Geotechnical investigation for a proposed waste management facility

Introduction

This case study summarises a geotechnical investigation aimed at assessing the prevailing subsurface geotechnical and geological conditions for the proposed development of a waste management facility located in South Africa. The geotechnical investigation was structured to satisfy the Environmental Impact Assessment (EIA) protocols as well as inform conceptual and detailed designs of the Class A facility, in accordance with Section 3(1) of the National Norms and Standards for Disposal of Waste to Landfill (GN R636, 2013). The project scope included development of waste disposal cells, contamination and leachate ponds as well as appurtenant infrastructure associated with a waste management facility. The investigation focused on soil and rock profiling and logging, identifying geotechnical constraints, and providing recommendations for clay liner construction, foundation design, excavation stability, and material suitability including mitigation for any geotechnical constraints.

The site investigation encompassed an initial desktop and site reconnaissance assessment for planning and optimisation of the site activities, followed by a comprehensive geotechnical investigation that included fifty-nine (59No.) test pits, thirteen (13No.) rotary core boreholes, twelve (12No.) in-situ double ring infiltrometer tests, eighteen (18No.) Dynamic Cone Penetrometer (DCP) tests, and laboratory testing of samples for material classification and determination of shear strength parameters.

Challenge

The investigation identified varying soil types, ranging from transported expansive soils of pedogenic origin and significant quantities of cohesionless aeolian deposits, as well as residual soils of Siltstone and Sandstone origins with associated weathered bedrock across proposed development area. In accordance with the laboratory test results, the pedogenic and residual soils comprised varying plasticity indices which are generally susceptible to expansion due to moisture fluctuations. Due to the variability in composition and occurrence of clayey soils across the site, the residual soils were deemed unviable for use during clay liner application. Furthermore, the residual and weathered bedrock materials, as per the laboratory test results, predominantly classified as poorer than G10 quality and were therefore not suitable for use as layerworks for internal roads and hardstand areas. In areas where these materials occur at earthworks level, it was recommended that subgrade pre-treatment be undertaken for internal roads/hardstands, which would likely entail removal and substituting with better quality material to suit the design.

Perched groundwater conditions were encountered in just two test pits, noting that the investigations were undertaken during the dry summer months. A shallower more widespread water table, as well as surface run off, was observed during the drilling investigation in the rotary cored boreholes, with higher than normal rainfall experienced during this time. Essentially, the presence of a defined transported/pedogenic horizon across the site comprising matrix supported gravel fragments and ferricrete, as well as mottled and blotched colouration in the residual soils, suggests a potential for perched water table conditions during periods of sustained rainfall, and/or during the wet winter months. It was therefore anticipated that certain areas across the site may potentially develop shallow perched water tables and it was recommended that de-watering be undertaken during construction.

Solution

Excavation assessments from test pits revealed that the transported and residual soils are generally easy to excavate, classifying as “Soft Excavation” according to SANS 1200D. Bedrock conditions are generally highly weathered, classifying as “Intermediate Excavation” according to SANS 1200D, which may require heavy ripping during bulk earthworks. As part of the investigation, a materials balance assessment was also undertaken which essentially provided estimated volumes of the prevailing subsurface materials to a depth of 5m below ground level. This allowed for the provision and estimation of stockpiling materials during earthworks.

Shallow conventional foundations were recommended in areas with stable strata, while deep foundations or ground improvement were recommended for regions with expansive and / weak soils. To manage groundwater and excavation conditions effectively, the implementation of sump pumping and cutoff drains in shallow groundwater zones were considered critical. Temporary shoring methods are to be employed in soft soil areas to prevent slumping, and wider footings or the use of a soil raft was considered necessary for foundations in weak founding horizons.

Impact

The investigation revealed variable subsurface conditions characterised by varying soil composition with the associated complexities relating to bearing capacity and appropriate foundation solutions across the site. The tailored foundation solutions, groundwater control measures, and stabilisation techniques outlined are crucial for ensuring site stability and suitability for the proposed development of the waste management facility.