Subsurface Investigation Methods, Core Drilling and Geophysical Surveys

An investigation plan is an experiment on the site model: choose observations that can discriminate between competing ground conditions. The resulting report should distinguish raw records, processed products, interpreted units and engineering parameter selection.

Outcomes and a practical engineering question

By the end of this unit, a student should be able to translate the geologic observations in investigation methods and geophysical interpretation into a documented alternative ground model, quantify at least one relevant sensitivity, and explain what additional evidence would change the decision. The goal is not memorization of terms but a defendable sequence: observation → interpretation → process model → engineering implication → uncertainty.

A usable field record contains the datum, location, measurement method, orientation or depth, date, observer, evidence strength and confidence. A drawing without those attributes is difficult to audit when field conditions change.

Four connected lessons

5.1 Walkover reconnaissance and field sampling

Core physical interpretation. Initial reconnaissance identifies exposures, geomorphology, seepage, human modification and access constraints. A sampling strategy should respond to geological variability, not only a regular grid.

Civil-engineering case. A proposed hillside roadway shows fresh scarps in satellite imagery but a dry-season visit shows no active seepage.

Common error. A single favorable season can conceal groundwater or slope hazards.

Recommended observational method. Map photographs with coordinates, scale, azimuth, weather and field inference confidence.

From field observation to a working ground model. Begin by recording what could actually be seen or measured when investigating walkover reconnaissance and field sampling. In a coastal embankment, the team may encounter salinity-sensitive pore fluid while tidal groundwater variation changes boundary conditions. Divide the record into physical evidence, a preferred interpretation, and at least one plausible competing explanation. For example, a curved reflectivity anomaly, a change in drilling return or a wet joint might be consistent with multiple geological situations. A good model therefore predicts several independent observables rather than treating one indirect signal as a unique material label.

Scale and engineering decision. The scale of the question for walkover reconnaissance and field sampling must match the scale of the decision. A hand specimen can establish mineral type or intact strength but cannot prove the continuity of a weak layer over a bridge footprint. Conversely, a regional map can reveal structural trends but rarely resolves a narrow fracture zone at a tunnel face. Define whether the decision concerns a meter-scale foundation, a tens-of-meters roadcut, or a kilometre-long alignment, then select mapping, boring, laboratory testing and monitoring accordingly. State the range of plausible boundary positions and explain how each affects stability, excavation, drainage and the construction sequence.

Measurement protocol. For walkover reconnaissance and field sampling, first state the expected range and diagnostic features, then calibrate instruments and confirm the reference datum. Photograph the observation with a scale and north indication, record adverse as well as favorable evidence, and label samples so the record can be reconstructed. Preserve original orientation data before transformation. Where the first interpretation is uncertain, obtain an independent check rather than selecting a value just because it fits a familiar textbook category. Compare day-to-day conditions when moisture or stress changes could affect what is measured.

Quantitative reasoning. Choose a calculation proportional to the physical process instead of using a formula because it is available. Write the dimensions, units, reference directions and boundary assumptions before substituting numbers. Work with ranges for the parameters least constrained by field evidence. A sensitivity plot can reveal which parameters matter most, but its smoothness is not evidence that the geological variation is smooth. Every plotted result should be traced to the original observation, the simplifying constitutive law, and a check against an independent physical limit.

Interpretation and risk. In coastal embankment settings, a mistaken assessment of salinity-sensitive pore fluid may be more consequential under tidal groundwater variation. Define a failure of the working hypothesis: for example, repeated water-level readings contradict the assumed flow direction, measured joint planes diverge from the mapped structural domain, or a test pit intersects a contact outside its expected depth interval. Explain the difference between an uncertainty that merely widens a construction cost range and one that introduces a new safety-critical failure mechanism. Assign new evidence collection to the latter first.

Alternative hypothesis exercise. Hypothesis A assumes walkover reconnaissance and field sampling follows the dominant regional trend; hypothesis B assumes a local feature interrupts that trend. Draw a vertical section for both, label what is observed versus extrapolated, and compare the expected outcome of a borehole, scanline or geophysical traverse. Record what measurement would falsify either interpretation. The useful result is a testable prediction—not a forced choice between unverified assumptions.

