A novel in situ test for the design of drilled foundations

DFI Journal - The Journal of the Deep Foundations Institute ISSN: 1937-5247 (Print) 1937-5255 (Online) Journal homepage: http://www.tandfonline.com/loi/ydfi20 A novel in situ test for the design of drilled foundations A. S. Bradshaw, B. Reyes, C. DeVillers & P. Sauco To cite this article: A. S. Bradshaw, B. Reyes, C. DeVillers & P. Sauco (2016) A novel in situ test for the design of drilled foundations, DFI Journal - The Journal of the Deep Foundations Institute, 10:1, 2-7, DOI: 10.1080/19375247.2016.1158383 To link to this article: http://dx.doi.org/10.1080/19375247.2016.1158383 Published online: 25 May 2016. Submit your article to this journal Article views: 5 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ydfi20 Download by: [University Of Rhode Island] Date: 27 May 2016, At: 11:41 A novel in situ test for the design of drilled foundations A. S. Bradshaw*1, B. Reyes1, C. DeVillers2 and P. Sauco3 Downloaded by [University Of Rhode Island] at 11:41 27 May 2016 This paper describes a novel in situ test concept called the Borehole Plug Test that could improve the design of drilled foundations. The test method involves casting a small-scale grout plug at the bottom of a typical borehole for site investigations and load testing the plug to obtain the load transfer behaviour and the maximum unit side shear resistance (or bond strength). The plug test results can then be used to simulate full-scale load tests for design purposes. A field trial is presented that demonstrated its feasibility and potential benefits. Additional research is needed both to study fundamental behaviour and to collect more field data to compare with full-scale static load test data in a variety of soil and rock types. Keywords: Borehole Plug Test, drilled foundation, Drilled shaft, Micropile, Ground anchor, In situ test, Grout plug, Load transfer, Bond strength, Unit side shear, Skin friction, t–z curve, Load test simulation Introduction This paper focuses on the design of drilled foundations, specifically cases where capacity is controlled by friction on the sides of the foundation (i.e. side shear). This could include the following: (i) any drilled foundation element that is subject to uplift (e.g., ground anchors), (ii) largediameter ‘floating’ drilled shafts where significant foundation movements may be required to mobilise the end bearing resistance in compression and (iii) micropiles where it is common practice to neglect the end bearing resistance because it is small in comparison to the side shear resistance (e.g., Juran et al. 1999; Sabatini et al. 2005). The design process for drilled foundations generally involves three main steps. First, a site investigation is performed to characterise the subsurface conditions and determine soil properties for design. Next, geotechnical and structural calculations are performed to establish the minimum diameter, embedment depth, and internal reinforcement that are needed for the specified design loads. The foundation is then constructed in the field and may be statically load tested to verify that the foundation has sufficient capacity and does not exceed the specified tolerable movement. Due to issues with soil disturbance, the capacity and deformation analyses performed during design commonly utilise in situ test data obtained from the Standard Penetration Test (SPT) and the Cone Penetration Test (CPT). The test data are used either to empirically estimate 1 University of Rhode Island, Kingston, RI 02881 Northern Drill Service, Inc. 130 East Main Street, Bldg. A, Northborough, MA 01532. 3 Rhode Island Department of Transportation, Two Capitol Hill, Providence, RI 02903 2 *Corresponding author, email abrads@uri.edu 2 © 2016 Deep Foundations Institute Published by Taylor & Francis on behalf of the Institute Received 15 October 2015; accepted 22 February 2016 DOI 10.1080/19375247.2016.1158383 fundamental soil properties (e.g., friction angle) or foundation capacity, or movement directly. Lutenegger and Miller (1994) proposed an approach to predict the uplift capacity of small-diameter drilled shafts using a combination of the pressuremeter test to estimate the lateral stresses and the borehole shear test to obtain the soil friction angle. A new in situ test concept is proposed herein that aims to improve drilled foundation design by simulating, to the extent possible, the load transfer behaviour between an axially loaded deep foundation element and a site-specific soil. The test involves casting a small-scale grout ‘plug’ in a typical borehole for site investigations (∼10-cm diameter), and load testing the plug to obtain the load