Piles and Foundation
- It is not necessary to design nominal reinforcement to piles. Is it true?
In BS8110 and BS5400 Pt.4, they require the provision of nominal reinforcement for columns. However, for pile design the requirement of nominal reinforcement may not be necessary. Firstly, as piles are located underground, the occurrence of unexpected loads to piles is seldom. Secondly, shear failure of piles is considered not critical to the structure due to severe collision. Moreover, the failure of piles by buckling due to fire is unlikely because fire is rarely ignited underground.
However, the suggestion of provision of nominal reinforcement to cater for seismic effect may be justified. Reference is made to J P Tyson (1995).
- How do rock sockets take up loads?
The load transfer mechanism is summarized as follows:
When a socketed foundation is loaded, the resistance is provided by both rock socket wall and the socket base and the load distribution is a function of relative stiffness of foundation concrete and rock mass, socket geometry, socket roughness and strength. At small displacements the rock-socket system behaves in an elastic manner and the load distribution between socket wall and socket end can be obtained from elastic analysis. At displacements beyond 10-15mm, relative displacement occurs between rock and foundation and the socket bond begins to fail. This results in reduction of loads in rock-socket interface and more loads are transferred to the socket end. At further displacements, the interface strength drops to a residual value with total rupture of bond and more loads are then distributed to the socket end.
- In designing mini-piles, should the strength of grout be neglected during assessment of loading carrying capacity?
In designing min-piles, there are two approaches available:
- In the first approach, the axial resistance provided by the grout is neglected and steel bars take up the design loads only. This approach is a conservative one which leads to the use of high strength bars e.g. Dywidag bar. One should note that bending moment is not designed to be taken up by min-piles because of its slender geometry.
- In the second approach, it involves loads to be taken up by both grout and steel bars together. In this way, strain compatibility requirement of grout and steel has to be satisfied.
- What are the considerations in determining whether casings should be left in for mini-piles?
Contrary to most of pile design, the design of min-piles are controlled by internal capacity instead of external carrying capacity due to their small cross-sectional area.
There are mainly two reasons to account for designing mini-piles as friction piles:
- Due to its high slenderness ratio, a pile of 200mm diameter with 5m long has a shaft area of 100 times greater than cross-sectional area. Therefore, the shaft friction mobilized should be greater than end resistance.
- Settlements of 10%-20% of pile diameter are necessary to mobilize full end bearing capacity, compared with 0.5%-1% of pile diameter to develop maximum shaft resistance.
Left-in casings for mini-piles have the following advantages:
- Improve resistance to corrosion of main bars;
- Provide additional restraint against lateral buckling;
- Improve the grout quality by preventing intrusion of groundwater during concreting;
- Prevent occurrence of necking during lifting up of casings during concreting.
- What is the purpose of post-grouting for mini-piles?
Post-grouting is normally carried out some time when grout of the initial grouting work has set (e.g. within 24 hours of initial grouting). It helps to increase the bearing capacity of mini-piles by enhancing larger effective pile diameter. Moreover, it improves the behaviour of soils adjacent to grouted piles and minimizes the effect of disturbance caused during construction. In essence, post-grouting helps to improve the bond between soils and grout, thereby enhancing better skin friction between them.
During the process of post-grouting, a tube with a hole at its bottom is lowered into the pile and grout is injected. The mechanism of post-grouting is as follows: the pressurized grout is initially confined by the hardened grout and can hardly get away. Then, it ruptures the grout cover and makes its way to the surrounding soils and into soft regions to develop an interlock with harder soil zones. In order to enhance the pressurized grout to rupture the initial grout depth, a maximum time limit is normally imposed between the time of initial grouting and time of post-grouting to avoid the development of high strength of initial grout. Consequently, the effect of soil disturbance by installation of casings and subsequent lifting up of casings would be lessened significantly.
- In designing the lateral resistance of piles, should engineers only use the earth pressure against pile caps only?
