DEPTH EFFECTS AND VIBRATIONS ASSOCIATED WITH DYNAMIC COMPACTION CASE STUDY

Transcription

1 DEPTH EFFECTS AND VIBRATIONS ASSOCIATED WITH DYNAMIC COMPACTION CASE STUDY Derek Avalle 1 and Jaime Tabucanon 2 1 Keller Ground Engineering, PO Box 7974, Baulkham Hills NSW 1755, Australia. davalle@kellerge.com.au 2 Advanced Geomechanics, Monash Avenue, Nedlands WA 6009, Australia. JaimeT@ag.com.au Ground improvement using dynamic compaction (DC) was undertaken for the foundation of a new water storage tank in Carabooda, north of Perth, WA. The tank site was located on sloping ground underlain by sand over limestone. The condition of the sand deposits varied from very loose to medium dense. The depth to limestone varied significantly and the subcrop was characterized by pinnacles, between which the sand was found to be very loose. DC involved the delivery of high compaction energy to densify the site soils. The configuration designed for the application of the DC included primary and secondary phases of compaction. In order to measure the performance of the DC works testing was carried out both pre-treatment and post-treatment, using electrical friction-cone penetrometer (CPT), flat dilatometer (DMT) and plate loading. Vibration monitoring was carried out throughout the DC works. This paper presents the results of the performance testing program, and its assessment in relation to the ground improvement specification for the tank site. Idiosyncrasies in relation to the limestone pinnacles are discussed. Vibration data are presented to augment the current body of information in this regard. Keywords: Dynamic compaction, Sand, Depth effects, Vibrations, Tank foundation. 1. INTRODUCTION A new 60 ML water storage tank has recently been constructed on a site in Carabooda, north of Perth, Western Australia. One of the challenges associated with the tank foundation design was related to the presence beneath the site of a variable thickness of very loose to medium dense sand above a limestone subcrop characterised by pinnacles. Ground improvement using dynamic compaction (DC) was designed and undertaken on the site to improve the density and stiffness of the sandy soils above the limestone and meet the tank foundation design specifications. During the dynamic compaction works, ground vibration monitoring was undertaken in relation to nearby residential areas and the results proved to be within acceptable limits. Proceedings of the International Conference on Ground Improvement and Ground Control Edited by Buddhima Indraratna, Cholachat Rujikiatkamjorn and Jayan S. Vinod Copyright 2012 by Research Publishing Services. All rights reserved. ISBN: :: doi: /

2 1064 Proceedings of the International Conference on Ground Improvement and Ground Control North Nearest residences Proposed water storage tank Scale (approx.) 100m Figure 1. Carabooda Water Reservoir Site. 2. THE TANK The water storage tank at Carabooda (approximately 45 km north of Perth city centre) is an 88 m diameter steel structure, 11 m high, constructed on a reinforced concrete base with an outer concrete ring beam. It is located on sloping ground, with a fall of approximately 5.5 m across the tank pad. There are residences relatively close to the western side of the development site, as can be seen in Figure 1. Prior to construction, the site was an open area with a sandy surface. 3. GEOTECHNICAL CONDITIONS Several geotechnical reports had been prepared for the site, including GHD (2006 and 2008) and Golder Associates (2011). The geology of the area consists of a variable thickness of Tamala Sand, overlying Tamala Limestone. Outcrops and pinnacles of limestone were reported near the top of Carabooda Hill, to the east of the tank site. The following generalised stratigraphy was inferred for the tank site: Sand: fine to coarse grained, overall thickness up to 8 m. Generally loose to medium dense to a depth of approximately 5 m below ground level, gradually becoming medium dense below this depth. However, the sand in between the limestone pinnacles, particularly the 1 m thick zone just above the limestone, was generally in a loose to very loose condition. Limestone: encountered as calcarenite, siliceous calcarenite and calcirudite. Generally slightly to extremely leached and variably cemented, with pinnacles up to 6 m in height. Groundwater was reported at depths exceeding 20 m below ground level.

