Maintenance of Waterfront Facilities by Department of the Army - HTML preview

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section, spalling and featheredges can be avoided by

having chippers work from both faces. All interior

corners should be rounded to a minimum radius of 1

inch.

The construction and setting of forms are

important steps in the procedure for satisfactory

concrete replacement where the concrete must be

placed from the side of the structure. Form details for

walls are shown in Figure 3-6. To obtain a tight,

acceptable repair the following requirements must be

observed:

(1)

Front forms for patches more than 18

inches high should be constructed in horizontal sections

so the concrete can be conveniently placed in lifts not

more than 12 inches high. The back form can be built in

one piece. Sections to be set as concreting progresses

should be fitted before concrete placement is started.

(2)

For irregularly shaped holes, chimneys

may be required at more than one level. In some cases,

such as when beam connections are involved, a

chimney may be necessary on both sides of the wall or

beam. In all cases the chimney should extend the full

width of the hole.

(3)

Forms should be substantially constructed

so that pressure can be applied to the chimney cap at

the proper time.

(4)

Forms should be mortartight at all joints

between adjacent sections, between the forms and

Figure 3-5. Excavation of irregular area of defective

concrete, and at the tie-bolt holes to prevent the loss of

concrete where top of hole is cut at two

mortar when pressure is applied to the concrete during

levels.

the final stages of placement. Twisted or stranded

caulking cotton, folded canvas strips, or similar material

Figure 3-5). Where a hole passes through a structural

should be used as the forms are assembled.

element, it may be necessary to fill the hole from both

sides. In this case the slope of the top of the cut should

Immediately before placing the front section of

be modified accordingly.

form for each lift, the surface of the old concrete (at the

sides which will be covered by new concrete) should be

(4)

The bottom and sides of the hole should be cut

coated with a 1/8-inch-thick layer of mortar. This mortar

sharp and approximately square with the face of the

should have the same sand and cement content and the

wall. When the hole goes entirely through the concrete

3-11

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sectional area of the hole is greater than 36 and less

than 72 square inches for reinforced concrete repair or

144 square inches for nonreinforced concrete repair, a

no-slump concrete should be placed, thoroughly

vibrated, and power tamped in 3-inch layers. If

practicable, the new concrete should be preshrunk by

letting it stand as long as practicable before it is tamped

into the hole. The mix proportions and the aggregate

gradation should be selected for minimum water

content. The W/C weight ratio should be less than 0.40.

Casting concrete in open-top forms, as used for

the reconstruction of the top of bulkheads and pier-deck

curbs, is a comparatively simple operation. The W/C

weight ratio should not exceed 0.45. No special

features are required in the forms, but they should be

mortartight when vibrated, and should give the new

concrete a finish similar to that of the adjacent areas.

The slump should be as low as practicable, and the

amount of air-entraining agent increased as necessary

to ensure the maximum permissible percentage of

entrained air, despite the low slump. Top surfaces

should be sloped so as to provide rapid drainage.

Manipulation in finishing should be held to a minimum,

and a wood-float finish is preferable to a steel-trowel

finish. Edges and corners should be tooled or

chamfered. Water should not be used to aid in

finishing.

Forms for repairs involving cast-in-place

Figure 3-6. Details of forms for concrete replacement in

concrete can usually be removed the day after casting

bulkheads.

unless form removal would damage the newly placed

concrete. The projections left by the chimneys should

same W/C ratio as the mortar in the replacement

normally be removed the second day. If the trimming is

concrete. The surface should be damp, but not wet.

done earlier, the concrete tends to break back into the

The mortar can be applied by means of an air-suction

repair. These projections should always be removed by

gun, by brushing, or by being rubbed into the surface

working up from the bottom because working down from

with the hand encased in a rubber glove. Concrete

the top tends to break concrete out of the repair.

placement should follow immediately. If the cross-

3-12

3.4.4 SHOTCRETE. Shotcrete is satisfactory for

repairing a deteriorated waterfront structure is: (1)

repairing minor damage to concrete piles and framed

remove all defective concrete, (2) clean all rust off

structures and should be considered whenever there is

exposed reinforcement, (3) roughen all smooth surfaces

enough repair work to justify the cost of the equipment.

