Introduction
Characteristics of tall buildings
-
The building weight, and thus the vertical load to be supported by the foundation, can be substantial. Moreover, the building weight increases non-linearly with height, and so both ultimate bearing capacity and settlement need to be considered carefully.
-
High-rise buildings are often surrounded by low-rise podium structures which are subjected to much smaller loadings. Thus, differential settlements between the high- and low-rise portions need to be controlled.
-
The lateral forces imposed by wind loading, and the consequent moments on the foundation system, can be very high. These moments can impose increased vertical loads on the foundation, especially on the outer piles within the foundation system. The structural design of the piles needs to take account of these increased loads that act in conjunction with the lateral forces and moments.
-
The wind-induced lateral loads and moments are cyclic in nature. Thus, consideration needs to be given to the influence of cyclic vertical and lateral loading on the foundation system, as cyclic loading has the potential to degrade foundation capacity and cause increased settlements.
-
Seismic action will induce additional lateral forces in the structure and also induce lateral motions in the ground supporting the structure. Thus, additional lateral forces and moments can be induced in the foundation system via two mechanisms:
-
Inertial forces and moments developed by the lateral excitation of the structure;
-
Kinematic forces and moments induced in the foundation piles by the action of ground movements acting against the piles.
-
-
The wind-induced and seismically induced loads are dynamic in nature, and as such, their potential to give rise to resonance within the structure needs to be assessed. The risk of dynamic resonance depends on a number of factors, including the predominant period of the dynamic loading, the natural period of the structure and the stiffness and damping of the foundation system.
-
The dynamic response of tall buildings poses some interesting structural and foundation design challenges. In particular, the fundamental period of vibration of a very tall structure can be very high (10 s or more), and conventional dynamic loading sources such as wind and earthquakes have a much lower predominant period and will generally not excite the structure via the fundamental mode of vibration. However, some of the higher modes of vibration will have significantly lower natural periods and may well be excited by wind or seismic action. These higher periods will depend primarily on the structural characteristics but may also be influenced by the foundation response characteristics.
Foundation options
Factors affecting foundation selection
-
Location and type of structure.
-
Magnitude and distribution of loadings.
-
Ground conditions.
-
Access for construction equipment.
-
Durability requirements.
-
Effects of installation on adjacent foundations, structures, people.
-
Relative costs.
-
Local construction practices.
Raft or mat foundations
Compensated raft foundations
Piled foundations
Piled raft foundations
-
As piles need not be designed to carry all the load, there is the potential for substantial savings in the cost of the foundations.
-
Piles may be located strategically beneath the raft so that differential settlements can be controlled.
-
Piles of different length and/or diameter can be used at different locations to optimise the foundation design.
-
Varying raft thicknesses can be used at different locations to optimise the foundation design.
-
Piles can be designed to carry a load approaching (or equal to) their ultimate geotechnical load, provided that the raft can develop an adequate proportion of the required ultimate load capacity.
-
Soil profiles consisting of relatively stiff clays.
-
Soil profiles consisting of relatively dense sands.
-
Profiles with very soft clays at or near the surface of the raft, where the raft can contribute only a relatively small proportion of the required ultimate load capacity.
-
Profiles which may be subjected to long-term consolidation settlement; in this case, the soil may lose contact with the raft and transfer all the load to the piles.
-
Profiles which may be subjected to expansive (upward) movements; in this case, the soil movements will result in increased contact pressures on the raft and the consequent development of tensile forces in the piles.
Compensated piled raft foundations
-
The soft clay often provides only a modest bearing capacity and stiffness for the raft, with the piles having to carry the vast majority of load.
-
If the soft clay is likely to undergo settlement, for example due to reclamation filling or dewatering, the soil may settle away from the base of the raft, again leaving the piles to carry the load.
