What Happened Last Time – Lessons Without Experience, Chapter 1

Harry Oussoren

In the proceedings of: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Soil Mechanics and Foundations (Advances in Laboratory Testing II)

ABSTRACT: What happens when there is not enough information about the ground conditions or when designers do not refer to records of past experience? Two case histories involving dewatering for excavations in unconfined aquifers adjacent to rivers are presented. The first concerns an addition to a sewage treatment plant where the geotechnical investigation was lacking and the relevance of a known condition was not investigated. This resulted in the redesign of the plant addition and the operation of a complicated dewatering system in conjunction with sheeting for a pumping station excavation. The second concerns an addition to a pumping station where significant lessons about past experience were not investigated. This led to a claim in which both contractor and owner blamed each other for past experience.

RÉSUMÉ: Que se passe-t-il quand il n'y a pas assez d'informations sur les conditions du sol ou que les concepteurs ne tiennent pas compte des expériences passées? Deux études de cas seront présentées pour l'assèchement de fouilles dans des aquifères à nappe libre adjacents à des rivières. La première étude concerne un ajout à une station d'épuration des eaux usées où il n'y pas eu d'étude géotechnique et où la pertinence des conditions connues n'a pas été étudiée. Il en est résulté une révision de l'ajout à l'usine et la nécessité d'exploiter un système d'assèchement compliqué en même temps que des fouilles blindées pour l'excavation d'une station de pompage. La seconde étude concerne une addition à une usine où des leçons significatives sur l'expérience passée n'ont pas été prises en compte. Cela a conduit à une réclamation dans laquelle l'entrepreneur et le propriétaire se blâmaient mutuellement pour l'expérience passée. 1 THE RAW WATER PUMPING STATION ON BEDROCK It was time to expand the sewage treatment plant which was located in a river valley in a northern city within the pre-Cambrian shield. The addition included a raw water pumping station plus new aeration tanks, clarifiers and miscellaneous structures. A geotechnical investigation was carried out which included putting down several boreholes using solids stem augers to advance the hole, split barrel sampling and, because the silty sands were heaving in the bottom of the borehole below 6 m in depth, cone penetration tests. The cones were driven to refusal consistently at a depth of 9 m which was the depth at which the new raw water pumping station was to be founded. As the water table was within 2 meters of ground surface, the hydrostatic forces acting on the structure were to be overcome by a number of rock anchors drilled below the base of the raw water pumping station and tied to the base slab. The excavation for the pumping station was to extend through water-bearing silty sand so the contractor chose to install double walls of interlocking steel sheet piles which were to be driven around the excavation to the bedrock at 9 m depth to retain the soil and to cut off the groundwater seepage. The contractor, which had considerable experience across the country with similar projects, was faced with the difficulty of sealing the gaps between the tips of the steel sheeting and the bedrock. To minimize the risk of loss of ground from under the sheet, it was decided to install a groundwater control system between the two sheet pile walls to temporarily lower the water table. In designing a dewatering system, it must be realized that a limitation of a system of wells, is its ability to control water between the wells where there is a perched condition (where sand overlies bedrock for example). If the aquifer (the silty sand) did not extend below the base of the excavation, then there was bound to be some perched sheeting. At this particular site the ground conditions had only been sampled to two-thirds of the depth of the proposed excavation. Therefore, it was not possible to design an efficient and effective groundwater control system without knowing the hydrogeological characteristics of the soil, particularly at the bottom and below the proposed pumping station excavation. It was decided to put down two test wells so that a pumping test could be conducted from which the aquifer parameters (transmissivity, hydraulic conductivity and storativity) could be determined. The deep wells were installed (Figure 1) using a dual rotary drill and 150 mm steel casing with wire- wells were drilled to a depth of 56 meters all the while encountering silty sand and no bedrock. Pumping tests on several wells screened at about 10 m depth indicated that the

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