and either wire brush or abrasive blast the exposed

Piers, navigation locks, wooden piling, concrete piling,

underlying concrete surfaces, (4) instill wire fabric and

and steel piling are typical applications for shotcrete

ensure that the laps do not project more than 3/4 inch

where waterfront repairs are necessary.

from the surface of the underlying concrete, (5) fix the

profiles, (6) fill out with shotcrete to the original face of

The advantages of shotcrete, compared with

the structure, and (7) apply not less than 2 inches of

either regular concrete or prepacked concrete, are: (1)

shotcrete (the final coat should not be less than 1/2 inch

ease of placement, (2) minimum need for formwork

thick).

and plant equipment, and (3) high strength. The

comparative disadvantages of shotcrete are: (1)

3.4.5 PREPACKED CONCRETE. Prepacked concrete

susceptibility to wide structural variation (composition is

is used on large repair jobs, particularly underwater

dependent on the skill of the nozzle man), (2) drying

placement or where placement of regular concrete

shrinkage rate and coefficient of thermal expansion can

would be either difficult or impossible. This method is

be considerably different than those of the original

used also in restoring old concrete and masonry

concrete in the structure being repaired, and (3)

structures. The advantages of either regular concrete or

relatively high porosity.

prepacked concrete, compared with shotcrete, are:

greater density, greater uniformity, lesser permeability,

Repairs and restorations accomplished by the

lower shrinkage, less dependence on personal skills of

shotcrete method are economical and successful where

equipment operators, less dust, less clean-up work, and

deterioration is shallow and the repaired area is large

more economical. The comparative disadvantages of

and irregular. In regions of severe exposure, periodic

these two methods are that all work on vertical surfaces

protective applications are necessary to seal cracks that

requires formwork, and for extensive restoration the

allow the entry of water. More information can be found

plant required could be considerably more expensive

in References 3-2, 3-3, and 3-4.

than that required for shotcrete placement.

With shotcrete, only that amount of water

Prepacked concrete entails placing coarse

necessary for hydration is added to the mixture of

aggregate in the form and filling the voids in the

aggregate and cement. Thus, shotcrete can be more

aggregate mass with intrusion grout that consists of

dense than regular concrete, an important factor in the

portland cement, a high grade pozzolan, sand, water

resistance of concrete to weathering. The ratio of

and an intrusion aid. The intrusion aid is a chemical

cement to aggregate should never be greater than 1 to

admixture that suspends the solid particles in the grout,

3.5; these proportions will result in a ratio of about 1 to

provides fluidity at low W/C ratios, and undergoes slight

2.5 (by weight) after gunning the shotcrete in place.

expansion before final set. The volume of air entrained

in the intrusion grout is about 9%. The amount in the

The recommended shotcrete procedure for

3-13

hardened concrete depends on the ratio of grout to

It is general practice to use a steel tremie, but a

coarse aggregate, but usually is about 4%. Bonding

rigid rubber hose could be substituted. An aluminum

strengths of prepacked to regular concrete are between

alloy tremie should never be used because an adverse

70% and 100% of that attainable in regular concrete.

chemical reaction may occur to produce inferior

This makes it possible to restore deteriorated concrete

concrete [3.5]

members to near their original strengths or to enlarge

existing members to take additional loads.

The size of the tremie depends on the

maximum size of gravel and on the quantity of concrete

Weakened material should be removed to

to be emplaced; the usual range in diameter is from 8 to

expose sound concrete, and the surfaces of sound

18 inches. Records of underwater construction show

concrete should be roughened by either chipping or

rates of lineal flow between 3/4 and 1 foot per second.

heavy sandblasting before repairing. Space must be

The slump of tremie concrete must be maintained

provided for the replacement or addition of at least 3 to

between 6 and 7 inches.

