A seismic wave appears to a pile foundation as a soil movement forcing the piles to move with the soil.
The movement is resisted by the pile cap, bending and shear are induced in the piles, and a horizontal force develops in the foundation, starting it to move in the direction of the wave. A half period later, the soil swings back, but the pile cap is still moving in the first direction, and, therefore, the forces increase.
This situation is not the same as one originated by a static force.
Seismic lateral pile design consists of determining the probable amplitude and frequency of the seismic wave as well as the natural frequency of the foundation and structure supported by the piles. The first requirement is, as in all seismic design, that the natural frequency of the foundation and structure must not be the same as that of the seismic wave (a phenomenon called "resonance"). Then, the probable maximum displacement, bending, and shear induced at the pile cap are estimated. Finally, the pile connection and the pile cap are designed to resist the induced forces.
In the past, seismic design consisted of assigning a horizontal force equal to a quasi-static load as a percentage of the gravity load from the supported structure, e.g., 10 %, proceeding to do a static design.
Often this approach resulted in installing some of the piles as inclined piles to resist the load by the horizontal component of the axial force in the inclined piles. This is not just very arbitrary, it is also wrong. The earthquake does not produce a load, but a movement, a horizontal displacement. The force is simply the result of that movement and its magnitude is a function of the flexural stiffness of the pile and its connection to the pile cap. The stiffer and stronger the stiffness, the larger the horizontal load.
Moreover, while a vertical pile in the group will move sideways and the force mainly be a shear force at the connection of the piles to the pile cap, the inclined pile will rotate and to the extent the movement is parallel to the inclination plane and in the direction of the inclination, the pile will try to rise. As the pile cap prevents the rise, the pile will have to compress, causing the axial force to increase. As a result of the increased load, the pile could be pushed down. Moreover, the pile will take a larger share of the total load on the group. The pile inclined in the other direction will have to become longer to stay in the pile cap and its load will reduce — the pile could be pulled up or, in the extreme, be torn apart. Then when the seismic action swings back, the roles of the two inclined piles will reverse. After a few cycles of seismic action, the inclined piles will have punched through the pile cap, developed cracks, become disconnected from the pile cap, lose bearing capacity — essentially, the foundation could be left with only the vertical piles to carry the structure, which might be too much for them. If this worst scenario would not occur, at least the foundation will be impaired and the structure suffer differential movements. Inclined piles are not suitable for resisting seismic forces. If a piled foundation is expected to have to resist horizontal forces, it is normally better to do this by means other than inclined piles.
An analysis of seismic horizontal loads on vertical piles can be made by static analysis. However, one should realize that the so-determined horizontal force on the pile and its connection to the pile cap is not a force casing a movement, but one resulting from an induced movement — the seismic displacement.
7.22.4 Pile Testing
A pile design should consider the need and/or value of a pile test. A “routine static loading test”
involving only loading the pile head to twice the allowable load and recording the pile head movement is essentially only justified if performed for proof testing reasons. It is rarely worth the money and effort, especially if the loading procedure involves just a few (eight or so) load increments of different duration and/or an unloading sequence or two before the maximum load. The only information attainable from such a test is that the pile capacity was not reached. If it was reached, its exact value is difficult to determine and the test gives no information on the load-transfer and portion of shaft and toe resistance.
In contrast, a static loading test performed building up the applied load by a good number of equal load increments with constant duration, no unloading, to a maximum load at least equal to twice the allowable load and on an instrumented pile (designed to determine the load-transfer) will be very advantageous for most projects. If performed during the design phase, the results can provide significant benefits to a project. When embarking on the design of a piling project, the designer should take into account that a properly designed and executed pile test can save money and time as well as improve safety. The O-cell test will meet all objectives of a properly designed test. A dynamic test with proper analysis (CAPWAP) will also provide valuable information on load-transfer, pile hammer behavior, and involve testing of several piles.
For detailed information on pile testing and analysis of pile tests, see Chapter 8.
