Figure 8-29. - Rubble-mound breakwater or jetty.

Four crew members to place the piles in the leads.

One or two crew members to prepare the piles. This is based on the further assumption that the pile driver can pick up and place the piles in the leads. If this cannot be done because of the location of the undriven piles, you must allow for an additional crane and increase the total man-days required by 15 percent.

The time in man-days required to drive each pile depends on the type of pile and its length. Precast concrete bearing piles drive slower than wood or steel ones; and logically, the use of a longer pile usually means that you plan to drive it deeper, which will take more time. Under average conditions, it takes 3.0 man-hours to drive a 25-foot wood pile, but for a steel pile, it takes 4.0 man-hours, and for a precast concrete pile, it takes 13.2 man-hours. These estimates take into account pile preparation, placing it in the leads, driving, and cutoff, if required.

When estimating the man-days required to complete a pile-driving operation, you cannot forget to include time for the assembly of the leads and hammer, preparing the equipment for driving, cutting holes in steel piling to facilitate handling, and disassembly of the equipment upon completion, if required. You must also allow time for pile extraction if it is a required part of the project.

WATERFRONT STRUCTURES

Waterfront structures may be broadly divided into three types as follows: (1) harbor-shelter structures, (2) stable-shoreline structures, and (3) wharfage structures.

HARBOR-SHELTER

STRUCTURES Harbor-shelter structures are offshore structures that are designed to create a sheltered harbor. Various types of these structures are covered next.

A BREAKWATER is an offshore barrier, erected to break the action of the waves and thereby maintain an area of calm water inside the breakwater. A JETTY is a similar structure, except its main purpose is to direct the current or tidal flow along the line of a selected channel.

The simplest type of breakwater or jetty is the rubble-mound (also called rock-mound) type shown in figure 8-29. The width of its cap may vary from 15 to

Figure 8-30. - Composite breakwater or jetty.