Part 2 – Earthworks and Land Stability

2.6    Planning and Design

2.6.1    Landform

The final choice of landform should represent the most desirable compromise between taking account of the factors referred to in Section 2.5 and the preservation of natural features and the natural quality of the landscape including the retention of natural watercourses. 

The choice of a suitable landform is dependent on many factors which may be specific to a particular site. In general unnecessary earthworks should be avoided but considerations which may justify the carrying out of earthworks include:

1. Minimising the possibility of damage to property occurring through ground movement in the form of slips, subsidence, creep, erosion or settlement.
2. Minimising the possibility of damage to property occurring through flooding, or surface water run-off.
3. The development of a more desirable roading pattern with improved accessibility to and within the site and the creation of a better sense of orientation and identity for the area as a whole.
4. Efficient overall land utilisation including the quality of individual sites and amenity areas around buildings, the economics of providing engineering services, and the standard of roading and on-site vehicular access.
5. The need to create suitably graded areas for playing fields and other community facilities.
6. The enhancement of the general environmental character of the area by softening the landscape or by artificially creating or emphasising landforms of visual significance, particularly on flat sites or on areas devoid of landscape features.
7. The safety of the site by incorporating CPTED (Crime Prevention through Environmental Design) principles.

2.6.2    Soils and Investigations

Where appropriate the general nature and shape of the ground should be studied and particular note taken of:

1. The geological nature and distribution of soils.
2. Existing and proposed drainage conditions and the likely effects on ground water.
3. The previous history of ground movements in similar soils in the area.
4. The performance of comparable cuts and fills (if any) in adjacent areas.
5. The existence of peat soils including consistency, depth and extent.

Soil data should be obtained for areas which are intended to:

1. Form in situ bases for fills.
2. Yield material for construction of fills.
3. Be exposed as permanent batters.

Sufficient borings, probings, or open cuts should be made to:

1. Classify the soil strata by field and visual methods.
2. Evaluate the likely extent and variation in depths of the principal soil types.
3. Establish the natural ground water levels.

The soil information thus obtained should form the basis for:

1. Further sampling and testing which may be required on representative soil types.
2. Relating subsequent soil test properties to relevant strata over the site.

The appropriate test data for different areas shall be determined by the soils engineer.

2.6.3    Stability Criteria

Settlement

The most important factor in ensuring satisfactory performance of stable fills is the limiting of post-construction differential settlements. The design and construction of fills should be such that these settlements are kept within acceptable limits.

Bearing Capacity

The strength of the ground resisting general shear failure (and resulting gross deformation) under the footings of a house is a local phenomenon distinct from settlement. Fill constructed to minimise settlement in accordance with this Manual will have adequate shear strength.

Shrinkage and Expansion

Where peat soils are present in the area of the subdivision then special provisions shall be made to limit drainage of the peat which would lead to shrinkage.

Slope Stability

In most cases, it is unnecessary or impracticable to measure quantitatively the factor of safety of a slope against shear failure.  Maximum slopes of cuts and fills may be determined by the geotechnical engineer from experience and from observation of slopes in the vicinity which have a long-standing history of stability, are of similar height to the proposed slope, and are of apparently similar geological formation. 

Where necessary or where a precedent is not available, a special soils engineering investigation should be carried out by the geotechnical engineer to determine acceptable limits to cut and fill slopes.  In assessing slope stability, account should be taken of possible future changes in ground water level or other conditions.