7.22.5 Pile Jetting
Where dense soil of limitations of the pile driving hammer hampers the installation of a pile, or just to speed up the driving, jetting is often resorted to. The water jet serves to cut the soil ahead of the pile toe.
For hollow piles, pipe piles and cylinder piles, the spoils are let to flow up inside the pile. When the flow is along the outside of the pile, the effect is a reduction of the shaft resistance, sometimes to the point of the pile sinking into the void created by the jet. The objective of the jetting may range from the cutting of the dense soil ahead of the pile toe or just to obtain the lubricating flow along the pile shaft. When the objective is to cut the soil, a small diameter jet nozzle is needed to obtain a large velocity jet. When the objective is to obtain a "lubricating" flow, the jet nozzle is larger to provide for the needed flow of water.
It is necessary to watch for the water flowing up along the pile does not become so large that the soil near the surface can erode resulting a crater that makes the pile loose lateral support. It is also necessary to ensure that the jetting cutting is symmetrical so that the pile will not drift to the side. For this reason, outside placement of jetting pipes is risky as opposed to inside jet placement, say, in a center hole cast in a concrete pile.
Water pumps for jetting are large-volume, large-pressure pumps that provide small flow at large pressure and large flow at small pressure. The pumps are usually rated for 200 gal/min to 400 gal/min, i.e., 0.01 m3/s to 0.02 m3/s to account for the significant energy loss occurring in the pipes and nozzle during jetting. The flow is simply measured by a flow meter. However, to measure the pump pressure is difficult. The flow rate (volume/time) at the pump and out through the jet nozzle and at any point in the system is the same. However, the pressure at the jet nozzle is significantly smaller than the pump pressure due to energy losses. The governing pressure value is the pressure at the jet nozzle, of course. It can quite easily be determined from simple relations represented by Eq. 7.31 (Toricelli's relation) and Eq. 7.32 (Bernoulli's relation) combined into Eq. 7.33. The relations are used to design the jet nozzle as appropriate for the requirements of volume (flow) and jetting pressure in the specific case.
(7.31) Q=μAv
where Q = flow rate (m3/s) à = jetting coefficient ≈0.8 A = cross sectional area of nozzle v = velocity (m/s)
(7.32)
γ p g v = 2
2
which converts to: Eq.7.32a
ρ v2 = 2p
where v = velocity (m/s) g = gravity constant (m/s2)
p = pressure difference between inside jet pipe at nozzle and in soil outside γ = unit weight of water (KN/m3)
= unit density of water (kg/m3) (7.33)
p A Q
μ 2
= ρ
where A = cross sectional area of nozzle Q = flow rate (m3/s)
à = jetting coefficient ≈0.8
= unit density of water (kg/m3); ≈1,000 kg/m3
p = pressure; difference between inside jet pipe at nozzle and in soil outside When inserting the values (0.8 and 1,000) for à and into Eq. 7.33a, produces Eq. 7.34.
(7.34)
p A=28 Q
For example, to obtain a cutting jet with a flow of 1 L/s (0.001 m3/s; 16 us gallons/minute) combined with a jet pressure of 1.4 KPa (~200 psi), the cross sectional area of the jet nozzle need to be 7 cm2. That is, the diameter of the nozzle needs to be 30 mm (1.2 inch).
During jetting and after end of jetting, a pile will have very small toe resistance. Driving of the pile must proceed with caution to make sure that damaging tensile reflections do not occur in the pile.
The shaft resistance in the jetted zone is not just reduced during the jetting, the shaft resistance will also be smaller after the jetting as opposed to the conditions without jetting. Driving the pile after finished jetting will not improve the reduced shaft resistance.