In some design lateral loads are assumed to be resisted by earth pressure exerted against the side of pile caps only. However, it is demonstrated that the soil resistance of pile lengths do contribute a substantial part of lateral resistance. Therefore, in designing lateral resistance of piles, earth pressure exerted on piles should also be taken into consideration.
In analysis of lateral resistance provided by soils, a series of soil springs are adopted with modulus of reaction kept constant or varying with depth. The normal practice of using a constant modulus of reaction for soils is incorrect because it overestimates the maximum reaction force and underestimates the maximum bending moment. To obtain the profile of modulus of subgrade reaction, pressuremeter tests shall be conducted in boreholes in site investigation. Reference is made to Bryan Leach (1980).
- In some codes, they limit the ratio of weight of hammer to weight of pile for pile driving. What is the reason behind this?
When a hammer with initial motion collides with a stationary pile, the transfer of energy is most efficient when the two masses are comparable. That is the reason why some codes limit the ratio of weight of hammer to the weight of pile to be more than 0.5. If the weight of hammer is too low, most of energy during hammer driving is distributed to the hammer and this causes tension induced in hammer and results in inefficient transfer of energy.
- Should engineers rely solely on Hiley’s formula in the design of H-piles?
About 90% of H-piles adopt Hiley’s formula for design. However, this formula is only applicable to pile lengths less than 30m and is suitable for course-grained materials (not suitable to fine-grained soils) as suggested by GEO (1996). In Hiley’s formula, by observing the penetration of piles after the hammer impact, the pile capacity could be readily obtained from the response of the impacting force. Therefore, the individual pile capacity could be obtained by this dynamic method.
However, in normal foundation, groups of H-piles are present and the soil foundation may not be able to support these H-piles simultaneously even though individual piles are proven to have sufficient capacity by using dynamic method. In this case, static method should be employed to ascertain if the soil foundation could support these H-piles.
- What is the function of drilling fluid in rotary drilling in site investigation?
Drilling fluid in rotary serves two main purposes:
- Facilitate the rotation of drilling tube during rotary drilling;
- Act as a cooling agent to cool down heat generated during drilling operation.
Traditionally, water is normally employed as drilling fluid. However, it suffers from the following drawbacks:
- It affects the stability of nearby ground with the introduction of water into the borehole (borehole for soil; drillhole for rock);
- It affects the quality of sample by changing the water content of soil samples collected from the borehole/drillhole.
Substitutes are available in market to replace water as drilling fluid (e.g. white foam).
- What are the differences in function between rock anchors and rock sockets?
Rock anchors are used primarily for resisting uplift forces. On the contrary, rock sockets serve three main purposes:
- Rock socket friction and end bearing to resist vertical load;
- Passive resistance of rock sockets contribute to resistance of lateral load; and
- Socket shaft friction is also used for resisting uplifting forces. But only 70% of this capacity should be used because of the effect of negative Poisson ratio.
Note: Rock anchors, which may consist of a high tensile bar or a stranded cable, are provided for tension piles when there are insufficient soil covers to develop the required uplifting resistance.
- What are the functions of cap block, drive cap and pile cushion in driven piles?
Cap block is installed between the hammer end and the drive cap to control the hammer blow in order to protect both the hammer and the pile from damage. When the hammer hits the cap block, it compresses elastically and reduces the peak forces, thereby lengthening the time of hammer blow. Moreover, it should be capable of transmitting the hammer energy effectively to the piles.
Drive cap is inserted at hammer tip to enhance uniform distribution of hammer energy to the pile. Pile cushion is positioned between the drive cap and the pile top. It intends to protect the pile from driving stress induced during hammer blows. Moreover, it also serves to provide a uniform driving load on top of the pile.
12.What is the significance of driving sequence of driven piles?
For basement construction, if piles are driven from the centre to the perimeter, there is a tendency of soils to move outwards. Such lateral movement of soil may cause damage to nearby structures and utilities.
