Geotechnical Investigations Overview

Site Investigation

• A site investigation is the process of collecting information, appraising data, assessing, and reporting. This is essential to understand hazards in the ground beneath a site.
• Site investigations are conducted to determine the physical properties of soil and rock. This information helps in designing earthworks, foundations for structures, and for repairing damage from subsurface conditions.
• Site investigations are also used to measure the thermal resistivity of soils. This is crucial for underground utilities like transmission lines, pipelines, and radioactive waste disposal facilities.
• Geotechnical investigations generally include surface exploration and subsurface exploration of a site.
• Geophysical methods are used to collect data about sites in some circumstances
• Surface exploration includes geologic mapping, geophysical methods, and photogrammetry. Geotechnical professionals can also observe conditions and physical characteristics by walking around a site.

Surface Exploration

• Site surface conditions can be observed though basic geological mapping, geophysical methods, and photogrammetry. Physical site observations by geotechnical professionals are also helpful.
• Understanding what is found on the surface is important but some sites may require subsurface exploration for complete condition information.

Objectives of Site Investigation

• The objective of a site investigation is to gather information needed for risk assessment. This enables the assessment of contamination on land.
• Soil samples can be collected at various depths using different equipment, from basic hand tools to rig-mounted drilling systems.
• Investigations may also include sampling water and gases found in the ground.
• The collected information is used to create a risk assessment by characterizing contaminants and their pathways and receptors.
• The information requirements for a robust risk assessment need to be identified upfront prior to designing the investigation.

Stages of Site Investigation

• The site investigation process is divided into stages, including:
  • Desk study
  • Site reconnaissance
  • Ground investigation (includes trial pits, boreholes, sampling)
  • Reporting

Desk Study

• A desk study is completed prior to site work to plan the ground investigation.
• Researching the site to gather geological and historical information is essential for planning of site investigations.

Sources of Information for Desk Study

• Geological maps and memoirs are useful in determining the type of ground.
• Aerial photographs provide valuable information.
• Records from previous site investigations are helpful.
• Service records are critical for accurate site planning.

Site Reconnaissance

• Site reconnaissance is normally a walk over survey.
• Collect information on the overall site layout, topography, and basic geology.
• Observe climate, stream flows, groundwater conditions, site utilization, and timing for the investigation to account for environmental conditions.
• Maintain photographic records.

Evidence to Look For During Reconnaissance

• Hydrogeology: look for wet marshy ground, springs, seepage, ponds, streams, and wells.
• Slope instability: look for bent trees, hummocks on the ground, and displaced fences or drains.
• Mining: look for subsidence, disused mine shafts, open cast mine evidence, and diverted streams.
• Access: consider limitations on access due to overhead cables and watercourses.

Ground Investigation

• Ground investigation is the process of collecting information beyond the data gathered from the walk-over survey.
• Trial pits are small excavations to a maximum depth of 6 meters.
• Trial pits are used for surface work like block sampling, and to look for underground services.

Types of Samples from Trial Pits

• Disturbed samples: the soil may not retain in-situ properties.
• Block samples: not undisturbed but do retain some in-situ properties.
• Push in tube samples: tube samples of the soil in the pit.

Boreholes

• Used to determine ground conditions below 6 meters in depth
• Sometimes undisturbed samples can be taken from deeper locations than the trial pits.
• Using in-situ testing methods when possible may be necessary.
• Results should be compared to the information gathered from the desk study and site reconnaissance.
• The number of bore holes will impact the reliability of the information.

Sampling

• SPT test: measures soil density.
• Core samples require sealing with paraffin to maintain water conditions and end sealed to avoid physical interference.
• Bulk samples are usually taken from trial pits in situations where cohesion is minimal.

Reporting

• Site investigation reports should address all questions from the planning stage.
• Viability of the proposed route and possible alternatives are part of the report.
• A location map of all boreholes, trial pits, and other relevant data should be included with supporting logs. These logs provide information about the soil and rock structure.

Soil Sampling

• Borings (usually large and small diameter varieties) are used to examine the soil or rock stratigraphy in-situ.
• Small diameter borings are an option for soil or rock examination or to gather samples at depth.

Categorization of Soil Samples

• Soil samples are categorized as either "undisturbed" or "disturbed".
• Undisturbed samples are not truly undisturbed but are often close enough to represent in-situ conditions to perform tests of structural properties.
• Disturbed samples are used to represent properties of soil grains (size distribution and Atterberg Limits)

Types of Soil Samplers

• Shelby tubes (thin walled, seamless steel tubes): used to obtain undisturbed samples for routine site investigations.
• Piston samplers: used when soil is sensitive to disturbance or consists primarily of silts or silty sands. Provide high-quality samples.
• Scraper bucket: used when soils are mixed with pebbles, making split spoon methods less effective.
• Standard sampler (Split Spoon): a common technique with an inside diameter of 35mm and an outside diameter of 50mm, typically used for visual examination and classification testing.

Dam Site Investigations

• The primary purpose of geological investigations for a dam project is to provide information necessary to design a safe structure and estimate the cost.
• The goal of dam designers is to minimize cost while meeting safety standards.

Investigation Required at Dam Sites

• Regional geological mapping
• Dam axis surveys (left and right)
• Landslide possibility survey
• Environmental Impact assessments

Dam Site Investigation Methods

• Geological mapping of surface rock outcrops.
• Geophysical surveys (like seismic refraction) to determine the depth of overburden.
• Excavation of trenches and pits using equipment such as bulldozers and backhoes.
• Diamond core drilling to collect relatively undisturbed cylindrical core samples (50 mm in diameter) to considerable depths. Other drilling techniques may also be used in some situations

Site Investigation for Bridges

• Site investigation helps plan, conduct, and report investigations.
• Objectives include describing stages of work and required information for bridge foundation design.

This quiz covers the essential aspects of site investigations in geotechnical engineering. Participants will learn about the processes of data collection, assessing soil and rock properties, and the importance of understanding subsurface conditions. Key methods such as geophysical exploration and thermal resistivity measurement are also discussed.