4 inches of new prepacked concrete. Forms are then

well-anchored to the old concrete, filled with coarse

3.4.7 PUMPED CONCRETE. Pumping freshly mixed

aggregate (of proper gradation for the thickness being

concrete is the most expeditious means of placing

placed), and the grout intruded. When the forms are

concrete in spaces of limited accessibility. The pumping

filled, a closing pressure of about 10 psi is held for

method offers several advantages: (1) High quality

several minutes to drive out all air and water through a

concrete is required because the mixture must be

vent at the highest point. The forms are removed one

workable in order to pass through the pump; (2)

or two days later, and the new concrete is properly

Workable mixtures containing relatively small coarse

cured.

aggregate particles tend to provide a dense concrete;

(3) The pump pressure helps to coat the aggregate

3.4.6 TREMIE CONCRETE. One method of placing

particles more uniformly and, thus, increase the density

concrete underwater, especially at easily accessible

of the resultant concrete; (4) Concrete can be

locations, involves a tremie (a steel tube having a

transferred from a barge directly into wooden forms at

hopper for filling at its upper end). A plug, consisting of

the patching site; and (5) Pumped concrete can be used

either a rubber ball or a wad of burlap that fits snugly

to fill the forms from the bottom upwards, displacing the

inside the tremie, is inserted below a loading hopper

seawater as additional concrete is forced in at the

located at the upper end of the tremie. The freshly

bottom.

mixed concrete, introduced at the hopper, forces the

plug down and displaces the seawater. The tremie is

The pumping method also has some

continually replenished with concrete while the lower

disadvantages: (1) The slump must be carefully

end is kept embedded in the newly deposited concrete.

controlled to prevent segregation as excessively wet

Tremie concrete must be quite workable so that it flows

mixtures will sometimes segregate; (2) Coarse

readily into place.

aggregate should consist of rounded particles as

crushed stone mixtures are comparatively difficult to

3-14

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airentrained concrete will cause little reduction in air

content.

The pipeline should be either horizontal or

vertical rather than inclined, wherever possible. With an

inclined pipeline any water bleeding from the freshly

mixed concrete within the pipeline will collect above the

concrete and run down the inside of the pipeline.

Delays as long as 1-1/2 hours can generally be

tolerated if the mixture is moved several feet at least

every 10 minutes (while in the hose or pipeline) until

continuous pumping is resumed.

The concrete should be pumped as near to its

final underwater position as possible. The diver who

Figure 3-7. Typical squeeze-type concrete pump.

has control of the discharge end should not permit

lateral flow within the open-top form of more than 2 or 3

pump because the angular particles tend to interlock;

feet. The discharge end of the line has to be buried in

and (3) Porous aggregates (e.g., expanded clay, foamed

the mass of fresh concrete; otherwise, segregation will

slag, pumice, and many corallin, materials) should be

occur at the point where the concrete comes out.

avoided, if denser aggregates are available.

Aluminum pipe should not be used because an adverse

chemical reaction with the concrete will occur. Rubber

The squeeze-type pump (Figure 3-7) is

hose should only be used for discharge lines or for very

preferred for pumping freshly mixed concrete into the

short pumping distances. The pipeline should be

form because few of the pump parts contact the

protected from any excessive heat (solar included).

concrete. This pump is easy to clean and does not

place the concrete under great pressure.

3.4.8 EPOXY RESIN. Cracks and joints in concrete

waterfront structures must be sealed against the

When air-entraining agents are required as

adverse effects of a marine environment as a means of

described in 3.1.2.4, they are dissolved in the mixing

prolonging the lives of such facilities. Various

water before it enters the concrete mixer. Quantities

formulations of epoxy resin compounds are used for

needed per bag of cement are specified by the

sealing, grouting, patching, and waterproofing cracks

manufacturer and are shown on the containers.

and joints in concrete, and as adhesives for bonding

Normally about 2 fluid ounces per bag of cement are

freshly mixed concrete or precast concrete to old

used.

concrete. No formulation can serve as an all-purpose

material for these applications, and so each epoxy

Water-reducing admixtures will also improve the

formulation should be used only for its intended

pumpability of the concrete. If admixtures are used, do

purpose. Proper methods of treating the surfaces of

not decrease the cement composition; to do so would

concrete and

probably cause blockage in the pipeline. Pumping

3-15

reinforcing steel preparatory to applying the epoxy

compound, and correct procedures for using epoxy

compounds are described in detail in References 3-6 through

3-8.

3.4.9 PROTECTIVE COATINGS. Coating hardened

concrete surfaces (e.g., the decks of piers and wharves) with

protective water repellents may be a good precautionary

measure. A useful guide to coatings for protecting concrete

is found in Reference 3-9.