However, if piles are driven from the outside perimeter inwards, there are little soil lateral movements. This results in a well-compacted centre with an excess pore water pressure built up to resist the loading of piles. Consequently, shorter pile lengths than the original designed ones may result. However, some time after the pile driving operation, the excess pore water pressure is dissipated and the shorter driven piles may not be able to take up the original design loads. In this situation redriving is required to drive the piles to deeper depths after dissipation of excess pore water pressure.
- What is the function of followers in driven H-piles?
A follower is an extension between the pile head and the hammer that transfers the blow to the pile in which the pile head cannot be reached by the hammer or is under water .For construction of driven piles, the piling frame and hammer are normally erected on existing ground level but not at the base of pile caps. However, H-piles are designed to be terminated near the base of pile caps. If piles are driven at ground level, a certain length of H-piles is wasted and cut when constructing pile caps. In this connection, pile followers are used so as to save the wasted section of H-piles because followers can be removed during subsequent construction of pile caps.
- What are the advantages of using top-down approach in basement construction?
The advantages of top-down approach are listed below:
- The structures above ground can be carried out simultaneously with the structures below ground. This greatly reduces the time for construction.
- By using this approach, settlement can be reduced.
- Since the permanent columns and slabs can be utilized to support loadings during construction, it saves the cost of formwork.
Note: Top-down approach means construction of basement is carried out from ground level downwards.
- What are the methods to tackle negative skin friction?
- Use slender pile sections (e.g. H-pile or precast pile) because smaller pile area when subject to the same working load would produce higher deformation, thus increasing the relative downward movement of piles.
- In a certain region of H-piles for ground water table fluctuation, painting is applied on the surface of H-piles because the rise and fall of water table contribute to the corrosion of H-piles. On the other hand, to reduce the effect of additional loads
brought about by negative skin friction, bitumen is applied on the pile surface corresponding to the region of soils that has negative skin friction. However, bitumen should not be applied to the whole section of H-piles because it would be unable to derive the designed frictional reaction from soils.
(iv) Design the piles as end-bearing so that they can take up more load.
- In piling works, normally founding levels of bored piles are defined by using total core recovery or rock quality designation (RQD). Are there any problems with such specification?
The use of total core recovery to determine the founding level may not be able to indicate the quality of rock foundation for piles because it depends on the drilling technique and drilling equipment (GEO (1996)). For instance, if standard core barrels are used to collect samples, it may indicate sufficient core recovery for samples full of rock joints and weathered rock. On the other hand, if triple tube barrels are used for obtaining soil samples, samples with joints and weathered rock can also achieve the requirements of total core recovery.
In case RQD is adopted for determining founding levels, it may also result in incorrect results. For instance RQD does not indicate the joints and infilling materials. Moreover, as it only measures rock segments exceeding 100mm, rock segments exceeding 100mm is considered to be of good quality rock without due consideration of its strength and joint spacing.
- What are the head details of H-piles under compression and subject to bending moment?
For steel sections referred to in BS5950, universal bearing pile is characterized by having equal flange and web thickness while universal column has different flange and web thickness. Universal columns can also be used as bearing piles.
In the design of the head details of H-piles, there are three typical cases to be considered, namely compression piles, tension piles and piles with bending moment at the head in addition to tension or compression. The design of these piles recommended by G. M. Cornfield (1968) is listed below:
(i) Compression piles
For this type of piles, H-piles should be embedded 150mm in concrete pile caps and it is not necessary to use any dowels and capping plates in their connection.
(ii) Tension piles
A number of hook-ended bars are welded to the top of H-piles.
(iii) Piles with bending moment at their head (tension or compression)
The depth of embedment of piles into pile caps is substantially increased and loads are transferred by horizontal bars welded to piles’ flanges.
- In deep excavation, adjacent ground water table is drawn down which may affect the settlement of nearby buildings. What is the remedial proposal to rectify the situation?
One of the methods to control settlement of nearby buildings due to excavation work is by recharging. Water collected in wells in deep excavation is put back to the top of excavation in order to raise the drawn-down water table. The location of recharge should be properly selected to ensure the soil is sufficiently permeable to transfer the pumped water back near the affected buildings.