The design process
Stages of design
Design issues and criteria
Ultimate capacity
Load combinations
Cyclic loading considerations
Serviceability—settlement and differential settlement
Quantity | Value | Comments |
---|---|---|
Limiting tolerable settlement (mm) | 106 | Based on 52 cases of deep foundations |
Observed intolerable settlement (mm) | 349 | Based on 52 cases of deep foundations |
Limiting tolerable angular distortion (rad) | 1/500 | Based on 57 cases of deep foundations |
Limiting tolerable angular distortion (rad) | 1/250 (H < 24 m) to 1/1000 (H > 100 m) | From 2002 Chinese code H = building height |
Observed intolerable angular distortion (rad) | 1/125 | Based on 57 cases of deep foundations |
Level of motion | Acceleration (m2/s) | Effect |
---|---|---|
1 | <0.05 | Humans cannot perceive motion |
2 | 0.05–0.1 | Sensitive people can perceive motion. Objects may move slightly |
3 | 0.1–0.25 | Most people perceive motion. Level of motion may affect desk work. Long exposure may produce motion sickness |
4 | 0.25–0.4 | Desk work difficult or impossible. Ambulation still possible |
5 | 0.4–0.5 | People strongly perceive motion, and have difficulty in walking. Standing people may lose balance |
6 | 0.5–0.6 | Most people cannot tolerate motion and are unable to walk naturally |
7 | 0.6–0.7 | People cannot walk or tolerate motion |
8 | >0.85 | Objects begin to fall and people may be injured |
Design for ground movements
-
The foundations are subjected to additional movements which must be considered in relation to the serviceability requirements.
-
Additional axial and shear forces and bending moments are induced in the piles.
Dynamic loading
-
The natural frequency of the foundation system should be greater than that of the structure it supports, to avoid potential resonance phenomena. The natural frequency depends primarily on the stiffness of the foundation system and its mass, although damping characteristics may also have some influence.
-
The amplitude of dynamic motions of the structure-foundation system should be within tolerable limits. The amplitude will depend on the stiffness and damping characteristics of both the foundation and the structure.
Earthquake loading
-
Increases in pore pressure;
-
Time-dependent vertical ground movements during and after the earthquake;
-
Time-dependent lateral ground movements during the earthquake.
Structural design—soil–structure interaction issues
Factoring of resistances
Stiffening effect of the superstructure
Estimation of pile load distribution
Durability
Preliminary design tools
Detailed design tools and computer programs
Analysis requirements
-
Non-homogeneous and layered soil profiles;
-
Non-linearity of pile and, if appropriate, raft behaviour;
-
Geotechnical and structural failure of the piles (and the raft);
-
Vertical, lateral and moment loading (in both lateral directions), including torsion; and
-
piles having different characteristics within the same group.
-
Pile–pile interaction, and if appropriate, raft–pile and pile–raft interaction;
-
flexibility of the raft or pile cap;
-
some means by which the stiffness of the supported structure can be taken into account.
Commercially available packages
Other packages
-
Pile Group Settlement (PIGS): PIGS is a proprietary FORTRAN program that analyses the settlement and load distribution within a group of piles subjected to axial and moment loading and can also consider (in an approximate manner) the effects of externally imposed vertical ground movements such as those due to swelling or consolidation of the soil profile. Different pile types can be specified within the pile group, as can varying soil profiles. The underlying principles of this program are described by Poulos [59].
-
Combined loading analysis of piles (CLAP): this proprietary program is a development of the commercially available program DEFPIG and can consider all six components of loading, rather than only vertical loading and horizontal and moment loading in one direction. Nonlinear pile behaviour is allowed for so that the program can be used to assess the overall stability of a pile group or a piled raft. It can also be used to compute single pile stiffness values and pile to pile interaction factors.
-
General analysis of rafts with piles (GARP) is a proprietary program based on a finite-element analysis of the raft and a boundary element analysis of the piles. Small and Poulos [73] describe the basis of the GARP analysis. The contact stress that acts between the raft and the soil is assumed to be made up of a series of uniform blocks of pressure that act over each element in the raft. Each of the piles is assumed to apply a reaction to the raft at a point (corresponding to a node in the raft). The raft can have different thicknesses assigned to the elements that make up the mesh to represent areas of varying raft thickness. The deflections, shear forces and moments in the raft and the vertical loads on the piles due to the loading can be assessed. Because it can take raft (or pile cap) flexibility into account, it is suitable for assessing serviceability requirements. It is also useful for obtaining the axial stiffness of the piles within the group, which can then be passed on to the structural designer for incorporation into the overall structural analysis. In this way, it is possible to obtain more reliable bending moments and shears within the raft than is obtained directly from GARP, since account is taken of the stiffness of the supported structure.
Summary of design analysis process
Case | Purpose | Factor applied to geotechnical strength parameters | Load case | Comment |
---|---|---|---|---|
1 | Geotechnical design capacity | ϕg
| ULS | Geotechnical reduction factor, ϕg, applied to strength parameters to assess overall stability of the pile group |
2 | Structural design capacity | 1.0 | ULS | Unfactored geotechnical strength parameters are adopted to assess maximum pile axial load and pile bending moment using short term pile modulus |
3 | Serviceability | 1.0 | SLS | Unfactored geotechnical strength parameters are adopted to assess pile head deflections and rotations |
-
The geological complexity of the site;
-
the extent of ground investigation;
-
the amount and quality of geotechnical data;
-
experience with similar foundations in similar geological conditions;
-
the method of assessment of geotechnical parameters for design;
-
the design method adopted;
-
the method of utilising the results of in situ test data and pile installation data;
-
the level of construction control and
-
the level of performance monitoring of the supported structure during and after construction.
