--- title: "Foundation settlement causes and underpinning" subject: "Building Technology" chapter: 2 key: "semester_6/building_technology:settlement-underpinning-067as" source: https://www.bctengineer.com/q/semester_6/building_technology/settlement-underpinning-067as site: BCT Engineering --- # Foundation settlement causes and underpinning Building Technology · Chapter 2: Foundations - Asked in: 2067 Asar (old course), 8 marks - Video: [How to Identify and Repair Foundation Settlement](https://www.youtube.com/watch?v=v_tnb7cIhXk) — 58 Foundations & Waterproofing - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/settlement-underpinning-067as - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2 ## Question What are the various causes of foundation settlement? Define underpinning and explain different methods of underpinning with neat sketches. ## Short answer Settlement is caused by consolidation, weak or non-uniform soil, water table changes, overloading, adjacent excavation, tree roots, erosion, earthquakes and faulty design. Underpinning strengthens and deepens an existing foundation to a stronger stratum or enlarges its base. Methods include pit (needle), pile, pretest, jet/chemical grouting, micropiles, and beam and needle method. ## Answer #### Causes of foundation settlement 1. Consolidation of compressible clay or silt under load. 2. Weak, loose, or filled soil, poorly compacted. 3. Non-uniform soil or unequal loading. 4. Change of water table; shrinkage and swelling of clays. 5. Overloading or addition of floors. 6. Adjacent excavation, vibrations, or construction. 7. Tree roots, erosion, leakage and underground cavities. 8. Earthquake and liquefaction. 9. Faulty design: footing too small or too shallow. #### Underpinning **Underpinning** is the process of strengthening and deepening an existing foundation by extending it down to a stronger stratum, or enlarging its base, so that it supports the structure safely. It is used to stop settlement, to add floors, to lower basements, or when deep excavation is done nearby. #### Methods of underpinning 1. **Pit (needle) method**: The wall is divided into short sections (1 to 1.5 m). Pits are dug under alternate sections down to the firm layer, one at a time, and filled with concrete or masonry up to the base of the old footing, packed tight with dry mortar and wedges. Then the adjacent sections are done. This is the simplest method, for shallow depths. 2. **Pile underpinning**: Piles are driven or bored on both sides of the wall (or through it), and a needle beam or cap transfers the load to them; used when the firm layer is deep. 3. **Pretest method**: The new piles or piers are pre-loaded with hydraulic jacks before the load is transferred to them, so that their settlement takes place in advance. 4. **Jet grouting / chemical grouting**: Grout is injected to strengthen the soil. 5. **Miniature (micro) piles**: Small-diameter piles drilled and grouted through the existing footing. 6. **Beam and needle method**: Needle beams under the wall pass the loads to piers on either side. ```text wall |==== old footing ====| ||pit 1|| ||pit 2|| (concrete filled pit, alternate sections) ``` During underpinning, the structure is shored, and cracks are monitored. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is underpinning? A. Building a new foundation beside the old one for a new structure B. Strengthening and deepening an existing foundation to a stronger stratum or enlarging its base C. Removing the old foundation and rebuilding the wall D. Waterproofing the plinth of a building Answer: B. Underpinning strengthens or deepens an existing foundation so it can support the structure safely. (Key point: Definition of underpinning) 2. In the pit (needle) method, how are pits dug? A. Outside the building away from the wall B. Only at the corners of the building C. Under alternate short sections (1 to 1.5 m), one at a time D. All sections together along the whole wall Answer: C. Short alternate sections are excavated and filled one by one so the wall stays supported. (Key point: Pit method procedure) 3. Which cause of settlement is linked to clay under load? A. Wide plinth beam B. Use of rich mortar C. Faulty roof slope D. Consolidation of compressible clay or silt Answer: D. Compressible clay or silt consolidates slowly under load and causes settlement. (Key point: Causes of settlement) 4. When is pile underpinning chosen? A. When the firm layer is very near the surface B. When only plaster is cracked C. When the wall is a partition D. When the firm layer is deep Answer: D. Piles carry load through a needle beam or cap to a deep firm layer. (Key point: Pile underpinning) ## Related in Building Technology - [Foundation settlement causes and treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afoundation-settlement-treatment) — Building Technology, chapter 2 - [Foundation settlement, effects, black cotton soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asettlement-black-cotton-062ba) — Building Technology, chapter 2 - [Causes of foundation failure](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afoundation-failures) — Building Technology, chapter 2, asked 2× ## Related in other subjects - [Modes of failure due to settlement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asettlement-modes-sketch) — Foundation Engineering, chapter 6 - [Proportioning footings for uniform settlement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aproportion-footings-uniform-settlement) — Foundation Engineering, chapter 6, asked 2× - [Clay 10 m, 2.54 cm sample, half settlement](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-10m-half-settlement-time-ratio) — Soil Mechanics, chapter 9 - [Differential settlement allowed, clay vs sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Adifferential-settlement-clay-sand) — Foundation Engineering, chapter 6