transfer curve for side shear (i.e. t-z curve) and the maximum value of unit side shear resistance (i.e. grout-to-ground bond strength). Testing of small-scale grout plugs to estimate side shear resistance has been utilized for many years in a variety of foundation systems. For example, Crapps (1986) performed pullout tests on 14-cm diameter by 61-cm high grout plugs to determine the side shear resistance of Florida limestone for drilled shaft design. It is also common practice in the design of ground anchors and soil nails to perform pullout tests on a portion of the element to isolate the bond zone capacity (e.g., Sabatini et al. 1999; Lazarte et al. 2015). The novel aspect to the proposed approach is that the test is performed in a typical borehole, which can be implemented during the site investigation phase of a project. It therefore, does not require the mobilization of fullscale foundation drilling equipment nor elaborate and extensive load testing setups. The data that are collected using the Borehole Plug Test can be directly incorporated into the design process. For example, the measured maximum unit side shear acting at The Journal of the Deep Foundations Institute 2016 VOL 10 NO 1 Bradshaw et al. Downloaded by [University Of Rhode Island] at 11:41 27 May 2016 the interface between the grout plug and the soil/rock could be used to estimate the side shear capacity of the full-scale foundation. The load transfer curve measured in the test could also be used to develop load transfer curves to analyze the axial load-deformation response (i.e. settlement) of the full-scale foundation. Currently, there is some uncertainty in the scaleability of the Borehole Plug Test results particularly for large-diameter drilled foundations and further research is needed. In concept the Borehole Plug Test can be performed in soil or rock as long as the stability of the borehole can be maintained. The remainder of this paper describes the details of Borehole Plug Test itself including the equipment and data analysis. A micropile project in Rhode Island provided an opportunity to perform a field trial of the borehole plug test. The case study is used to demonstrate its feasibility and provide a basis for discussing the potential benefits of the test. Description of the borehole plug test A schematic of the borehole plug test concept is shown in Fig. 1. In the proposed approach, a typical borehole used for geotechnical site investigations (∼10-cm diameter) is drilled into a potential soil or rock bearing stratum. Ideally, the borehole drilling method should be similar to the A novel in situ test for the design of drilled foundations method that is used to construct the full-scale foundation so that representative interface conditions and comparable levels of soil disturbance are achieved. For example, a drilling slurry could be used if the full-scale foundation utilizes drilling slurry to obtain comparable interface conditions (i.e. formation of a filter cake). If the bearing layer is bedrock a rock-drilling bit can be used to create ‘sockets’ at the bottom of the borehole. Once the borehole is completed to the required depth a small grout plug is cast at the bottom of the hole (Fig. 1). Ideally, grout should be placed using the same methods that might be used to construct the full-scale foundation. For example, the grout could be placed under gravity head through a tremie pipe to simulate the installation of a Type A micropile. Grout could be pressurised through a casing to simulate the installation of a Type B micropile. The optimal height of the grout plug has yet to be established. However, it will be a balance between the need to increase the plug height to minimize end effects, and the desire to shorten it to minimize the size of the test equipment that is needed. A structural bar is connected to the grout plug so that it can be load tested at ground level (Fig. 1). To minimize the potential for the bar to pull out of the plug during testing, the bar could be attached to a circular steel plate that is lowered to the bottom of the borehole before grouting. The plate would also help to centralize the bar within the grout plug. After the grout cures a pull-out test is performed on the plug using a test frame similar to the schematic shown in Fig. 1. The test can be performed by applying uniform load increments, typically in tension, and measuring the resulting displacements, or by applying a constant rate of displacement and measuring the resulting loads. The loads can also be applied and maintained to investigate the creep behaviour, as might be performed during a fullscale load test. After the load test is completed, calculations