Ground investigation and characterization
Ground information for geotechnical model development
-
They provide a means of identifying the stratigraphy between boreholes;
-
they can identify localised anomalies in the ground profile, for example cavities, sinkholes or localised pockets of softer or harder material;
-
they can identify bedrock levels;
-
they provide quantitative measurements for the shear wave and compression wave velocities. This information can be used to estimate the in situ values of soil stiffness at small strains and hence to provide a basis for quantifying the deformation properties of the soil strata.
Assessment of geotechnical design parameters
Key parameters
-
the ultimate skin friction for piles in the various strata along the pile.
-
The ultimate end bearing resistance for the founding stratum.
-
The ultimate lateral pile-soil pressure for the various strata along the piles.
-
The ultimate bearing capacity of the raft.
-
The stiffness of the soil strata supporting the piles, in the vertical direction.
-
The stiffness of the soil strata supporting the piles, in the horizontal direction.
-
The stiffness of the soil strata supporting the raft.
Empirical correlations
Correlations with SPT
-
Raft ultimate bearing capacity:$$ p_{\text{ur}} = K_{1} \cdot N_{\text{r}}\,\, {\text{kPa}}. $$(10)
-
pile ultimate shaft resistance:$$ f_{\text{s}} = a \cdot \left[ {2.8\,N_{\text{s}} + 10} \right] \,\,{\text{kPa}} $$(11)
-
pile ultimate base resistance:$$ f_{\text{b}} = K_{2} \cdot N_{\text{b}}\,\, {\text{kPa}} $$(12)
-
soil Young’s modulus below raft:$$ E_{\text{sr}} = 2N\,\,\,{\text{MPa}} $$(13)
-
Young’s modulus along and below pile (vertical loading):$$ E_{\text{s}} = 3N\,\,\,{\text{MPa,}} $$(14)
Soil type |
K
1 (raft) |
K
2 displacement piles |
K
2 non-displacement piles |
---|---|---|---|
Sand | 90 | 325 | 165 |
Sandy silt | 80 | 205 | 115 |
Clayey silt | 80 | 165 | 100 |
Clay | 65 | 100 | 80 |
-
Small strain shear Modulus, G 0:
Correlations with CPT
-
Ultimate square or circular raft (or footing) bearing capacity [43]:
Soil type | Condition |
a
1
|
a
2
|
---|---|---|---|
Clay, silt | All | 0.32 | 0.35 |
Sand, gravel | Loose | 0.14 | 0.35 |
Medium | 0.11 | 0.50 | |
Dense | 0.08 | 0.85 | |
Chalk | – | 0.17 | 0.27 |
-
Pile ultimate base capacity [20]:$$ f_{\text{b}} = k_{\text{b}} \cdot q_{\text{c}} $$(18)
Pile type | Clay and silt | Sand and gravel | Chalk | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Soft | Stiff | Hard | Loose | Med. | Dense | Soft | Weathered | |||
Drilled | ||||||||||
k
s
| – | – | 75a
| – | 80 | 200 | 200 | 200 | 125 | 80 |
f
sl (kPa) | 15 | 40 | 80 | 40 | – | – | 120 | 40 | 120 | |
Drilled removed casing | ||||||||||
k
s
| – | 100 | 100b
| – | 100b
| 250 | 250 | 300 | 125 | 100 |
f
sl (kPa) | 15 | 40 | 60 | 40 | 80 | – | 40 | 120 | 40 | 80 |
Steel-driven close-ended | ||||||||||
k
s
| – | 120 | 150 | 300 | 300 | 300 |
c
| |||
f
sl (kPa) | 15 | 40 | 80 | – | – | 120 | ||||
Driven concrete | ||||||||||
k
s
| – | 75 | – | 150 | 150 | 150 |
c
| |||
f
sl (kPa) | 15 | 80 | 80 | – | – | 120 |
Soil type |
q
c (MPa) |
k
b
|
k
b
|
---|---|---|---|
Clay silt
| |||
A | 0.40 | 0.55 | |
Soft | <3 | ||
B | |||
Stiff | 3–6 | ||
C | |||
Hard | >6 | ||
Sand gravel
| |||
A | 0.15 | 0.50 | |
Loose | <5 | ||
B | |||
Medium | 8–15 | ||
C | |||
Dense | >20 | ||
Chalk
| |||
A | |||
Soft | <5 | 0.20 | 0.30 |
B | |||
Weathered | >5 | 0.30 | 0.45 |
Correlations with unconfined compressive strength
Parameter | Correlation | Remarks |
---|---|---|
Ultimate bearing capacity (raft) |
p
ur = a
0
q
u
|
a
0 Can vary from about 0.1 for extremely poor quality rock to 24 for intact high-strength rock [45]. A value of 2 is likely to be reasonable and conservative in many cases |
