Development approval process

Vertical subsidence by itself is seldom a significant factor in the design of individual buildings.  

Structures will be left at a lower level, but this normally has no adverse effect on them except in the case of buildings that are in close proximity to watercourses and waterbodies. Mine subsidence may expose such houses to an increased flooding risk. In these cases, habitable floor levels should be designed to be comfortably above relevant flood level design requirements additional to vertical subsidence requirements.
 

Subsidence design requirements for strain refer to normal strain, which is the differential horizontal movement of the ground.  The design must account for both tensile strains (expressed as positive) and compressive strains (expressed as negative). Strains can be oriented in multiple directions.  

Ground strains can be transferred into structures where the foundations do not form a flat plane and are keyed into the ground surface. Foundations that are keyed into the ground include integral retaining walls, drop edged slabs and slabs with thickened edge beams.  The greater the depth of the foundation, the greater the likelihood that strain will be transferred into the building structure. 

The transfer of strain can be reduced by: 

• Making the foundations as shallow as possible or over a slip layer, such as sand bedding and plastic membranes which separate the footing structure from the founding material where improvements are constructed on a level plane, such as a slab on ground.
• Including some form of separation between the foundations (for example piers and ground beams) and the building superstructure where deeper foundations are required, such as piers. 
• The excavation of trenches around the structure that extend just below the underside of the foundations which are backfilled with sufficiently compressible material.

Curvature is the rate of change of tilt and is typically expressed as a radius of curvature in units of km or km^-1. Curvature can be oriented in multiple directions and unless otherwise specified, the design should be able to accommodate both upward (hogging) and downward (sagging) ground curvatures. Ground curvature results in differential settlement across the building structure. 

The effects of ground curvature can be reduced by panelling and articulating walls to move without developing strains or cracks or causing doors and windows to jam. 

Vertical articulation joints spaced at appropriate intervals and in areas where the wall stiffness changes such as above windows and doors reduces the likelihood of damage. An additional articulation measure is to eliminate brickwork above windows, doorways and arches. If such details are included, special attention must be paid to the provision of bond seams and strengthening panels that incorporate arches.

A tilt caused by active underground mining refers to a change in the slope of the land surface between two points. Tilt is defined as a change in slope, or difference in vertical subsidence at 2 points, divided by the horizontal distance between them and is typically expressed in units of mm/m.

Whilst ground tilts will introduce additional bending moments in structures, the additional loads due to tilt rarely result in significant impacts on the structure. Tilt impacts are more often noticed to adversely affect the serviceability of structures.  

Small tilts generally do not affect the usage of a building and can be catered for by providing such things as generous falls for services. Construction tolerances are typically in the order of 3 to 4 mm/m and drainage structures are usually built with grades that are greater than 10 mm/m.
 

Sinkholes or potholes occur in areas that have been undermined by shallow historical mine workings. Potholes are holes in the ground surface resulting from subsidence and usually occur in areas where old inactive mine workings are very shallow. The majority of potholes that occur in previously developed areas are less than 5m in diameter. 

Properties that have been assessed as being subject to a sinkhole risk have been assigned a Surface Development Guideline 1. Some properties that have been assigned a Guideline 7 may also be subject to a sinkhole risk. 

Guideline 1 allows for a single storey brick veneer or 2 storey lightweight residential building. The guideline describes a standard sinkhole foundation design. 

Alternatively, Guideline 1 also allows for a design made to engineering principles by a qualified structural engineer that can accommodate a pothole of maximum diameter of 5m forming at any point beneath the structure. 

Specific design and performance requirements and restrictions are outlined in the guideline, and it should be read in detail to ensure compliance prior to carrying out the design. 

Structures that do not conform with Guideline 1 or have been assigned a Guideline 7 are assessed under Subsidence Advisory NSW’s Merit Assessment Policy. 

For applications that exceed the guideline, it is recommended that the engineer consult with Subsidence Advisory prior to undertaking detailed design. Specific design or sinkhole mitigation requirements may vary, dependent upon the building type and the assessed level of geotechnical risk.