7.22.6 Bitumen Coating
When the drag load (plus dead load) is expected to be larger than the pile structural strength can accept, or the soil settlement at the neutral plane (settlement equilibrium) is larger than the structure can tolerate, the drag load (the negative skin friction) can be reduced by means of applying a coat of bitumen (asphalt) to the pile surface. Resort to such reduction of shaft shear is messy, costly, and time-consuming. In most cases, it is also not necessary, when the long-term conditions for the piles and the piled foundation are properly analyzed. Moreover, other solutions may show to be more efficient and useful. However,
bitumen coating is efficient in reducing negative skin friction and the drag load, as well as in lowering the neutral plane. Note, a bitumen coat will equally well reduce the positive shaft resistance and, hence, lower the pile capacity. The coat can be quite thin, a layer of 1 mm to 2 mm will reduce the negative skin friction to values of 25 % through 10 % of the value for the uncoated pile. The primary concern lies with making sure that the bitumen is not scraped off or spalls off in driving the pile. The bitumen is usually heated and brushed on to the pile. In a cold climate, the coat can spall off, i.e., loosen and fall off in sheets "sailing" down from the piles, which risks for severe damage to people down below. In a hot climate, the coat may flow off the pile before the pile is driven. A dusty pile surface — be it a concrete pile or steel pile — may have to be primed by "painting" the surface with very thin layer of heated, hard bitumen before applying the shear layer. Fig. 7.26 illustrate brushing the shear layer onto a primed surface of a concrete pile. Figure 7.27 shows the coated pile when driven through a protective casing.
Note that the bitumen has flowed and formed a belly under the pile after the coating was applied.
Fig. 7.26 n coat to a concrete pile.
Fig. 7.26 View of a bitumen-coated pile driven through a protective casing. The left side of the pile was the pile underside in storage.
View of work under way to apply a bitume
A functional bitumen coat on a pile to reduce shaft resistance can be obtained from a regular bitumen supplier. The same bitumen as used for road payment can be used. Be careful about roofing bitumen as often some fibers have been added to make it flow less. Note also, that driving through coarse soil will scrape off the bitumen coat—even a "thick one"—and preboring, or driving through a pre-installed casing, or another means to protect the bitumen, may be necessary. Moreover, in hot weather, it may be necessary to employ a two-layer bitumen coat to ensure that the bitumen will not flow off between coating the pile and driving it. The inner coat is the about 1 mm to 2 mm "slip coat" and an outer coat of about the same thickness of very stiff bitumen is then apply to cover the inner coat to keep it in place.
The range of bitumen to use depends a bit on the climate of the site location. The ground temperature is about equal to the average annual temperature of the site. Therefore, a harder bitumen is recommended for use in tropical climate than in a cold climate. For most sites, a bitumen of penetration 80/100 (ASTM D946) is suitable (Fellenius 1975).
7.22.7 Pile Buckling
Buckling of piles is are often thought to be a design condition, and in very soft, organic soils, buckling could be an issue. However, even the softest inorganic soil is able to confine a pile and prevent buckling.
The corollary to the fact that the soil support is always sufficient to prevent a pile from moving toward or into it, is that when the soils moves, the pile has no option other than to move right along. Therefore, piles in slopes and near excavations, where the soil moves, will move with the soil. Fig. 7.28 is a 1979 photo from Port of Seattle and shows how 24-inch prestressed piles supporting a dock broke when a hydraulic fill of very soft silt flowed against the piles.
Fig. 7.28 View of the consequence of a hydraulic fill of fine silt flowing against 24-inch piles
7.22.8 Plugging of open-two pipe piles and in-between flanges of H-piles
Plugging of the inside of a pipe pile driven open-toe is a common occurrence. The question often raised is how the presence of a plug should be considered in a design analysis. First, there is the question if the soil inside the pipe is really a plug, or just a soil column over which the pipe slides. If it is a column, clearly, as the pile moves against the column, shaft resistance develops along the interface between the
pipe inside and the column. However, while the "outside" shaft resistance can be, and should be, calculated using the overburden effective stress, the "inside" shaft resistance is harder to determine. The push on the column from the shaft resistance mobilized along the upper portion of the column will be the source of an increase of stress from the column that, through the "Poisson's Ratio effect", will increase the lateral stress against the inside further down the column. If electing to use total stress analysis, one avoids that problem, but not the problem of determining the magnitude of the inside shaft resistance. The general view appears to be that the inside shear force is smaller than the outside. The issue of set-up is also different and unknown.