Field-report exercise. Draft a one-page section called “Evidence and limits for Walkover reconnaissance and field sampling.” Include the measurement conditions, the most consequential model uncertainty, the observation that would reduce it, and the implication for a civil-engineering decision. Explain why the quantities should not be interpreted as final design parameters without project-specific evidence and qualified review.

Three contrasting investigation scenarios (synthetic input records).

• Scenario 5.1.1: A proposed coastal embankment exposes a geological contact at 19 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 23° relative to the local structural reference and a second observation is 6 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.1.2: A proposed coastal embankment exposes a geological contact at 17 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 43° relative to the local structural reference and a second observation is 13 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.1.3: A proposed coastal embankment exposes a geological contact at 25 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 56° relative to the local structural reference and a second observation is 14 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.

Local checkpoint. Explain the difference between a feature shown on a regional geological map and a site-specific characteristic verified at an excavation. State one parameter that can be calculated from the observations and one that cannot be estimated responsibly.

5.2 Boreholes, trenches and test pits

Core physical interpretation. Drill type, borehole diameter, core orientation, recovery and disturbance affect what is observed. Logs must separate measured strata from inferred boundaries and include groundwater observations.

Civil-engineering case. A drilling method washes away weak seams and creates a deceptively competent core record.

Common error. A low recovery interval is a signal of uncertainty, not automatic proof that it is a void.

Recommended observational method. Select sample and logging methods according to the anticipated materials and question of interest.

From field observation to a working ground model. Begin by recording what could actually be seen or measured when investigating boreholes, trenches and test pits. In a hillside road cut, the team may encounter daylighting structural surfaces while short-duration rainfall changes boundary conditions. Divide the record into physical evidence, a preferred interpretation, and at least one plausible competing explanation. For example, a curved reflectivity anomaly, a change in drilling return or a wet joint might be consistent with multiple geological situations. A good model therefore predicts several independent observables rather than treating one indirect signal as a unique material label.

Scale and engineering decision. The scale of the question for boreholes, trenches and test pits must match the scale of the decision. A hand specimen can establish mineral type or intact strength but cannot prove the continuity of a weak layer over a bridge footprint. Conversely, a regional map can reveal structural trends but rarely resolves a narrow fracture zone at a tunnel face. Define whether the decision concerns a meter-scale foundation, a tens-of-meters roadcut, or a kilometre-long alignment, then select mapping, boring, laboratory testing and monitoring accordingly. State the range of plausible boundary positions and explain how each affects stability, excavation, drainage and the construction sequence.

Measurement protocol. For boreholes, trenches and test pits, first state the expected range and diagnostic features, then calibrate instruments and confirm the reference datum. Photograph the observation with a scale and north indication, record adverse as well as favorable evidence, and label samples so the record can be reconstructed. Preserve original orientation data before transformation. Where the first interpretation is uncertain, obtain an independent check rather than selecting a value just because it fits a familiar textbook category. Compare day-to-day conditions when moisture or stress changes could affect what is measured.

Quantitative reasoning. Choose a calculation proportional to the physical process instead of using a formula because it is available. Write the dimensions, units, reference directions and boundary assumptions before substituting numbers. Work with ranges for the parameters least constrained by field evidence. A sensitivity plot can reveal which parameters matter most, but its smoothness is not evidence that the geological variation is smooth. Every plotted result should be traced to the original observation, the simplifying constitutive law, and a check against an independent physical limit.

Interpretation and risk. In hillside road cut settings, a mistaken assessment of daylighting structural surfaces may be more consequential under short-duration rainfall. Define a failure of the working hypothesis: for example, repeated water-level readings contradict the assumed flow direction, measured joint planes diverge from the mapped structural domain, or a test pit intersects a contact outside its expected depth interval. Explain the difference between an uncertainty that merely widens a construction cost range and one that introduces a new safety-critical failure mechanism. Assign new evidence collection to the latter first.

