Lecture 8 - Desk Study and Reconnaissance

Site Investigation

• Aims to foresee problems that may arise and figure out possible ways to address them. Always a prerequisite for design of engineering projects.

• It can focus on various concerns depending on project and site.

• It also happens even during construction and operationalization – to check if the findings/assumptions prior still holds true.

Geological Investigation

• It aims to identify problems that may arise in relation to the geology of a site and the proposed structure, as well as possible solutions.

• Geological investigation, more often, go hand-in-hand with geotechnical investigation. Geotechnical Engineering is the field of Civil Engineering that deals with applications in relation to the soils and may include rocks to a degree.

• Exploration hopes to have a confident and complete understanding of the subsurface. This is often limited by time and cost.

• Realistically, investigations are satisfied by determining properties relevant to a project in the site. Engineers should recognize this limitation and will dictate the level of conservation in the design.

• “A distinctive and challenging part of engineering geology is the reality that site geology will be exposed during construction and that geologic predictions will be tested by ground truth.”

• Data needed:

  • Soil and rock underlying the site, their kinds and properties

  • Stratigraphy and structures

  • Groundwater level and mobility

  • Extent of weathering

  • Hazards (landslide, earthquake, etc.)

• Special prob need to be considered based on the data available.

  • Certain rocks/regions may prove to be susceptible to

    • Weakness planes

    • Solubility

    • Sinkholes

  • Certain soils may have issues with

    • Large settlement over time

    • liquefaction

    • collapsibility

• Complexity depends on the site itself as well as the project/structure

• Has 3 general phase:

  • Desk Study

  • Preliminary Reconaissance

  • Site Exploration

Desk

• Aims to gather and study all available literature and data related to the site, including but not limited to:

  • Geologic maps

  • Topographical maps

  • Soil maps

  • Hazard maps

  • Possible loads

  • Structure importance

  • Structure tolerance to deformation

• The amount of effort put into desk study must be based on several factors, including but not limited to:

  • Project complexity

  • Project extent/size

  • Site conditions (possible issues based on material on site)

• For simple projects with relatively simple site conditions, basic geologic maps and topographic maps may suffice. For more complicated ones, more literature may prove to be useful.

• The desk study should provide you with a list of what data is still needed and what exploration tests will be needed to get them. This ensures efficiency.

Geologic Maps

• These are maps that show the geology of an area including data related to the rock units and distribution in the area, geologic structures, age relationships of the rocks, as well as cross section view of the map.

• A geologic map usually covers very large areas. MGB maps range from scales of 1:50,000 to 1:1,000,000. 1:50,000 means that 1cm in the map is 50,000cm or 500m). Scales depend on the information and purpose of the map.

• May provide other information like shallow deposits, sources of materials, etc.

• Does not necessarily have quantitative properties that we want to know about our site.

• Drift Edition

  • Map of surface geology – shows data about material occurring at ground surface including alluvium, mud, and other materials.

  • Called as such as it shows materials that have been “drifted” and covers the more solid units underneath

• Solid Edition

  • Map with no drift shown (like looking at ground if drift material are removed), only the SOLID geology underneath.

• Studies must be done so that both drift and solid geologies are considered as they are both important. There are maps called “Solid and Drift” editions for places with very little drift such that the boundaries between solid and drift parts are clearly seen.

• In reading geological maps, some of the key information you must understand from the maps are:

  • AGE OF UNITS (in relation to one another)

    • A stratigraphic column shows the relative ages of strata in a map and is usually found at the margin of the map.

    • There may be materials surrounded by other materials which may be younger or older than the others. Some materials are also proof of erosion/deposition or uplift events.

  • STRUCTURAL RELATIONSHIP OF UNITS:

    • A layer can be determined to overlying/underlying another layer based on outcrop boundary shape, outcrop width, strata dip, and faults.

Geologic Mapping

• Scientific process to map geologic features in the area connected and create maps of it

  • Includes details like rock units, types, age, and distribution, as well as other geologic features like faults.

  • Data are collected in anticipation of making a 3D model of the site’s geology.

• The first step is the traditional field visit where a geologist walks around and takes field notes which will then be used to make geologic maps. These data can be supplemented by aerial images and remote sensing and other technologies.

