SITEWORKS

AASHTO

(The American Association of State Highway and Transportation Officials) – A soil classification system of natural material before stabilisation.

Bank cube

The volume of undisturbed ground before it is excavated (in situ in cubic metres).

Clay

A cohesive material made up of microscopic particles (less than 0.002mm) and vary widely in properties; some have more moisture movement than others.

Cofferdam

A temporary watertight enclosure around an area of water bearing soil or an area of water from which the water is removed (via pumping) allowing construction to commence in a water free area.

Compaction

The act of compressing soil to increase its density.

Demolish

Pull or knock down an existing structure (building).

Dewatering

Pumping subsurface water from an excavation to maintain dry and stable working conditions.

Excavate

Dig out or extract material from the ground

Gravel

Hard rock material in particles larger than 6.4mm in size but smaller than 76mm.

Groundwater

Water that exists below the surface of the earth and passes through the subsoil.

Plant and Equipment

Site assessment

Before any construction work can begin, the building site must be thoroughly analysed and understood to ensure an efficient construction process. The more common site assessment processes are described below.

Site surveying

Site surveying is a method of accurately determining points and lines of direction (bearings or co-ordinates) on the earth’s surface and preparing from them maps or plans. Boundaries, areas, elevations, construction lines, and geographical or artificial features are determined by the measurement of horizontal and vertical distances and angles and by computations based on geometry and trigonometry. The height of points in relation to a datum line (usually mean sea level) is measured with a levelling instrument consisting of a telescope fitted with a spirit level and usually mounted on a tripod. It is used in conjunction with a levelling rod placed at the point to be measured and sighted through the telescope. The transit is used to measure vertical and horizontal angles and may be used also for levelling; its chief elements are a telescope that can be rotated (transited) about a horizontal and about a vertical axis, spirit levels, and graduated circles supplemented by vernier (small movable graduated scale for obtaining fractional parts of subdivisions on a fixed main scale) scales.

Known also as a transit theodolite, or transit compass, the transit is a modification of the theodolite, an instrument that, in its original form, could not be rotated in a vertical axis. The stadia method of measuring distance, a rapid system useful in surveying inaccessible terrain and in checking more precise measurements, consists in observing through a telescope equipped with two horizontal cross hairs or wires (stadia hairs) the interval delimited by the hairs on a calibrated stadia rod; the interval depends on the distance between the rod and the telescope. Surveys based on photographs are especially useful in rugged or inaccessible country and for reconnaissance surveys for construction, mapping, or military purposes. In air photographs, errors resulting from tilt of the airplane or arising from distortion of ground relief may be corrected in part by checking against control points fixed by ground surveys and by taking overlapping photographs and matching and assembling the relatively undistorted central portions into a mosaic. These are usually examined stereoscopically (this is where two photographs of the same view, but taken at slightly different angles, are viewed together, creating an impression of depth). Maps or plans that represent the natural and artificial features of the earth’s surface, like hills and rivers, roads and houses and so on are commonly referred to as topographical maps or plans.

Environmental Assessment

The coordinated efforts, of building owners, architects, landscape architects, civil engineers, other professionals and construction companies are required to ensure that the structure being built will optimize site utilization, work well within existing natural systems, and minimize environmental damage. An Environmental Impact Assessment (EIA) of the building site could be called for by the local authority; which would have typically formed part of the plan approval phase. We highlight in table 10.1 below the Site Sensitivity levels covered in the National Building Regulations as described in SANS 10400: Part A.

* The inherent importance of the site in environmental or heritage terms, as defined by the National Heritage Resources Act, 1999 (Act No. 25 of 1999), the National Environmental Management Act, 1998 (Act No. 107 of 1998), and the Local Government: Municipal Systems Act, 2000 (Act No. 32 of 2000).

In addition, to assess potential or existing environmental contamination, an environmental site assessment could also be required if the site is on reclaimed land, close to an industrial area or mine. The environmental site assessment looks for contamination to soil, subsurface materials, and the existence of hazardous substances.

