ejge paper 2004 -0434

 

 

Lateral Load Tests on Small Diameter Drilled Piers

 

Sanjeev Kumar

Department of Civil and Environmental Engineering,
Southern Illinois University — Carbondale, Carbondale, Illinois 62901

Denton Kort

Regional Manager, Loadtest, Inc., 2631-D NW 41st St., Gainesville, FL 32606

Alyass Hosin

Graduate Student, Department of Civil and Environmental Engineering,
Southern Illinois University— Carbondale, Carbondale, Illinois 62901

and

Say Chong Ng

Former Graduate Student, Department of Civil and Environmental Engineering,
Southern Illinois University — Carbondale, Carbondale, Illinois 62901

 

ABSTRACT

The lateral load tests on drilled piers described in this paper were performed as a part of a comprehensive drilled pier testing program initiated to determine if the coal combustion products (CCPs) from burning of Illinois pulverized coal can be used to construct drilled pier foundations without jeopardizing their structural capacity. A total of four piers were tested. Two piers were constructed using conventional concrete and two piers were constructed using concrete composites having different amounts of pulverized coal combustion (PCC) fly ash and bottom ash. All the piers had nominal diameter of 13 inches and were 25 feet deep. Load was applied by using one test pier as a reaction pier for the other. The piers were installed and tested at a site located in the Carterville campus of Southern Illinois University Carbondale. Results presented show that the lateral load-deflection response of the piers constructed using PCC fly ash and bottom ash was similar to that of piers constructed using conventional concrete.

KEYWORDS: Bottom Ash, Drilled piers, Flexural Behavior, Lateral Load

INTRODUCTION

Deep foundations are being used for more than 2000 years to resist heavy loads from structures. The early deep foundations were always made of wood and thus were limited in length, capacity, and durability. However, in last 200 years, concrete and steel piles, and more recently, concrete drilled piers have become a part of every other construction project in the world. Deep foundations are subjected to compression loads mainly due to dead and live loads from superstructure, and uplift and lateral loads due primarily to wind and earthquake.

It is a well known fact that concrete is very strong in compression compared to soil and therefore, under axial compression loads on concrete drilled piers, soils reach their ultimate capacity much before the concrete. However, due to weak tensile and flexural strength of concrete, lateral load-deflection behavior of drilled piers depends significantly on material properties of the piers. Therefore, lateral load-deflection of drilled piers is considered as one of the most important factors to evaluate field performance of any material used to construct them. The most precise approach to determine the capacity of deep foundations is to install a full-size prototype pile or pier at the site and load it to failure (Coduto, 1994). However, because of the cost to perform field tests on full-size piles or piers, these tests are not performed routinely. Piles and piers are designed using load test data and design methods available in literature. Therefore, availability of data demonstrating the performance of any type of piers made from new concrete composites, under actual loading conditions is necessary.

Many of the developing and developed countries use coal to generate energy. Due to readily available supply of coal, it continues to be the dominant fuel source for electricity in the United States. Use of coal in generation of electricity has resulted in production and accumulation of large quantities of coal combustion products (CCPs). During 2001, about 900 million metric tons (Mt) of coal was burned in the United States and about 107 Mt of coal combustion products were generated by electric utilities and non-utilities (Kalyoncu 2003). According to Dube (1994), only 10% of the total ash produced was being used in early 90s. In 2001, approximately 33 percent of the total CCPs were used in various applications (Kalyoncu 2003), whereas in 2000 approximately 29 percent (29 Mt) of CCPs were used (Kelly and Kalyoncu 2002). In the Year 2002, an overall CCP production was 128.7 million tons and approximately 45.5 million tons (35.4 percent) was used in various applications (ACAA 2004).

The major utilization of CCPs has been in construction-related applications. Within the past 50 years, the concrete industry has given special attention to the safe and economical utilization of these residues (Helmuth 1987). Current research on the beneficial use of CCPs as building and highway construction materials has identified several promising uses for these materials. In addition to use in concrete, CCPS have been successfully used in the agricultural industry, blasting grit and roofing material, cement clinker raw feed, flowable fill, grout, mineral filler, mining applications, snow and ice control, wallboard, roller compacted concrete, structural fill, embankments, and soil stabilization (Kalyoncu, 2003). Several case histories of utilization of coal combustion products in construction projects are available (ACAA, 2001; GAI, 1988; Golden, 1986; Korcak, 1998; Kumar et al., 2001; Kumar and Stewart, 2003a and b; Kumar and Vaddu, 2004a and b; Lovell et al. 1997; Naik et. al., 1997; Schroeder, 1994; Seals et al. 1972; Tikalsky and Carrasquillo, 1989).

