Geotechnical investigation practice: select the correct statements regarding the use and spacing of inclined (oblique) boreholes during site exploration.

Introduction / Context:
Site investigations often require flexibility in boring alignments to reach target strata or to sample beneath obstacles. Inclined (oblique) borings are a practical tool in an engineer’s toolkit.

Given Data / Assumptions:

• Existing structures or services may obstruct vertical access.
• Geometric arrangements can combine vertical and inclined holes for coverage.
• Objective: correct conceptual uses of inclined borings.

Concept / Approach:
Inclined borings allow sampling under foundations, abutments, or pavements without disturbing them. They can supplement or, in constrained sites, replace some vertical borings. Thoughtful spacing—such as aligning one hole’s bottom under another’s collar—provides three-dimensional coverage of subsurface variability.

Step-by-Step Solution:
Confirm use beneath existing works → (a) is true.Recognize occasional substitution for vertical holes → (b) is true.Acknowledge spacing strategies for proper overlap and profiling → (c) is true.Therefore, 'all the above' is correct.

Verification / Alternative check:
Transportation and bridge projects frequently leverage inclined borings to sample beneath piers or retaining walls where vertical access is limited.

Why Other Options Are Wrong:

• Each individual statement is correct; choosing only one would be incomplete.

Common Pitfalls:

• Assuming vertical borings are the only acceptable approach; oblique borings are widely used when justified.

Final Answer:
all the above