CONSTRUCTION OF ROCK SOCKETED PILES IN SYDNEY SANDSTONE TO MEET PERFORMANCE REQUIREMENTS 1

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1 CONSTRUCTION OF ROCK SOCKETED PILES IN SYDNEY SANDSTONE TO MEET PERFORMANCE 1 Pawan L. Sethi 1, Ingrid Geng 2 and Patrick K. Wong 3 1 Associate, 2 Graduate Geotechnical Engineer, 3 Senior Principal, Coffey Geotechnics Pty Ltd, Sydney, Australia ABSTRACT Construction aspects are critical to ensure that the design intent / requirements are achieved through good construction methodology and quality control. This translates into ensuring the pile shaft/sidewalls are to a large extent free of crushed or smeared rock. Similarly, the base should be clear of any debris also. Presence of soft material / debris on the base of the pile will alter the load deflection characteristics. In order to meet the performance requirements through construction a strategy was formulated. The strategy included an advance programme of proof core drilling, use of appropriate drilling tools, reference to the available site investigation information and full-time geotechnical supervision. 1 INTRODUCTION Stage 1 of the Barangaroo Development accounts for approximately 4 ha of the total 22 ha of land currently occupied by the former shipping and overseas passenger terminal, located at the southern extent of the site, alongside Hickson Road, Millers Point. Barangaroo South, Stage 1 development includes the construction of three high rise commercial towers and associated structures including podiums. The foundation system consists entirely of piled foundations for the towers. All piles are required to be socketed into the underlying sandstone rock. The core and tower column piles are more heavily loaded than the podium piles. Consequently, core and tower piles require to be socketed into sandstone Class II or better Sandstone based on the Sydney rock classification system proposed by Pells et al. (1998). Some of the podium piles have relatively large uplift loads and consequently also require to be socketed into Class II or better Sandstone. 2 GEOLOGICAL SETTING AND STRATIGRAPHY The site locality is underlain by fill and Quaternary alluvium overlying Triassic age Hawkesbury Sandstone. The fill and alluvium are highly variable in nature and in thickness. The sandstone is described as medium to coarse grained quartz sandstone with very minor shale and laminite lenses. Rises and falls in sea level have resulted in incised valleys within the sandstone known as palaeochannels, that have been in-filled with alluvium and fill. There are two main joint sets within the Hawkesbury Sandstone in the Sydney CBD region, with one set trending north north east and an orthogonal set trending west north west. The joints are typically sub-vertical, relatively widely spaced and may not be continuous through the sandstone beds. There are a number of sub-vertical fault zones and joint swarms that have been identified trending roughly north east across the Sydney CBD that can result in more closely spaced jointing and may impact on excavation stability to a greater extent than the more widely spaced joints of the main joint sets. The site stratigraphy could be described as follows: heterogeneous fill of highly variable thickness, overlying alluvial and estuarine sediments, overlying residual soils and Hawkesbury sandstone bedrock. 1 This paper is included as a non refereed project paper Australian Geomechanics Vol 47 No 3 September

2 CONSTRUCTION OF ROCK SOCKETED PILES IN SYDNEY SANDSTONE TO MEET PERFORMANCE 3 GENERAL PILE CONSTRUCTION METHODOLOGY All the contract piles on the project are rotary bored pile type. Due to the high groundwater level and the variable and significant depth of fill on site a combination of techniques is being employed to temporarily support the pile bore. The type of temporary pile bore support method adopted varied for the different pile diameters. Double wall temporary segmental casing (1180 mm and 1500 mm piles) Single wall temporary casing (1000 mm diameter piles) Single wall temporary casing + bentonite (2400 mm diameter piles) The casings are being installed either all the way to the required toe level through the rock using double all segmental casing or just embedding into the rock with single walled (16 mm thick) conventional casing. In order to advance the casing through the hard soils and rock the double walled starter casings (cutting shoes) are fitted with changeable teeth. The teeth are being used to provide an overcut of between 5 mm and 10 mm i.e. to provide the required socket roughness. Figures 1 and 2 show casings with teeth. Figure 1: Casing with changeable teeth Figure 2: Cutting edges of double wall starter casing Where the casings were not advanced all the way to the toe level (1000 mm diameter), the drilling in the rock was carried out using rock tools with appropriate teeth fitted to achieve the socket roughness. An example of such a tool is shown in Figure 3. Figure 3: Progressive rock auger The largest piles (2400 mm diameter) were drilled using single walled casings for the first 6 m to 7 m depth and then the bores were supported by drilling fluid, bentonite in this case. 96 Australian Geomechanics Vol 47 No 3 September 2012