Alternative hypothesis exercise. Hypothesis A assumes boreholes, trenches and test pits follows the dominant regional trend; hypothesis B assumes a local feature interrupts that trend. Draw a vertical section for both, label what is observed versus extrapolated, and compare the expected outcome of a borehole, scanline or geophysical traverse. Record what measurement would falsify either interpretation. The useful result is a testable prediction—not a forced choice between unverified assumptions.

Field-report exercise. Draft a one-page section called “Evidence and limits for Boreholes, trenches and test pits.” Include the measurement conditions, the most consequential model uncertainty, the observation that would reduce it, and the implication for a civil-engineering decision. Explain why the quantities should not be interpreted as final design parameters without project-specific evidence and qualified review.

Three contrasting investigation scenarios (synthetic input records).

• Scenario 5.2.1: A proposed hillside road cut exposes a geological contact at 11 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 24° relative to the local structural reference and a second observation is 7 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.2.2: A proposed hillside road cut exposes a geological contact at 18 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 44° relative to the local structural reference and a second observation is 8 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.2.3: A proposed hillside road cut exposes a geological contact at 26 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 57° relative to the local structural reference and a second observation is 15 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.

Local checkpoint. Explain the difference between a feature shown on a regional geological map and a site-specific characteristic verified at an excavation. State one parameter that can be calculated from the observations and one that cannot be estimated responsibly.

5.3 Seismic refraction and wave velocities

Core physical interpretation. Travel-time curves can reveal contrasts in seismic velocity, but layered refraction requires faster material below slower material and assumes tractable geometry. Hidden layers and lateral variation challenge interpretation.

Civil-engineering case. A simple refraction survey infers a shallow stiff layer while core drilling later reveals weathered rock with irregular depth.

Common error. Velocity is influenced by saturation, fractures and stress; it is not a direct RQD or foundation capacity measurement.

Recommended observational method. Calibrate seismic interpretation with borehole information and state resolution limits.

From field observation to a working ground model. Begin by recording what could actually be seen or measured when investigating seismic refraction and wave velocities. In a urban excavation, the team may encounter buried fill and utility corridors while pumping-induced drawdown changes boundary conditions. Divide the record into physical evidence, a preferred interpretation, and at least one plausible competing explanation. For example, a curved reflectivity anomaly, a change in drilling return or a wet joint might be consistent with multiple geological situations. A good model therefore predicts several independent observables rather than treating one indirect signal as a unique material label.

Scale and engineering decision. The scale of the question for seismic refraction and wave velocities must match the scale of the decision. A hand specimen can establish mineral type or intact strength but cannot prove the continuity of a weak layer over a bridge footprint. Conversely, a regional map can reveal structural trends but rarely resolves a narrow fracture zone at a tunnel face. Define whether the decision concerns a meter-scale foundation, a tens-of-meters roadcut, or a kilometre-long alignment, then select mapping, boring, laboratory testing and monitoring accordingly. State the range of plausible boundary positions and explain how each affects stability, excavation, drainage and the construction sequence.

Measurement protocol. For seismic refraction and wave velocities, first state the expected range and diagnostic features, then calibrate instruments and confirm the reference datum. Photograph the observation with a scale and north indication, record adverse as well as favorable evidence, and label samples so the record can be reconstructed. Preserve original orientation data before transformation. Where the first interpretation is uncertain, obtain an independent check rather than selecting a value just because it fits a familiar textbook category. Compare day-to-day conditions when moisture or stress changes could affect what is measured.

Quantitative reasoning. Choose a calculation proportional to the physical process instead of using a formula because it is available. Write the dimensions, units, reference directions and boundary assumptions before substituting numbers. Work with ranges for the parameters least constrained by field evidence. A sensitivity plot can reveal which parameters matter most, but its smoothness is not evidence that the geological variation is smooth. Every plotted result should be traced to the original observation, the simplifying constitutive law, and a check against an independent physical limit.

Interpretation and risk. In urban excavation settings, a mistaken assessment of buried fill and utility corridors may be more consequential under pumping-induced drawdown. Define a failure of the working hypothesis: for example, repeated water-level readings contradict the assumed flow direction, measured joint planes diverge from the mapped structural domain, or a test pit intersects a contact outside its expected depth interval. Explain the difference between an uncertainty that merely widens a construction cost range and one that introduces a new safety-critical failure mechanism. Assign new evidence collection to the latter first.