are performed to obtain geotechnical engineering information such as the average unit side shear on the plug given by the following: f = P−W pBH (1) where f is the average unit side shear on the plug, P is the applied tensile load, W is the weight of the grout plug, structural bar, and base plate, B is the nominal diameter of the grout plug and H is the length of the grout plug. The movement of the plug is calculated from the following equation: w = wm − 1 Schematic illustrating the Borehole Plug Test configuration PLa AE (2) where w is the rigid body movement of plug at a measured load of P, wm is the movement of the top of the bar at a load of P, La is the apparent free length of the bar (e.g., PTI 2004), A is the cross-sectional area of the bar and E is the elastic modulus of the bar. Equations (1) and (2) can be used to construct the t–z curve of the plug that can also be used to identify the ultimate unit side shear ( fu). Since the diameter and length of the full-scale foundation is larger than the plug it is The Journal of the Deep Foundations Institute 2016 VOL 10 NO 1 3 Bradshaw et al. A novel in situ test for the design of drilled foundations Downloaded by [University Of Rhode Island] at 11:41 27 May 2016 necessary to consider the effect of scale on the t–z behaviour. Given the limited data available on the scaleability of small-scale plug tests in soil, additional research is needed. However, until such scaling effects can be quantified the preliminary approach proposed herein is to present the Borehole Plug Test results in dimensionless form. Randolph and Wroth (1978) and Randolph (2003) indicate that movements due to side shear around a pile will initially be proportional to the pile radius but will reach a limiting value that is controlled by the shear strength at the groutground interface. Consistent with this concept, O’Neill and Reese (1999) present empirical load transfer curves for drilled shafts in dimensionless form where the side shear is normalized by the ultimate side shear and the movement is normalized by the shaft diameter. In concept if the load transfer curve for the plug is presented in the same manner, the curve can be ‘scaled up’ to a full-scale foundation based on its diameter. 2 Load-movement curves for the measured and simulated fullscale micropile load tests Field trial A field trial of the Borehole Plug Test was performed at the Great Island Bridge in Narragansett, RI. At the time of the demonstration test, the Rhode Island Department of Transportation (RIDOT) was replacing an existing bridge at which micropiles were being used at the abutments. Two full-scale test piles were installed, one at each abutment, and were statically load tested to twice the design load. The plug was installed and, and testing of the plug was performed approximately 15.2 m (50 ft) from one of the test micropiles after all of the production piles were installed. This allowed a side-by-side comparison between the Borehole Plug Test and full-scale micropile load test data. Grout plug installation and testing A geotechnical borehole was drilled into the silty fine sand layer to a depth of 17.1 m (56 ft). The hole was drilled using the rotary wash method with casing and no drilling slurry. Two SPTs were performed with an automatic hammer (93% efficiency) at the same depth that the plug would be cast and yielded blow counts of 32 and 31 blows/ft, respectively. This corresponds to N60 values of 50 and 48 blows/ft, respectively. After completing the borehole a 15.9-mm (5/8-in) diameter high strength (621 MPa yield strength), fully threaded bar with a 9.5-cm (3.75-in) diameter steel end plate was placed within the borehole. The plate was used to ensure that the load was transferred to the grout plug without having to rely on the bar-to-grout bond strength, which may have been insufficient. The bars were assembled in 6.1-m (20-ft) long sections with couplings. Portland cement grout was prepared at the same water/ cement ratio that was used in the full-scale micropile test. After placing the bar, the grout was tremied into the bottom of the hole. The casing was then extracted by 0.61 m (2 ft) to allow the grout to flow to the soil at the bottom of the hole and form a bond zone. A ‘U’ fitting was then attached to the bottom of the tremie pipe, shown in Fig. 3, which was used to flush out the extra grout to an elevation just below the bottom of the casing. The grout was allowed to cure for 7 days at which point the top of the grout plug was sounded with a rod and verified a plug height of 0.53 m (1.75 ft). The water table was also measured at a depth of 3.4 m (11 ft). There were a few feet of very soft ‘muck’ that had formed on top of the grout plug that was likely soil and cement particles that had settled out of the water column. It was anticipated that this soft material had negligible influence on the plug test results. A load test frame was designed and fabricated to apply a tensile load to the grout plug of up to 89 kN (20 kips). The Soil conditions and full-scale micropile test details The soil conditions in the vicinity of the micropile test location generally consisted of 11.6 m (38 ft) of loose to medium dense sand and gravel, underlain by very dense silty fine sand. The test micropile was installed to a depth of 24.8 m (80 ft) using the rotary percussive duplex method with a bit diameter of 203 mm (8 in). No drilling slurry was utilised. The length of the bond zone was of 9.1 m (30 ft) within the very dense silty fine sand layer. The outer diameter of the casing was 194 mm (7.625 in) with a wall thickness of 13 mm (0.5 in) and the reinforcing bar had a diameter of 44 mm (1.75 in). The grout consisted of Portland Type I/II cement prepared to a water/cement ratio of 0.44 by weight. Tests performed on grout cubes indicated a 7-day compressive strength of approximately 48 MPa (7000 psi). The micropile was load tested in tension 7 days after installation and the results are plotted in Fig. 2. As shown in the figure loads were applied incrementally with a holding time of 2.5 or 5 minutes with two cycles of unloading/ reloading. To investigate the creep behavior the load was held for 10 minutes at 133% of the design load (DL) and for 60 minutes at the maximum test load of 1156 kN at 200% of the DL. Significant creep deformations were observed at the maximum test load. 4 The Journal of the Deep Foundations Institute 2016 VOL 10 NO 1 Bradshaw et al. Downloaded by [University Of Rhode Island] at 11:41 27 May 2016 3 A novel in situ test for the design of drilled foundations Photograph of the ‘U’ fitting used to flush out the extra grout to the desired elevation 5 Load-movement curve measured at the top of the bar 4 Photograph of the load test set-up used in the field trial load test set-up is shown in Fig. 4 and was based on the approach specified in ASTM D 3689 for tensile testing of piles shown conceptually in Fig. 1. As shown in Figure 4 the test frame was quickly and easily assembled by hand with no heavy equipment. The loads were applied incrementally using a dual piston hydraulic jack with a through-hole load cell placed between the bar and the jack. Each loading increment was held for a period of 3 minutes. Load was also assessed from the jack pressure. Bar displacements were measured relative to a fixed reference beam using both a mechanical dial gauge and an electronic linear variable differential transducer (LVDT). A laptop computer and data acquisition system was used to record the load cell and LVDT data. Plug test results The load-movement data for the top of the bar are plotted in Figure 5. As shown in the figure significant movements occurred during the first two load increments partly due to slack in the loading system. Beyond this point the load-movement relationship increased almost linearly. At a test load of 37.8 kN (8.5 kips) a small ground tremor was heard and the load quickly dropped. This was an indication that the plug had plastically slipped. The jack had to be continuously actuated for the load to be reapplied. The unit side shear resistance and movement of the grout plug were calculated from the load and displacement measurements using equations (1) and (2) and the results are plotted as a ‘t–z’ curve in Fig. 6a. The effect of the slack in the loading system can be seen in the early part of the t–z curve. The curves were adjusted by extending the t–z curve down to intersect the movement axis at zero unit side shear. A value of about 4.7 mm was determined at the intersection point which was used to shift the t–z curve to the origin (Fig. 6a). Also the data point corresponding to the load drop was removed. As shown in Fig. 6 the load transfer behaviour was almost linear reaching a maximum unit side shear value of 198 kPa (29 psi) at a movement of 1.8 mm (0.07 in). Since the maximum unit side shear of 198 kPa could not be sustained for the 3 minute hold time, the bond strength was taken to be 183 kPa (27 psi) based on the previous step. The adjusted t–z curve in Fig. 6a was then used to develop the normalized t–z curve shown in Fig. 6b. As shown in this figure the peak normalized resistance of unity was reached at a movement of 1.7% of the plug diameter. Based on the behavior an elasto-plastic t–z model was fit to the data. Attempts were made to pull the grout plug out of the borehole for inspection