3 CONSTRUCTION OF ROCK SOCKETED PILES IN SYDNEY SANDSTONE TO MEET PERFORMANCE Following the completion of drilling the reinforcement cages were suspended and the pile bores filled with concrete with tremie pipes. Figures 5 and 6 show the reinforcement cages being installed and preparation for concreting respectively. Figure 4: Cage Placement 4 Figure 5: Installation of Tremi pipe for concreting DESIGN PHILOSOPHY AND PERFORMANCE The foundation pile design has been carried out in accordance with AS Piling Code, addressing both ultimate and serviceability limit states. The pile sockets have been designed for load sharing between shaft and the base. No contribution from the material other than rock (OTR) has been allowed in the design. The fundamental design criterion stipulated by the structural engineer was to limit the pile toe settlement to 0.3% of the pile diameter. We note that due to the varying pile lengths caused by the large variations in the rock level across the site, the majority of the long-term foundation settlement would result from compression of pile shafts, including longterm concrete shrinkage and creep effects. The structural engineers were concerned about differential settlement, and in particular lateral sway of the tall buildings under high wind loading. Therefore, although the stringent design criteria was not strictly required for satisfactory performance of the structure, it was set to limit additional differential settlement from the foundation, and also to allow for uncertainties in design predictions. Table 1, below shows the various diameters and their typical serviceability limit state loads (SLS) load range. Table 1: Pile Diameters with Corresponding Limiting Settlement Criteria Pile Diameter (mm) Typical SLS Loads (MN) 12 to to to 93 The adopted parameters for the design of the rock sockets are presented in Table 2. \ Australian Geomechanics Vol 47 No 3 September

4 CONSTRUCTION OF ROCK SOCKETED PILES IN SYDNEY SANDSTONE TO MEET PERFORMANCE Table 2: Adopted Pile Design Parameters Soil & Rock Class(1) Elastic Modulus (MPa) Ultimate Shaft Friction Ultimate End Bearing (MPa) Pressure (MPa) Fill and Alluvium 20 Ignored Not used Class V Sandstone 100 Ignored Not used Class IV Sandstone Not used Class III Sandstone 1, Class II or better Sandstone 2,000(2) (1) Rock Class based on Pells et al. (1998) (2) Initial tangent modulus at low stress level; secant modulus back-analysed from O-Cell testing were approximately 1,500 MPa and 1,200 MPa at load levels of 40 MPa and 59 MPa respectively. The project piling specification calls for: Sockets roughness between R3 and R4. Base cleanliness 90% of the base to be clear of any debris. Given the stringent toe settlement criterion in conjunction with construction requirements relating to the socket roughness and base cleanliness the need for good construction methodology and quality control was paramount. 5 FORMULATION AND EXECUTION OF STRATEGY TO ACHIEVE PERFORMANCE Construction aspects are critical to ensure that the design intent / requirements are achieved through good construction methodology and quality control. This translates into ensuring the pile shaft/sidewalls are to a large extent free of crushed or smeared rock. Walker and Pells highlighted that crushed rock or soil smeared on the sidewalls of sockets may attain a thickness of 20 mm or more and will form an infill on the grooves and undulations in the rock. The presence of sidewall smear will limit skin friction on the sidewall of the socket so that it may be less than that assumed in the design. Similarly, the base should be clear of any debris over 90% of the base area. Presence of soft material/debris on the base of the pile will alter the load deflection characteristics. Therefore, in order to meet the performance requirements, it was essential to ensure: Confirmation of founding material - Sockets are formed in the required (or better) sandstone class Minimum design sockets are achieved. Specified socket roughness was achieved. Clean pile bases with 90% of the base area to be free from debris. In order to verify the founding conditions an advance programme of proof coring was undertaken. The piles nominated with relatively high loads within the core were targeted. Not all the piles were proof cored except the large 2.4m diameter piles, which were all proof cored. The proof coring grid was developed by selecting sufficient core numbers to provide adequate coverage. The proof cores were drilled 3 to 5 times the diameter beyond the estimated design pile toe levels. A full-time geotechnical engineer was present on site to verify the founding conditions. Due to the drilling process it was not always straight forward to identify the rock classes. The distinction between Class V and IV Sandstone was comparatively easy, whereas distinction between Class III Sandstone and better classes of rock was not as simple. Therefore, information from the existing bore holes, proof cores and actual site observations were used to finalise the pile toe levels on site. In order to ensure the design side wall friction is achieved the socket walls need to be substantially free from crushed rock / smeared rock. Furthermore, appropriate drilling tools are employed to achieve the required socket roughness. Walker and Pells (1998) commented that there is no universal classification of roughness and a commonly adopted simple classification system for socket sidewalls, as given by Pells, Rowe and Turner (1980), is reproduced in Table 3 below. 98 Australian Geomechanics Vol 47 No 3 September 2012

