Site Analysis

Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.

Ground Stability

Proper investigation of ground conditions is a crucial part of the site assessment process. You’ll need good information about ground stability and bearing pressure to determine whether the site is suitable for building, where on the site to locate any building, and the type and size of foundations you specify for the type of construction proposed.

Cutting corners when assessing ground stability can lead to serious problems such as some degree of building failure. If in doubt, call in expert help from a structural or geotechnical engineer.
What to look for

In your site visit, consider:

* general landforms – can you see any clues which point to a risk of landslip, soil creep, or subsidence?
* what are the soil types present – some types of soil can bear more load than others, and some expand and contract with changes in moisture levels (see soil types below)
* drainage/runoff, water table, and presence of natural springs or waterlogged soils
* proximity to mine works, quarries, exposed banks and landfill sites
* bearing capacity of the soil
* presence of expansive clays.

Look out for warning signs such as:

* slumps/hollows
* cracks in the ground
* sudden changes in slope
* leaning fence posts, power poles and trees
* remnants of buildings
* hollows in the centre of a flat area of ground
* undercut banks
* banks cut steeper than the angle of repose for the soil type – repose angles are in the order of 33º for sand, 25-40º for dry sediment, 30º average for soil
* evidence of slips.

You’ll also need to consider past use of the site, including the possibility of buried structures, contamination, and earthworks and uncompacted fill that may affect ground stability.
Determining bearing pressure

The allowable bearing pressure of soil is a measure of its ability to carry the load of the building without excessive (more than 25 mm) settlement. Bearing pressure must be measured at the base of the foundations and will depend on soil type.

For buildings within the scope of NZS 3604 Timber-framed buildings the requirement is for an ultimate bearing pressure of 300 kPa. Indications of good ground include:

* the foundations of adjacent buildings are satisfactory
* there have been no landslips in the vicinity
* all fill complies with NZS 4431 Code of practice for earth fill for residential development
* there is no organic soil, peat or soft clay.

If the ground does not meet these requirements, a comparatively simple soil strength test method referenced in NZS 3604 using a scala penetrometer may be employed. See BRANZ Bulletin 438: Using a Scala Penetrometer for more information, or NZS 4402 Determination of the penetration resistance of a soil.

If it the ground does not comply, or the building is outside the scope of NZS 3604 you’ll need to have the ground condition verified by an engineer experienced in soil investigation.
Soil types
Peat subsoil

Peat, which occurs in low-lying areas, is compressed dead vegetable matter which has been preserved from decay by acidic groundwater. The surface of the ground may appear stable and dry, but the peat can be present in a deep layer which can compress appreciably under the weight of a building. If you suspect peat is present, you’ll need to consult a specialist soils engineer. The extent and depth of the peat will need to be determined by drilling a number of bore holes. Where the peat layer is thin it may be able to be removed to expose firmer soils below, or a specifically designed raft foundation and floor slab may be needed.
Sand

Sands vary in their particle size and in compaction. Some types of sand have low bearing capacity so you’ll need to use piles, driven down to a firmer layer, in conjunction with a specifically designed slab.
Expansive clay

Some soils contain substantial amounts of clay material, which increases markedly in volume when wet and shrinks when dry. Certain trees can add to the severity of shrinkage through their uptake of moisture.

If these soils may extend to a significant depth below the surface, and especially if they occur at a level where the water content fluctuates periodically, substantial uplift of the surface may occur during wet periods, followed by subsidence during a dry season. The amount of this uplift varies with the clay content of the soil but can be as much as 50 mm in some areas.

Remember that putting a building on a clay soil will change the moisture content, and result in a new pattern of expansion and contraction. Moisture content will also be reduced by large paved areas such as sealed driveways and patios, tree planting, and subsoil drainage.

The effect of the clay movement is influenced by the moisture content of the site at the time of construction. It is important that, once the excavation has been carried out, the slab is laid without delay to minimise drying out of the area which will be beneath the slab.

In areas of potential movement of subsoil, the use of rigid elements such as brick veneer and concrete block should be avoided unless the slab and foundations are specifically designed.
Fill

If the building site contains areas where the levels have been shaped by earth fill, either excavated from within the site or imported, it is important to ascertain that this fill complies with NZS 4431 Code of practice for earth fill for residential development.

A certificate showing compliance should be obtained. If such a certificate is not available, have tests done on the area to determine its bearing capacity and to ensure that it does not contain unsuitable material. If the bearing capacity of the fill is inadequate, the foundations must penetrate through to solid bearing.
Sloping ground subject to slipping

The stability of a sloping site is difficult to define. Slips can occur on slight slopes which have no obvious warning signs on the surface, particularly when soil becomes saturated. Look at the condition of similar slopes in the area, and also look for cracks, slumping, leaning trees, tilted fence posts and other evidence of subsidence.
Water table

A very high water table can lead to extra costs by making construction more difficult and because of the need for pumping of excavations and provision of land drainage to remove the water.

A high water table implies that water pressure in the soil is high, which means the soil will be correspondingly weaker. High water pressure will adversely affect the stability of any slope and increase the loading on the retaining structures. Certain types of expansive clay soil will increase markedly in volume when they are wet.

If the building site is surrounded by large areas of higher ground, underground water will tend to flow to the site. This may cause pressure beneath a concrete floor slab or increased moisture levels beneath a timber floor; it can drive water into timber piles. Subsoil drainage may be necessary.

Initial indications of a high water table include:

* reeds
* surface water or boggy ground
* springs.

The history of the site (obtained from documents such as the certificate of title) may confirm that the subsoil is suitable for the type of building proposed. If there is any doubt, its suitability can be established by digging trial holes.

For sites where a low rise building is proposed, digging some trial holes by a mechanical back hoe or by hand can provide useful information.

When digging trial holes, a record should be kept containing:

* date of excavation
* location of hole on the site
* relative level of hole if the ground is not flat
* overall dimensions and depth of hole
* excavation system used
* ease of excavation
* rainfall that occurred while the holes were being dug
* groundwater conditions and water table level (if found)
* soil descriptions and depth of each layer
* positions from which samples were taken.

During excavation of trial holes, precautions must be taken to support the sides of the hole so that it can be safely entered for taking samples. On completion, the trial hole should be backfilled by compacting the material in 150 mm layers so that future work in the area will not be affected.
Finding information

The first step in assessing ground conditions is to get the background information from the local and regional councils (see the site analysis checklist), the certificate of title and a site visit.

Wherever there are doubts about ground stability, always consult a geotechnical engineer.
For your clients

To help your clients understand the hazards that might affect a site and how to identify them, refer them to www.smarterhomes.org.nz/siting-landscaping/hazards.