The objective of this portion of the study was to demonstrate the suitability of concrete composites made with PCC fly ash and bottom ash for construction of drilled pier foundations by conducting a set of lateral load-deflection tests on field-size drilled piers. Four drilled piers were constructed using conventional concrete and PCC fly ahs and bottom ash at a site in Carterville, Illinois and tested under lateral loads. The proportions of PCC concrete composites were selected based on extensive laboratory testing. The two composites selected were: (1) 100 percent replacement of natural fine aggregate with PCC bottom ash and 10 percent replacement of portland cement with PCC fly ash; and (2) 50 percent replacement of natural fine aggregate with PCC bottom ash and 20 percent replacement of portland cement with PCC fly ash. Test results presented show that the field performance of the piers constructed using concrete composites containing fly ash and bottom ash was similar to that of piers constructed using an equivalent conventional concrete.

SOIL AND GROUNDWATER CONDITIONS

The site used for conducting the tests is located in the Carterville campus of Southern Illinois University Carbondale (SIUC). The site was selected due to its proximity to the coal research laboratory. Two borings were drilled at the site to determine the subsurface conditions. One of the borings was drilled to a maximum depth of 34 ft and the other boring was drilled to a depth of approximately 21 ft. The borings were drilled using a truck mounted CME 75 rotary drill. Standard Penetration Tests (SPT's) were performed using an automatic hammer. Split spoon samples and relatively undisturbed Shelby tube samples were obtained at various depths. In general, the soil stratigraphy at the site consisted of approximately 18 in thick layer of gravel fill underlain by medium stiff to stiff, brown silty clay to depths of approximately 21 ft. The silty clay is underlain by very stiff to hard, sandy clay shale to the maximum depth explored, i.e. 34 feet. Both the borings were terminated at spoon refusal (more than 50 blows required to penetrate first 6-inches of the split spoon sampler).

Laboratory testing was performed on soil samples to estimate pertinent engineering and index properties of the soil. Moisture contents were determined for cohesive samples and Atterberg limit tests were accomplished on selected cohesive samples. Unconfined compression tests were performed on selected Shelby tube samples. The results of field and laboratory testing indicated that the compressive strength of the silty clay ranged between 0.5 to 2 tsf. The moisture content of the silty clay generally varied between 19 and 23 percent.

Groundwater at the site was measured by installing a piezometer. The groundwater table was observed to be at depths between 4 and 6 ft below the ground surface. An artificial pond existed within 100 feet of the test site. The water level in the pond was observed to be 4 feet below the bank surface which generally corresponded with the groundwater level at the test site. Figure 1 shows the typical subsurface profile at the site.


Figure 1. Typical Soil Profile at the Test Site

DRILLED PIER CONSTRUCTION

In order to identify and test viable concrete mixtures suitable for construction of drilled piers, several preliminary trial mixtures were prepared and tested in the laboratory in compression by replacing 50 or 100 percent of the natural sand with PCC bottom ash and 0 to 30 percent of cement with PCC Class C fly ash. Based on the results of the laboratory testing, two concrete composites were selected for construction of drilled piers in the field. These composites were (1) 100 percent replacement of natural aggregate with Illinois PCC bottom ash and 10 percent replacement of Portland cement with PCC Class C fly ash (F10B100), and (2) 50 percent replacement of natural aggregate with Illinois PCC bottom ash and 20 percent replacement of Portland cement with PCC Class C fly ash (F20B50). The compressive strength data of the concrete composites and conventional concrete used for construction of drilled piers is given in Table 1. The mix designation CM corresponds to an equivalent conventional concrete, i.e. control mix.

 

Table 1. Compressive Strengths (f'c) of the Concrete
Composites and Control Mix Used

Mixture
Designation		 Compressive Strength (psi)
Curing Age (Days)
7 28 60 90 180
F10B100 2941 4647 5612 6069 7071
F20B50 3154 5113 7051 7174 7869
CM 4555 6342 7180 7222 7581

 

The piers were constructed by drilling 13-inch nominal diameter and 25-foot deep holes and filling the holes with either one of the two concrete composites or conventional concrete. All holes were drilled with a high capacity, rotary drill rig CME 750 mounted on an All Terrain Vehicle (ATV). The pier reinforcement consisted of a pair of No. 6 threaded bars in the direction of the loading. Additional reinforcement was provided by a pair of 1/2-inch diameter, black iron pipes which were used as telltales. It is important to note that presence of a gravel layer near the ground surface caused difficulties in the construction of piers, and therefore, the diameter of the piers near the ground surface was greater than 13 inches.

The drilling for the piers was performed in dry. After the hole was drilled to required depths, the reinforcing steel and attached instrumentation was lowered into the hole and concrete was placed into the pier by direct discharge. Due to high groundwater table, every effort was made to place concrete in the holes immediately after drilling of each hole was completed to avoid seepage of water.