Ultimate shaft friction, f
s
|
f
s = a (q
u)
b
|
a Generally varies between 0.20 and 0.45; b in most correlations is 0.5 |
Ultimate end bearing, f
b
|
f
b = a
1 (q
u)
b1
|
a
1 Generally varies between 3 and 5, b
1 in most correlations is 1.0, although Zhang and Einstein [80] adopt b
1 = 0.5 |
Young’s modulus for vertical loading, E
sv
|
E
sv = a
2 (q
u)
b2
|
a
2 Varies between about 100 and 500 for a wide range of rocks, b
2 is generally taken as 1.0 |
Parameters for lateral pile response
Laboratory testing
Triaxial and stress path testing
Resonant column testing
Constant normal stiffness (CNS) testing
In-situ testing
Penetration testing
Pressuremeter testing
Geophysical testing
Derivation of secant values of soil modulus for foundation analysis
-
The secant modulus for axial loading may be about 20–40 % of the small-strain value for a practical range of factors of safety;
-
The secant modulus for lateral loading is smaller than that for axial loading, typically by about 30 % for comparable factors of safety.
Pile load testing
Introduction
-
Provide information on the design issues;
-
Be able to be undertaken on pre-production piles;
-
Be able to be undertaken on any of the production piles without special preparation;
-
Be relatively inexpensive;
-
Provide reliable and unequivocal information which can be applied directly to the design process.
Static vertical load test
Static lateral load test
Dynamic load test
-
The principles of the dynamic load test are now very well-established [24, 69]. The test is now accepted as a routine procedure, especially for quality control and design confirmation purposes. Despite its widespread use, the dynamic pile load test has a number of potential limitations, including the fact that the load-settlement behaviour estimated from the test is not unique, but is a best-fit estimate. Two measurements (strain and acceleration versus time) are taken, and from these, the complete distribution of resistance along the pile, as well as the load-settlement behaviour, are interpreted. Also, the load is applied far more rapidly than in most actual situations in practice, and hence time-dependent settlements are not developed during the test. Fortunately, under normal design load levels, the amount of time-dependency (from both consolidation and creep) is relatively small as most of the settlement arises from shear deformation at or near the pile–soil interface. Hence, the dynamic test may give a reasonable (if over-estimated) assessment of the pile head stiffness at the design load. However, it may be expected to be increasingly inaccurate as the load level approaches the ultimate value.
Bi-directional (Osterberg cell) test
-
It is applicable primarily to bored piles;
-
the cell must be pre-installed prior to the test; and
-
there is interaction between the base and the shaft, and each will tend to move less than the “real” movement so that the apparent shaft and base stiffnesses will tend to be larger than the real values.
Statnamic test
-
the test is quick and easily mobilised.
-
High loading capacity is available.
-
The loading is accurately centred and can be applied to both single piles and pile groups.
-
The test does not require any pre-installation of the loading equipment.
-
The test is quasi-static and does not involve the development of potentially damaging compressive and tensile stresses in the test pile.
-
Certain assumptions need to be made in the interpretation of the test, especially in relation to the unloading of the pile.
-
It cannot provide information on time-dependent settlements or movements. While this may not be of great importance for single piles, it can be a major limitation when testing pile groups, especially if compressible layers underlie the pile tips.
