Foundation design

This section provide knowledge for foundation analysis and design.

 The topics include:

• Bearing capacity
• Settlement analysis
• Spread footing design
• Combined footing design
• Strap footing design
• Lateral earth pressure
• Cantilever retaining wall design
• Sheet pile wall design
• Braced excavation, tie back wall (under construction)
• Pile foundation (under construction)
• Drilled pier and caisson (under construction)

Bearing Capacity

Allowable soil bearing capacity

Qa = Qu / F.S.

Qa: Allowable soil bearing capacity

Qu: Ultimate soil bearing capacity determined by various soil bearing capacity equations.

F.S.:  Factor of safety, normal from 2.5 to 3.

• Terzaghi’s bearing capacity equations

• Meyerhof’s general bearing capacity equations

• Bearing capacity from SPT numbers

• Effect of water table on soil bearing capacity

Settlement Analysis

There are two type of foundation settlements:

1. Immediate settlement – occurs immediately, usually within a week or two, small.
2. Consolidation settlement – long term, may be more than 10 years, occurs due to saturated weak clay layer under footing, can be very large.

Determine Vertical soil pressure under footing

• 2 to 1 method

• Newmark’s influence chart

Consolidation settlements calculation

Spread Footing Design

Topics:

• Determine size of spread footing
• Determine depth of spread footing
• Design reinforcement for spread footing
• Design of square footing
• Design of rectangular footing
• Design of wall footing

Introduction

One of the most common ways to support the column is spreading the column load over a large footing area.  There two parts in spread footing design:

Service load design:

1. The footing area should be large enough so that soil pressure under the footing is not larger than allowable soil bearing pressure
2. The weight of footing should be able to out-weigh column uplift due to wind or seismic forces safely.

Reinforced concreter design:

3. The footing depth should be deep enough to resisting punching shear and direct shear transferred from column load
4. The reinforcement in the footing should be designed to resist bending moments.
5. The dowels at the column-footing interface should be sufficient to transfer column loads.

Combined and Strape footings  

Combined footings and strap footings are normal used when one of columns is subjected to large eccentric loadings.  When two columns are reasonably close, a combined footing is designed for both columns as shown in Figure 3.1.  When two columns are far apart, a strap is designed to transfer eccentric moment between two columns as shown in Figure 3.1.  The goal is to have uniform bearing pressure and to minimize differential settlement between columns.