circumstances it is helpful to grade-off the topsoil, then level the subsoil and do some drainage operations if needed, then put back the topsoil as a uniform layer.

Subsoiling

Subsoiling is a practice originating from agriculture. Much can be learned from observing a farmer carrying out the task.

Advantages:

A deep, well fi ssured1 soil is a requirement for the best rooting possible; in fact it is essential for many plants and trees. Therefore this is a technique well employed in preparation for planting, on certain sites.

Soils with poor surface drainage characteristics are greatly improved when subsoiling has been carried out properly. However, this is not a successful substitute treatment for land which has inadequate natural drainage.

Single or double-bladed instruments, with sharpened leading edges and angled lifting shoes, are used. The depth at which they are used can vary from 30cm to 60cm.

The objective is to shatter and disturb the subsoil and is best done in dry conditions so as not to ruin the soil structure. Ideal times tend, therefore, to be in the summer (often July or August).

It is desirable to ensure that the subsoil and topsoil are not mixed together. However, there may be a benefi t to the soil structure. A silty, clay soil overlaying sand could benefi t from subsoiling by altering the surface texture. It does take many years for lumpy, clay soils to mix into sand to make a loam-like texture.

Subsoiling effectively reduces adverse effects from a number of conditions and soil types. Such improvements include:

1. Compaction and smearing of soil following cultivations when the soil moisture content is too high.

1 Fissures are deep cracks in the subsoil which aid the drainage of surplus water to a deeper water table.


2. Impervious layers such as pans are broken up. Panning is common on heavy textured soils.

3. Poorly structured soils following inappropriate soil cultivations may recover their structure.

4. Gravel soils, silty and clay-based soils suffer most from compaction. Often this has been brought about artifi cially by machinery and subsoiling can rectify this situation.

Subsoil Benefi ts

A variety of improvements can be seen, which enhance tree growth and development, including the following:

1. Root growth is faster and better. Roots can invade to a deeper level due to improved soil porosity with both micro and macro-pores. Tree roots can provide better anchorage.

2. Water infi ltration is more effi cient - faster.

3. Improved water availability, because there is an increase in soil micro-pores and soil depth.

4. Improved soil permeability and natural drainage.

5. Coarse porosity. This ensures greater stability, tolerance to drought and resistance to disease attack.

Subsoil Problems

1. Waterlogging can occur if the soil beneath that which has been subsoiled has inadequate natural drainage or artifi cial piped drainage.

2. Damage to soil structure can occur if carried out when the soil has a high moisture content.

3. Damage to existing piped or mole drains can occur when endeavouring to remove an impermeable layer.

Angle of Repose

The ‘angle of repose’ is the name given to a concept whereby the downhill movement of soil and loose, unconsolidated sediments is caused by the force of gravity and is resisted by friction. When these two forces balance, at the angle of repose, the maximum slope angle that unconsolidated materials can maintain is formed. At any angle steeper than this, the friction cannot counter gravity and so mass wasting occurs (like a landslide). At any angle


less than the angle of repose, gravity cannot overcome friction and sediments will accumulate to form steeper slopes.

When tasked with construction works, this angle must be taken into consideration. This means that it is no good deciding that a large mound is needed if it is not going to hold steady! A degree of common sense is needed to ensure that the construction is performed in an effective way. It is also considered in relation to health and safety as workers cannot safely work on a growing mound if it is likely to slide and injure people.

Soil Storage

Inevitably any construction and groundwork is going to mean that soil needs to be moved around. What is to be done with it when it has been removed but cannot be put back for some time?

Ideally, soil is stripped from one part of a site and immediately spread over a worked area elsewhere on site (the direct, progressive restoration method). This would save both handling costs and storage problems.

Soil storage necessitates additional handling that can lead to a greater risk of compaction. Risk to the soil during storage also arises through the potential mixing/dilution of the soil types and potential chemical and biological degradation.

Stored soil may remain in the stockpiles for periods ranging from a few months to several years. Very long-term storage presents the danger of the stored soil being assimilated into the landscape and becoming a forgotten resource. Careful site planning and record-keeping should ensure the soil remains available for restoration.

Where to Store the Soil?

It should be sited on dry ground, not in hollows and should not disrupt local surface drainage. If this is likely to cause a problem, a cut-off ditch will have to be made with attached water discharge facilities.

What Condition should it be in?

Soil should not be put into store during winter months when the weather is unsuitable and the soil too wet to be handled without being damaged. A wet soil is likely to suffer more in a stockpile than a dry soil. A wet clay is not only considerably heavier than a dry clay, thereby increasing the load on all soil


below it, but also has a lower shear strength and is thus less able to resist any applied load. A wet, sandy soil is more likely to slake (the effect of inter-particle lubrication allowing them to move and pack more closely) in storage than a dry, sandy soil. Conversely, the construction of stored soil mounds in dry weather can create dust problems.