Test interpretation
Ultimate axial capacity
Ground modulus values
Typical high-rise foundation settlements
Foundation type | Founding condition | Location | No. of cases | Settlement per unit pressure (mm/MPa) |
---|---|---|---|---|
Raft | Stiff clay | Houston | 2 | 227–308 |
Limestone | Amman; Riyadh | 2 | 25–44 | |
Piled raft | Stiff clay | Frankfurt | 5 | 218–258 |
Dense sand | Berlin; Niigata | 2 | 83–130 | |
Weak rock | Dubai | 5 | 32–66 | |
Limestone | Frankfurt | 1 | 38 |
Case 1—La Azteca building Mexico
Case | Computed average final settlement (mm) | Ratio of settlement to settlement of compensated raft |
---|---|---|
Raft alone, no compensation | 2342 | 2.37 |
Raft alone, with compensation | 988 | 1.0 |
Piled raft, no compensation | 1084 | 1.10 |
Piled raft, with compensation | 283 | 0.29 |
Case 2—The Burj Khalifa, Dubai
Introduction
Geotechnical investigation and testing program
-
Phase 1 (main investigation) 23 boreholes, in situ SPT’s, 40 pressuremeter tests in three boreholes, installation of four standpipe piezometers, laboratory testing, specialist laboratory testing and contamination testing—1st June to 23rd July 2003;
-
Phase 2 (main investigation) Three geophysical boreholes with cross-hole and tomography geophysical surveys carried out between three new boreholes and one existing borehole—7th to 25th August, 2003;
-
Phase 3 Six boreholes, in situ SPT’s, 20 pressuremeter tests, installation of two standpipe piezometers and laboratory testing—16th September to 10th October 2003;
-
Phase 4 One borehole, in situ SPTs, cross-hole geophysical testing in three boreholes and down-hole geophysical testing in one borehole and laboratory testing.
-
The drilling was carried out using cable percussion techniques with follow-on rotary drilling methods to depths between 30 and 140 m below ground level. The quality of core recovered in some of the earlier boreholes was somewhat poorer than that recovered in later boreholes, and, therefore, the defects noted in the earlier rock cores may not have been representative of the actual defects present in the rock mass. Phase 4 of the investigation was targeted to assess the difference in core quality and this indicated that the differences were probably related to the drilling fluid used and the overall quality of drilling.
-
Conventional tests, including moisture content, Atterberg limits, particle size distribution, specific gravity, unconfined compressive strength, point load index, direct shear tests, and carbonate content tests.
-
Sophisticated tests, including stress path triaxial, resonant column, cyclic undrained triaxial, cyclic simple shear and constant normal stiffness (CNS) direct shear tests. These tests were undertaken by a variety of commercial, research and university laboratories in the UK, Denmark and Australia.
Geotechnical conditions
Foundation design
Stratum | Sub-strata | Subsurface material | Level at top of stratum (m DMD) | Thickness (m) | UCS (MPa) | Undrained modulus* E
u (MPa) | Drained modulus* E′ (MPa) | Ult. comp. shaft friction f
s (kPa) |
---|---|---|---|---|---|---|---|---|
1 | 1a | Medium dense silty sand | +2.50 | 1.50 | – | 34.5 | 30 | – |
1b | Loose to very loose silty sand | +1.00 | 2.20 | – | 11.5 | 10 | – | |
2 | 2 | Very weak to moderately weak Calcarenite | −1.20 | 6.10 | 2.0 | 500 | 400 | 350 |
3 | 3a | Medium dense to very dense sand/silt with frequent sandstone bands | −7.30 | 6.20 | – | 50 | 40 | 250 |
3b | Very weak to weak calcareous sandstone | −13.50 | 7.50 | 1.0 | 250 | 200 | 250 | |
3c | Very weak to weak calcareous sandstone | −21.00 | 3.00 | 1.0 | 140 | 110 | 250 | |
4 | 4 | Very weak to weak gypsiferous sandstone/calcareous sandstone | −24.00 | 4.50 | 2.0 | 140 | 110 | 250 |
5 | 5a | Very weak to moderately weak calcisiltite/conglomeritic calcisiltite | −28.50 | 21.50 | 1.3 | 310 | 250 | 285 |
5b | Very weak to moderately weak calcisiltite/conglomeritic calcisiltite | −50.00 | 18.50 | 1.7 | 405 | 325 | 325 | |
6 | Very weak to weak calcareous/conglomerate strata | −68.50 | 22.50 | 2.5 | 560 | 450 | 400 | |
7 | Weak to moderately weak claystone/siltstone interbedded with gypsum layers | −91.00 | >46.79 | 1.7 | 405 | 325 | 325 |
Analysis method | Loadcase | Settlement (mm) | |
---|---|---|---|
Rigid | Flexible | ||
FEA | Tower only (DL + LL) | 56 | 66 |
REPUTE | Tower only (DL + LL) | 45 | – |
PIGLET | Tower only (DL + LL) | 62 | – |
VDISP | Tower only (DL + LL) | 46 | 72 |
-
The FE, REPUTE and PIGLET models take account of the pile-soil-pile interaction, whereas SOM modelled the soil as springs connected to the raft and piles using an S-Frame analysis.
