--- title: "Factors affecting bearing capacity; water table" subject: "Foundation Engineering" chapter: 6 key: "semester_5/foundation_engineering:bearing-capacity-factors-water-table" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/bearing-capacity-factors-water-table site: BCT Engineering --- # Factors affecting bearing capacity; water table Foundation Engineering · Chapter 6: Bearing Capacity and Settlement of Shallow Foundations - Asked in: 2081 Bhadra, 4 marks; 2080 Baisakh, 2+2 marks; 2075 Asoj, 6 marks; 2074 Asoj, 4 marks; 2075 Chaitra, 2 marks (asked 5 times) - Video: [Factors Affecting Bearing Capacity|Shape|Size|Water Table |Foundation Explain |](https://www.youtube.com/watch?v=OHZbq0HCsK4) — DIPAK MANDAL - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/bearing-capacity-factors-water-table - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-6 ## Question What are the factors (size, shape, water table) affecting the bearing capacity of soil? Explain how fluctuation of the water table affects the bearing capacity of foundation when the water table lies above and below the base of the footing, with neat sketch. ## Short answer Bearing capacity depends on footing size, shape, depth, soil strength, and water table position. A rising water table reduces effective stresses, decreasing capacity. When above the base, it reduces both the surcharge (qN_q) and width (0.5γBN_γ) terms; when below the base, only the width term is affected, with full submergence halving the γBN_γ term. ## Answer Bearing capacity of a footing depends on the soil strength and on the geometry of the footing, and it is reduced when the soil is submerged. Terzaghi's equation for a strip footing is $q_u = cN_c + \gamma D_f N_q + 0.5\gamma B N_\gamma$. #### Factors affecting bearing capacity - **Size (width B):** in cohesionless soil the third term $0.5\gamma BN_\gamma$ grows with $B$, so a wider footing has a larger ultimate capacity. In clay ($\phi = 0$, $N_\gamma = 0$) the capacity does not depend on $B$. A wider footing, however, stresses a deeper zone and settles more, so settlement often governs the design. - **Shape:** a square or circular footing mobilises three-dimensional resistance. Terzaghi uses shape factors: strip $1.0 / 0.5$, square $1.3 / 0.4$, circular $1.3 / 0.3$ (for the $cN_c$ and $\gamma BN_\gamma$ terms). A rectangle lies between a strip and a square. - **Depth of foundation:** a deeper base increases the surcharge $q = \gamma D_f$ and so the $N_q$ term. - **Soil properties:** $c$, $\phi$ (which fix $N_c, N_q, N_\gamma$) and unit weight $\gamma$. Loose or soft soil fails in local shear and gives a lower capacity. - **Water table:** it reduces the effective unit weight of the soil (see below). - **Eccentricity and inclination of load:** these reduce the effective width and the capacity. #### Effect of water table position Submergence reduces the effective stress, so the shear strength of the soil falls. This can be allowed for in either of two ways (use one, not both): - (a) use the submerged unit weight $\gamma'$ for the soil below the water table, or - (b) use the bulk unit weight $\gamma$ in Terzaghi's equation with the water table correction factors $$ q_u = cN_c + \gamma D_f N_q R_{w1} + 0.5\gamma B N_\gamma R_{w2} $$ with $R_{w1} = 0.5\left(1 + \dfrac{Z_{w1}}{D_f}\right)$ and $R_{w2} = 0.5\left(1 + \dfrac{Z_{w2}}{B}\right)$. Here $Z_{w1}$ is the depth of the water table below the ground surface (taken as $D_f$ if the water table is at or below the base), and $Z_{w2}$ is the depth of the water table below the base (zero if it is at or above the base, and not more than $B$). ```text Case 1: WT above base Case 2: WT below base GL ______________________ GL ______________________ WT ~~~~~~~~~~~~~ (Zw1) | gamma (moist) | gamma' (submerged) Base ====================== Base ==================== WT ~~~~~~~~~~~~~~ (Zw2 below base, < B) | gamma' (submerged) | gamma' (submerged) ``` 1. **WT above the base** ($Z_{w1} < D_f$, $Z_{w2} = 0$): the soil beside the footing (below the water table) and the soil below the footing are submerged. With method (a), $\gamma'$ is used in the $N_\gamma$ term and the surcharge becomes $q = \gamma Z_{w1} + \gamma'(D_f - Z_{w1})$. With method (b), $R_{w2} = 0.5$ and $R_{w1}$ lies between $0.5$ and $1$. If the water table is at the ground surface, both terms are about halved, so the capacity of a cohesionless soil falls by about half. 2. **WT at base level:** $R_{w1} = 1$ and $R_{w2} = 0.5$, so only the $N_\gamma$ term is halved. 