Subsurface investigation is an essential preliminary step for any civil engineering project to understand subsurface conditions. It involves sampling and examining subsurface materials like soil and rock to provide data for design recommendations. The investigation process includes planning explorations, executing them using techniques like boreholes and test pits, laboratory testing of samples, and reporting findings with descriptions, test results, analyses, and recommendations. The stages are reconnaissance, data collection, in-depth investigation, and laboratory testing to characterize subsurface conditions like bearing capacity. This informs foundation selection and predicts issues like settlement.

1. Site subsurface Investigation

2. • A good knowledge about a site
including its subsurface conditions is
very important in its safe and
economical development.
• It is therefore an essential
preliminary to the construction of
any civil engineering work.

3. What is subsurface investigation?

4. A subsurface
investigation is
one of the
fundamental
assessments
performed at
the beginning
of engineering
projects.
The subsurface
investigation is the
primary method
for gathering the
data to generate a
report of
geotechnical
engineering
recommendations

5. Sampling
Examination
provide a “picture” of
what is beneath the
ground which is critical
for designers and
construction teams alike
to help assure the
successful completion
of any construction
project.
of the below surface
materials including
soil, rock,
groundwater and
any manmade
materials.

6. What is the importance
of Subsurface
investigation?

7. For new structures
• 1. To assess the general suitability
of the site.
• 2.To select the type and depth of
foundation.
• 3.To determine the bearing capacity
of the selected foundation.
• 4.To predict the settlement of the
selected foundation.
• 5.To determine the ground water
level and possible corrosive effect
of soil and water on foundation
material.

8. • 6. To foresee and provide against
difficulties that may arise during
construction due to ground and
other local conditions.
• 7. To find out the sources of
construction material and
selection of sites for disposal of
water or surplus material.
• 8. To ensure the safety of
surrounding existing structures.

9. For existing structures
1. To investigate the
safety of the structure.
2. To predict the
settlement.
3. To design for the failed
structures or remedial
measures for the
structures deemed to be
unsafe.

10. Bearing capacity
The presence of
groundwater
Estimate of settlements

11. How is the subsurface
investigation done?

12. Planning
• To minimize cost of explorations and yet give reliable data.
• Decide on quantity and quality depending on type, size and
importance of the project and whether investigation is preliminary or
detailed.

13. Execution
• Collection of distributed and/or
undistributed samples of
subsurface strata from field.
• Conducting in-situ tests of
subsurface material and obtaining
properties directly or indirectly.
• Study of ground water conditions
and collection of sample for
chemical analysis
• Laboratory testing on samples.

14. Report writing
• Description of site conditions-topographic features, hydraulic
conditions, existing structures supplemented by plans/drawings.
• Description of field and lab tests carried out.
• Analysis and discussion of data collected.
• Preparation of charts, graphs and tables.
• Calculations performed.
• Recommendations.

15. Stages of Site Investigation
Site investigation can be broadly classified into four stages:
1- reconnaissance 2-data and map study
3-In-depth investigation 4-laboratory testing.

16. reconnaissance
Simply scouting around the site can sometimes provide a lot of information such as topography, vegetation, geological
features and utility lines.
Topography of the land including water bodies,
estuaries.
Slopes' angles and orientation
Presence of structures, heritage structures .
Presence of hazardous industries or waste
disposal sites
Observations made during site reconnaissance include:

17. 2-data and map study
Geological maps, old plans and photographs can be obtained from the survey
or records department. The study includes:
• Presence of licensed water abstraction and discharge consents, landfills, and waste disposal sites
• History, if any, of hazardous incidents
• Presence of coal and other mines
• Historical mapping
• Buried and overhead supply and utility lines

18. Disk study
• Previous use of land, such as a
history of contamination, can
make an impact on its future
use. Details obtained from the
reconnaissance survey are
thoroughly studied in sync
with the desk study in order to
understand the site
topography and decide the
next steps of investigation.

19. 3-In-depth investigation
• locations for trial pits and boreholes can
be selected. The selection should be such
that a complete geologic sub-surface
perspective can be obtained. At least
three points should be selected with
spacing of 10 to 30 meters depending on
site conditions and type of project.
Commonly boreholes, probes and trial
pits are used to undertake in-depth
investigation.

20. Laboratory Testing
• The soil recovered during
the in-depth soil
investigation is tested in the
lab at this stage. The
material obtained is
classified and characterized,
and based on the project,
geotechnical parameters
are provided for the design
phase.

21. The basic tests are:
• Classification Test
For cohesive soils, the tests are
done for moisture content,
plasticity index, particle size
distribution and bulk density. For
granular soils, tests are done for
particle size distribution and
bulk density

22. Shear Strength Test
• For cohesive soils, tests are done for short-term stability, long-
term stability and residual shear strength properties using the
shear box test. For granular soils, the shear box test is done for
both short-term and long-term stability analysis.
•
Direct sheer
books video
click to open

23. - Conventional Soil Boring
- Cone Penetrometer Testing
- Test Pits

24. Conventional Soil Borings
Small diameter (
2
-
2.5
) soil and rock cores are obtained using a drill rig.
Provides Standard Penetration Test (SPT) values which correlate to soil strength and can be used to
determine soil bearing capacity and estimate settlements for foundation design.

26. Ok

27. BENTONITE
H

28. https://youtu.be/2n4DL-HINMk

29. Cone Penetrometer Testing
Cone penetrometer testing (CPT) allows for rapid exploration of the subsurface materials. Soil and rock samples are not obtained.
Instead different sensors gather information on the soil as a cone shaped tip is pushed through the soil. The information
gathered includes:
Tip resistance which is the force required to push the tip of the cone through the soil.Sleeve friction which is the force required to
push the sleeve through the soil.
Pore Water Pressure
Seismic shear wave velocity
The ratios of the above CPT parameters provide additional information regarding the physical soil properties and pore fluids.

30. G

31. D

32. D

33. https://youtu.be/paRRQmoyI4U

34. References :
• https://www.trenchlesspedia.com/understanding-the-4-stages-of-site-investigation/2/4167
• https://www.slideshare.net/Shahnaseerrush/site-investigation-123177256
• https://earthengineering.com/geotechnical-investigations/subsurface-
investigation/#:~:text=The%20subsurface%20investigation%20is%20the,groundwater%20and%20any%20man
made%20materials
• https://geosystems.ce.gatech.edu/Faculty/Mayne/papers/NHI%202002%20Subsurface%2
0Investigations.pdf
• https://www.trenchlesspedia.com/definition/4220/desk-study-site-investigation
• https://tunnel.ita-
aites.org/media/k2/attachments/public/1_Site%20Investigation%20by%20Clayton%20FREE.pdf
Books (pdf) :