-
The HCL FE analysis modelled the soil/rock using non-linear responses compared to the linear spring stiffnesses assumed in the SOM analysis.
-
The specified/assumed superstructure stiffening effects on the foundation response were modelled more accurately in the SOM analysis.
Overall stability assessment
Liquefaction assessment
Independent verification analyses
Stratum number | Description | RL range DMD | Undrained modulus E
u (MPa) | Drained modulus E′ (MPa) | Ultimate skin friction (kPa) | Ultimate end bearing (MPa) |
---|---|---|---|---|---|---|
1a | Med. dense silty sand | +2.5 to +1.0 | 30 | 25 | – | – |
1b | Loose-v. loose silty sand | +1.0 to −1.2 | 12.5 | 10 | – | – |
2 | Weak-mod. weak calcarenite | −1.2 to −7.3 | 400 | 325 | 400 | 4.0 |
3 | V. weak calc. sandstone | −7.3 to −24 | 190 | 150 | 300 | 3.0 |
4 | V. weak–weak sandstone/calc. sandstone | −24 to −28.5 | 220 | 175 | 360 | 3.6 |
5a | V. weak–weak–mod. weak calcisiltite/conglomerate | −28.5 to −50 | 250 | 200 | 250 | 2.5 |
5b | V. weak–weak–mod. weak calcisiltite/conglomerate | −50 to −70 | 275 | 225 | 275 | 2.75 |
6 | Calcareous siltstone | −70 and below | 500 | 400 | 375 | 3.75 |
-
The commercially available computer program FLAC was used to carry out an axisymmetric analysis of the foundation system for the tower. The foundation plan was represented by a circle of equal area, and the piles were represented by a solid block containing piles and soil. The axial stiffness of the block was taken to be the same as that of the piles and the soil between them. The total dead plus live loading was assumed to be uniformly distributed. The soil layers were assumed to be Mohr–Coulomb materials, with the modulus values as shown in Table 14, and values of cohesion taken as 0.5 times the estimated unconfined compressive strength. The main purpose of this analysis was to calibrate and check the second, and more detailed, analysis, using the computer program for pile group analysis, PIGS [59].
-
An analysis using PIGS was carried out for the tower alone, to check the settlement with that obtained by FLAC. In this analysis, the piles were modelled individually, and it was assumed that each pile was subjected to its nominal working load of 30 MN. The stiffness of each pile was computed via the program DEFPIG [54], allowing for contact between the raft section above the pile and the underlying soil. The pile stiffness values were assumed to vary hyperbolically with increasing load level, using a hyperbolic factor (R f) of 0.4.
-
Finally, an analysis of the complete tower-podium foundation system was carried out using the program PIGS, and considering all 926 piles in the system. Each of the piles was subjected to its nominal working load.
FLAC and PIGS results for the tower alone
-
FLAC analysis, using an equivalent block to represent the piles: 72.9 mm.
-
PIGS analysis, modelling all 196 piles: 74.3 mm.
PIGS results for tower and podium
Cyclic loading effects
-
Cyclic triaxial laboratory tests;
-
Cyclic direct shear tests;
-
Cyclic constant normal stiffness (CNS) laboratory tests;
-
Via an independent theoretical analysis carried out by the independent verifier.
Pile load testing
-
Static load tests on seven trial piles prior to foundation construction.
-
Static load tests on eight works piles, carried out during the foundation construction phase (i.e., on about 1 % of the total number of piles constructed).
Preliminary pile testing program
Pile no. | Pile diameter (m) | Pile length (m) | Side grouted? | Test type |
---|---|---|---|---|
TP1 | 1.5 | 45.15 | No | Compression |
TP2 | 1.5 | 55.15 | No | Compression |
TP3 | 1.5 | 35.15 | Yes | Compression |
TP4 | 0.9 | 47.10 | No | Compression (cyclic) |
TP5 | 0.9 | 47.05 | Yes | Compression |
TP6 | 0.9 | 36.51 | No | Tension |
TP7A | 0.9 | 37.51 | No | Lateral |
-
The effects of increasing the pile shaft length are as follows;
-
The effects of shaft grouting,
-
the effects of reducing the shaft diameter,
-
the effects of uplift (tension) loading,
-
the effects of lateral loading and
-
the effect of cyclic loading.
