9 Harbor, River, and Estuary Structures 9.1 General These structures have their roots in ancient civilizations. The Babylonians and Chinese built bridges; the Phoenicians built breakwaters and crude quays using timber pile cribs filled with stone. The Romans built an artificial harbor at Ostia, for which they may have used pozzolanic cement for underwater concrete. We know for certain that they did build timber cribs and cofferdams for bridge piers as well as pile-supported trestles. Included in this chapter are harbor structures and wharves, river structures such as locks and low-level dams, piers for overwater bridges, submerged prefabricated tunnels (tubes), and storm-surge barriers. 9.2 Harbor Structures 9.2.1 Types Currently, the dominant structural type of harbor structures is the quay or marginal wharf for the unloading and loading of containers. Finger piers, extending normal to the shore, are used for transfer of petroleum products and trestles are built to furnish access to loading platforms and wharves. 9.2.2 Pile-Supported Structures These consist of either steel or concrete piles, driven into the soft clays and sands of the seafloor. They typically support a reinforced concrete deck. To obtain economy, the piles are generally on a spacing of 7–10 m. Thus, the design capacity per pile typically ranges from 100 to 250 tn. per pile (1.0–2.5 MN) although somewhat larger capacities are now emerging. Both steel and prestressed concrete piles are utilized for the harbor structure addressed in this section. They are also used in larger sizes for bridge piers (see Section 9.5). 9.2.2.1 Steel Piles These are either tubular pipes of 400–600 mm diameter, or H piles. The pipe piles present less surface area for corrosion and hence, are more readily protected. Although in many soils, steel H piles penetrate more readily; in stiff clays the soil plugs between the flanges and the pile act as a partially end-bearing pile of square cross section. The steel pipe pile 319 q 2006 by Taylor & Francis Group, LLC 320 Construction of Marine and Offshore Structures also plugs, as does its larger diameter counterpart in the offshore. The plugging occurs when the internal skin friction exceeds the ultimate bearing value of the soil at the tip. Since the water depths for modern container and cargo ships are about 16 m, and the design pile capacity is only of the order of 200–400 tn., in most cases piles will range from 30 to 40 m in length. The trend is now towards container ships of deeper draft and greater beam requiring 20 m depth at the berth; piles may be up to 40–50 m in length to provide greater capacity for the support of the water-side crane load and 1000 mm in diameter to provide the stiffness for lateral loads. Petroleum terminals require greater depth at the face of the wharf, usually 23 m, and hence larger and more heavily loaded piles. Steel piles can be picked (pitched) readily, and placed into the leads. At deck level, the pile may be centered by a template or a mechanical gate, while at the top a hydraulically operated arm may swing around the pile to center and align it in the leads. 9.2.2.2 Concrete Piles Today, these piles are almost always prestressed concrete piles. The sizes up to 500 mm are typically square in cross section, those of 600 mm size are octagonal, and from 900 to 2100 mm diameter are round with a hollow core. The precompression in the pile, after losses, is 7–10 MPa. Experience indicates that values less than 7 MPa do not perform as well under driving impacts as those with the higher values. In pitching (upending) concrete piles, because of their weight and bending capacity, multiple pickup points are usually required. Their location in the fabrication plant is usually determined on the basis of vertical lifts on a horizontal pile. In the case of pitching into the leads (up-ending), the lines (or slings) will lead at angles that will vary as the pile changes from horizontal to vertical. The vertical and horizontal components of the various lines will change with the leads of the lifting lines and the pile’s inclination. Usually (but not always), the critical case is the initial lift from the horizontal. The reinforcement, conventional and prestressed, must be adequate for both transport and pitching (see Figure 9.1). 