3-16

CHAPTER 4 - STONE MASONRY STRUCTURES

SECTION 1. INTRODUCTION

4.1.1 BACKGROUND. Throughout the 19th century,

(Figure 4-1). These structures usually incorporate

stone masonry was generally used in constructing

massive gravity walls, the stability of which is a function

graving docks, quay walls, and wharves. As late as the

of their mass. The designers of masonry waterfront

1850s, the cut stones of granite were set in lime mortar;

structures specified greater mass, proportional to the

after that, they were set in portland cement mortar.

expected loads, than is customary with mass concrete

used today. Granite masonry usually develops no

4.1.2 DESIGN. In most instances the quarried and

maintenance problems except at the joints.

trimmed building stone used in graving docks is granite

SECTION 2. METHOD OF INSPECTION

4.2.1 VISUAL. The stone blocks in these old waterfront

width of cracks in adjacent paved areas atop the earth

structures have been subjected to weathering,

behind the walls. If leaks are detected, note the rate of

extraordinary loads, abrasion, and seawater. The best

discharge and whether or not material is suspended in

visual indication of how well they have resisted

the water. Divers should investigate for scour and

weathering is their general appearance. Blocks of high-

undermining, especially outside the closure (Figure 4-2).

quality stone retain their sharp edges and corners and

their delicate tool marks for many years. After a century

4.2.2 SETTLEMENT. If settlement of the structure is

of service, these distinguishing attributes may no longer

suspected, establish points for a level survey; locate

be present.

these points on both sides of each suspected joint and

at both ends of the masonry structure; these points

Empty graving docks should be inspected for

should be related to permanent bench marks

leaking groundwater through the joints in their stone

established previously by the U.S. Coast and Geodetic

floors and sidewalls and for leaking seawater around the

Survey. For a graving dock, these points should be

seals of the closure to the basin. All joints should be

located at the tops of the sidewalls, at the floor adjoining

examined for cracks and erosion. The earth behind the

the sidewalls, along the longitudinal centerline of the

sidewalls should be inspected periodically for

floor, and along the outer rail of the crane track. Note

settlement. Movement of the sidewalls of a graving

any condition that could reveal settlement.

dock or quaywalls is usually revealed by an increase in

4-1

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Figure 4-1. Masonry graving dock.

Figure 4-2. Types of deterioration that will require repair.

4-2

4.2.3 DOCUMENTATION . Periodically record and/or

should include the data and a scalar guide to enable the

photograph cracks and related defects to ensure

viewer to better understand the magnitude of the defect.

documentation of progressive failure; each photograph

SECTION 3. METHODS OF REPAIR

4.3.1 ENGINEERING INVESTIGATION . If a masonry

in vertical joints. If the masonry exhibits signs of

quaywall or graving dock has moved as the result of

leakage, hairline cracking can also be present in the

sliding at the foundation, a structural analysis to

horizontal beds of mortar. Spalled mortar can be

determine the cause is necessary before any restoration

caused by many cycles of alternate freezing and

is attempted. An investigation of the settlement may

thawing.

involve pumping dyed water through cracked or leaky

joints to determine the extent of hidden cavities or

4.3.4 TUCK-POINTING. Defective joints can usually be

voids; the existence of a cavity would be confirmed by

repaired by tuck-pointing with portland cement mortar; a

finding the colored water at some drainage outlet. After

skilled stone mason is required. Tuck-pointing only the

such an investigation has yielded the location, depth,

obviously defective joints does not ensure that the

and extent of the cavity or void, a program of grouting

untreated joints will not leak; therefore all joints, vertical

must be planned.

and horizontal, in the face of the wall should be tuck-

pointed. This procedure requires removing and

If any portion of the masonry structure is

replacing all mortar to a depth of at least 5/8 inch

damaged, or if any stone blocks are loose, repairs

throughout every joint. Each joint is raked to a depth

should ensure that the bond between the blocks is

not greater than 1 inch, unless the old mortar is so

restored. If a masonry quaywall or sidewall of a graving

defective that removal to a greater depth is necessary.

dock is cracked due to unequal settlement, restoration

The depth of old mortar removed should be such that

should be delayed until the cause of settlement has

sound mortar will serve as the base for the new mortar.

been corrected.