3. **WT below the base** (at depth $Z_{w2} < B$ below it): $R_{w1} = 1$ and $R_{w2}$ lies between $0.5$ and $1$. The effect is only on the $\gamma B N_\gamma$ term, because the failure wedge below the base is partly submerged. 4. **WT deeper than $B$ below the base:** $Z_{w2} \ge B$, so $R_{w2} = 1$ and the water table has no effect. In clay with $\phi = 0$, the water table has little effect on $q_u$ because $N_q = 1$ and $N_\gamma = 0$, except through the change in surcharge. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does footing width B affect ultimate capacity? A. Increases in sand via 0.5 gamma B N_gamma; independent of B in clay B. Decreases always C. Independent in sand; increases in clay D. No effect in any soil Answer: A. In clay N_gamma = 0 so B has no effect on q_u; in sand the third term grows with B. (Key point: Effect of width) 2. What changes if the water table is above the footing base? A. Nothing B. Gamma' used in the N_gamma term and in the submerged part of surcharge C. Capacity doubles D. Only N_c changes Answer: B. Submergence reduces effective unit weight: gamma' is used below the water table and in the surcharge. (Key point: WT above base) 3. When does the water table not affect capacity? A. At the base level B. Above the base C. When Z_w2 >= B below the base (R_w2 = 1) D. Never Answer: C. If water is deeper than B below the base, the failure wedge is dry and R_w2 = 1. (Key point: WT depth beyond B) 4. What are R_w1 and R_w2 at the ground surface? A. 1 and 1 B. 0 and 0 C. 1 and 0.5 D. 0.5 and 0.5 Answer: D. R_w = 0.5(1 + Z/D), so with water at the surface both are 0.5, halving capacity in cohesionless soil. (Key point: Correction factors) ## Related in Foundation Engineering - [Rectangular footing 3 m × 4 m, water table cases](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Arect-footing-3x4-backfilled) — Foundation Engineering, chapter 6 - [Circular footing on stiff clay, water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acircular-footing-stiff-clay) — Foundation Engineering, chapter 6 - [Strip footing, change in capacity with water rise](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Astrip-footing-dry-sand-water-rise) — Foundation Engineering, chapter 6, asked 2× - [Strip footing FOS, three water table cases](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Astrip-footing-fos-water-table) — Foundation Engineering, chapter 6 - [Footing 2 m × 3 m, chart factors, eccentricity](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Afooting-2x3-eccentric-skempton) — Foundation Engineering, chapter 6 - [Rectangular footing size, 6000 kN](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Afooting-6000kn-rectangular-sand) — Foundation Engineering, chapter 6 ## Related in other subjects - [Factors affecting foundation design, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afactors-design-improve-bc) — Building Technology, chapter 2 - [Below and above grade water proofing](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awaterproof-below-above-grade) — Building Technology, chapter 1 - [Soil stabilization factors; mechanical; cement-soil base](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asoil-stabilization-factors-mechanical-cement-soil) — Transportation Engineering II, chapter 3 - [Soil moisture forms; factors affecting CWR](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-cwr-factors) — Irrigation and Drainage Engineering, chapter 2