3 Depth Effects and Vibrations Associated with Dynamic Compaction Case Study FOUNDATION REQUIREMENTS The specification for ground improvement works (CB&I, 2011; Water Corporation, 2011) stipulated that the soil beneath the tank site shall meet the following criteria after successful completion of the ground improvement work: An allowable bearing capacity of at least 110 kpa; and Settlement limits: maximum allowable edge settlement of 25 mm after piping is connected; total immediate settlement up to 50 mm during hydro-test; no more that 10 mm overall out-of-plane settlement of the tank shell over 10 m arc; no more than 180 mm total dishing settlement; and no more than 500 mm tilting settlement across the tank diameter. Without any ground improvement, the predicted settlements of the proposed tank (Golder Associates, 2011) exceeded the specified settlement criteria. It was recommended that ground improvement of the sand by dynamic compaction (DC) would be required in order to satisfy the tank settlement and bearing capacity criteria. 5. GROUND IMPROVEMENT DESIGN AND IMPLEMENTATION To comply with the foundation design criteria, the design of the ground improvement by DC (Keller Ground Engineering 2011a) was based on a target minimum Young s Modulus (E) of 50 MPa for the sand. It was considered in the DC design that a minimum CPT tip resistance q c of 10 MPa was required to achieve the minimum E (i.e. E = 5q c, Mitchell & Kay, 1985; Lunne et al., 1997). The DC design resulted in the following requirements: The use of 2.4 m diameter, 25t, octagonal pounder, performed in two phases: A primary pounding phase carried out on a 5 m square grid pattern, with pounder drop heights in the range of 15 m to 25 m; and A secondary pounding or ironing phase, to compact the upper zone of the soil mass previously loosened by the primary pounding, carried out in a contiguous or overlapping grid with drop heights in the range of 5 m to 15 m. A testing regime carried out to verify the improvement of the subsurface conditions using cone penetration tests (CPT), dilatometer tests (DMT) and plate load tests. The sloping site was benched to form four level platforms in order to facilitate the execution of the DC works using a 120t crane. The DC works commenced with compaction trials on Bench 1 to establish the optimum drop height and number of blows required to achieve the target requirements. A CPT rig was on site to facilitate measurement and optimisation of the DC process during the trial phase, as well as during the production DC works across the site.

4 1066 Proceedings of the International Conference on Ground Improvement and Ground Control Figure 2. DC works in progress. Figure 3. DC. Craters resulting from the first phase The DC works were undertaken during March and April, Figure 2 shows the DC works in progress and Figure 3 shows the resulting surface craters that were formed by the primary pounding phase. 6. PERFORMANCE TESTING AND RESULTS In total, 19 pre-compaction and 21 post-compaction CPTs, one pre-compaction and four post-compaction DMTs, and two post-compaction plate load tests were undertaken (Keller Ground Engineering, 2011b). The pre-compaction CPTs generally reflected q c values of between 5 MPa and 10 MPa in the top 2 m of the subsurface profile, decreasing to 5 MPa or less with depth, particularly between the limestone pinnacles, which were consistent with the previous test results (GHD 2006 and 2008, and Golder Associates, 2011). The compaction trials to establish the optimum number of pounder blows resulted in the adoption of 10 pounder blows for the upper Benches 1 and 2, and 12 pounder blows for the lower Benches 3 and 4 at each grid point. In the shallow zones to depths up to 1.1 m, the observed post-compaction CPT q c values were less than the target q c of 10 MPa. These lower q c values were assessed to be related to the lack of confining pressures at these shallow depths and disturbance of the soils due to the DC works itself. Post-treatment surface compaction was later undertaken using conventional vibratory rolling techniques after completion of all DC works. At deeper locations within the sand zones, the post-compaction q c values were generally greater than 10 MPa. The most significant exception to this was in the zone 0.5 m to 1.7 m immediately above the limestone rock and in between limestone pinnacles, where q c values were less than 10 MPa (at six of the 21 test locations). The difficulty in effectively applying DC to compact these deep sand zones, even with an increased number of pounder blows, was believed to be related to energy absorption by the limestone pinnacles, thus dissipating the compaction energy away from the loose sand. Overall, the average q c over the full depth of sand was greater than 10 MPa, even at locations where the loose sand zones were encountered at depth. Based on the DMT and CPT results, the correlation between Young s Modulus (E) and CPT q c pre-compaction were E = 6q c from0.5mto3mdepth,ande = 4q c to 5q c from