but it was not possible due to capacity limitations of the bar. The exact failure mode was uncertain but the measured bond strength was on the higher end of the range of 97–214 kPa (14–31 psi) specified by AASHTO for very dense silty sand (Sabatini et al. 2005). It is interesting to note that the results were also very consistent with the bond strength of 179 kPa (26 psi) that was assumed by the micropile design engineer. The Journal of the Deep Foundations Institute 2016 VOL 10 NO 1 5 Bradshaw et al. A novel in situ test for the design of drilled foundations Downloaded by [University Of Rhode Island] at 11:41 27 May 2016 time of 3 min. The simulated load–movement curve was in very good agreement at the lower load levels (<400 kN). However, at the higher load levels the simulated curve showed a stiffer response with up to 7 mm less movement. Some of the differences could be attributed to the assumption of a linear elastic axial pile stiffness in the model. The axial response of concrete and steel composite piles is non-linear with a secant modulus that degrades with strain (Fellenius 2015). This would have resulted in a softer pile response at the higher load levels. Differences could also be explained by inaccuracies in the t–z curves that were developed from the Borehole Plug Test data. The load transfer behavior of the grout plug may not be representative of the micropile behavior due to differences in geometry, scale, and level of soil disturbance. The simulated micropile test had an ultimate resistance of 1310 kN. The actual micropile showed an apparent plunging at the maximum test load of 1156 kN but this was attributed to creep deformations from the 60 min holding time. The significant creep could suggest, however, that the pile is approaching an ultimate resistance possibly comparable to the simulated test pile that was derived from the bond strength of the grout plug. Discussion of potential benefits The case described above is used as a basis for discussing potential benefits of the Borehole Plug Test. First, the test provides a means to make site-specific and thus more reliable measurements of the bond strength early in the design phase of a project. Current practice relies, to a great extent, on judgment and is likely conservative. A direct measurement of bond strength using the results from the Borehole Plug Test could reduce the potential for over-conservative designs. The downside is the higher initial cost of performing a Borehole Plug Test. Based on the field trial the cost to perform one plug test would be about $4500. However, the cost can be reduced to about $2500 if the test is incorporated into the site investigation programme. The second potential advantage could be the reduction or elimination of the number of static load tests that are both costly and time consuming. For example, the bid cost for performing two pile load tests on the Great Island project was $66 000, which took 1 week to complete. The Borehole Plug Test is unique in that it provides information about the load transfer behaviour in the site-specific soils. Therefore, as demonstrated above it is possible to simulate a load test that could be used to assess pile capacity and settlement during the design process. If static load tests are not utilised, however, this will require the development of resistance factors specifically for the Borehole Plug Test. Relative to a static load test, the Borehole Plug Test would have no impact on the construction schedule and have much lower cost. 6 a Load transfer curve interpreted from the Borehole Plug Test results, and b normalized load transfer curve Load test simulation and comparison of results The normalised load transfer curve obtained in the borehole plug test (Fig. 6b) was used as a basis for performing a load test simulation of the adjacent test micropile. The simulation was performed using a self-developed ‘t–z’ analysis program that is based on the finite element method (e.g., Armaleh and Desai 1987). The elastic modulus of the grout was estimated at 28 GPa (4060 ksi) based on the reported unconfined compressive strength and guidelines presented in Sabatini et al. (2005). The modulus of the steel was 200 GPa (29 000 ksi). The axial stiffness of the pile (AE) was calculated for both the cased and bond sections of the micropile using the composite cross-sectional properties. The normalised t–z curve shown in Fig. 6b was used to construct a series of t–z curves for multiple pile segments along the bond zone. The bond strength measured in the plug test was at a depth of 55 ft. Since the bond zone was located in a granular soil layer, the β method was used to extrapolate the bond