Home Courses Articles Watch! Tools Downloads Request ----------------------------------------------------- Soil Mechanics > Physical Properties of Soil > Soil Phase Relationships > Lateral Earth Pressure > At-Rest State > Rankine's Lateral Earth Pressure > Coulomb's Lateral Earth Pressure > Slope Stability > Effects of Water on Slope Stability > Infinite Slope Analysis > Circular Arc Failure of Slope Analysis > Critical Failure Surface Foundation Analysis and Design > Settlement of Shallow Foundations > Settlement types, definitions, and general equation > Immediate settlement computations > Primary Consolidation > Secondary Compression > Bearing Capacity of Shallow Foundations > Terzaghi's Method > Meyerhof's Method > Brinch Hansen's Method > Bearing from SPT Number > Effect of Ground Water Table > Bearing Capacity of Piles (Deep Foundations) > Ultimate Tip Resistance of Piles > Shaft Resistance of Piles > Sheet-pile Walls: Cantilevered and Anchored > Cantilever Sheet Pile Walls Penetrating Sandy Soils > Anchored Sheet Pile Walls Penetrating Sandy Soils > Factors of Safety for Cantilevered Sheet Pile Walls Load Calculation > Structural Loading > Dead Load vs Live Load > Load Combinations Reinforced Concrete Design > General Topics > What Is Concrete? > Concrete Properties > Section Properties of Reinforcing Bars & Cement Types > Load Combinations of Concrete Design > Design of Concrete Members > Reinforced Concrete Beam Design > Flexural Design of Reinforced Concrete Beams > Serviceability of Reinforced Concrete Beams > Shear Design of Reinforced Concrete Beams Structural Steel Design Construction > Elements of construction > Construction Site Layout Planning Elements > All Types of Roofs And Their Details > Types of Foundations From Construction Point of View > All Types of Foundation Materials Timber Design > Design of Timber Members > Design of Sawn Timber Beams or Joists > Design of Sawn Timber Columns and Compressive Members Masonry Design Finite Elements Method > Basics of Finite Elements > Introduction to Finite Elements And the Big Picture > One-dimensional Bars/Springs > Plane Trusses Introduction Concepts & Formulas Videos Solved problems Download Files Effect of Ground Water Table Courses > Foundation Analysis and Design > Bearing Capacity of Shallow Foundations > Effect of Ground Water Table
Introduction on Effect of Ground Water Table :
If the water table is located at an elevation such that part of the slip mechanism for the footing would be below the water table, higher pore pressures will exist in that zoen, implying lower shear strength and consequently lower bearing capacity. The effect of the water table is best accounted for by suitably choosing the value of the weight for use in the bearing capacity equation as the buoyant unit weight, the wet (or material) unit weight, or some number in between.
Concepts and Formulas of Effect of Ground Water Table:
The buoyant unit weight must be used if the groundwater table is at or above the base of the footing (that is z_{w} < D), for in this case the whole of the potential slip mechanism below the footing base is under water. Since the depth of the slip mechanism is of the order of B below the base of the foundation (that is, a total depth of D + B), if the water table is below this depth, the material unit weight must be used. A simple interpolation can be used for water table depths between D and D + B.
definitions:
z_{w} = depth to the water table from ground
B = footing width
D = depth of embedment
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Solved sample problems of Effect of Ground Water Table:
Example 1: Determination of equivalent unit weight of soil to calculate soil bearing capacity with the effect of ground water table (English units)
Given:
Moist unit weight of soil above ground water table: 120 lb/ft^{3} .
Moist content = 20%
Friction angle, f = 25 degree
Cohesion of soil above ground water table: 1000 lb/ft^{2} .
Cohesion of soil below ground water table: 500 lb/ft^{2} .
Footing: 8 feet wide square footing, bottom of footing at 2 ft below ground surface.
Location of ground water table: 6 ft below ground water surface.
Requirement: Determine equivalent unit weight of soil to be used for calculating soil bearing capacity.
Solution:
Determine equivalent unit weight:
Dry unit weight of soil, g _{dry} = g _{m} /(1+ w ) = 120/(1+0.2) = 100 lb/ft^{3} .
Volume of solid for 1 ft^{3} of soil, V_{s } = g _{dry} / (G_{s} g _{w} ) = 100 / (2.65*62.4) = 0.6 ft^{3} .
Volume of void for 1 ft^{3} of soil, V_{v} = 1-V_{s} =1-0.6=0.4 ft^{3} .
Saturate unit weight of soil, g _{sat} = g _{dry} + g _{w} V_{v} = 100+62.4*0.4=125 ft^{3} .
Buoyant unit weight of soil = g _{sat} - g _{w} = 125-62.4=62.6 ft^{3} .
z_{w} = 6
D = 2
B = 8
D < z_{w} < D + B
Thus, equivalent unit weight of soil:
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