Piled raft foundation for the W-TOWER Tel Aviv

Transcription

1 Piled raft foundation for the W-TOWER Tel Aviv Prepared by A. Lehrer, S. Bar. 1. Introduction. Development of the world's largest cities dictated the need for high building housing in different soil conditions, as complicated as they are. In the 80's of the last century, due to the increasing load on the foundation piles and the need for planning groups of piles with a common head, began to develop a method of analysis of the combined action of the heads of piles or raft and piles. The combined action of piles and raft (Figure 1) allows the transfer of larger loads than on any individual system and reducing the settlement. Figure 1. Ongoing activities of the piles and the raft (Katzenbach, Arslan, Moormann, 1999). 1

2 Analysis of the interaction between piles and raft is a complex process: -Loading of the foundations causes a settlement. -During the settlement the loads transfer to the soil below the raft. Piles below the raft are designed to: -Reduce settlements. -Increase the bearing capacity of foundation system. Analysis of the abovementioned cases is slightly different. Following article focuses only on the analysis of the system of piles that reduce settlement. Piles raft system behavior characterized by β coefficient, which describes the distribution of load between piles and raft. Coefficient β is defined as follows: β = n i= 1 For the same soil conditions and the area of the raft, the coefficient β is a function of number of piles and their dimensions. R pile, i S tot Figure 2. Settlement of the foundations as a function of the coefficient β (Katzenbach, Arslan, Moormann, 1999). 2

3 Since the piles under the raft intended only to reduce the settlement and are not required to increase bearing capacity, it is clear that the piles can be used up to the loads approach their bearing capacity. 2. Methods of analysis for the raft with piles designed to settlement reduction Method for calculating the settlements of a raft on piles Equivalent Pile (Poulos and Davis, 1980). A raft with piles can be represented by an equivalent pile, diameter d e. For a raft with an area A r, de can be set as follows: d e = 1.13 A r Settlement of equivalent pile, S, can be calculated by the formula for calculating the single pile (Randolph and Wroth, 1974): S = τ (d G 2) 0 e 2r Ln( d m e ) τ0 - shear stress acting along the pile G - shear module of the soil r m - pile effect radius Equivalent raft (Tomlinson, 1986). The method is described in Figure 3. Settlement, S, of the equivalent raft is calculated as follows: mi m0 q B S = E s 3

4 m i, m 0 - coefficients dependent on the foundation geometry, foundation depth and thickness of compressive soil layer. B - width of the raft. Figure 3. Description of the method of equivalent raft An approximate method to evaluate the distribution of load between the raft and piles. Load distribution coefficient between the raft and piles, β, is a function of the number of piles, n, and the stiffness of the raft and piles. 1 β = = 0.2 K r 1+ K K r p K p p total K p = n Kps (1 R i fp β pu total ) K r = Kr (1 R i fr total (1-β) ) ru p - load transferred to piles total - total load 4

5 K p - piles group stiffness Kps i - initial stiffness of single pile K r - raft stiffness Kr i - initial raft stiffness pu - piles group bearing capacity ru - raft bearing capacity Accurate calculation method. Accurate analysis of the foundation system soil-pile-raft - is only possible by using advanced computer programs, such as finite element software Plaxis 3D three-dimensional. Solution takes into account the characteristics of all system elements, such as raft stiffness, raft impact load under friction along the pile, nonlinear behavior of piles and so on. Calculation process includes: -Determination of the nonlinear model soil behavior. -Imaging of single pile behavior. -Calculation of a raft with piles behavior. -Measuring the actual behavior of the raft-piles system during the construction process. The software allows the combination of piles of various sizes under the same raft and changing distance between the piles in different areas of the raft. The influence of the distance between the axis of the pile on load distribution and settlements can also be checked. 5

6 3. W-TOWER Tower in Tel Aviv Project description. W-TOWER Tower was built in the Bavli district in Tel Aviv. The tower has 48 floors and above 4 basement floors. Overall height of the tower about 156 m. Typical floor area is about 1100 square meters. Project Architect - Yashar Architects Office. Construction plans - Israel David Office. Project Manager - Waxman Govrin Office. Contractor - U. Dori. Soil profile consists of layers clayey sand, sand with fines covered with kurkar (Figure 4). Groundwater is at ±0.0 m (A.S.L). Nspt z, [m ] Figure 4. Soil profile and a standard penetration test results for W-TOWER. 6

7 3.2. Selection of foundation type. During the designing were tested several methods of foundation. Raft foundation. Overall average effort under the tower is on the order of 95 tons/m. Under the influence of forces of wind or earthquakes the raft effort will be, of course, larger. Under elasticity sunsets above efforts are on the order of 50 m. To reduce sunsets permitted values (3-4 cm) is required to increase significantly barge area. Deep foundation. Transferred loads are up to 3,500 tons per pile. Load on the piles that are in the building is about 1000 tons per pile. Assuming general effort allowed for the head foundational element is tons/m, obtained space required to transfer loads is about 165 square meters. Foundation elements depth estimated at m. Raft foundation combined with piles to reduce the settlements. According to preliminary simplified calculation, involving of cm-diameter pile allow to transfer to piles 50% -70% of total load. Settlement of all foundation system will be up to about 5 cm. This method was found and selected as the most economical solution for pile foundation system The analysis process with D3 PLAXIS software Choosing a soil model. The Hardening Soil model (Schang, 1998) was chosen for calculations. The model is based on elasticity theory, the theory of plasticity, the phenomenon of dilatation. The model considers changes in soil stiffness as a function of strain. The main idea of the model is hyperbolic relationship between the vertical strain, ε 1, and deviatory stress, q, (see Fig. 5). 7