5 Depth Effects and Vibrations Associated with Dynamic Compaction Case Study 1067 Figure 4. Typical Vibration Test Results. 4 m to 8 m depth; and E = 8q c to 9q c on each bench post-compaction, except for E = 6.5q c from3mto6mdepthonbench2.thepost-compaction correlations were greater than the target value of E = 5q c recommended for the treated ground. This resulted in an inferred average E over the full depth of the sand of more than 50 MPa. The post-compaction plate load tests were undertaken using a 1.2 m diameter steel plate. The back analysed values of Young s Modulus of 6.4 MPa and 14.7 MPa for the initial loading stage, and 18 MPa and 22.5 MPa for the unload-reload cyclic loading stage were lower than the 50 MPa adopted for the settlement analysis (Golder Associates, 2011). However, the results of the plate load tests only represented the average deformation response of the sand zones within a depth of about 2.5 plate diameter or nominally 3 m depth, where they were influenced by low confining pressure and disturbance of the ground due to the DC, and they did not account for the subsequent surface compaction. For these reasons, the plate load test results were not utilized in the assessment of soil modulus from CPT q c data. Ground vibrations were monitored at locations close to the nearest residences at the boundary of the site, approximately 100 m away from the DC works. A maximum peak particle velocity (PPV) of approximately 4.5 mm/s was measured (see Figure 4), which was well below the nominated site limit of 20 mm/s. 7. CONCLUSIONS It was concluded that the DC works provided adequate improvement of the foundation soils beneath the storage tank site to meet the requirements stipulated in the ground improvement specification for the project. The depth effect of DC was verified to be generally achieving improvement to depths between 6 m and 8 m. Some loose sands at depth, between limestone pinnacles, remained in a loose condition after the DC treatment, which led to the conclusion that the delivered compaction energy at this depth was dissipated by absorption through the stronger limestone pinnacles.

6 1068 Proceedings of the International Conference on Ground Improvement and Ground Control The DC treatment resulted in an average q c over the full depth of the sand of greater than 10 MPa. Post-compaction correlation factors ranging from E = 8q c to E = 9q c were derived from extensive field testing. These factors were higher than the commonly adopted factor for sandy soils of E = 5q c. These higher factors resulted in an average E value over the full depth of the sand beneath the tank site of greater than the required minimum of 50 MPa. Ground vibrations were monitored during the DC works for the tank site. The results from this site demonstrated that ground vibrations from dynamic compaction in sandy soils could dissipate significantly with distance; in this case a distance of 100 m away from the DC works. ACKNOWLEDGMENTS The authors acknowledge permission from the Water Corporation, owners of the site, and CBI Constructors Pty Ltd, Principal Contractor, to utilise and present the information contained in this paper. REFERENCES 1. Advanced Geomechanics (2011). Carabooda Water Storage Tanks Site Verification of Ground Improvement by Dynamic Compaction. Ref: AGTN-1616 Rev 1 (unpublished). 2. Arslan, H., Baykal, G. and Ertas, O. (2007). Influence of tamper weight shape on dynamic compaction. Ground Improvement. 11, No. 2, CB&I, Inc. (2011). Specification for Ground Improvement. Document No: CV-SP (unpublished). 4. GHD (2006). Carabooda 120ML Reservoir, Preliminary Geotechnical Assessment. Ref: 61/16846/06/56401 (unpublished). 5. GHD (2008). Carabooda Sump Site, Additional Geotechnical Investigation. Ref: 61/21068/74989 (unpublished). 6. Golder Associates (2011). Geotechnical Investigation, Carabooda Water Tank. Report Number: R-Rev0 (unpublished). 7. Keller Ground Engineering (2011a). Dynamic Compaction Design. Ref: E (unpublished). 8. Keller Ground Engineering (2011b). Dynamic Compaction Design Completion Report. Ref: E (unpublished). 9. Lukas, R. G. (1995). Dynamic Compaction. US Dept. of Transport, Federal Highway Administration. Pub. No. FHWA-SA Lunne, T., Robertson, P. K. and Powell, J. J. (1997). Cone Penetration Testing in Geotechnical Practice. Spon Press, London. 11. Mitchell, P. W. and Kay, J. N. (1985). Screw Plate and Cone Penetrometer as a Field Testing System. Proc. 11 th Int. Conf. on Soil Mechanics and Foundation Engineering, Rollins, K. M. and Kim, J.-H. (1994). U.S. Experience with Dynamic Compaction of Collapsible Soils. ASCE Geotechnical Special Publication No Terzaghi, K. and Peck, R. B. (1967). Soil Mechanics in Engineering Practice. John Wiley and Sons, New York. 14. Water Corporation (2009). Specification for the Design and Construction of a 60ML Water Storage Tank and Ancillary Works at Carabooda. Ref: (unpublished).