Allowable bearing capacity of granite (order of magnitude) What is the typical safe bearing capacity range (in kg/cm²) adopted in practice for foundations resting on hard granite rock, assuming sound, unweathered material and normal safety factors?

Introduction / Context:
Foundation design on rock depends on rock quality designation, discontinuities, weathering, and surface preparation. For strong, massive rocks such as granite, the controlling factors are often discontinuities and bearing area rather than compressive strength of intact rock, which is very high. Engineers therefore use empirical safe bearing values for preliminary design before site-specific tests.

Given Data / Assumptions:

• Sound, unweathered granite.
• Joints and seams properly evaluated; foundation area prepared.
• Safe bearing capacity refers to working (allowable) values with usual safety margins.

Concept / Approach:
Granite has very high compressive strength (intact strengths can exceed thousands of kg/cm²), but allowable bearing pressure for foundations is limited by mass behaviour, joints, and settlement criteria. Typical tabulated values for hard rocks are in the tens of kg/cm², with granite commonly cited around the higher end of such ranges. Among the options, 40–45 kg/cm² reflects this order of magnitude for safe bearing in practice.

Step-by-Step Solution:

Verification / Alternative check:
Preliminary values in many handbooks list hard rock at several MPa (equivalently tens of kg/cm²). Final design should be based on plate load tests or rational analysis when critical.

Why Other Options Are Wrong:

• 5–10, 15–20, 30–35 kg/cm²: conservative for hard, sound granite; more typical of weaker rock or heavily jointed conditions.
• None of these: incorrect because 40–45 kg/cm² is reasonable.

Common Pitfalls:
Ignoring joint orientation and persistence; extrapolating allowable pressure from intact strength; neglecting drainage and weathering at the rock surface.

Final Answer:
40 to 45 kg/cm²