The stored soil should be in the form of stable structures, from which water will run off without causing erosion or instability. Slopes in excess of 45° are usually regarded as unstable (back to the angle of repose). However slopes at angles of less than 25° may reduce drainage from the stored soil.

Soil type and climate are factors to consider when determining the size and cross-section for each pile of soil.

The use of equipment to construct a storage soil mound is a tricky one since it must be operated safely, effectively, without causing any undue compaction and yet it must maintain the stability of the soil structure. For instance, the construction of stored soil mounds using motor-scrapers, which build the height up in shallow incremental layers, ensures stability but results in compact mounds.

The magnitude of the compacting and consolidating forces will depend on the height of the storage mound and the length of time the soil is stored, but the effect of these forces will depend on the texture, density and moisture content of the soil within it.

For example, if the stored soil is to remain for more than a few months or over the winter period, then it should be vegetated and kept weed-free. The seed mix details and method of sowing may be different to normal grass-sowing techniques – mowing should occur at least twice a year (dependant upon seed mix), and spraying undertaken to prevent a build-up of weed seeds. The vegetation enables unauthorised movement of soil to be noticeable and assists in preventing soil erosion.

Destruction of soil structure can occur when soil is stored inappropriately. Stacks of topsoil which will be reused should be no higher than 1 metre.

Biosecurity Measures

The aim of biosecurity measures is to preserve the health and vitality of living things, particularly by excluding, detecting and responding to pests and disease organisms and pathogens.


The International Plant Protection Convention (IPPC) is an international treaty concerned with such measures. Their purpose is to secure a common and effective action to prevent the spread and introduction of pests of plants and plant products, and to promote appropriate measures for their control. It looks at natural fl ora and plant products, direct and indirect damage by pests and weeds and the provisions extend to cover conveyances, containers, storage places, soil and other objects or material capable of harbouring plant pests. They therefore give some advice on bio-security measures in relation to soil.

Sterilising soil, boots, equipment and so on reduces the risk of spreading pests and diseases. It is important to consider how a secure place for soil storage can be achieved where no risk of spread of pests and diseases is likely. A quarantine-type situation helps.

Just a thought rather than as a deep study – in a garden the problems that can linger (or spread) include:

Pests:

Scale insects

Dreaded slugs and indeed other slugs but the subterranean ones can be very persistent

Soil-inhabiting eelwoms – potato cysts and onion stem

Vine weevil

Diseases:

Potato deep scab

Wart disease of potatoes

Strawberry red core disease

Armillaria mellia – honey fungus

Onion white rot

Violet root rot

Fusarium and verticillium wilt

Contamination:

From old stocks of paints, oils, explosives, heavy metals, herbicides, fertilisers, night soil, even the needles and other drug users’ debris may be important.


3. CONSTRUCTION OF DRAINAGE SYSTEMS

By the end of this section, you will be able to understand the factors which determine the type of drainage system required in various situations.

Describe the construction of an intercept or French drain to collect run-off, a pipe drain system to lower the water table and a soakaway to drain a localised wet area.

Drainage systems were discussed in a previous lesson. Please refer back for more information.

As discussed in the previous lesson, when drainage systems are set up they must slope slightly so that the water can be carried away to another destination. This section demonstrates how to construct a drainage system with this ‘fall.’

Transferring Levels from Plans to Achieve Falls

When wanting to construct a pipe drain system, the drainage plan may be quite clear and it may be possible to peg out where the various pipe runs are to be created. The drainage pattern itself can be marked out on the ground using sand or spray paint (e.g. in a herringbone pattern or whatever is on the design).

On a large scale, the trenches for the pipes are cut out by machines which can be controlled by a laser. This enables the bottom of the trench to be at the precise depth from the datum line – an imaginary horizontal plane of light visible by the machine and projected by the laser. It is harder on a small scale, but not impossible. Small trenching machines with endless belt trenchers are a boon for small schemes. Trenches can be dug out by hand, but this is clearly the hardest way of going about it. Unfortunately this is likely to be the situation in most gardens.

One method of transferring the plan to paper is to peg out the drainage lines with 50 x 50mm square pegs, approximately 600mm in length. Starting with the outfall (or the highest point) establish what amount of fall is actually possible. It can be hard if the site is nearly fl at because some of the trench cutting will be quite deep. A target of 1:100 is a good one because the drainage pipes tend to stay free from silt. This means that for every 100 metres of pipe or ditch, the level would fall by 1 metre.


Dig out the outfall and create a ‘sighting profi le’.

Figure 7 Figure 7 A trench being prepared to receive the drainage pipes

A pair of these fi xed sight profi les provides a temporary and convenient line to sight from and ensures that the pipes can be set at the right depth.

Figure 8 A fi xed sight profi le

Fixed profi le at the next outfall at, say, 20m along the trench line

Ground level

Traveller

Adjustable peg

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