5 CONSTRUCTION OF ROCK SOCKETED PILES IN SYDNEY SANDSTONE TO MEET PERFORMANCE Table 3: Roughness Classification Roughness Class R1 R2 R3 R4 Description Straight, Smooth sided socket, grooves or indentations less than 1 mm deep Grooves of depth 1-4 mm, width greater than 2 mm, at spacing 50 mm and 200 mm Grooves of depth 4-10 mm, width greater than 5 mm, at spacing 50 mm and 200 mm Grooves or undulations of depth > 10 mm, width >10 mm at spacing 50 mm to 200 mm. It has been reported that sockets in sandstone need to be R2 or rougher to avoid brittle failure of the interface. Peak shear strength will be mobilised at a small displacement and will be maintained. As reported in the earlier section, construction methodology based on using augers/casing with protruding teeth has been deemed to be adequate in achieving the required socket roughness between R3 and R4. Walker and Pells (1998) have reported that sidewall shear is usually worst when drilling in moist weathered shale or sandstone using flight auger. However, the majority of the sockets on this site are being constructed in sandstone Class III / II or better rock. To limit the design toe settlement to 0.3% of the pile diameter, it was imperative to ensure that the assumed stiffness in the design was achieved in practice by ensuring the pile base was substantially clean and free of debris. Walker and Pells (1998) have highlighted that debris at the base of pile alters the load deflection characteristics and could result in working load deflections exceeding those assumed in the design. Consequently, it was essential to inspect the base of every single pile. The piling specification calls for the 90% of the base area to be free from muck and rock debris. The primary method employed by the contractor to clean the pile base was a cleaning bucket. However, it was found that multiple passes (lowering and raising of cleaning bucket) were required to reach an acceptable level of base cleanliness. Each pass of cleaning bucket was followed by sounding of the pile base with a tape measure, which had a metal weight attached to it in the form of 200 mm long reo bar. This exercise was carried out numerous times along the circumference of the pile bore to detect any potential soft spots. As the contract progressed, another base cleaning tool in the form of a hydraulic plunger was introduced by the piling subcontractor to supplement the cleaning bucket for base cleaning. It was very important to minimise the delay between cleaning of the pile base and concreting of piles. This was considered critical to ensure that the suspended particles in the water in the pile bore did not settle at the pile base. However, inevitably delays do occur on sites during placing of reinforcement cages or receiving concrete deliveries. Therefore, the adopted approach on site was to clean the pile base using the conventional cleaning bucket and using the hydraulic plunger just ahead of placing reinforcement cages and concreting operation. Figures 5 and 6 show the cleaning bucket and hydraulic plunger respectively. Figure 6 Cleaning Bucket Figure 7 Hydraulic Plunger In the case of larger 2.4 m diameter piles it was not possible to sound the base of the pile to assess the presence of any debris. The properties of the bentonite stabilising fluid was sampled from just above the centre of the base. The sand content was reduced to acceptable levels through the de-sanding process. Verification of construction methodologies to achieve the specified socket roughness and base cleanliness was demonstrated by load testing. Two Osterberg Cell load tests were carried out. To verify the adequacy of construction techniques the load test piles were constructed using the same procedure as the contract piles. The toe and shaft \ Australian Geomechanics Vol 47 No 3 September

6 CONSTRUCTION OF ROCK SOCKETED PILES IN SYDNEY SANDSTONE TO MEET PERFORMANCE behaviour demonstrated by the loads test (Wong and Oliveira, 2012) confirmed that the pile construction methods being employed appear to be reasonable and appear to be achieving the design intent. 6 CONCLUSION Appropriate construction methodology and adequate supervision are essential to maximise the possibility of achieving the design intent. The drilling methods employed must be able to achieve the required / specified socket roughness assumed in the design. In the absence of measures which allow direct measurement of socket roughness, the verification must be achieved indirectly through assessing the drilling tools being used and load tests. The bases must be cleaned using the appropriately designed buckets or other tools. For the Barangaroo South Project, the design and construction intents were successfully achieved by the implementation of a program of proof coring, full-time independent geotechnical presence to validate that appropriate founding material has been reached and pile base is sufficiently cleaned. A program of prototype pile load testing was carried out (Wong and Oliveira, 2012) to validate pile capacity design requirements and to enable back-analysis of pile stiffness to be assessed. 7 REFERENCES Barangaroo Stage 1 Development (2012 )Coffey Geotechnical Report 15 October PELLS, P J N, ROWE, R K and TURNER, R M. (1980) "An Experimental Investigation into Side Shear for Socketed Piles in Sandstone", Int Conf Structural Foundations on Rock, Sydney, A.A. Balkema, pp WALKER, B.F. and PELLS, P.J.N, (1998) The Construction of Bored Piles Socketed into Shale and Sandstone, Australian Geomechanics Vol 33 No 3 Dec WONG,P.K and OLIVEIRA, D (2012) Class A Prediction versus Performance of O-Cell Pile Load Tests in Sydney Sandstone, Australian Geomechanics Vol 47 No 3 September Australian Geomechanics Vol 47 No 3 September 2012