- What is the significance of quality of bentonite slurry in the construction of diaphragm walls?
The quality of slurry plays an important role in the quality of diaphragm walls. Firstly, if a thick slurry cake is formed in the interface between slurry and in-situ soil, it has a tendency to fall off during concreting works and it mixes with freshly placed concrete. Moreover, large thickness of slurry cake would reduce the concrete cover and affect the future durability performance of diaphragm walls.
- During concreting of diaphragm walls, three tremie pipes are used in one time. However, only one concrete truck is available. How should the concreting works be carried out
The most ideal situation is to supply each tremie pipe with a single concrete truck. However, if only one concrete truck is available, all the fresh concrete in the truck should not be placed in one single tremie pipe. With all fresh concrete placed in one single tremie pipe while the others left void, then due to the huge supply of concrete to the tremie pipe, a small concrete hump may form at the base of the tremie pipe and it is likely that it may collapse and trap the slurry inside the diaphragm walls. Therefore, the fresh concrete should be evenly shared among the tremie pipes to avoid such occurrence.
- What is the purpose of conducting load test for piling works?
Pile load test provides information on ultimate bearing capacity but not settlement behavior. In essence, it can determine if the load is taken up by the stratum designed or if the centre of resistance is at the design location in piles as suggested by Robert D. Chellis (1961).
After conducting load tests, the curve of movement of pile head (Settlement against load) and the curve of plastic deformation can be plotted. By subtracting the curve of plastic deformation from the curve of pile head movement at each load, the curve of elastic deformation can be obtained. For piles of end-bearing type unrestrained by friction, the theoretical elastic deformation can be calculated from e=RL/AE where e is elastic deformation, L is pile length, A is area of pile, E is Young’s Modulus of pile material and R is the reaction load on pile. By substituting e in the formula, the elastic deformation read from the curve of elastic deformation, L can be obtained which shows the location of the centre of resistance corresponding to that load.
- Why are vibrators not used in concrete compaction in piling works?
Concrete for piles should be a high-slump self-compacting mix which is capable of flowing between reinforcement cage with ease. Since concrete is designed to be self-compacting, vibrators are not used for providing further compaction. Moreover, the concrete in piles is compacted by energy derived from free falling. However, if vibrators are used, the vibrated concrete may be compacted to the sides of the concrete casings and hinders the lifting up of casings. Reference is made to GEO (1996).
- In Hiley’s formula for driven piles i.e. R=E/(s+0.5c), why is a coefficient of 0.5applied for the term elastic deformation of piles and soil?
Hiley’s formula is based on the principle of energy conservation in which the energy brought about by hammers during the action of hitting are transferred to piles in ground. When the hammer force and displacement is plotted, the energy absorbed by piles is the area under the curve. Since the curve of elastic component is linear with a positive slope, the area i.e. energy should be the area of triangle (0.5xRxc) where R is reaction force and c is elastic compression due to helmet, piles and soil system. For settlement, it is of horizontal line in force-displacement diagram and hence the energy transferred to pile-soil system is (Rxs).
- For a rigid pile cap with vertical piles at the middle and raking piles at the sides, what is the pattern of load distribution of piles in such arrangement?
Due to the effect of interaction of individual piles, the central piles tend to settle more than the edge piles when the pile cap is under a uniform load. Therefore, raking piles at the edge take up a higher fraction of total loads and are subject to higher axial and bending loads in case the pile cap is stiff. In the extreme case, failure of these raking edge piles may occur.
- What are the problems associated with prestressed concrete piles (Daido)?
The origin of Daido piles comes from Japan where these prestressed concrete piles are used as replacement plies. Holes are pre-formed in the ground and Daido piles are placed inside these pre-formed holes with subsequent grouting of void space between the piles and adjacent ground. However, in Hong Kong Daido piles are constructed by driving into ground by hammers instead of the originally designed replacement method. Since the installation method of Daido piles is changed, construction problems like deformation of pile tip shoes, crushing of concrete at pile tip etc. occur. Reference is made to B. W. Choy (1993).
