Geotechnics of Landfill

Introduction

When waste is deposited at a landfill site, air is trapped in the voids within the waste and soil. This causes the decomposition of biodegradable materials within the uncontrolled fill and the breakdown of the fill anaerobically by microorganisms. There are a number of potential serious problems that affect the construction of infrastructure on such areas, most of which are a result of significant volume changes. The following issues can be problematic for proposed developments.

• ?Decomposition of Biodegradable Fill

A significant cause of settlement within the uncontrolled fill is volume reduction resulting from the decomposition of organic matter. The decomposition of the fill may occur over a very long period of time.

As a result of the decomposition processes voids/pockets within the soil can occur where waste products that once existed have now been decomposed. This creates issues for proposed construction with potential for excessive settlement, differential settlement and areas of low shear strength.

• Variable Strength

Shear strength of the soil within uncontrolled fill is highly variable as it is generally not compacted upon disposal and the presence of voids as a result of the decomposition of waste materials. This has an important impact on the construction of infrastructure as it controls the bearing capacity of foundations.

• Surcharge/Fill/Building Induced Settlement

If the uncontrolled fill comprises coarse granular material with waste products throughout, the coarse granular fills are relatively free draining, and excess pore pressures are unlikely under normal loading conditions. Therefore, much of the settlement of the existing landfill will be immediate caused by an increase in effective stress from earthwork fill and/or the weight of the building being constructed. Given the likely variability in density and voids within the soil, induced loads could cause unacceptable amounts of settlement and damage to structures.

• Collapse Settlement

The application of increased loads to waste products within the fill can cause immediate physical compression resulting in crushing, distortion, bending and reorientation of the materials. This resultant movement of materials can create voids and cause immediate settlement directly above the affected materials. This is particularly pertinent with items like empty drums, cans, jars etc.

Possible Remedial Solutions

In order to address the geohazards highlighted above, over the former landfill area, it is likely that some form of remedial solution or alternative foundation solution will be required. These could include:

• Remove & Replace
• Preloading/Surcharging
• Dynamic Compaction/Rapid Impaction Compaction
• Piled Foundation
• Raft Foundation

An adequate geotechnical investigation is required in order to provide the most suitable remedial options. This should include sufficient depth of investigation, i.e. all the way through the landfill waste and into the underlying natural soils in order to enable pile designs to be completed. It must also include adequate assessment of the composition and strength of the landfill waste.

But how do I describe the materials?

BUT one of the biggest issues is the description of the materials. Accurate and consistent description of Fill or anthropogenic soils/materials is of importance as it may provide vital indication of the material’s likely geotechnical behaviour, the potential for contamination and/or the potential for ground gas generation. However, Eurocodes and Australian Standards provide only limited guidance on the description of anthropogenic “soils”, and this has led to an inconsistent approach.

The AGS Contaminated Land Working Group have produced a Guidance Note, in which a standard framework for the geo-environmental description of anthropogenic “soils” is set out.

They stress the need for detailing the proportions of inclusions within Fill, be it through standardised descriptors (rare, occasional, numerous, abundant) or by listing the approximate percentage of the inclusions by volume (e.g. “grey clayey sand of ash with textile fragments (20%), polythene (10%) and paper (10%)”).

This is important as it will govern geotechnical behaviour.

As with anything geotechnical – if you want to adequately assess risks you MUST have good quality ground investigation data. This should start with having accurate complete and useful material descriptions.

Click on the below for the link to the AGS guide.