Three of the four piers were instrumented and constructed with a 2.77-inch outer diameter inclinometer casing at the center, which was used to house tilt sensors to measure the displacement profile of the piers and rotation of the pier head, if any. Instrumentation for the piers consisted of two levels of two vibrating wire strain gages, in addition to tilt sensors installed into the inclinometer casing. The tilt sensors were comprised of 4 levels of unidirectional fixed inclinometers, used to measure the angular displacement between the levels. These measurements enabled generation of a displacement profile along the laterally loaded test pier, with respect to a fixed pivot point located 5 feet below the lowest level of sensors. These four levels of tilt sensors were installed in-line with the direction of loading. One tilt sensor was installed in the uppermost level and oriented in the direction, corresponding to the direction 90 degrees counterclockwise from the direction of loading. Figure 2 shows the reinforcement detail of the drilled piers. Figure 3 shows the construction of a set of piers in progress. Figure 4 shows the inclinometer casing and the top of a hole drilled for construction of a pier.

 


Figure 2. Typical Reinforcement of the Piers

 


Figure 3. Construction of Piers in Progress

 


Figure 4. Inclinometer Casing and the Top of a Hole
Drilled for Construction of a Pier

 

TESTING, RESULTS, AND DISCUSSION

The drilled piers were tested 90 days after their construction to allow enough time for the piers to cure under field environmental conditions. After 90 days, the piers were tested in general compliance with ASTM D3966. The reaction was provided by either a reaction pier (a pier without instrumentation and inclinometer pipe) or by another test pile. For all tests, the load was applied using a 450-kip Osterberg Cell (Hayes et al. 2004) and the loads were measured using pressure-load calibration of the hydraulic jack. In addition, a vibrating wire load cell was used to measure the load for verification purposes. Pier head deflections were measured using digital dial indicators attached to a reference beam and connected to the data acquisition system. The loading increments were applied in general accordance with the Standard Loading Schedule as per ASTM D3966. Figure 5 shows the testing of drilled piers in progress. As evident from Figure 5, all the piers tested were free head. Detailed information on the tests on drilled piers and results are presented elsewhere (Hosin, A. 2001; Kumar et al. 2001; Ng, S. 2001).


Figure 5. Testing of the Piers in Progress

Figure 6 shows the load-deformation response of the piers constructed from concrete composites and conventional concrete. From Figure 6, it is clear that the load-deflection response of piers constructed with concrete composite ‘F20B50’ is similar to that of pier constructed with conventional concrete ‘CM’. Although, all three piers were constructed in a similar manner, and had similar diameter and reinforcement, the pier constructed using concrete composite ‘F10B100’ showed slightly higher resistance to lateral loads compared to other two piers. As discussed previously, due to presence of a gravel layer near the ground surface, the diameter of the piers near the ground surface was larger than the nominal diameter of the piers. Field observations showed that the head diameter of the pier constructed using concrete composite ‘F10B100’ was slightly larger than that of the other piers. Therefore, it was concluded that slightly higher resistance observed in the pier constructed with concrete composite “F10B100” was due to slightly larger pier head diameter.

Figure 7 shows the displacement profile of all the piers obtained from measurement made using the inclinometer. As discussed earlier, four levels of unidirectional fixed tilt sensors were installed into the inclinometer casing to measure the deflections at various depths and the angular displacement between the levels, if any. The results presented in Figure 7 show that the displacement in the piers increased significantly above Inclinometer Level 3, which indicates that a crack may have developed in all the piers above the location of Inclinometer Level 3. However, it is clear from Figure 7 that the displacement profile of piers constructed using concrete composites was similar to that of the pier constructed using an equivalent conventional concrete.


Figure 6. Comparison of Lateral Load –Deflection Response of Drilled Piers
Constructed using Concrete Composites (F10B100 and F20B50) and Control mix (CM)


Figure 7. Comparison of Displacement Profiles of Drilled Piers
Constructed using Concrete Composites (F10B100 and F20B50) and Control mix (CM)

Based on the results presented in Figures 6 and 7, it was concluded that if the piers are constructed in a similar manner, and have similar dimensions and reinforcement, piers constructed using concrete composites containing Illinois PCC bottom ash and Class C fly ash, developed as a part of the overall project, are expected to behave similar to the piers constructed using an equivalent conventional concrete.

CONCLUSIONS

Several concrete mixtures containing PCC Class C fly ash and Illinois PCC dry bottom ash were tested as a part of the overall project. Based on the laboratory test results, two concrete composites were selected and used to construct the drilled piers along with piers constructed using an equivalent conventional concrete. Lateral load-deflection response of the piers showed that the piers constructed using concrete composites containing Illinois PCC bottom ash and Class C fly ash was similar to that of the pier constructed using an equivalent conventional concrete. Based on the results obtained, it was concluded that the piers constructed using concrete composites containing Illinois PCC bottom ash and Class C fly ash are expected to behave similar to the piers constructed using an equivalent conventional concrete, provided the piers are constructed in a similar manner, and have similar dimensions and reinforcement.

ACKNOWLEDGEMENTS

This project was completed with support, in part by grants made possible by the Illinois Department of Commerce and Economic Opportunity (DCEO) through the Office of Coal Development (OCD) and the Illinois Clean Coal Institute (ICCI). Help received from Dr. Nader Ghafoori (formerly with Southern Illinois University Carbondale) and Dr. Vijay K. Puri (Southern Illinois University-Carbondale) is also acknowledged.

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