Soil Classification

Soils are classified by the sizes of their particles and their physical properties. Most soils are a mixture of the following five types of soils:

1. Gravel – a hard rock material with particle sizes larger than 6.4mm in diameter but smaller than 76mm.
2. Sand – consists of fine rock particles smaller than 6.4mm in diameter to 0.05mm.
3. Silt – consists of fine sand particles smaller than 0.05mm and larger than 0.002mm..
4. Clay – is a very cohesive material with microscopic particles less than 0.002mm..
5. Organic matter – is partly decomposed animal and vegetable matter..

The process of classification is to place a soil sample in a group or class that can be internationally accepted and understood. Soil classification systems are based on particle sizes found in the soil mass and where the system recognises three main types of soil:

1. Coarse grained* / cohesion-less soil, e.g. sand
2. Fine grained/cohesive soil, e.g. clay
3. Organic soil, e.g. peat

* Grained refers to the individual mineral properties in the soil.

There is a basic difference in behaviour between coarse granular soils and fine cohesive soils. The percentage of silt and clay size particles (or fines smaller than 0.06 mm) in a soil is important since, when this percentage is high, the soil will cease to behave as a granular soil. The critical magnitude of the fines content is expected to be in the range of 10–25%, and typically about 15%.

Soil Bearing Capacities

A field test can be conducted in determining the bearing capacity of an open foundation trench by taking a deformed reinforcing bar/rod of around 1m in length with a diameter of 16mm (minimum) and then try pushing this bar into the ground by hand (don’t use a hammer). If the bar can be pushed in easily for more than 300mm to 1000mm then the bearing capacity is inadequate. One can excavate further until a suitable stratum is found and the bar can no longer be pushed into the ground. For buildings not exceeding two storeys the local authority or building contractor would usually be able to assist with determining whether the soil in the area is capable of carrying these types of loads – For multi-storey buildings an engineer must always be consulted.

Backfill is required around the foundations after foundation brickwork has been completed and to the underside of the surface bed (before the concrete is placed). Normally the material excavated from the foundation trenches will be used for refilling, but if it is of a type difficult to compact and where potential settlement could occur, it may be necessary to import other fill material; likewise if there is insufficient fill for filling under the surface bed, fill would need to be imported.

This fill is either builders’ rubble (hardcore) or a soil that is suitable and safe to use for a particular application. Your local sand and stone supplier would usually be able to offer guidance in this regard and undertake to transport this filling material to site.

No special consideration is needed where fill has been placed under carefully controlled conditions since the load-bearing characteristics of well compacted fill are likely to be superior to many naturally occurring soils. However, where the fill is poorly compacted, the allowable bearing pressure of the surface soil is likely to be limited by the potential for long term settlement in the fill, especially where there is a flow of water to wash out any granular backfill material.

Remember it is essential that the fill material should be placed and spread in regular layers of not more than about 200 mm in thickness and each layer adequately compacted before placing the next.

It may also occur that fill is needed to either level a site or fill depressions, and where compaction is not necessary.

The compaction of soil refers to increasing its density by mechanically forcing the soil particles closer together by forcing out the air and water, using rollers, tampers and vibrators. The amount of compaction that can be gained depends on the physical and chemical properties of the soil as well as its moisture content, the method of compaction, and the thickness of the soil layers being compacted.

As compaction creates a denser soil mass, the effect of compaction can be measured in terms of dry density, i.e. the mass of solid material per unit volume of the soil. Bulk density and the moisture content of the soil are used to calculate dry density.

The level of compaction can be measured both in the field as well as the laboratory. Laboratory tests determine the degree of compaction achievable when using a standard amount of compaction effort, with a view of understanding the probable degree of compaction obtainable in the field.

There are two methods of ensuring compaction to a required specification are achieved:

• The compaction procedure method – specifying the type of compaction plant to be used as well as the number of passes for each layer. This specification is usually provided by the civil or structural engineer when required.
• The end result method – specifying the ‘in-place’ dry density of the compacted soil by using tests like MOD ASSHTO1 or CBR1. How this compaction is achieved is left entirely to the contractor’s discretion, provided the ‘end result’ as specified is achieved. – 1These are the most commonly used tests to determine the strength and density of the soil; the mod AASHTO [] mod=”Modified” ] and CBR (California Bearing ratio) tests. (which are not discussed here)

• The strength of the material is increased
• The durability and resistance to the effects of water are improved
• Wet soils can be dried out
• The workability of clayey materials can be improved

Blasting

Many parts of South Africa have considerable rock formations extending below the surface of the ground. Before a foundation or services like sewerage pipes or other subsurface work can begin, it is sometimes necessary to blast out rock located in an excavations path. Drilling and blasting is expensive and a careful analysis of the site must be made during the tender phase of the project. When an area known to contain rock, soil testing must be done to determine whether blasting would be required before construction work commences.

