--- title: "Pile capacity in sand, group effect, load test" subject: "Foundation Engineering" chapter: 8 key: "semester_5/foundation_engineering:pile-capacity-sand-group-load-test" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/pile-capacity-sand-group-load-test site: BCT Engineering --- # Pile capacity in sand, group effect, load test Foundation Engineering · Chapter 8: Pile Foundations - Asked in: 2073 Shrawan, 10 marks - Video: [Load carrying capacity of pile using Static Method](https://www.youtube.com/watch?v=nsiRsFCFSmo) — WIT Solapur - Professional Learning Community - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/pile-capacity-sand-group-load-test - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8 ## Question Give a method to determine the bearing capacity of a pile in sandy soil. What is group effect and how will you estimate the capacity of a pile group in sand with neat sketch? Explain the application and limitations of pile load test. ## Short answer In sand, use static formula Q_u = Q_p + Q_s (end bearing with N_q* plus shaft friction) or SPT correlation Q_p = 40 N (L/D) A_p. Group effect: overlapping stress zones change capacity; efficiency η from Converse-Labarre gives group capacity η n Q_u, settlement larger. Pile load test applies incremental load, finds safe load (50% of 10%D settlement), but is costly, tests single pile, omits time effects. ## Answer #### Bearing capacity of a pile in sand The ultimate capacity is the sum of end bearing and shaft friction, $Q_u = Q_p + Q_s$. **Static formula** $$ Q_p = A_p\,q'N_q^{*} \le A_p\,q_l,\qquad q_l = 50N_q^{*}\tan\phi\ (\text{kN/m}^2) $$ $$ Q_s = \sum K\sigma'_v\tan\delta\,(\pi D)\,\Delta L $$ where $q'$ is the effective overburden pressure at the tip (limited to the critical depth, $15D$ to $20D$), $K$ the earth pressure coefficient (about 1 for loose, 2 for dense sand), $\delta \approx 0.75\phi$ the pile–soil friction angle, and $\sigma'_v$ the average effective stress over each layer. The allowable load is $Q_u/FS$ with $FS = 2.5$ to $3$. **SPT (Meyerhof) method:** $Q_p = 40N\dfrac{L}{D}A_p \le 400NA_p$ and $f_s = 2\bar N$ (kN/m², driven piles), $Q_s = f_sA_s$. #### Group effect When piles are close together, the stress zones overlap, so the group behaves differently from the sum of single piles. The **efficiency** is $\eta = \dfrac{\text{capacity of group}}{n \times \text{capacity of one pile}}$. ```text Single pile Pile group (overlapping zones) | | | | | | | | | | | | | | / \ / \ / small\ / wide and \ / bulb \ / deep zone \ ``` - In **loose sand** a driven group *compacts* the soil, so $\eta \ge 1$ (taken as 1 for design); in **dense sand** or bored piles in sand, $\eta \le 1$. - Group capacity: $Q_g = \eta\,nQ_{u}$, with $\eta$ found from the Converse–Labarre formula $\eta = 1 - \theta\dfrac{(n-1)m + (m-1)n}{90mn}$, where $\theta = \tan^{-1}(D/s)$, or taken as 1 for driven piles at $s \ge 2.5D$. - Settlement of a group is larger than that of a single pile at the same load per pile, because a larger zone of soil is stressed. #### Pile load test: application and limitations **Application:** carried out by applying load in increments (usually through a hydraulic jack against a kentledge or anchor piles) and recording the settlement (IS 2911 Part 4). - It gives the actual ultimate load (the load at which settlement reaches about 10% of the pile diameter, or the point of a break in the curve) and the safe load, i.e. the smaller of 50% of the load at 10% $D$ settlement or two-thirds of the load at 12 mm settlement. - It checks the design assumptions, and verifies the quality of the installed pile. - Used to determine the settlement under working load, lateral load capacity (horizontal test) and uplift capacity (pullout test). **Limitations:** - It is costly and time-consuming (the test takes days). - It shows the behaviour of a single pile; a group has a different capacity (deeper stress zone), and group settlement is not found. - Time effects (set-up, consolidation in clay) may not be reflected unless the test is carried out after a rest period. - The test depends on the correctness of the test set-up; it covers only the test location. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Ultimate capacity of a pile in sand is: A. Qp + Qs B. Qp - Qs C. Qp only D. Qs/FS Answer: A. End bearing plus shaft friction. (Key point: Capacity components) 2. Effective overburden q' at the tip is limited to: A. 15D to 20D critical depth B. 1D C. Pile length always D. Zero Answer: A. Beyond critical depth stress is taken constant. (Key point: Critical depth) 3. Group efficiency is: A. Group capacity / (n x single pile capacity) B. n / capacity C. Single / group D. Spacing / D Answer: A. It compares group to sum of individual piles. (Key point: Efficiency) 4. A driven group in loose sand has efficiency: A. Greater than or equal to 1 (taken as 1) B. Zero C. Much less than 1 D. Exactly 0.5 Answer: A. Driving compacts the sand. (Key point: Driven group in loose sand) 5. A limitation of the pile load test is: A. Costly, slow, and shows single pile behaviour only B. Inaccurate always C. Gives group settlement D. Needs no setup Answer: A. Group capacity and settlement are not found. (Key point: Load test limitation) 6. Safe load from test is the smaller of: A. 50% of load at 10% D settlement or 2/3 of load at 12 mm B. Any load C. Load at failure D. Twice test load Answer: A. As per IS 2911. (Key point: Safe load) ## Related in Foundation Engineering - [Allowable load of 4×4 pile group, layered sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-4x4-figure-dcr) — Foundation Engineering, chapter 8 - [Bearing capacity from plate load curve](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abc-from-load-settlement-sand) — Foundation Engineering, chapter 6 - [Approaches to estimate pile capacity](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-approaches) — Foundation Engineering, chapter 8 - [Allowable load of 400 mm pile, layered soil](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-400mm-layered-soil-chart) — Foundation Engineering, chapter 8 - [Settlement of pile group in sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-settlement-sand) — Foundation Engineering, chapter 8 - [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 ## 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 - [Soil exploration and improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asoil-exploration-improving-bc-nepal) — Building Technology, chapter 2 - [Sub-soil exploration, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asubsoil-exploration-improve) — Building Technology, chapter 2 - [Methods of improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aimproving-bc) — Building Technology, chapter 2, asked 2×