If a plug has formed, the issue is easier, as a definite plug that moves down with the pile, will simply cause the pile to act as a closed-toe pile. The soil mass inside will be of importance for the driving, but not for the static shaft resistance and the plug will provide the pile with a larger toe resistance than had instead a column formed. For design, it is a matter of whether to trust this toe resistance to be available in the long-term, of course. In case of an open-toe pipe driven through soft or loose soil and into a competent dense soil therein forming a base, the so-obtained toe resistance can be trusted in most conditions.
Plugging can also occur in-between the flanges of an H-pile. If an H-pile has developed a "column", then the shaft resistance can be calculated on the total circumference, the "H", If a plug, the shaft resistance should be calculated on the "square". The similar approach applies to the toe of the H-pile, but a bit more cautiously. Analyzing an H-pile is harder than analyzing an open-toe pipe pile, because the plug/column can occur along different length of the pile, Indeed, also at different times during the short interval of the driving impact.
The issue of plug/column is not a major one for land-piles or short marine piles. However, it is a major issue for offshore piles used for support of offshore platforms. However, such foundations are not part of this brief text.
7.22.9 Sweeping and bending of piles
Practically all piles, particularly when driven, are more or less out of design alignment, and a perfectly straight pile is a theoretical concept, seldom achieved in practice. It should be recognized that the deviation from alignment of a deep foundation unit has little influence on its geotechnical capacity.
Assigning a specific tolerance value of deviation, say, a percentage of inclination change only applies to the pile at the pile cap or cut-off location (as does a specific deviation of location).
When long piles are driven into any type of soil, or shorter piles driven through soils containing obstructions, the piles can bend, dogleg, and even break, without this being recognized by usual inspection means after the driving. Pipe piles, and cylinder concrete piles, that are closed at the toe provide the possibility of inspection of the curvature and integrity given by the open pipe. A closed-toe pile that was filled with soil during the driving can be cleaned out to provide access to the inside of the pile. It is normally not possible to inspect a precast concrete pile or an H-pile for bending. However, by casting a center tube in the precast concrete pile and a small diameter pipe to the flanges of the H-pile before it is driven, access is provided for inspection down the pile after driving.
The location of a pile and its curvature can be determined from lowering an inclinometer down the pile, if access is provided by the open pipe or through a center pipe. Fig. 7.29 show an example of deviations between the pile head and pile toe locations for a group of 60 m (200 ft) long, vertically driven prestressed piles in soft soil (Keehi Interchange, Hawaii. The piles were made from two segments spliced with a mechanical splice. The main cause of the deviations was found to be that the piles were cast with
the pile segment ends not being square with the pile. When this was corrected, the piles drove with only small deviations).
Fig. 7.29 Example of deviations determined by inclinometer measurements in 60 m long prestressed piles. Principle of the curvature probe
For a pipe pile, inspection down the open pile is often only carried out by lowering a flashlight into the pipe, or center tube, to check that the pile is sound, which it is considered to be if the flash light can reach the bottom of the pile while still being seen from above. However, dust and water can obstruct the light, and if the light disappears because the pile is bent, there is no possibility to determine from this fact whether the pile is just gently sweeping, which is of little concern, or whether the pile is severely bent, or doglegged. In such a case, a specially designed, but simple, curvature probe can be used to vindicate undamaged piles, and to provide data for aid in judging and evaluating a suspect pile.
The curvature probe consists of a stiff, straight pipe of dimensions so chosen that it, theoretically, will 'jam' inside the pipe, or center tube, at a predetermined limiting bending radius expressed in Eq. 7.35 (Fellenius 1972).
(7.35)
) (
8
8 1 2
2 2
D D
L t
R L
= −
=
Where R = Bending radius
L = Probe length
t = Annulus D1 - D2
D1 = Inside diameter of the pile or center tube D2 = Outside diameter of the curvature probe
The principle of the use of the curvature probe are illustrated in Fig. 7.30.