Alternative hypothesis exercise. Hypothesis A assumes seismic refraction and wave velocities follows the dominant regional trend; hypothesis B assumes a local feature interrupts that trend. Draw a vertical section for both, label what is observed versus extrapolated, and compare the expected outcome of a borehole, scanline or geophysical traverse. Record what measurement would falsify either interpretation. The useful result is a testable prediction—not a forced choice between unverified assumptions.

Field-report exercise. Draft a one-page section called “Evidence and limits for Seismic refraction and wave velocities.” Include the measurement conditions, the most consequential model uncertainty, the observation that would reduce it, and the implication for a civil-engineering decision. Explain why the quantities should not be interpreted as final design parameters without project-specific evidence and qualified review.

Three contrasting investigation scenarios (synthetic input records).

• Scenario 5.3.1: A proposed urban excavation exposes a geological contact at 12 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 25° relative to the local structural reference and a second observation is 8 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.3.2: A proposed urban excavation exposes a geological contact at 19 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 45° relative to the local structural reference and a second observation is 9 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.3.3: A proposed urban excavation exposes a geological contact at 27 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 58° relative to the local structural reference and a second observation is 16 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.

Local checkpoint. Explain the difference between a feature shown on a regional geological map and a site-specific characteristic verified at an excavation. State one parameter that can be calculated from the observations and one that cannot be estimated responsibly.

5.4 Electrical resistivity and integrated evidence

Core physical interpretation. Electrical resistivity responds to saturation, pore-fluid chemistry, clay content and temperature. Similar anomalies can have different geological causes, making inversion nonunique.

Civil-engineering case. A low-resistivity zone could represent clay, saline groundwater or a weathered fault depending on local context.

Common error. Do not label an anomaly as groundwater or karst without independent constraints.

Recommended observational method. Design a verification plan combining boreholes, surface mapping and repeat surveys.

From field observation to a working ground model. Begin by recording what could actually be seen or measured when investigating electrical resistivity and integrated evidence. In a bridge approach, the team may encounter buried channel lenses while seasonal river stage changes boundary conditions. Divide the record into physical evidence, a preferred interpretation, and at least one plausible competing explanation. For example, a curved reflectivity anomaly, a change in drilling return or a wet joint might be consistent with multiple geological situations. A good model therefore predicts several independent observables rather than treating one indirect signal as a unique material label.

Scale and engineering decision. The scale of the question for electrical resistivity and integrated evidence must match the scale of the decision. A hand specimen can establish mineral type or intact strength but cannot prove the continuity of a weak layer over a bridge footprint. Conversely, a regional map can reveal structural trends but rarely resolves a narrow fracture zone at a tunnel face. Define whether the decision concerns a meter-scale foundation, a tens-of-meters roadcut, or a kilometre-long alignment, then select mapping, boring, laboratory testing and monitoring accordingly. State the range of plausible boundary positions and explain how each affects stability, excavation, drainage and the construction sequence.

Measurement protocol. For electrical resistivity and integrated evidence, first state the expected range and diagnostic features, then calibrate instruments and confirm the reference datum. Photograph the observation with a scale and north indication, record adverse as well as favorable evidence, and label samples so the record can be reconstructed. Preserve original orientation data before transformation. Where the first interpretation is uncertain, obtain an independent check rather than selecting a value just because it fits a familiar textbook category. Compare day-to-day conditions when moisture or stress changes could affect what is measured.

Quantitative reasoning. Choose a calculation proportional to the physical process instead of using a formula because it is available. Write the dimensions, units, reference directions and boundary assumptions before substituting numbers. Work with ranges for the parameters least constrained by field evidence. A sensitivity plot can reveal which parameters matter most, but its smoothness is not evidence that the geological variation is smooth. Every plotted result should be traced to the original observation, the simplifying constitutive law, and a check against an independent physical limit.