All exposed sound mortar must have a clean, square-cut

surface. All dust and dirt within the raked joint should be

4.3.2 WEEP HOLES. If there is evidence that water is

washed out by a jet of water. Wherever old mortar is

collecting behind the quaywall, and if weep holes are

raked out deeper than 1 inch, the hollow spots must be

part of the installation, the holes should be cleared to

filled with new mortar first so that a uniform line is

allow drainage. If this procedure is insufficient to relieve

created. The cleaned joints are tuck-pointed with the

the pressure, additional weep holes should be drilled.

portland cement mortar while the masonry is still damp

(not wet) from washing out the raked joints.

4.3.3 SHRINKAGE CRACKS. Shrinkage cracks in joint

mortar appear as hairline cracks; they are usually found

4-3

The mortar is mixed at least 1 hour before use to ensure

provided the cement-base stabilizing mixture contains

prehydration, which stabilizes the plasticity and

an intrusion aid. The consistency of the intrusion

workability of the mortar and minimizes any tendency to

mixture is that of a smooth slurry. This mixture is

shrink after insertion into the joint opening. A suitable

pumped into holes previously drilled at various intervals

mortar incorporates a special masonry cement (Type II

to various depths without damaging the integrity of the

portland cement and a plasticizer), silica sand, and

structure. Before the intrusion grout is pumped, the

freshwater. It has a somewhat stiff consistency to

holes are tested by pumping water to see if the drilling is

enable it to be tightly packed into place. The

adequate and to determine the correct consistency for

sand/cement ratio should be about 3 to 1 by volume.

the slurry repairs. Technical details relative to portland

Placement should be done as follows:

cement grouting are given in Reference 4-1.

(1)

Insert a base layer 3/16 to 1/4 inch thick

Details for the use of portland cement grout in

(2)

Insert the second layer when the base is

reducing leakage by more than 90% in a gravity-type

sufficiently set so that no fingerprint can be made (use a

dam of rubble masonry faced with ashlar are described

sample representative of the first layer)

in Reference 4-2. It is shown that (1) isolated grouting

(3)

Allow the second layer to set for several

tends to hinder the flow of grout injected later through

hours before the joint is finally packed to its full depth

adjacent drill holes, and (2) pumping the grout

and tooled at the face

simultaneously into a multiple series of drill holes

ensures good penetration into any fissures within the

After tuck-pointing, the masonry should be maintained in

structure.

a damp (not wet) condition for at least 72 hours. Note:

modern epoxy-based materials should be permitted as

The pressurized injection of latex siliconate, a

an alternative to portland cement mortar.

method of grouting developed during the 1960s, has

been successfully used in England for making masonry

4.3.5 GROUTING. Leaky stone masonry structures can

structures watertight [4-3].

be sealed effectively with less cost by using grout,

4-4

CHAPTER 5 - RUBBLE-MOUND STRUCTURES

SECTION 1. STRUCTURAL COMPONENTS

5.1.1 MOUND. A mound is an artificial embankment or

Since the rubble used as riprap must be

ridge composed of sand, gravel, and cobbles,

available in large sizes, the quality, condition, and shape

constructed on the oceanic floor by dumping the

of stone are important. Each piece should be devoid of

material from scows and barges. The dumping

planes of weakness, have a specific gravity not less

operation continues until the mound emerges a certain

than 2.6, and have excellent resistance to abrasion and

distance above the mean seawater level. Wave action

weathering. Massive, compact, fine-grained igneous

on the mound gives the sides a natural stable slope.

rock is the best source of rubble.

Since wave action decreases as the oceanic depth

increases, the natural side slopes of the mound normally

5.1.3 RIPRAP. Riprap is a mixed assemblage of rubble,

are steeper in deeper water; this is a function of top

either dumped indiscriminately (as a foundation for the

elevation of the mound, bottom configuration, and tidal

waterfront structure) from scows and barges or

range. Rubble-mound structures are used extensively,

deposited on the surface of a mound to protect the

because they are adaptable to most any depth of water

mound against erosion by waves and scouring by tidal

in the vicinity of harbors and can be repaired readily. If

action and underwater currents. Where it can be

the oceanic floor is not rocky, rubble can protect the

procured in large quantities at low cost, riprap can be

floor against scouring that otherwise might occur at the

useful as a filter blanket over a sandy bottom, as fill

foot of the mound.

behind moles and quaywalls, and as protection for the

sloping sides of mounds. Riprap submerged en masse

5.1.2 RUBBLE. Rubble is irregularly shaped, rough

weighs no more than earthen fill and rests at a steeper

stones, ranging in size up to 1,000 cubic feet each and

slope (approximately 1-1/4 to 1) than does earth.

in weight up to nearly 90 tons each. The stones are in

the same condition as when quarried but without any

The riprap in older breakwaters consists of large

preparation (i.e., dressing) other than removing very

cubical or rectilinear blocks of quarried stone. Since

sharp angles and any objectionable protruding points.

