Geology 229Engineering and

Environmental Geology

Lecture 29Engineering and Environmental Site Investigation (West, Ch. 19)

Engineering & Environmental Site Investigation1. The purposes of site investigations2. Fundamental steps3. Approaches and tools

Problems in Engineering and Environmental Site Investigations

• Dams, levees, and bridges;• Highways and railroads;• Heavily loaded buildings;• Landfills;• Aquifers and groundwater;• Contamination plumes;• Liquefaction and paleo-liquefaction;• Archeological artifacts;• Underground voids, conduits, and

fractures;

Engineering investigationvs Environmental investigation

Eng: development Env: conservation– Mechanic properties - chem/bio properties– Seismic dominated -EM dominated– water table critical -water table critical– Bedrock depth critical -bedrock depth critical– More matured -in development

Purpose of Site Investigation

Establishment of the subsurface model at the site under investigation as accurate as possible.

3 Steps in Site Investigation

• 3-step approach: 1. Literature review, do your homework;2. Site visit and reconnaissance;3. Subsurface exploration:– direct and invasive techniques (boring

and coring);– Indirect and non-invasive techniques

(geophysical surveys,G228, G277).

1. Literature review• Review of designing plans and preliminary

plans:– How much is the impact (load, stress, demands)?

How deep it will go of the foundation?• Review of engineering reports:

– Reports of previous projects on the same site;– Reports of previous projects with similar nature of

the proposed one;– Review of public information or USGS open file

reports.

New tools for literature search

• World wide web (WWW)– Example 1: google earth– Example 2: site map:

http://terraserver.microsoft.com

2. Field reconnaissance• Visit the site by a group of experts:

– Project manager;– Designer;– Project geologist;– Project engineer;– Construction inspection personnel;– Town zoning officer.

• Purposes of Field reconnaissance: – To allow an experienced observer to view the site and

record information;– Preparing subsurface exploration (step 3);

2. Field reconnaissance (cont.): What to observe?

• Proposed location of the structure;• Topography and vegetation;• Surface soils, gullying (ditches and

trenches), and natural slopes;• Surface and subsurface water;• Surface and subsurface geology.

3. Subsurface exploration

– First need to run indirect and non-invasive techniques (geophysical surveys). This procedure can also provide more information for direct exploration;

– direct and invasive techniques (boring and coring, and other excavations).

Geophysical techniques

• Seismic refraction and reflection;• DC resistivity and self potential;• Induced Polarization;• Magnetic and magnetic gradiometry;• Electromagnetic Induction (Time domain

and frequency domain);• Ground penetrating radar;• Nuclear magnetic resonance;• Geophysical well logging.

Direct and invasive techniques• Sounding (direct push, probing, manually to find

the refusal (bedrock or large boulder), if the target depth is shallow;

• Drilling (churning (chiseling), rotary, percussion (jack-hammer); not much intact samples reserved;

• Boring and Coring, Geoprobe;• Multi-level sampling (groundwater chemistry);• Trenching (time consuming and expansive);• Pumping tests;• Tracer tests;

Figure 19.1 Truck-mounted rotary drilling using hollow-stem augers (West, Ch. 19).

Figure 19.4 Log of test boring (West, Ch. 19).

Figure 19.5 Symbols for different categories of soil and rock (West, Ch. 19).

Economical considerations

• Most site investigations are carried out by contractors in private sector industrial firms: money and time are the major constraints.

• Doing site investigation is doing business: – maximize the profit, minimize the expenses,

here the profit is the acquisition of information, as accurate as possible, with the least expense of monetary and man power consumptions.

from Geotimes, May 1995

200x200 20x2050x50 10x10 5x5

Noise free

10% Gaussian noise added

Extrapolation from known to unknown:

• GPR survey at a cemetery site in New Milford, CT.

Choose the right tools: GPR vs Magnetic surveys

• There is not a tool universally working for all the engineering and environmental site investigation problems;

• The most expensive tool may not be the one you need for a particular site problem;

• Not New London, CT

Five torpedo boats can be clearly seen floating in the Thames River, New London, CT in this 1930s aero photo

Complementary surveys: Seismic refraction and GPR

• Mansfield town hall, CT

Proposed Storrs Community Center Site

Seismic Refraction ResultsProfile Parallel to the Tennis Courts

GPR Profile Parallel to Tennis Courts

GPR Profile Normal to Tennis Courts

Combination of direct/indirect methods:

DC resistivity monitoring tracer test

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Wells from previous extraction systemHydraulic Test WellsWells from previous extraction systemHydraulic Test WellsWells from previous extraction systemHydraulic Test WellsProposed electrode location

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Wells from previous extraction systemHydraulic Test WellsWells from previous extraction systemHydraulic Test WellsWells from previous extraction systemHydraulic Test WellsProposed electrode location

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Use Multiple Surveys to Synergize the most comprehensive model

• Case example:– UConn’s Willimantic River Wellfield– GPR, Resistivity, Seismic refraction

tomography

Final geological model including all information from seismic, GPR, resistivity, and well logs