--- title: "Soil improvement techniques; explain two" subject: "Foundation Engineering" chapter: 10 key: "semester_5/foundation_engineering:soil-improvement-explain-two" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/soil-improvement-explain-two site: BCT Engineering --- # Soil improvement techniques; explain two Foundation Engineering · Chapter 10: Foundation Soil Improvements - Asked in: 2082 Bhadra, 4 marks; 2080 Bhadra, 4 marks (asked 2 times) - Video: [Principles of Ground Improvement | Shallow Foundations](https://www.youtube.com/watch?v=2e79-3b6MFM) — Structural Guide - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/soil-improvement-explain-two - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-10 ## Question What are the different soil improvement techniques? Explain any two of them. ## Short answer Compaction densifies loose soil by mechanical energy (rolling, dynamic compaction, vibroflotation) to increase strength and reduce settlement. Preloading with vertical drains places a surcharge and drains to accelerate consolidation of soft clay, achieving most settlement before structure construction. ## Answer Soil improvement techniques include compaction, preloading with vertical drains, stone columns, grouting, chemical stabilisation, replacement, dewatering, geosynthetic reinforcement and soil nailing. Two of them are explained below. #### 1. Compaction (dynamic compaction and vibroflotation) Compaction densifies loose granular soil by reducing its voids using mechanical energy, which increases the strength and reduces settlement and liquefaction potential. - **Surface compaction:** rollers (smooth, sheepsfoot, vibratory) compact thin layers of fill to a specified density at optimum moisture content. - **Dynamic compaction:** a heavy weight (10 to 40 t) is dropped from 10 to 30 m onto the ground in a grid, which compacts the soil to depths of 5 to 10 m. - **Vibroflotation:** a vibrating probe (vibroflot) is sunk by water jet into the sand; the sand around it is densified and the hole is backfilled with sand/gravel. - Suitable for loose sand and gravel with little fines. Quality is checked by SPT or CPT after compaction. #### 2. Preloading with vertical drains For soft saturated clay, a surcharge is placed over the area to force the water out of the clay and speed up the consolidation settlement before the structure is built. - A sand blanket is laid on the surface, and **vertical sand drains or prefabricated band drains** are installed at 1.5 to 3 m spacing to shorten the drainage path, so consolidation is completed in months rather than years. - The fill (surcharge) is left until about 90% consolidation, monitored by settlement plates and piezometers, and then removed. - It also increases the shear strength of the clay, and is used under embankments, roads, airport runways, tanks and warehouses. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does dynamic compaction densify the ground? A. Lime is mixed into the soil B. A heavy weight (10 to 40 t) is dropped from 10 to 30 m in a grid C. Sand drains are installed D. Water is jetted into clay Answer: B. Repeated impacts of a heavy weight densify loose granular soil to about 5 to 10 m depth. (Key point: Dynamic compaction) 2. Which soil suits dynamic compaction and vibroflotation? A. Soft saturated clay B. Peat C. Intact rock D. Loose sand and gravel with little fines Answer: D. These methods work on loose granular soils with few fines, checked afterwards by SPT or CPT. (Key point: Suitability of compaction) 3. What is the purpose of vertical drains with a sand blanket in preloading? A. To bear the structure load B. To prevent any settlement C. To reduce the shear strength of clay D. To shorten the drainage path so consolidation takes months rather than years Answer: D. Drains at 1.5 to 3 m spacing shorten the path water travels, speeding consolidation. (Key point: Drains with preloading) 4. Besides reducing later settlement, what else does preloading do to soft clay? A. Reduces its shear strength B. Raises its water content C. Makes it expansive D. Increases its shear strength Answer: D. Expelling water during consolidation increases the clay's shear strength. (Key point: Preloading side benefit) ## Related in Foundation Engineering - [Methods of soil improvement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-methods-list) — Foundation Engineering, chapter 10, asked 6× - [Purposes of foundation soil improvement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-purposes) — Foundation Engineering, chapter 10 ## Related in other subjects - [Soil exploration, safe bearing capacity, improvement](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexploration-bc-065s) — Building Technology, chapter 2 - [Linear solution techniques; CG two iterations](https://www.bctengineer.com/question.md?key=semester_8%2Fcomputational_techniques_in_civil_engineering%3Asolution-techniques-cg-numerical-two-iter) — Computational Techniques in Civil Engineering, chapter 2 - [Effects of soil pollution and its control](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asoil-effects-control) — Engineering Chemistry, chapter 3 - [Hybrid multiple access techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-multiple-access) — Wireless Communication, chapter 7