Question 14.2: Solve Prob. 14.1 assuming the retaining wall is restrained. ...
Solve Prob. 14.1 assuming the retaining wall is restrained. Compare this answer to the value based on the equation k_{o} = 1 – \sin \phi.
In Fig. 10.5, the soil is described as “sand backfill.” In Table 14.10, for restrained wall, use at-rest pressure. The at-rest pressure is 60 psf/ft for both well-graded clean sands and poorly graded clean sands. Hence use this value of equivalent fluid pressure (EFP) of 60 psf/ft:
P_{H}=1 / 2( EFP )(H)^{2}=1 / 2(60)(20)^{2}=12,000 lb / ft \text { and } P_{V}=0Using: k_{0}=1-\sin \phi=1-\sin 30^{\circ}=0.5
P_{H}=1 / 2\left(k_{0}\right)\left(\gamma_{t}\right)(H)^{2}=1 / 2(0.5)(110)(20)^{2}=11,000 lb / ft \text { and } P_{V}=0
| TABLE 14.10 Lateral Soil Load | |||
| Description of backfill material^{C} | Unified soil classification system | Design lateral soil load^{ a } (pounds per square foot per foot of depth) | |
| Active pressure | At-rest pressure | ||
| Well-graded, clean gravels; gravel-sand mixes | GW | 30 | 60 |
| Poorly graded clean gravels; gravel-sand mixes | GP | 30 | 60 |
| Silty gravels, poorly graded gravel-sand mixes | GM | 40 | 60 |
| Clayey gravels, poorly graded gravel-and-clay mixes | GC | 45 | 60 |
| Well-graded, clean sands; gravelly sand mixes | SW | 30 | 60 |
| Poorly graded clean sands; sand-gravel mixes | SP | 30 | 60 |
| Silty sands, poorly graded sand-silt mixes | SM | 45 | 60 |
| Sand-silt clay mix with plastic fines | SM-SC | 45 | 100 |
| Clayey sands, poorly graded sand-clay mixes | SC | 60 | 100 |
| Inorganic silts and clayey silts | ML | 45 | 100 |
| Mixture of inorganic silt and clay | ML-CL | 60 | 100 |
| Inorganic clays of low to medium plasticity | CL | 60 | 100 |
| Organic silts and silty clays, low plasticity | OL | Note b | Note b |
| Inorganic clayey silts, elastic silts | MH | Note b | Note b |
| Inorganic clays of high plasticity | CH | Note b | Note b |
| Organic clays and silty clays | OH | Note b | Note b |
| For SI: 1 pound per square foot per foot of depth = 0.157 kPa/m. 1 foot = 304.8 mm. a. Design lateral soil loads are given for moist conditions for the specific soils at their optimum densities. Actual field conditions shall govern. Submerged or saturated soil pressures shall include the weight of the buoyant soil plus the hydrostatic loads. b. Unsuitable as backfill material. c. The definition and classification of soil materials shall be in accordance with ASTM D 2487 (Unified Soil Classification System). Source: Table 1610.1 of the International Building Code (2012). See also Table 14.9. |
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| TABLE 14.9 Retaining Walls | |
| Topic | Discussion |
| Introduction | The main provisions in the International Building Code (2012) for retaining walls are Sec. 1610 (Soil Lateral Loads) and Sec. 1807 (Foundation Walls, Retaining Walls and Embedded Posts and Poles). |
| General regulations | General provisions concerning retaining walls are presented in Sec. 1807 of the International Building Code (2012), as follows: “Section 1807.2.1 General. Retaining walls shall be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift. Where a keyway is extended below the wall base with the intent to engage passive pressure and enhance sliding stability, lateral soil pressures on both sides of the keyway shall be considered in the sliding analysis. Section 1807.2.2 Design lateral soil loads. Retaining walls shall be designed for the lateral soil loads set forth in Section 1610. Section 1807.2.3 Safety factor. Retaining walls shall be designed to resist the lateral action of soil to produce sliding and overturning with a minimum safety factor of 1.5 in each case. The load combinations of Section 1605 shall not apply to this requirement. Instead, design shall be based on 0.7 times nominal earthquake loads, 1.0 times other nominal loads, and investigation with one or more variable loads set to zero. The safety factor against lateral sliding shall be taken as the available soil resistance at the base of the retaining wall foundation divided by the net lateral force applied to the retaining wall. Exception: Where earthquake loads are included, the minimum safety factor for retaining wall sliding and overturning shall be 1.1.” |
| Regulations for lateral loads | Concerning lateral soil loads on retaining walls, the International Building Code (2012) states: “Section 1610.1 General. Foundation walls and retaining walls shall be deigned to resist lateral soil loads. Soil loads specific in Table 1610.1 [see Table 14.10] shall be used as the minimum design lateral soil loads unless determined otherwise by a geotechnical investigation in accordance with Section 1803. Foundation walls and other walls in which horizontal movement is restricted at the top shall be designed for at-rest pressures. Retaining walls free to move and rotate at the top shall be permitted to be designed for active pressure. Design lateral pressure from surcharge loads shall be added to the lateral earth pressure load. Design lateral pressure shall be increased if soils at the site are expansive. Foundation walls shall be designed to support the weight of the full hydrostatic pressure of undrained backfill unless a drainage system is installed in accordance with Sections 1805.4.2 and 1805.4.3. Exception: Foundation walls extending not more than 8 feet (2438 mm) below grade and laterally supported by the top by flexible diaphragms shall be permitted to be designed for active pressure.” |
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