Miljøprojekt, 1032; Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Fjernelse af jordforurening ved og under huse - Priskatalog

Soil Cleanup Close to or Under Buildings

Summary and conclusions

In connection with the assessment of a number of cases of contamination caused by leakage from oil tanks, a need has emerged for a common basis for assessing the technical/economic aspects of mechanical removal of contamination under or close to buildings. Such a basis will also be applicable in other pollution remediation projects.

The feasibility of soil cleanup under a building, without demolition of the building, depends on a number of conditions, which are described in this report.

Soil and groundwater conditions influence the spreading of contamination and determine the nature of the foundations of the building in question, and thereby affect the conditions for the execution of the remediation project.

Generally, the bearing capacity of foundations in moraine clay is independent of the depth of the foundations, while the bearing capacity in sand will increase with depth. The stability in clay is high but low in sand. Excavations of short duration in clay can, thus, be carried out with very steep sides, while excavation in sand requires gradients of less than 1:1. Clay is less suitable as fill soil, because clay tends to settle with time; sand is, however, suitable, as well-compressed sand will not settle.

The hydraulic conductivity (permeability) is low in clay and high in sand, and often it is not necessary to lower the groundwater table in clay soil, while the opposite is the case with sand soils and with soft ground.

The design of the permanent buildings and the condition of the constructions influence the costs of a cleanup project. The height/number of storeys is of significance for the stability of the building and for safety during the excavation works. Higher buildings will require more extensive and more expensive safety measures.

Furthermore, the height of the individual storeys and presence and height of basements will affect the accessibility for contractor equipment. A basement will often provide a better contact to solid ground.

The extent and the quality of secondary buildings and non-permeable cover layer/paving of the contaminated area also influence the economy of the cleanup project.

The horizontal spreading of the contamination under the building determines the number of walls to be pulled down or to be supported, and determines the size of structural floor/basement floor to be broken up and re-established.

The vertical spreading of the contamination in relation to the foundation depth determines the need for any work to support the foundation.

Furthermore, the horizontal as well as the vertical spreading of the contamination are of significance for the total amount of contaminated soil.

The cheapest and most frequently applied method for removal of contamination is open excavation carried out with digging machines. This method will, however, often leave residual pollution close to buildings, if used alone.

When excavating inside buildings, walls and ceilings limit space and such excavation requires breaking up of the structural floor/basement floor. For this purpose, a small mini digger and conveyor belt or a mini dumper to transport the soil to a container or truck is used.

Drilling up soil with an ordinary auger bit in steel casings is more expensive and slower than open excavation, but at the same time a relatively common and simple method, which allows dimensions up to 1.6 m. The advantage of drilling is the vertical, stable walls of the drilled hole. This method can also be used in sand above the groundwater level.

A new method to remove contaminated soil is vacuum excavation. This method seems relatively cost-saving, and is quick to use inside buildings and in other inaccessible "corners". The cleanup consists of a simple "vacuum cleaning" of soil directly to a tank lorry. This method renders the use of conveyor belts and digging machines in the building superfluous. Furthermore, the vacuum cleaning causes less spreading of dust and associated pollution to the environment. The method is especially applicable in loose types of soils, but it is uncertain whether it will be economical or applicable in the case of hard clay.

Excavation close to and under foundations requires supporting measures. This also applies where excavation with free slopes will not fulfil the purpose. Excavations can be carried out using spread shoring support or using trench (excavation) boxes and system walls. The supporting structure must be strong enough to carry the construction load. This can be obtained by piling – either the so-called "Københavner-væg" (typically a wall construction of vertical steel girders supporting horizontal wooden boards), traditional steel piling or secant walls, which is a concrete wall consisting of alternate bored and closely placed cast piles. The piles are inserted to a depth under the planned bottom of the excavation in order to be well secured in the ground. A reinforced beam can be cast at the top to secure the wall.

As an alternative to a single line of piles, 2-3 lines of larger (ø 1.0 – 1.6 m) piles can be placed behind one another. This will be advantageous if for example it is impossible to bore deeply into the soil under the bottom of the excavation due to the groundwater level.

Finally, the foundations of the building itself can be supported either by sectioned support structures with reinforced concrete or by anchoring the foundation to two support beams carried/supported by steel beams at both ends.

Chapter 4, "Price catalogue", presents calculations for four main situations based on different building types; low building with slab foundation, low building with basement or crawl space, low building with self-supporting floor and piles, and apartment buildings with more than two storeys and basement.

For three of the main situations/building types, three scenarios with different soil characteristics; moraine clay, sand with a deep groundwater level and sand with a high groundwater level, have been calculated, while for the main situation "building with piling", calculations have been carried out for only one soil type, i.e. soft ground. This gives a total of 10 price tables.

For each situation, cleanup prices have been calculated dependent on the location of the contaminated soil in relation to the building. The contamination is defined in relation to a cross-section through the building, a vertical dividing line is placed at the outside of the building and an approximate horizontal line is placed under the foundation level or similar. The sections are referred to as section A, B, C, and D.

In the price table, the assumptions relating to the prices for the individual items are shown. In the case of significant deviations from the above mentioned assumptions, an addition or deduction in the calculated price can be made according to the catalogue.

The prices are expressed as an interval based on experience. The interval indicates that the prices depend on the following: Large or small quantities, spatial conditions, variations in the quality of building parts and paving to be broken up and re-established, and variations in the contractors' pricing of similar services. The prices are exclusive of V.A.T.

A user-guide for the price catalogue is provided in the report, and chapter 5 gives four examples of how to use of the price catalogue, and furthermore a model for estimation of the costs for cleanup projects is presented. Conditions that can be expected to have a significant influence on cleanup costs are described, including poor foundations and damaged building constructions as well as expensive building parts, which will have to be taken down and re-established.

The calculation model is presented in appendix A, and the price tables for each main situation are shown in appendices B-E.

Using the price catalogue given in this report, it is possible to make a realistic estimate of the costs for the cleanup of a case of contamination without having to prepare a project plan and a detailed estimate. The estimates can be made on a common basis, which will generate a greater degree of certainty and uniformity in the discussions between the participants in the project. The price tables can be directly applied in most situations. In special cases, additions or deductions to the prices stated in the tables are given for deviations from the standard scenarios. Even though the price catalogue is prepared for oil and petroleum contamination, it may be applied to other pollutants as well. The catalogue states where and at which price the cleanup costs have been calculated. In the case of deviations for the pollutant concerned, the unit prices may be adjusted accordingly.