Ultimate axial load capacity
Ultimate shaft friction
Ultimate end bearing capacity
Load-settlement behaviour
Pile number | Working load (MN) | Max. load (MN) | Settlement at W. load (mm) | Settlement at max. load (mm) | Stiffness at W. load (MN/m) | Stiffness at max. load (MN/m) |
---|---|---|---|---|---|---|
TP1 | 30.13 | 60.26 | 7.89 | 21.26 | 3819 | 2834 |
TP2 | 30.13 | 60.26 | 5.55 | 16.85 | 5429 | 3576 |
TP3 | 30.13 | 60.26 | 5.78 | 20.24 | 5213 | 2977 |
TP4 | 10.1 | 35.07 | 4.47 | 26.62 | 2260 | 1317 |
TP5 | 10.1 | 40.16 | 3.64 | 27.45 | 2775 | 1463 |
TP6 | −1.0 | −3.5 | −0.65 | −4.88 | 1536 | 717 |
-
The measured stiffness values were relatively large and were considerably in excess of those anticipated;
-
As expected, the stiffness was greater for the larger diameter piles;
-
The stiffness of the shaft grouted piles (TP3 and TP5) was greater than that of the corresponding ungrouted piles.
Effect of reaction piles
Pile axial stiffness predictions
Uplift versus compression loading
Cyclic loading effects
Pile number | Mean load/P
w
| Cyclic load/P
w
| No. of cycles (N) |
S
N
/S
1
|
---|---|---|---|---|
TP1 | 1.0 | ±0.5 | 6 | 1.12 |
TP2 | 1.0 | ±0.5 | 6 | 1.25 |
TP3 | 1.0 | ±0.5 | 6 | 1.25 |
TP4 | 1.25 | ±0.25 | 9 | 1.25 |
TP5 | 1.25 | ±0.25 | 6 | 1.3 |
TP6 | 1.0 | ±0.5 | 6 | 1.1 |
Lateral loading
Works pile testing program
-
The pile head stiffness of the works piles was generally larger than for the trial piles.
-
None of the work piles reached failure, and indeed, the load-settlement behaviour up to 1.5 times the working load was essentially linear, as evident from the relatively small difference in stiffness between the stiffness values at the working load and 1.5 times the working load. In contrast, the relative difference between the two stiffnesses was considerably greater for the preliminary trial piles.
Summary of pile testing outcomes
Settlement performance during construction
Summary
Case 3—Incheon 151 tower, South Korea
Introduction
Ground conditions and geotechnical model
Strata |
E
v (MPa) |
E
h (MPa) |
f
s (kPa) |
f
b (MPa) |
---|---|---|---|---|
UMD | 7–15 | 5–11 | 29–48 | – |
LMD | 30 | 21 | 50 | – |
Weathered soil | 60 | 42 | 75 | – |
Weathered rock | 200 | 140 | 500 | – |
Soft rock (above EL-50 m) | 300 | 210 | 750 | 12 |
Soft rock (below EL-50 m) | 1700 | 1190 | 750 | 12 |
Foundation layout
Loadings
Assessment of pile capacities
-
Minimum socket length in soft rock = 2 diameters;
-
Minimum toe level = EL-50 m.
Material | Ultimate friction f
s (kPa) | Ultimate end bearing f
b (MPa) |
---|---|---|
Weathered rock | 500 | 5 |
Soft rock | 750 | 12 |
Assessment of vertical pile behaviour
Predicted settlements
Design stage | Method | Predicted settlement (mm) | Remarks |
---|---|---|---|
1 (Preliminary) | Equivalent pier | 75 | Average settlement |
2 (Detailed) | Program GARP | 67 | Maximum, taking account of all eight ground profiles |
3 (Final) | Program PLAXIS3D | 56 | Maximum, adopting a single representative ground profile |
Assessment of lateral pile behaviour
-
3D finite-element computer program PLAXIS 3D Foundation;
-
Computer program DEFPIG developed by Sydney University in conjunction with Coffey;
-
Coffey’s in-house computer program CLAP.
-
3D finite Element Structural Analysis Programs (Midas Set, Etabs, Safe) that included the effect of soil structure interaction.