1950, a number of precast concrete armor units have

Hard rock, which is more desirable, usually consists of

been developed; the prevalent types are tetrapods,

either granite or traprock (fine-grained igneous rock).

tribars, and dolosse (Figure 5-1 ). Smaller concrete

Limestone, dolomite, and sandstone are undesirable

armor units can often be substituted for larger quarry

because of their lesser hardness, toughness, and

stones and still obtain comparable protection of the

durability.

5-1

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Figure 5-1. Concrete armor units.

5-2

mound of rubble. No reinforcing steel or steel lifting

during placement and storms than the various other

eyes are used in dolosse and tetrapods; consequently,

types of concrete armor units. The Army Engineer

corrosion is not a problem, and unit cost is minimized.

Waterways Experiment Station considers the dolos

Dolos and tetrapod units are less vulnerable to damage

armor unit the most efficient [5-1].

SECTION 2. CAUSES AND TYPES OF DETERIORATION

5.2.1 TYPES OF DAMAGE. The three principal types

5.2.2.2 Groin. Correctly designed, located, and

of deterioration in rubblemound structures are: (1)

constructed groins seldom undergo damage by wave

sloughing of side slope in the riprap, (2) slippage of base

action, because the littoral drift tends to fortify the

material as the result of scour by offshore currents, and

structures (Figure 5-2). Ideally, the shoreline remains

(3) dislodgment of stones, especially capstones covering

stable as long as the rates of deposit and erosion are

the crown of the structure, from their original position as

equal. If the erosion rate the deposition rate, the shore

the result of intense wave action.

decreases in area, and the groin is then subjected to

gradual destruction.

Scour at or near the base of a rubblemound

structure does not normally occur if the structure is

5.2.2.3 Breakwater. Breakwaters are often subjected to

correctly designed and the floor is stabilized by means

extreme wave action that dislodges riprap and washes

of a properly designed filter blanket and ample riprap.

out portions of the mound. During violent storms,

However, if one or more groins should be subsequently

sections of a breakwater can occasionally be broken

installed at incorrect locations nearby, then radical

through. A typical rubble-mound breakwater is shown in

changes in currents and their velocities could adversely

Figure 5-3.

influence the base of the structure.

5.2.2.4 Jetty. Jetties are designed to direct the flow of

5.2.2 STRUCTURAL DAMAGE.

currents and tides through the entrance channel so as to

ensure a minimum velocity. Though to all outward

5.2.2.1 Seawall. A seawall can suffer loss of riprap; this

appearances they may be satisfactorily sustaining the

successively leads to erosion, by subsequent wave

pressure, the flowing water can gradually scour the base

action, of the toe of the structure and later to

material on the channel side and eventually cause either

undermining of the base.

subsidence of a portion of the jetty or sloughing of the

riprap comprising the side slopes.

5-3

index-64_1.png

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Figure 5-2. Cross section of semipermeable rubble-mound groin.

Figure 5-3. Rubble-mound breakwater.

5-4

SECTION 3. INSPECTION

Rubble-mound structures are always subjected

operator at the surface to maneuver the apparatus

to wave action and, consequently, to deterioration. The

readily.

inspection should provide for detecting beginning

weaknesses in the bases of these structures (e.g.,

5.3.2 DRAINAGE. A rubble-mound mole requires

washout of small stones in the riprap and any core

proper drainage of backfill to prevent a pressure

material in the mound).

