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Site Preparation

As a landscaper you will be faced with many challenges. You may feel very comfortable with talking to your clients and convincing them on a specific design, but then, when it comes to measuring slopes, angles and distances on the ground so that you can accurately plot them onto a map, you struggle to do so accurately. What about determining the amount of soil to be dug out and/or to be brought in and the volumes of soil to be moved in a landscape job?

John Mason, Principal of ACS Distance Education, draws from years of experience to deliver some concise advice on site preparation for the landscape contractor.

Surveying

The first task at hand is to survey the site. Surveying is the process of measuring and recording dimensions, layout and content of a garden site; including noting of site factors that may be relevant to planning development.

Surveying can be made difficult by the following things:

· The view along site lines being blocked by something such as buildings or plants

· Inaccessible boundaries (e.g. if part of a boundary is: in water, a hill, muddy ground, or covered by overgrown weeds)

· Old survey pegs cannot be found

· Inaccurate prior surveys (e.g. if you have an earlier plan that shows a fence, water pipes, or something else in one location, but you in fact find them in a different location).

· Features which are supposed to exist cannot be located (e.g. a water meter).

What do I need to survey?

Different projects will require the surveying of different site factors. Almost any survey will require the surveying of property boundaries.

Other factors that may need to be surveyed and recorded might be:

· Contours

· Orientation

· Aspect

· Altitude

· Exposure

· Shelter

· Soil type and condition

· Drainage

· Access points

· Overhead and underground services (ie. Electricity, Gas, Water, etc)

· Internal views

· External views

You may also need to measure and record tabulated and referenced lists of plants or other features, to allow you to locate and indicate features on a plan, such as:

· Lawns

· Garden beds

· Feature trees

· Hedges

· Screens

· Paths

· Driveways

· Steps

· Ramps

· Patios

· Terraces

· Walls

· Fences

· Gates

· Pergolas

· Statues

· Ponds

· Streams

· Fountains

· Rock gardens

· Any other feature or structure

Preparing a plan or map

Angles and distances on the ground can be measured so that they can be accurately plotted onto a map. The first step in preparing a plan is measuring the site.

A simple, yet accurate, way to map is by triangulation, described below:

· Establish an initial base line. It might be the distance between two survey pegs marking one of the boundaries, or between two trees; the only requirements are that the base line be long, and marked at the ends by permanent fixtures.

· To fix the position of a feature near the base line (such as a tree), measure from two points on the base line to the tree. The two dimension lines to the tree should meet an approximate right angle (excessively acute or obtuse intersections mean loss of accuracy).

· Besides accurately fixing the position of the tree, you now have a new base line from which to plot other features.

· Finally, to plot the positions of the features on the base plan, use a sharp pencil compass to draw arcs from the two ends of the base lines, with radii corresponding to the measured distances. For small sites, a scale of 10 mm to 1 m is appropriate.

How do I deal with slopes?

Before you design your garden in detail, you should know the various levels and slopes with which you are dealing. The most useful way of showing levels on a plan is by contours. A contour is a line comparable with the edge of a pond, because it follows a truly horizontal course. If you picture your land with successive tidemarks each 30 cm higher than the last, you have a contour map with a vertical interval of 30 cm between contours.

Try using one of the following options to determine the contours of your site:

Direct Contouring

This is accomplished by sighting through a hand level (which will give an accurate, horizontal line‑of‑sight) and moving a boning rod about the site to find places (at various distances from the hand level) where the top of the boning rod corresponds with the line of sight. These positions are pegged. A line joining the pegs is then a truly level line ‑ a contour. The operation requires two people, one sighting through the hand level and the other holding the boning rod in various positions.

The Grid System

In the grid system, the ground is pegged out at regular intervals. No two points marked are necessarily level ‑ but this does not matter. A calibrated rod is used instead of a boning rod, and the distance from the horizontal sight line to the top of each peg is recorded. These distances are then used to calculate the relative level of each peg. When the peg heights are marked on a plan, contour lines can be inferred from them and drawn in on a grid.

What is levelling?

Levelling refers to determining the relative heights or points on the earth's surface. A level surface in this context is one which is at all times normal (90 degrees) to the direction of gravity.

Datum Line ‑ Two basic types exist.

(a) Ordnance datum. Mean sea level. In the UK this is an official datum above which all levels shown on ordnance survey maps are calculated. From this datum, a series of Ordnance Survey benchmarks (O.S.B.M.) are situated all over the UK, the commonest ones being found on walls, fences and buildings indicated by a tripod, ie. an inverted V with a centre line.

(b) Level datum. A local datum is chosen by the surveyor purely for convenience and may be any arbitrary plane, ie. step, post, wall, etc. and given an assumed value.

Reduced Level ‑ reduced level of a particular point is its height above or below the adopted datum line. Reduced levels can be represented on a drawing in 3 ways, namely spot levels, sections and contours.

