The question asks about the primary purpose of pressure grouting in building foundation treatment work. Let's explore this by understanding what pressure grouting is and its purposes.

Pressure Grouting: Pressure grouting, also known as injection grouting, involves injecting a grout material under pressure into cavities or voids to fill them and solidify the surrounding soil or structure. This technique is widely used in foundation work, tunneling, and other civil engineering applications.

Main Purposes of Pressure Grouting:

1. Improving Soil Strength: One of the primary uses of pressure grouting is to enhance the structural integrity and load-bearing capacity of the soil. By filling voids and compacting the soil, the density and strength of the foundation are increased, providing better support for structures.
2. Reducing Water Penetration: Grouting fills cracks and voids, which helps in preventing water seepage into the foundation. This is crucial for the stability and longevity of structures, especially in areas with high water tables or prone to flooding.

Given these explanations, the correct answer is:

Improving soil strength and reducing water penetration below foundation structures

Let's now evaluate the incorrect options:

• Supporting internal wall finishes against mechanical damage: This is not related to pressure grouting, which is primarily concerned with foundational strength and waterproofing.
• Applying protective surface coating for appearance improvement: Surface coatings are cosmetic treatments, while pressure grouting addresses structural issues.
• Increasing thermal insulation performance: While insulation is important, pressure grouting is unrelated to thermal performance. It focuses on structural integrity and moisture control.

In summary, pressure grouting is mainly used for strengthening soil and reducing water ingress in foundation structures, making option one the correct choice.