strength to other depths within the bond zone such that fu = bs′v where β is an empirical factor and s′v is the vertical effective stress. A β value of approximately 0.95 was calculated from the plug test results. It was assumed that there was no soil resistance in the unbonded zone. The simulated load–movement curve at the pile head is compared with the micropile load test results in Fig. 6. Note that the simulated data corresponds to a holding 6 The Journal of the Deep Foundations Institute 2016 VOL 10 Summary and conclusions A simple and novel in situ test concept was proposed for the design of drilled foundations. The Borehole Plug Test NO 1 Downloaded by [University Of Rhode Island] at 11:41 27 May 2016 Bradshaw et al. involves the load testing of a short concrete or grout plug within a standard geotechnical borehole used for site investigations. The plug measurements can be used to assess engineering properties such as the maximum unit side shear as well as the load transfer (t–z) behaviour. The Borehole Plug Test method was described and a field trial was performed in very dense silty fine sand at a bridge site in Rhode Island. The field trial showed that the test is feasible. A simulation of a full-scale micropile load test was also performed based on the load transfer curve measured in the Borehole Plug Test. The simulated and measured load–movement curves at the pile head were in good agreement at the lower load levels but the simulated response was stiffer at the higher load levels. The test pile was not brought to geotechnical failure so an assessment of the bond strength was not possible. The two primary benefits to the test are that it can provide more reliable design information earlier during the design process, and could possibly reduce or eliminate the number of static load tests on a project, which are costly. The results are encouraging but there is a need for further research to address uncertainties related to plug geometry and scale effects, soil and rock type as well as soil disturbance during installation. Additional research should include fundamental studies as well as additional plug testing at sites where static load test data may already be available. Acknowledgements The authors would also like to thank Antonio Marinucci for his input and thoughtful review comments that greatly improved the quality of the paper. The authors gratefully acknowledge funding provided by the Rhode Island Department of Transportation and the University of A novel in situ test for the design of drilled foundations Rhode Island Transportation Center (Project 0000212). U.S. patent pending no. 62/233,213. No. References Armaleh, S. and Desai, C. S. 1987. Load deformation response of axially loaded piles, Journal of Geotechnical Engineering, 113(12), 1483– 500. Crapps, D. K. 1986. Design, construction and inspection of drilled shafts in limerock and limestone. Proceedings of the 35th Annual Geotechnical Conference, ASCE/AEG, University of Kansas, Lawrence, 38 pp. Fellenius, B. H. 2015. Basics of foundation design. Electronic Edition, April 2015. Accessed from www.fellenius.net Juran, I., Bruce, D. A., Dimillio, A. F. and Benslimane, A. 1999. Micropiles: the state of practice. part II: design of single micropiles and groups and networks of micropiles, Proceedings of the ICE – Ground Improvement, 3(3), 89–110. Lazarte, C. A., Robinson, H., Gomez, J. E., Baxter, A., Cadden, A. and Berg, R. 2015. Soil nail walls – reference manual. Publication No. FHWA-NHI-14-007, Washington, DC: Federal Highway Administration, 425 pp. Lutenegger, A. J. and Miller, G. A. 1994. Uplift capacity of small-diameter drilled shafts from in situ tests, Journal of Geotechnical Engineering, ASCE, 120(8), 1362–80. ONeill, M. W. and Reese, L. C. 1999. Drilled shafts: construction procedures and design methods. Technical Report FHWA-IF-99-025, Virginia: Federal Highway Administration. PTI. 2004. Recommendations for prestressed rock and soil anchors. 4th edn. Farmington Hills, Michigan: Post Tensioning Institute. Randolph, M. F. 2003. RATZ load transfer analysis of axially loaded piles. Randolph, M. F. and Wroth, P. C. 1978. Analysis of deformation of vertically loaded piles, Journal of the Geotechnical Engineering Division, 104(GT12), 1465–87. Sabatini, P. J., Pass, D. G. and Bachus, R. C. 1999. Geotechnical engineering circular No. 4: ground anchors and anchored systems. Publication No. FHWA-IF-99-015, Washington, DC: Federal Highway Administration, 304 pp. Sabatini, P. J., Tanyu, B., Armour, T., Gronek, P. and Keeley, J. 2005. Micropile design and construction. FHWA-NHI-05-039. The Journal of the Deep Foundations Institute 2016 VOL 10 NO 1 7