Martin L. King

ABSTRACT

An underground high voltage cable, in which pressurised mineral oil is used as an insulating medium, was known to be leaking oil at one or more locations along its 2.5 kilometre length. It was impractical and even dangerous for the most part to dig along the cable route in an attempt to find the location of this leak or leaks. It was known that a significant quantity of mineral insulating oil had left the cable and entered the soil at the site of the leak.

It was decided to trial ground penetrating radar by scanning along and over the buried cable to attempt to pinpoint the site of the oil leak.

Soil dielectric properties are largely determined by the moisture content so that where moisture is displaced by oil the soil dielectric properties will change. Soil stratigraphy seen using radar is due to a large extent to the variable moisture content in the layering of the soil.

Where oil is dispersed through the soil, it will tend to displace moisture. This dielectric property change makes the area sufficiently anomalous so that it can be detected utilising ground penetrating radar.

This principle has now been successfully used on a number of occasions in New Zealand

NON–INVASIVE GROUND–PENETRATING RADAR INVESTIGATION OF A FAILING CONCRETE FLOOR SLAB.

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(International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Sept 2003)

Martin L. King, Dany P. Wu and Dr. David C. Nobes

ABSTRACT

At a New Zealand wastewater treatment plant there are several large wastewater settlement tanks that are crucial to the operation of the plant. These concrete tanks are 50 metres in diameter and 8 metres deep with the base of each tank approximately seven metres below ground level.

It was discovered that the base of one of these tanks had become distorted resulting in its failure to carry out its function. This presented a major problem for two reasons:

1. The importance of the tank to maintaining sufficient throughput of the overall operation.
2. The danger of contamination of the subsurface water aquifers, which are the main potable water supply for the area, should a tank base failure occur.

Any plans made to repair this problem was faced with a major restriction. Due to the type of construction of these tanks the floor could not be cut into, or disturbed, to any significant extent without risking catastrophic failure.

Ground penetrating radar was used to accurately pinpoint the areas below the base of the tank where voids, which were the root cause of the tank floor distortion, had formed. The information provided using ground penetrating radar enabled grouting repairs, through the floor slab, to be carried out using precisely positioned small diameter holes thus avoiding the risk of catastrophic failure of the tank base.

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