8 The modulus value (limit), E50, depending on the initial elasticity module: E 50 = E 50 ' c cosφ σ3sinφ c cosφ p sinφ + m E50 - stiffness modulus erred to pressure p =100 KPa, c-cohesion, φ - angle of internal friction, 0.5 <m <1 - coefficient depending on soil type. For loading and unloading the following relation should be used: E ur = E ur ' c cosφ σ3sinφ c cosφ p sinφ + m Eur - stiffness modulus for reloading and unloading relating to pressure p =100 Kpa. Figure 5. Relation between the vertical strain, ε 1, and deviatoric stress, q. Another parameter that defines the behavior of matter model is the angle dilatation, ψ. For near-destruction conditions, the friction angle, φ, is a function of angle dilatation, ψ. In practice it can be assumed. ψ=φ

9 The soil model parameter for foundation system calculation now specified as follows: 50 E, [KPa] E ur, [KPa] C, [KPa] φ, [ 0 ] m ψ, [ o ] 50, , Single-pile model. A simulation of the loading the pile with diameter 100 cm and a length 20 m was done with the help of Plaxis 3D. Maximum load at the top of the pile is 1,500 tons Load, [ton] Settlement, [cm] Figure 7. Calculation result of the settlement of a single pile. The pile was modeled like a volumetric model, so you can get the load distribution along the pile. Based on the above result, it was possible to simplify the system model by using the foundation piles that are not volumetric, with known bearing capacity - embedded piles. 9

10 The raft model and determination the number of piles. For the analysis, parametric raft model was built, so only half a barge could be calculated. Pile dimensions were selected as 100 cm diameter and 20 m length. These piles were implemented with continues flying auger (CFA), so the work was fast and relatively low cost. After checking a system that includes a different number of piles (different distances between the axes), it was decided to plan a raft with 63 piles, i.e. the grid of 4.0/5.0 m β מספר כלונסאות n Figure 8. Relative amount of load passed to the piles, β, as a function of the number of piles. Figure 9. A raft with the piles. 10

11 Figure 10. The computer model. Development load on the piling during construction is interesting (Fig According to the model, for low loads level (9 floors), about 85% of the load goes to piles. Later, with the settlement development, part of the load passes to the raft, and on the final stage load goes to piles is about 68% of total load β Number of floors Figure 11. Load distribution changes with the progress of construction of the building. 11

12 4. Monitoring system behavior during construction Loads on piles. Strain gages were installed along the reinforcement of three piles. For each pile, the pair of strain gages was installed at level close to connection with the raft. It allows measuring the load delivered to the piles. The measured load versus the calculated load delivered to the piling is described on graphs below. Pile Pile reaction, [ton] Calculadet load Measured Load Pile Pile reaction, [ton] Calculadet load Measured Load 12

13 Pile Pile reaction, [ton] Calculadet load Measured Load Figure 12. The measured load versus the calculated load Settlements. The measured settlement of the tower at the end of the construction process is 22 mm. As we see (Figure 13), there is a good match between predicted and measured values. 0 Number of floors Settlement, [mm] Calculated Measured Figure 13. Calculated and predicted settlement of piled raft foundation. 13

14 5. Comparing costs for different foundation solutions Estimated cost of the raft foundation. Raft area needed to reach the allowable stress of 40 ton/ m 2 (no tensile stress in dynamic calculation), is about 2,600 m 2. Thickness of the raft is about 2.0 m. The estimated cost of the raft as follows: 2,600 [m 2 ] * 2.0 [m] * 250 [$/m 3 ] = 1,300,000 [$] 5.2. Estimated cost of deep foundation (piles). Assuming the depth of drilled elements is about 25 m and the overall stress to head foundation elements is about 600 t/m 2, the obtained cost of piles foundation is: 2,640,000 [ton] 600 [ton/m 2 ] * 25 [m] * 250 [$/m3]= 1,100,000 [$] 5.3. Estimated cost of piled raft foundation. The piles are designed only to reduce the settlement, so that their security factor is minimal. Raft cost 1100 [m2] * 2.0 [m] * 250 [$/m3]= 550,000 [$] Piles cost 63 * 15.7 [m3] * 250 [$/m3]= 247,000 [$] Total cost 797,000 [$] 14

15 The costs for different foundation solutions is summarizes in the following table. Foundation solution Raft cost Deep foundation cost Final cost Raft 1,300,000 1,300,000 Deep foundation 550,000 1,100,000 1,650,000 Raft and piles 550, , , Conclusions. Foundation system of raft with piles for settlement reduction was found as economic, especially for structures with high aspect ratio height/ floor area. For such structures, for example W-TOWER in Tel Aviv, raft foundation with an area of the raft more than the floor area or a deep foundation is requires. Economic priority of a piled raft foundation can be easily calculated. It is found that the match between the calculated and measured values for load delivered to the piles and the settlements are good. 15