differential. Visual inspection of the surface of the

backfill will reveal any discrepancies concerning

5.3.1 COMPONENTS. The crown of a rubble-mound

drainage. Dredging alongside a rubble-mound mole

structure is inspected visually on foot; the portions

must be restricted to depths not greater than

above water level are inspected visually from a dinghy

contemplated in the original design. If any portion of the

or small craft; and the portions below the water line are

base becomes undermined (e.g., dredging too closely to

inspected by divers or underwater TV cameras. The

the riprap) the structure is likely to slip and fail. If the

intended depth of the structure is determined from the

backfill in either a mole or seawall shows evidence of

design drawings; the as-built depth should be compared

settling (e.g., cracking and cave-in of paved surface) as

with depth data obtained by soundings taken at stations

illustrated in Figure 5-4, the condition of the backfill

that are located at equidistant intervals. As much of the

should be investigated to determine whether or not

structure as possible should be inspected at low tide. If

either improper surface drainage or loss of backfill

scouring or sloughing is apparent an engineering

through the riprap is the cause. Stabilization of the

investigation should be initiated. The inspection by the

backfill, either by replacing the lost fill with properly

divers when tidal conditions and wave actions permit

graded material in the filter blanket and in the core or by

may be able to verify the indicated deficiency or

careful grouting, may correct the problem (see Chapter

damage. Underwater television can be effective as a

6). After stabilizing the backfill, all defective pavements

visual means of inspection, but is frequently negated by

should be repaired to prevent any erosion of the

turbulence, suspended sediment, or inability of the

underlying backfill.

SECTION 4. METHODS OF REPAIR

5.4.1 AS-BUILT DRAWINGS. Drawings showing the

before undertaking new repairs.

construction as actually built, rather than as originally

designed, should be used in preparing plans for repair of

5.4.2 REPLACING COMPONENTS. Maintenance on

rubblemound structures. All drawings and records

rubble-mound breakwaters is greater than on any of the

pertaining to any previous repairs should be reviewed

other types. Proper grading of the seaward slope and

5-5

index-66_1.jpg

Figure 5-4. Cave-in, indicating settlement of mole.

use of the correct classes of stone for core and capping

pattern, the vertical joints between the stones may need

will, to a great extent, minimize the amount of annual

venting (by leveling the corners of individual stones) to

maintenance required. Any material lost through

permit entrapped air and water to escape. This reduces

scouring and washing must be replaced periodically with

the lifting action beneath the stones and improves their

materials of the same kind and size as used originally.

stability.

Adjustments in seaward slope may be necessary.

Material should not be replaced to the original slope if

The replacement of riprap (after replacement of

investigation shows that a change is in order. A change

any material washed out of the mound) should either

in the type of capping material also may be necessary

retard or prevent further scouring. If the scour is

with the passage of time, and use of concrete tetrapod,

produced only by wave action, the problem can be

tribar, or dolos armor units may improve the structure.

solved by fortifying the toe of the structure with a thick

If large facing and capping stones are set in a tight

5-6

index-67_1.png

layer of riprap which serves to stabilize the bottom; the

heavy rubble may be effective in deflecting the

rubble must be carefully emplaced so that the smaller

underwater current; in such an installation, the groin is

stones become wedged in the spaces between the

designed to accumulate waterborne material so that the

larger stones. Units weighing less than 1 ton each

floor around the foot of the structure builds up and

should constitute about 15% (by weight) of the additional

serves as a stabilizing influence.

riprap, and the maximum weight of each of the larger

stones should approximate 3 tons. The minimum

Rubble-mound breakwaters, jetties, and

dimension of any stone should be at least one-third of its

seawalls occasionally are repaired by adding crushed

maximum dimension.

stone to the crowns and seaward slopes and grouting

the new surfaces. Repairs of this type, which must be

If the scour is caused by offshore underwater

made in stages because of tides, must produce a thick

currents, installation of groins at strategic locations

(3 feet or more) protective layer or blanket of grouted

along the shoreline may be necessary. If the bottom is

stone. Figure 5-5 illustrates the general scheme of

scoured so extensively that the stability of the structure

repairs.

is endangered, an underwater groin consisting of very

Figure 5-5. The grouted sheathing of face of a breakwater, which is exposed to storms.