Technologies for Mapping

• Geographic Information System (GIS)

  • Technology that allows easier management of spatial data

    • Used for data capturing, storing, retrieving, editing, analyzing, comparing, and displaying.

  • Allows combining of several data from other fields that can be used together to create new and/or better data, maps, and results.

  • Other data that can be used in GIS are from remote sensing, aerial images, and other maps from geophysical methods (gravimetric maps, aeromagnetometry maps, etc.).

• Global Positioning System (GPS)

  • Allows determination of precise location.

  • This gives geologists the ability to assign a structure/unit of importance to a coordinate easily.

• Remote Sensing

  • Uses aircraft or satellites to measure reflected and emitted radiation to monitor physical characteristics of an area or structures in an area.

  • The data measured can take the form of acoustic waves, or other electromagnetic spectrum waves (gamma, radio, etc).

  • The results of remote sensing are images based on the data gathered by the receivers.

  • Some remote sensing systems:

    • Infrared Linscan (IRLS) - uses emitted electromagnetic radiation related to temperature of objects.

    • Side-looking Airborne Radar (SLAR) - Pulses of energy are shot from an aircraft, bounces off the ground, and is received by the aircraft again. This allows creation of photographs from recorded data. One of its advantages is that it is not affected by cloud cover.

    • Light Detecting and Ranging (LiDAR) - uses laser in determining the distance or range of an object by computing the time for it to return.

  • The images from remote sensing can:

    • Help in study of landforms, their characteristics, which may provide information as to what comprises them.

    • For places with limited maps, it can be useful in creation of other maps related to the terrain or geology, among others

  • Has a wide variety of applications in Civil Engineering field, including but not limited to:

    • Water resources studies and modelling

    • Disaster risk management related research for natural calamities

    • Environmental observation

    • Urban monitoring and studies

• Aerial Images

  • Image usefulness depends on its quality as well as what can be shown in the photograph

  • Can be used with a stereoscope to make a 3D ground surface model, which can then be used to make topographic maps.

  • Stereoscopy - 3D effect

  • Many landforms can be recognized easily:

    • Dry soils and rock may appear light toned while moist ones are darker.

    • Sinkholes appear as small closed depressions often with standing water or dark tones.

    • Landslides and debris flows are recognizable by their morphology and accumulate debris at toe

    • River meandering points out regions of impending instability.

    • Faults shown as disruption of alignment of topographic trenches and stream offsets.

    • Volcanic flows are usually flat or undulating surfaces of dark material.

    • Works of human beings.

Topographic Map

• shows mainly the elevation data related to the map. It uses contour lines to show which areas are at the same altitude/elevation and thus allows for visualization of flat areas vs slope areas and how steep slopes are.

Soil Maps

• Much like geologic maps, but these show data and distribution of soil in an area.

• Usually used in terms of farming application but may give a general idea about the site.

Hazard Maps

• Shows the site’s susceptibility to various hazards like earthquake or liquefaction.

Civil Engineering Considerations

• POSSIBLE LOADS – based on the expected structure, one can estimate how much load will be transferred to the ground

• IMPORTANCE – certain important structures have stricter rules in design and construction

• TOLERANCE TO DEFORMATION – some structures can have deformation like settlement of floor while some like factories that require precision (due to machine connections to each other) may have more stringent design considerations related to deformations.

Preliminary Reconnaissance

• Before a report is made for the desk study phase, the preliminary reconnaissance (site inspection or walkover survey) is also done to supplement the desk study and give a better report.

• During this, a lot of information is noted related to the site as it is like soil and rocks observed, vegetation, slopes, evidence of possible problems like cracks on the ground or rocks, etc.

• The surrounding areas are also looked at for possible effect on the site (ex. Is the site in the path of a possible failure of a slope, etc.)

• Geotagged photos of the existing site conditions with necessary information and description are taken.

• Interviews with the people from the area are also done. May be asked site conditions not observed during visit (i.e. previous structure, flooding, past earthquake or failure events, etc.)

Pre-Site Exploration Report

• A report is made summarizing all details related to the project and site, the possible issues (and solutions), as well as future work suggestions to further determine feasibility of project or rule-out possible issues and data to be used for design phase.

• This includes all the pertinent information determined from the sources previously mentioned. The report suggests the processes to be done in the exploration phase of investigation (Site Exploration program or Field investigation program).