Blasting requires the services of a suitably qualified person(s) to carry out such work and is regulated in terms of the National Building Regulations, SANS 10400 Part G, described as follows:

1. The distance from a blast hole to the closest point of a building, other than a historical monument or buildings with a foundation system that does not comply with the requirements of SANS 10400 – H, shall be not less than that given in table 10.9 below.
2. Detonating relays shall have at least a 20 millisecond delay interval.

Dewatering Techniques

Dewatering is the process of lowering the level of subsurface water on a site to allow excavation work to occur in a dry and stable environment. Sub-soil water can be detrimental to the foundations of a building and for this reason it must be addressed before any building operations start on the site. Dewatering usually begins before excavation and continues as the excavation proceeds. Subsurface water is removed by using one of the following techniques:

Sub soil drainage

In order to keep the ground floor under a building dry in waterlogged areas, a sub soil drainage system should be installed to drain the surplus water to a drainage sump. A network of pipes is laid under or adjacent to the building in a herringbone pattern to collect and drain the seepage water away from the footprint of the building. These pipes are usually manufactured from uPVC or pitch fibre with small holes drilled at approximately two thirds up on the outer circumference of the pipe (known as perforated pipes) to allow the water to filter into the pipe.

Retaining walls

Retaining walls are required in a number of civil work applications including smaller site works and excavations. These walls are specialised forms of construction with specific performance requirements and there is a large amount of technical literature dealing with retaining walls, and more specifically concrete retaining walls available from the likes of the CMA (Concrete Manufacturers Association). These applications are outside the scope of this publication. In this section a brief description is given of some relevant matters concerning the use and types of retaining walls.

The basic function of a retaining wall is to retain or support soil at an angle greater than it would naturally assume, usually at a vertical or near vertical position. The design of any retaining wall is basically concerned with the pressures of the retained soil and any subsoil water. Retaining walls must be designed to ensure that:

• Overturning does not occur
• Sliding does not occur
• The soil on which the wall rests in not overloaded
• The potential for the build-up of subsoil water behind the retaining wall is avoided by installing a natural drainage system behind the wall

Typical uses for small retaining walls are to provide near flat ‘terraces’ in a sloping site for platforms or gardens or access paths. Small is defined here as up to 2.00m. Retaining walls are generally constructed from masonry or concrete and may be reinforced or unreinforced. For small walls, calculations are not normally necessary and typical sections of masonry wall are accepted as suitable for the relatively small disturbing forces that will act on them in accordance with the National Building Regulations SANS 10400 Part K.

If any one or more of the seven factors described below apply to a retaining wall, the advice of a civil or structural engineer, or a similarly qualified person should be obtained.

1. The wall is higher than about 2.00 m above the top of the foundations
2. There is supporting backfill on which banked up soil, stored materials or buildings are to be placed close to the wall
3. The wall is higher than about 2.00 m above the top of the foundations supporting backfill on which vehicles or other heavy items will stand or pass close by
4. Retaining soil with a slope adjacent to the wall is steeper than 1 in 10
5. The wall is supporting a fence of any type other than a simple guard rail
6. The wall retains very wet earth, peat or water; for example a garden pond
7. The wall forms part of or adjoins a building

For large retaining walls, methods of calculation need to be established by a suitably qualified person to ensure adequate factors of safety against the common kinds of failure as illustrated in Figure 10.11 are avoided; which are outside the scope of this publication.

Types of retaining walls
There are various types of retaining walls – the more common being:

• Gravity retaining walls
• Cantilever walls
• Precast concrete retaining walls (CRB)

Gravity retaining walls – These walls are also often referred to as mass retaining walls as they rely on their own mass and friction (dead weight) on the underside of the wall to overcome the tendency to slide or overturn/rotate. These walls when small can be constructed from bricks, stone or reinforced concrete. Higher walls, especially those constructed using brickwork or stone must be reinforced with a reinforced concrete core.