Interpretation and risk. In bridge approach settings, a mistaken assessment of buried channel lenses may be more consequential under seasonal river stage. Define a failure of the working hypothesis: for example, repeated water-level readings contradict the assumed flow direction, measured joint planes diverge from the mapped structural domain, or a test pit intersects a contact outside its expected depth interval. Explain the difference between an uncertainty that merely widens a construction cost range and one that introduces a new safety-critical failure mechanism. Assign new evidence collection to the latter first.

Alternative hypothesis exercise. Hypothesis A assumes electrical resistivity and integrated evidence follows the dominant regional trend; hypothesis B assumes a local feature interrupts that trend. Draw a vertical section for both, label what is observed versus extrapolated, and compare the expected outcome of a borehole, scanline or geophysical traverse. Record what measurement would falsify either interpretation. The useful result is a testable prediction—not a forced choice between unverified assumptions.

Field-report exercise. Draft a one-page section called “Evidence and limits for Electrical resistivity and integrated evidence.” Include the measurement conditions, the most consequential model uncertainty, the observation that would reduce it, and the implication for a civil-engineering decision. Explain why the quantities should not be interpreted as final design parameters without project-specific evidence and qualified review.

Three contrasting investigation scenarios (synthetic input records).

• Scenario 5.4.1: A proposed bridge approach exposes a geological contact at 13 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 26° relative to the local structural reference and a second observation is 4 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.4.2: A proposed bridge approach exposes a geological contact at 16 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 46° relative to the local structural reference and a second observation is 10 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.
• Scenario 5.4.3: A proposed bridge approach exposes a geological contact at 28 m depth in one investigation point; an adjacent reconnaissance traverse is oriented 55° relative to the local structural reference and a second observation is 12 m higher in elevation. First decide which of these quantities is directly observed and which remains a hypothesis. Construct an A/B ground model and select a targeted follow-up measurement. Describe the effect of a wrong contact projection on construction planning and groundwater observation. Do not calculate apparent dip from these data without a measured true dip and a clear azimuth convention.

Local checkpoint. Explain the difference between a feature shown on a regional geological map and a site-specific characteristic verified at an excavation. State one parameter that can be calculated from the observations and one that cannot be estimated responsibly.

Integration of the two interactive laboratories

This unit uses Lab 09 and Lab 10. Both are stand-alone offline models. Start with the default case, predict the direction of change, perform an input sweep, and export the result. Critically compare the model to field observations: which assumptions are physically justified and which idealizations are merely convenient?

A laboratory is not complete until the student has submitted the predicted trend, the observed numerical response with units, a physical explanation, an estimate of sensitivity, and a paragraph on why the result cannot be applied directly to a real project.

Synthesis case: competing geological explanations

A team is examining a rock tunnel affected by fault-related alteration. The investigation was conducted while excavation sequence was relatively quiet, creating a risk that the observed state is not representative of the critical construction condition. The team has a preliminary map, two discrete observations and a short monitoring record. Neither spatial continuity nor transient behavior is fully constrained.

Construct three competing cross-sections: one with laterally continuous units, one with an abrupt structural boundary, and one with a buried erosional or weathering feature. For each cross-section, list the predicted drill response, the predicted groundwater response, and the construction problem that would arise if it were true. Rank only the value of information, not a numerical probability unsupported by data. Identify the most efficient next test and the observations that would cause you to revise the construction method.

Finally write a limitations paragraph for a geotechnical or engineering-geology report. This paragraph must say what was measured, what was inferred, the dates and groundwater conditions, and which assumptions need checking during construction. Separate a local observation from a regional geological generalization and make all units consistent.

Laboratory and assessment links

• [Simulation 09: Two-layer seismic refraction travel time](../labs/lab-09.html)
• [Simulation 10: Parallel-path bulk resistivity](../labs/lab-10.html)
• [Unit 05 assessment](../assessments/unit-05.html)

End-of-unit review

1. Identify two measurements that distinguish the alternative ground models presented in this unit.
2. Demonstrate an order-of-magnitude consistency check on one simulation output and explain each assumption.
3. Explain how sampling orientation, geological discontinuity and time-varying groundwater could each bias a conclusion.
4. Prepare a short site-report recommendation with one action that reduces the greatest uncertainty.