Horizontal load (MN) | Pile group disp. (mm) | Lateral pile stiffness (MN/m) | Lateral raft stiffness (MN/m) | Total lateral stiffness (MN/m) |
---|---|---|---|---|
149 (x direction) | 17 | 8760 | 198 | 8958 |
115 (y direction) | 14 | 8210 | 225 | 8435 |
Assessment of pile group rotational stiffness
Pile | Pile head angular rotation (rad) | Pile head rotational spring stiffness (MN m/rad) |
---|---|---|
3 | ||
Maximum | 0.094 | 2680 |
Minimum | 0.036 | 1380 |
27 | ||
Maximum | 0.144 | 1750 |
Minimum | 0.056 | 903 |
70 | ||
Maximum | 0.126 | 2000 |
Minimum | 0.049 | 1030 |
78 | ||
Maximum | 0.187 | 1350 |
Minimum | 0.073 | 700 |
Cyclic loading due to wind action
-
CASE A: 0.75(DL + LL).
-
CASE B: 0.75(DL + LL + WL x + WL y ),
Quantity | Value |
---|---|
Maximum half amplitude cyclic axial wind load S
c
* (MN) | 29.2 |
Maximum ratio η = S
c
*
/R
gs
*
| 0.43 |
Cyclic loading criterion satisfied? | Yes |
Pile load tests
Strata | Design value | Pile test | |||
---|---|---|---|---|---|
TP1 | TP2 | TP4 | Aver | ||
Soft rock | |||||
End bearing (MPa) | 4.0 | 6.3 | 9.0 | 9.2 | 8.1 |
Friction (kPa) | 350 | 743 | 897 | 663 | 767 |
Weathered rock | |||||
Friction (kPa) | 250 | 357 | 527 | 178 | 354 |
Design stiffness (MN/m) | Measured secant stiffness of test pile (MN/m) | |||
---|---|---|---|---|
Static | Dynamic | |||
0–900 kN | 900–1350 kN | 0–900 kN | 900–1350 kN | |
86–120 | 294 | 97 | 488 | 326 |
Summary
Case 4—tower on karstic limestone, Saudi Arabia
Introduction
Geological and geotechnical conditions
Geotechnical model
Depth at bottom of geo-unit (m) | Description of Geo-unit |
E
v (MPa) |
f
s (MPa) |
f
b (MPa) |
---|---|---|---|---|
20 | Coralline limestone (1) | 450 | 0.2 | 2 |
50 | Coralline limestone (2) | 600 | 0.2 | 9.8 |
70 | Coralline limestone (3) | 1200 | 0.35 | 9.8 |
100 | Coralline limestone (4) | 3000 | 0.4 | 9.8 |
Long-term Young’s modulus
Ultimate pile shaft friction and end bearing
Tower foundation details
Foundation analyses for design
Study of effects of cavities on foundation performance
Depth of cavity (m) | Max. raft displacement (mm) |
---|---|
0 | 55.7 |
20 | 55.5 |
40 | 56.7 |
50 | 58.0 |
60 | 58.4 |
70 | 55.9 |
80 | 55.8 |
90 | 55.7 |
100 | 55.7 |
Random cavities beneath the piled raft
Case | Cavity location (centre) | Depth below raft | Diameter of cavity (m) | Raft displacement (mm) | ||
---|---|---|---|---|---|---|
X (m) |
Y (m) | Top of cavity, Z
1 (m) | Bottom of cavity, Z
2 (m) | |||
1 | 1.875 | 0 | 40 | 43 | 3 | 72 |
−1.875 | −1.875 | 50 | 53 | 4 | ||
0 | 7.5 | 50 | 51.5 | 2 | ||
−9.25 | 0 | 43 | 45 | 2 | ||
−7.5 | −15 | 61.5 | 63 | 1.25 | ||
2 | 11 | 13 | 34 | 35 | 2 | 74 |
10 | 20 | 44 | 45 | 2 | ||
−2 | 4 | 49 | 51 | 4 | ||
−10 | −9 | 53 | 55 | 4 | ||
3 | 16 | 28 | 31 | 3 | ||
3 | −13 | 10 | 48 | 51 | 4 | 68 |
−7 | 2 | 23 | 25 | 3 | ||
13 | −10 | 41 | 44 | 3 | ||
16 | 11 | 69 | 71 | 1 | ||
16 | −2 | 44 | 47 | 2 | ||
4 | 2 | −7 | 59 | 62 | 2 | 65 |
15 | 7 | 39 | 41 | 4 | ||
−19 | −7 | 50 | 52 | 4 | ||
−6 | −12 | 66 | 68 | 2 | ||
0 | 4 | 38 | 39 | 1 |
Pile loads for random cavities
Moments in raft for random cavities
Problem | Maximum moment (k N-m/m run) | Minimum moment (kN-m/m run) |
---|---|---|
No cavities | 1140 | −23,120 |
Case 2 cavities | 1080 | −26,190 |