9.2.2.3 Installation After pitching, the steel or concrete pile is then allowed to run down under its own weight. The hammer and driving head are now lowered onto the pile with the pile line slacked and the pile penetrates farther under the weight of the hammer. Driving now commences. The FIGURE 9.1 Upending (pitching) a long prestressed concrete pile. Note six pickup points along pile. q 2006 by Taylor & Francis Group, LLC Harbor, River, and Estuary Structures 321 first blows may cause the pile to run a meter or more; hence the hammer line must be free in order to keep the hammer and driving head on the pile. Although vibratory hammers may be used to obtain initial penetration of steel piles in cohesionless soils, final seating is normally by impact hammer. Concrete piles are generally driven by impact hammer alone. Three types of impact hammers are in common use today, the single-acting steam hammer (which can also be activated by compressed air), the hydraulic hammer, and the diesel hammer. All three types depend on the mass of the ram impacting on the driving head, which in turn transmits the impulse to the pile. In the case of the single-acting steam hammer, the propelling force is gravity alone; in the case of the hydraulic hammer, the gravitational force is amplified by hydraulic pressure. With the diesel hammer, the force is applied sequentially; first the compression of the air by the falling ram “preloads” the pile, then the ram delivers its impact, and finally the thrust of the exploding fuel reacts downward while at the same time raising the ram for the next stroke. As a result, the hammer energies are computed differently; a rough rule of thumb equates the rated energy of a steam or hydraulic hammer to 1.6 times the rated energy of a diesel hammer insofar as the effectiveness in driving. This is not only due to the differing modes of impact but also to the proportionally lesser weight of the ram in the case of the diesel hammer, which is partially compensated by a higher velocity. The driving heads are designed to transmit the impact to the pile head. Hence, in the case of steel piles, the head is configured to match the pile section. The blow is transmitted steel to steel; no cushion material is interposed. The driving head serves to confine the pile head and prevent it from local buckling and crimping. In the case of the concrete pile, a cushion is required to attenuate the blow and extend its duration in order to prevent cracking under rebound tension. The driving head is configured to contain the pile head cushion. The pile head cushion is best made as a 200- to 400-mm-thick laminated softwood block. Plywood layers may be affixed top and bottom and inserted in the middle to help hold the block together during driving. Experi- ence, supported by dynamic measurements, shows that such a cushion will usually be adequate. The hammer blow creates a compressive wave, which travels down the pile at the speed of sound in the pile material. When the wave reaches the tip, it either causes the pile to penetrate, thus producing a tensile wave in the pile or, if the pile tip is on hard material such as rock, causes a rebound compressive stress wave of twice the intensity. It is this stress concentration which causes the tip of steel piles to buckle, tear, or accordion. The high rebound tensile waves, alternating with the input compression, may lead to low-cycle, high-amplitude fatigue of any welded splices in the steel piles. In the case of concrete piles, two special stress patterns must be considered in order to permit driving to the required penetration without damage. Unlike steel piles, which are most damaged by the high compression stresses in hard driving, concrete piles are subject to damage in soft driving, such as the initial blows, which produce tensile rebound stresses. Without the proper cushion block and hammer control, horizontal cracking may occur through the entire body of the pile, showing itself initially as a puff of dust at each hammer blow. Continued driving will lead to damage to the concrete at the crack and stresses in the steel beyond yield; the prestressing steel will eventually fracture. This phenomenon is also noted when, after driving through competent soils, which seize the pile in friction, the tip protrudes into weak material or a void below. Such a void may have been created by excessive jetting below the tip. This phenomenon is amplified underwater, where water is progressively sucked into the crack, then subject to high-impact pressure under succeeding blows. q 2006 by Taylor & Francis Group, LLC 322 Construction of Marine and Offshore Structures The second special stress pattern is due to Poisson’s effect: the lateral bursting of either the head or the tip under the high axial compression in hard driving. Fortunately, concrete that is well-confined can resist very high compressive forces. Hence, closely spaced spirals will prevent damage. To prevent vertical cracks due to bursting, spiral or hoop reinforce- ment should be proportioned so that the steel area at yield stress is greater than the concrete tensile strength in the area of concrete in the tensile zone; this will ensure that any crack formed by driving will be pulled closed.