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Foundations

A foundation transfers structural loads to the supporting soil or rock. Shallow foundations (isolated, combined, strip, and mat footings) spread the load near the surface; the soil bearing pressure is $q=P/A$. Deep foundations reach competent strata far below: a pile is a slender column driven or cast into the ground, a pier/caisson is a large-diameter shaft.

A pile carries load through end bearing ($Q_p$) at its tip plus skin friction ($Q_s$) along its shaft; the allowable capacity applies a factor of safety. When piles are closely spaced in a group, the group capacity is reduced by a group efficiency $\eta$.

$$q=\frac{P}{A}, \qquad Q_{\text{ult}}=Q_p+Q_s, \qquad Q_{\text{allow}}=\frac{Q_{\text{ult}}}{FS}$$
$$Q_{\text{group}}=\eta\,(n\,Q_{\text{allow,pile}})$$

★ Identifying a Deep Foundation Element

A slender structural member driven into the soil to transfer building loads to a deeper and stronger stratum is called what?

A pile. It carries load by end bearing on a firm layer and/or by skin friction along its length. A caisson/pier is a much larger-diameter shaft; shoring is temporary excavation support. Final answer: pile.

★ Soil Bearing Pressure Under a Footing

A column carries 960 kN to a square footing 2.0 m × 2.0 m. Determine the soil bearing pressure (neglect footing weight).

$$q=\frac{P}{A}=\frac{960}{2.0\times2.0}=\frac{960}{4}=240\ \text{kPa}$$

Final answer: 240 kPa.

★★ Sizing a Square Footing

A column load of 1500 kN must be supported on soil with an allowable bearing pressure of 200 kPa. Determine the required side of a square footing.

$$A_{\text{req}}=\frac{P}{q_{\text{allow}}}=\frac{1500}{200}=7.5\ \text{m}^2$$
$$B=\sqrt{7.5}=2.74\ \text{m}$$

Final answer: about 2.74 m × 2.74 m (use 2.75 m).

★★ Allowable Pile Capacity

A pile develops 520 kN of end bearing and 280 kN of skin friction at failure. Using a factor of safety of 2.5, determine the allowable load.

$$Q_{\text{ult}}=520+280=800\ \text{kN}$$
$$Q_{\text{allow}}=\frac{800}{2.5}=320\ \text{kN}$$

Final answer: 320 kN.

★★★ Pile Group Capacity

A 4 × 4 pile group (16 piles) uses piles each with an allowable capacity of 300 kN. If the group efficiency is 0.75, determine the allowable group capacity.

$$Q_{\text{group}}=\eta\,(n\,Q_{\text{allow,pile}})=0.75\,(16\times300)$$
$$=0.75(4800)=3600\ \text{kN}$$

Final answer: 3600 kN. Group action reduces the simple sum (4800 kN) by the efficiency factor.

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MSTE - Construction / Foundations / MSTE November 2019

It is nothing more than a column driven into the soil to support a structure by transferring building loads to a deeper and stronger layer of soil or rock.

  1. caisson
  2. shoring
  3. pier
  4. pile
A pile is a slender column driven into the ground to transfer structural loads through weak soil to a deeper, firmer stratum of soil or rock.
A caisson is a watertight box used for underwater work; shoring is temporary support against collapse; a pier is a raised platform extending over water.
$\boxed{\text{pile}}$