Determining causation is no easy matter in a court of law
For 75 years the 36-inch cast-iron water main under St Thomas Street, Southwark, served its function, taking in its stride the vicissitudes of life (Thames Water v London Regional Transport;
judgment 18 August 2004). Then, between 1995 and 1997, London Underground built the new Jubilee Line nearby. On 24 October 1999, the pipe suddenly failed, giving rise to claims that amounted to many millions of pounds, writes Susan Lindsey.
Thames Water blamed London Underground, alleging it had compromised the ground support for the main, causing damage to the pipe.
LU opposed the claim, saying the pipe was so loaded already by historic stresses and strains, that its actions were simply the straw that broke the camel's back. That, it said, was not sufficient to give rise to a legal liability.
Similar arguments might arise in the context of a building contract to carry out works to an existing building. For example, unbeknown to a contractor, previous works may have seriously compromised the structure. As soon as the builder starts work, damage results, triggered by the new work.
Other issues arise, including what the contractor is obliged to do and what it should and could have noticed before starting work. But how does a court approach questions of causation of damage in such cases?
In Thames Water, HHJ Wilcox took the wellknown starting point of what is called the 'but for' test. At first sight this appears a useful and straightforward test. If 'but for' the defendant's actions the damage would not have occurred, then the defendant is liable. But the test runs into problems, particularly when there is more than one possible cause of damage.
The judge referred, among other authorities, to Lord Hoffman's speech in Banque Bruxelles Lambert v Eagle Star, the well-known case about the extent of damages payable by a valuer who negligently over-values property that turns out to be inadequate security, in part because of a fall in the property value. Lord Hoffman gave the illuminating example of a mountaineer who, before setting out on a difficult climb, asks a doctor whether his knee is fit enough for him to undertake the journey. The doctor has a quick look, and declares the knee is fine. The mountaineer sets off, but suffers an injury that is unrelated to his knee, but which is a foreseeable result of mountaineering. 'But for' the doctor's advice, the mountaineer would have stayed at home, suggesting the doctor is liable. But common sense says that is wrong. There is no sufficient causal connection between the doctor's advice and the loss suffered by the mountaineer.
So the 'but for' test is a filter that can exclude things that are not a cause and identify things that are a potential cause, but it cannot be a reliable test of causation. Instead, the court has to decide what caused loss on a common sense basis, and within the context of what it was that the parties were supposed to be doing.
In applying this common-sense approach to the facts of Thames Water, the judge had to consider extensive evidence about the underground conditions, both in geological terms and in terms of man-made events that had altered the ground over time. He also heard about the condition of the pipe and the stresses and strains it had been subjected to, including as a result of London Underground's works. The documents referred to ranged from post-war bomb-damage maps, back through 18th century maps showing street realignment to archaeological records of Roman and mediaeval activity. The judge had the benefit of expert evidence on the significance of all this data in relation to the pipe failure, as well as expert assistance from metallurgy experts about the fracture damage to the pipe.
Applying a common-sense approach to this wealth of technical data, the judge concluded that LU's work caused differential settlement in the made ground around the pipe, triggering localised movement in the alluvium beneath the failed piece of pipe, thereby overloading the pipe joint.
Had the settlement caused by LU not happened, the pipe would have continued in use for many more years. Hence LU had caused the damage, and was liable.
Susan Lindsey is a barrister at Crown Office Chambers. Visit www. crownofficechambers. com