Height of Instrument ‑ this can be described as the height of the line of collimation above the chosen datum.

Back-sight ‑ a back-sight is the first reading taken after the level has been set up. It consists of taking a sight on to a point whose height is known or can be calculated, eg. O.S.B.M.

Fore-sight ‑ a foresight is the last sight taken before the instrument is moved. It consists of taking a sight on to a point whose height is needed to carry on the line of levels.

Intermediate sight ‑ an intermediate sight is any reading other than a backsight or foresight, taken when the instrument is in the same position.

Change point ‑ a change point occurs when the instrument is moved, the staff remains in the same position. The reading taken before moving the instrument is a foresight, and when the instrument is set up in the new position the first reading is a back-sight, ie. back to the position where the staff has been situated during the operation. Since these two readings are taken on to the same position, they are usually entered on the same line in the level book.

Levels ‑ a level is a piece of equipment which is basically a telescope mounted on a tripod and which can be made to rotate in any direction in a horizontal or level plane.

The Dumpy level ‑ this piece of equipment has been used for a number of years and is an improvement on the old 'Y' type level. The telescope is fitted firmly to the base plate and the instrument is adjusted in a horizontal plane by 3 or 4 levelling screws. In theory, once the instrument is levelled, no further adjustment is required. However, in practice it is found that the settings do not remain correct for very long. This necessitates regular corrections of the levelling screws to re‑ centralise the bubble in the spirit level.

The Quickset level ‑ Because of the problems of maintaining the adjustment of the Dumpy level, the quickset level was developed. This differs from the Dumpy in that the telescope is not rigidly fixed to the base plate, but is hinged centrally instead, an elevating screw being used for the final adjustment of the spirit level before a reading is taken.

The Cowley level ‑ Cowley level differs from other instruments in that neither lenses not bubble tubes are used in their construction. They are easy to use, requiring very little in the way of instructional training and are especially suited to the smaller site. The body of the instrument consists of two solidly constructed aluminium die castings, inside which is as unique system of mirrors, one of which is delicately but robustly pivoted on a pendulum. All the working parts are fully enclosed and, there are no knobs, screws or controls whatsoever.

Levelling Staff ‑ levelling staffs are usually produced in timber or aluminium and are constructed in telescoping sections. Graduations on the staff are marked off in metres, decimetres and centimetres, whilst millimetres are obtained by estimation.

With most types of level, the staff will appear inverted in the telescope this means that great care must always be shown when taking readings, otherwise errors can occur.

Reading the staff ‑ to read the staff, the telescope is directed towards the staff and the focusing screw is used to bring the staff into clear focus. If the quickset level is being used, the main spirit level is accurately centred using the elevating screw. With the diaphragm in sharp focus, the axial line will be clearly seen against the staff, the reading is then noted.

Levelling Procedure

To find the difference in level between two points:

1. Set up the instrument, preferably between the two points.

2. Adjust it until the telescope is horizontal.

3. Read the staff held at each point in succession.

4. By simple subtraction the difference in level can be found, ie. the difference between points A and B = 3.90 ‑ 0.50 = 3.40. It is essential that this simple example is understood otherwise more complicated calculations will not be followed.

Levelling a sloping site

1. Place the level at station A.

2. Put the staff at point A and take a back-sight reading.

3. Move the staff to point B and take a foresight reading.

4. Place the level at station 2 and take another back-sight reading on to point B.

5. Place the staff at point C and take a foresight reading.

6. Move the level to station 3 and take a back-sight onto point C.

7. Transfer the staff to point D and take another reading.

8. This process can be continued until the site is levelled.

In practice, a small error in reading is often unavoidable.

However, it should never be allowed to exceed 0.01 per change point. In order to check whether the levels are correct, the difference between the sums of the back-sights and foresights should equal the difference between the first and last reduced levels.

Earthworks Calculations

Calculations are frequently required in the landscape industry to determine the amount of soil to be dug out, to be brought in and the volumes of soil to be moved in a landscape job.

A rule of thumb used by estimators is that soil dug from the ground will expand by up to 30% due to aeration. This means, that from any measurement of volume in soil to be dug out, you should allow for a 30% increase of that soil should you need to transport it off the site.

Volumes of Irregular Solids

To calculate the volume of irregular solids as cuttings and trenches we use the prismoidal formula. When the sides of the solid are flat, the calculation is exact.

The prismoidal formula is

V = L (A + 4M + B)

6

where:

V is the volume

L is the length

A is the area of one end

M is the area of the middle section

B is the area of the other end

Sample of solids that this formula can be used on:

The landscaping game is competitive and you must be able to quote accurately for preparing the new site for landscaping. John Mason’s ideas in this article should help you get better at it. Start with surveying the site, make sure you feel comfortable with plotting the site on a map, and be aware of the various levels and slopes with which you are dealing. Work out what form of levelling you may need to use and then calculate the amount of soil to be dug out and/or to be brought in.

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