--- title: "Negative skin friction: causes, effects, remedies" subject: "Foundation Engineering" chapter: 8 key: "semester_5/foundation_engineering:negative-skin-friction-effects" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/negative-skin-friction-effects site: BCT Engineering --- # Negative skin friction: causes, effects, remedies Foundation Engineering · Chapter 8: Pile Foundations - Asked in: 2081 Bhadra, 4 marks; 2075 Chaitra, 4 marks; 2072 Chaitra, 4 marks; 2074 Asoj, 2 marks (asked 4 times) - Video: [Negative skin friction - Pile | Soil Mechanics](https://www.youtube.com/watch?v=_8O1RrAzfoY) — APSEd - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/negative-skin-friction-effects - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8 ## Question Define negative skin friction in piles. Explain a typical situation where it may occur, its causes, its effect on the load carrying capacity of the pile and the remedial/preventive measures. ## Short answer Negative skin friction is downward drag when surrounding soil settles more than the pile, adding load. It occurs in soft clay under fill, lowering capacity. Causes: fill, water table drop. Remedial measures: bitumen coating, casing, preloading to prevent drag. ## Answer **Negative skin friction (NSF)**, also called down-drag, is the downward drag force on a pile shaft when the surrounding soil settles more than the pile. The soil then moves down relative to the pile and acts as an extra load instead of supporting the pile. #### Typical situation A pile is driven through a soft compressible clay layer to a firm stratum (rock, dense sand). If a fill is placed over the clay after driving, the clay consolidates under the fill and settles, dragging the pile shaft down. Down-drag acts down to the **neutral point**, below which the pile moves more than the soil and normal (positive) skin friction acts. ```text Fill //////////////// GL --------------------- soil settles Soft | | <- soil moves down clay |P | NSF acts down - - -| |- - neutral point Firm | | positive friction layer|__| end bearing ``` #### Causes - Fill placed on soft clay after driving the piles. - Lowering of the water table (increase in effective stress, consolidation). - Consolidation of remoulded clay disturbed by pile driving. - Heavy surcharge or loads near the pile, and settlement of loose sand under vibration. #### Effects - Adds a downward load $Q_{nsf}$ on the pile, so the net capacity is reduced: $Q_{all} = \dfrac{Q_{ult} - Q_{nsf}}{FS}$ (or the working load plus $Q_{nsf}$ must be less than the capacity). - Excessive settlement or structural overstress of the pile, especially for end-bearing piles. - Tilting or cracking of the structure above, and settlement of the piles relative to the surrounding ground. #### Remedial and preventive measures - Coat the pile shaft over the compressible layer with **bitumen** (or use a sleeve) to reduce friction. - Use a **casing** or a larger-diameter driven hole filled with bentonite slurry. - Preload or drive the piles after the consolidation of the fill is complete. - Use lightweight fill, or avoid surcharging, and keep the water table from being lowered. - Increase the pile capacity (longer or larger piles) to carry the drag load. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Negative skin friction occurs when: A. Soil settles more than the pile, dragging it downward B. The pile moves up relative to soil C. Pile tip is in rock D. Soil swells Answer: A. Soil moving down relative to the pile acts as an extra downward load (down-drag). (Key point: Definition) 2. Which situation typically causes it? A. Fill placed on soft clay after driving piles B. Piles in dense sand only C. Excavation next to piles D. Lowering the pile cap Answer: A. The clay consolidates under the new fill and settles around the pile. (Key point: Typical situation) 3. What is the neutral point? A. The pile head B. The depth below which pile moves more than soil and friction becomes positive C. The water table D. The pile tip only Answer: B. NSF acts down to the neutral point; below it normal positive friction acts. (Key point: Neutral point) 4. Effect on allowable load? A. Increases it B. Reduces it: Qall = (Qult - Qnsf)/FS C. None D. Doubles settlement only Answer: B. The drag adds to the load, reducing net capacity. (Key point: Effect on capacity) 5. Which is a preventive measure? A. Coat the shaft with bitumen over the compressible layer B. Lower the water table C. Place more fill D. Use shorter piles Answer: A. Bitumen coating or sleeves reduce friction with the settling soil. (Key point: Remedy) ## Related in Foundation Engineering - [Negative skin friction on single pile and group](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Ansf-calculation-single-group) — Foundation Engineering, chapter 8, asked 2× - [Negative skin friction on 16-pile group](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aneg-skin-friction-group-16-piles) — Foundation Engineering, chapter 8 - [Pile capacity in sand, group effect, load test](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-sand-group-load-test) — Foundation Engineering, chapter 8 - [Pile spacing for 100% efficiency, 16 piles](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-spacing-100-efficiency-square-30cm) — Foundation Engineering, chapter 8, asked 2× - [Pile group 16 piles of 50 cm, efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-16-piles-50cm) — Foundation Engineering, chapter 8 - [Pile group 16 piles, 0.40 m, soft clay](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-16-soft-clay-040) — Foundation Engineering, chapter 8 ## Related in other subjects - [Water logging: causes, effects, remedies](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Awater-logging-causes-effects-remedies) — Irrigation and Drainage Engineering, chapter 9, asked 13× - [Soil erosion causes and preventive measures](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aerosion-causes-prevention) — Transportation Engineering I, chapter 4 - [Accident study objectives, causes, preventive measures](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aaccident-study-objectives-causes-prevention) — Transportation Engineering II, chapter 1 - [Remedial measures for slope failure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aslope-remedial-measures) — Soil Mechanics, chapter 11