5-7

CHAPTER 6- STRUCTURES INVOLVING SOIL

SECTION 1. SOIL DESCRIPTION

6.1.1 WATERFRONT USE. Earthworks, consisting of

Reference 1-10, Chapter 1, and/or References 6-1, 6-2,

soil materials generally enclosed within a protective

or 6-3); (2) its in-situ state, such as structure, density,

covering of coarse stone riprap, steel, or concrete skins,

shear strength, moisture content, etc.; and (3) its

are used for waterfront structures, such as dikes, levees,

mineralogic content. With reference to soils used in

breakwaters, causeways, groins, and seawalls. Soil

waterfront structures it is often sufficient to classify them

generally provides the backfill for quaywalls, caissons,

only according to size (i.e., clay, silt, sand, and gravel).

and other cellular structures. The most common cause

The density, plasticity, and moisture content are most

of deterioration and damage to such structures is

important for the finer-grained soils, while soundness

erosion of the soil by water movements, generally due

and gradation are most pertinent to the coarser-grained

to wind, tidal, or wave action (References 1-2 and 1-3).

soils and rock fills. The particle size, which marks the

Any breaching of or impairment to an earth structure

boundary between the fine-grained, generally cohesive

exposed to moving water sharply increases its

soils (silts and clays) and the coarse-grained, granular

susceptibility to damage. For this reason it is very

soils (sands and gravels), is approximately the smallest

critical that any required maintenance be identified and

sized particle that is large enough to be individually

carried out as quickly as possible.

discernible to the naked eye. This is the minimum size

retained on the no. 200 standard sieve (about 0.07 mm

6.1.2 DEFINITION. Soil is composed of particles that

in diameter). For the fine-grained materials the

differ physically in size and shape and vary in chemical

Atterberg Limits (ASTM D424 [6-4], D423 [6-5], and

composition. Organic matter, water, air, and bacteria

D427 [6-6] ) are very important as are the moisture

are usually present, but soil consists essentially of

contents. Organic soils, such as elastic silts and peats,

mineral matter that has originated from rocks by the

are never used in the construction or repair of

action of a series of weathering processes.

engineering structures, and, therefore, will not be

considered herein.

6.1.3 SOIL CLASSIFICATION . The complete

description of a soil includes: (1) its classification

according to the Unified Soil Classification System (see

6-1

SECTION 2. SOIL PLACEMENT

6.2.1 TYPES OF SOIL. Although most types of

58T) [6-8] tests. The latter method represents a larger

inorganic soil materials have been used for waterfront

compactive effort than the former and has been adopted

structures, the problems experienced with regard to

to account for the higher compactive efforts of current

placement, maintenance, and protection of the fill

construction equipment. However, the standard

generally increase markedly as the grain size

considered most pertinent for waterfront structures is

diminishes. Finer-grained soils in the cohesionless

still Procter. This is because the lower compactive

range are extremely susceptible to leaching and erosion,

effort results in a slightly higher optimum moisture

whereas fine-grained cohesive soils are more difficult to

content for maximum density, and the higher moisture

compact satisfactorily and may undergo undesirable

content is more compatible with the expected in-service

shrinkage and/or swelling behavior. With granular soils,

conditions of waterfront structures.

gradation (i.e., distribution of the various sizes) is

important. Uniformly graded soils with a narrow range

A typical requirement for many waterfront

of particle sizes are difficult to compact, are extremely

structures is to specify a compacted density of 95% of

porous, and obtain lower densities and strengths than

standard Procter, but this may vary with the type of

soils with a broader distribution of particle sizes.

structure and its present condition. It is generally

However, where compaction of sands and gravels is

desirable to place a soil material in a structure in as

involved, the presence of large, oversize cobbles can

close to its long-term stable condition as possible.

interfere with the compaction of the finer materials

Although clean, granular materials should be placed in

present. Such larger particles can be excluded from the

as saturated a condition as is practicable, fine-grained or

compacted fills and used as riprap or slope protection.

mix-grained soils may require moisture contents to be

maintained within a particular optimum range. Often,

6.2.2 COMPACTION CONTROL . The performance of

control of water content with respect to the optimum

soil embankments improves with density; thus, soil fill

value is left up to the discretion of the contractor, since

materials should generally be placed at as high a

he can elect to replace rigorous moisture control with

density as is economically feasible, particularly with the

increased compactive effort. In cases where excess

finer-grained soils. There is a particular moisture

compactive effort could result in damage to the

content for a particular soil at which the maximum

structure, such as in quaywalls or cofferdams, the

density is obtained under a specific compaction effort.

compactive effort should be minimized and the moisture

Thus, moisture control is an important factor toward

content should be controlled as well as is practicable.