JNTU B.Tech IV GROUND IMPROVEMENT TECHNIQUES Previous Question Paper (July/August - 2022 - R18)

Difficult soils are those that exhibit poor engineering properties, such as low shear strength, high compressibility, excessive permeability, or high sensitivity to moisture changes, making them unsuitable for supporting conventional civil engineering structures without prior modification. These soils often fail to meet the stability and settlement criteria required for safe construction.

The problems posed by difficult soils include:

• Excessive Settlement: High compressibility leads to large consolidation settlements under structural loads, causing damage to foundations.
• Low Bearing Capacity: Weak soils cannot support heavy structural loads, leading to shear failure or bearing capacity failure.
• Volume Change: Expansive soils (e.g., black cotton soil) undergo significant swelling and shrinkage with moisture fluctuations, causing structural cracking.
• Liquefaction: Loose, saturated cohesionless soils may lose shear strength entirely during seismic events.
• High Permeability: Excessive seepage in sandy soils can lead to piping or internal erosion.
• Corrosivity: Certain soils contain aggressive chemicals that can degrade buried concrete or steel foundations.

A soil site is identified as needing improvement when site investigation data reveals that the natural soil properties are inadequate to meet the performance requirements of the proposed structure. The identification process involves the following steps:

• Geotechnical Site Investigation: Conducting field tests such as Standard Penetration Test (SPT), Cone Penetration Test (CPT), and Vane Shear Test to determine in-situ soil density, strength, and consistency.
• Laboratory Testing: Analyzing soil samples for index properties (Atterberg limits, grain size distribution), compressibility (consolidation test), and shear strength parameters (triaxial or direct shear tests).
• Performance Criteria Comparison: Comparing the measured soil parameters against the design requirements. If the calculated settlement exceeds allowable limits, or if the ultimate bearing capacity is lower than the factored structural load, improvement is mandatory.
• Environmental and Moisture Sensitivity: Assessing the potential for volume change in expansive soils or the risk of liquefaction in loose, saturated sands through cyclic testing or mineralogical analysis.
• Economic and Safety Assessment: Determining if the cost of deep foundations or soil improvement is justified by the risk of structural failure or serviceability issues identified during the analysis.