- Which one is a better choice, a large diameter piles or a system of several smaller
piles with the same load capacity?
The choice of a large diameter pile suffers from the disadvantage that serious consequences would occur in case there is setting out error of the pile. Moreover, in terms of cost consideration, for the same load capacity the cost of a group of small diameter piles is generally lower than that of a large diameter pile. On the other hand, for small diameter piles i.e. mini-piles, they are advantageous in site locations with limited headroom and space. In addition, in some structures with only a few piles, it is uneconomic because of its high mobilization cost. Reference is made to Dr. Edmund C Hambly (1979).
- What is the difference between capping beams and ground beams for piles?
Capping beams for piles aim at transferring loads from closely spaced columns or walls into a row of piles. On the other hand, ground beams are beams provided between adjacent pile caps and they perform as compression struts or ties in an attempt to prevent lateral displacement or buckling of piles under uneven distribution of loads on pile caps. Both of them have to be specially designed to cater for differential settlement of piles.
Capping beam performs the same functions as pile caps. However, ground beams are structural elements to connect adjacent pile caps to improve the stability of foundation.
- In modeling a nonrigid mat foundation by using elastic springs, should a uniform
modulus of subgrade reaction be used along the whole base of mat?
By using a bed of springs to simulate the flexible behaviour of mat subject to loads, care should be taken in selection of the modulus of subgrade reaction. In fact, the modulus of subgrade reaction depends on many factors like the width of the mat, the shape of the mat, the depth of founding level of the mat etc. In particular, the modulus of subgrade reaction is smaller at the center while it is larger near the mat’s edges. If a constant modulus of subgrade reaction is adopted throughout the width of the mat, then a more or less uniform settlement will result when subject to a uniform load. However, the actual behaviour is that settlement in the center is higher than that at side edges. Consequently, it leads to an underestimation of bending moment by 18% to 25% as suggested by Donald P. Coduto (1994).
In general, a constant value of modulus of subgrade reaction is normally applied for structure with a rigid superstructure and the rigid foundation. However, a variable modulus of subgrade reaction is adopted instead for non-rigid superstructure and non-dominance of foundation rigidity to account for the effect of pressure bulbs.
29.What is the difference between direct circulation drilling and reverse circulation
For direct circulation drilling and reverse circulation drilling, the major difference in drilling method is related to the direction of movement of drilling fluid. For direct circulation drilling, the drilling fluid is circulated from the drill stem and then flows up the annulus between the outside of the drill stem and borehole wall. The drilling fluid that carries the drill cuttings flows to the surface and the subsequent settlement pits. Pumps are employed to lift the cuttings free fluid back to the drill stem.
For reverse circulation drilling, the direction of flow of drilling flow is opposite to that of direct circulation drilling. Drilling fluid flows from the annulus between the drill stem and hole wall to the drill stem. The drilling fluid is pumped to an nearby sump pits where cuttings are dropped and settled.
- What is the difference between “hammer efficiency” and “coefficient of restitution” when using Hiley’s formula in pile driving?
Hammer efficiency refers to the ratio of kinetic energy of the hammer to the rate energy (or potential energy). In essence, there is undoubtedly certain energy losses induced by the hammer itself prior to the actual impact on the driven pile. For instance, these losses may include misalignment of the hammer, energy losses due to guiding friction, inaccurate dropping height etc…
Coefficient of restitution refers to a value indicating the strain energy during collision regained after the bodies reverted back to their original shapes. If the coefficient of restitution is equal to unity, it means that the collision is elastic and all energy has been returned after the impact action. Hence, this is a index showing the degree the impact action in terms of elasticity.
In mathematical forms,
Coefficient of restitution = -(v1-v2)/ (u1-u2)
Where u=initial velocity and v=final velocity after impact