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GIVE US THE FACTS

Soil Consultants' geotechnical engineers Soil Consultants' geotechnical engineers examine the benefits of timely information examine the benefits of timely information to assist ground investigatio to assist ground investigatio ns ns Geotechnical and geoenvironmental engineers often decry the lack of information provided at the outset of projects. This can often be resolved by persistent requests for proposals, plans or the knowledge of site owners, occupiers and developers.

But inevitably, there are cases where such requests remain unanswered.

Yet schemes can benefit significantly and problems be resolved with the help of such timely information.

Historical records of subsidence sinkhole development in chalk strata, Alan Watson SC carried out an extensive ground investigation on a large site in Berkshire, which had been occupied for many years by a civil engineering contractor. The underlying geology comprises Lambeth Beds overlying Upper Chalk and a risk of chalk hazard development had been identified at an early stage.

To properly assess the potential impact on the proposed development, the interpretive reporting attempted to quantify the risks.

Fortunately, the former occupier was contactable and information on previous subsidence sinkhole development was available in the form of site plans. These showed locations of holes in the ground and dates of temporary measures (infilling) to deal with them.

The information spanned as far back as the 1960s and gave a suffi ciently reliable indication of the frequency of chalk hazard development within the prevalent ground conditions. On this basis SC was able to carry out simple calculations to give an expected frequency of events over the lifetime of the proposed development so that risks could be quantified.

Site owners who have occupied a site for decades have often accumulated a useful record of the history and development of the site. But there is frequently a perception that imparting information can be a bad thing, potentially affl icting the site prior to sale.

Overcoming such difficulties is not straightforward and can involve convincing the vendor that the desk study has revealed a certain level of a particular risk already, so the further revelations may only be fi e detail, albeit important background.

One approach that has proved successful is to state that in the absence of any factual records, overly conservative assumptions may have to be made that could jeopardise the land deal.

Clay heave and tree surveys, Hannah Azzopardi SC was instructed to do a ground investigation around a hospital building in the Birmingham area. The building was less than three years old but signifi cant horizontal movement and subsequent cracking had occurred and the cause of movement was not known.

A geotechnical investigation had been carried out, which gave an indication of the ground conditions and also mentioned that trees had originally been present on the site before building began.

SC was able to obtain an aerial photograph of the site from the Internet that was taken prior to construction.

The photograph showed that the trees were present near to and within the building footprint, and must have been removed prior to development.

But the tree species and water demand and the consultant were not known.

A window sample borehole investigation was carried out targeting the area of the building with the defects and this gave a general profi le of the ground conditions across the site.

Beneath a nominal covering of made ground was glacial till, underlain by weathered Mercia Mudstone.

Site workers excavated trial pits that showed the spread footings were founded at a depth of 2.5m. This was at odds to the 2m depth recommended in the original ground investigation reportand indicated that perhaps various factors were influential at the site.

There were also roots within the clay to a signifi cant 2.9m depth and undrained shear strength testing and moisture content profiling indicated desiccated clay soil of medium shrinkage potential to 2.9m depth in some areas.

There are established lime trees along one side of the site parallel to the area of cracking. If the trees formerly were also of a moderate water demand species then root penetration may not have been so great.

When SC's reporting was well advanced, it was told that a tree survey had been carried out prior to the construction of the building.

The consulting engineer was able to obtain a copy of the survey which indicated the trees previously in close proximity and within the footprint of the building were high water demand oaks. This information allowed SC to conclude that removal of the oaks was the most significant factor in the structural damage, which may have been caused by differential moisture recovery following earlier significant desiccation.

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