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1 LESSONS LEARNED FROM A STONE COLUMN TEST PROGRAM IN GLACIAL DEPOSITS Barry S. Chen 1, P.E., Member, Geo-Institute and Michael J. Bailey 2, P.E., Member, Geo-Institute ABSTRACT A stone column test program was conducted for ground improvement below an earth embankment project in western Washington. Stone columns were initially designed to mitigate soil liquefaction, improve shear strength, and reduce subgrade settlements to provide a stable foundation for the embankment. The test program includes four test areas of stone columns installed with varying control parameters such as diameter, spacing, and area replacement ratio. Each test area consists of 32 to 41 vibro-replacement stone columns. Stone columns were installed through the postglacial, loose to medium dense, silty sand and soft to medium stiff silt and embedded into glacially overridden, dense, silty sand and stiff to hard, sandy silt. Standard Penetration Tests (SPT) and Cone Penetration Tests (CPT) were used as verification tests in each of the four test areas before and after the installation of stone columns. Results indicated densification of the matrix in soil zones containing less than approximately 15 percent fines and little to no improvement in soil zones containing over approximately 15 percent fines. In some deeper zones where native soils were over-consolidated by glacial actions, disturbance in the stiff to hard silt actually caused a decrease in penetration resistance. INTRODUCTION For decades, vibro-replacement stone columns have been used as an effective ground improvement technique for increasing bearing capacity, reducing ground settlement, and mitigating soil liquefaction of foundation soils. These design goals are typically achieved by providing (1) densification of the soil in-between columns, (2) increase in shear strength and stiffness of the soil-stone matrix, and (3) dissipation of excess pore water pressure in the soil through the more permeable stone column. 1 Principal, Hart Crowser, Inc., 1910 Fairview Avenue East, Seattle, Washington , barry.chen@hartcrowser.com 2 Senior Principal, Hart Crowser, Inc., 1910 Fairview Avenue East, Seattle, Washington , mike.bailey@hartcrowser.com 1

2 Densification of soils can be easily verified using penetration tests such as Standard Penetration Test (SPT) and Cone Penetration Test (CPT) for construction quality control. However, it is also generally recognized that densification only occurs when the soil contains little to no fines (soil passing US No. 200 sieve). When the foundation soil contains more than 15 to 25 percent fines, the design often has to rely primarily on the increase in stiffness and sometimes increased capacity for dissipation of pore water pressure, but construction quality control is more difficult to verify in this case. In the stone column test program presented in this paper, the densification effect of soils in zones containing various amounts of fines was closely examined using penetration test data collected from four test areas. The purpose of the test program is to evaluate the feasibility of stone columns for subgrade improvement at this site, select final design geometry for the column installation, and evaluate the use of penetration tests for quality control during construction. PROJECT DESCRIPTION The project involves the design and construction of a 17-million-cubic-yard earth embankment located near Seattle. The earth embankment includes sections of 2 horizontal to 1 vertical (2H:1V) side slope and several mechanically stabilized earth (MSE) retaining walls. Ground conditions below most of the 2H:1V slope and MSE wall areas generally consist of 10 to 20 feet of alluvial deposits, colluvium, and recessional outwash underlain by glacially overridden soils. The alluvial deposits typically include interlayered sand, silt, clay, and occasional peat. The colluvium and recessional outwash generally consist of medium dense to dense, slightly silty to silty, slightly gravelly to gravelly sand. Variable depositional processes have produce a wide range in soil properties over short distances, as indicated by comparison of side-by-side explorations described below. The glacially overridden soils are usually present in the form of dense to very dense gravelly, silty sand and very stiff to hard silt. Groundwater in the area is generally within 5 to 10 feet below existing ground surface. Due to the presence of potentially liquefiable sand and compressible silt/clay in the foundation soils, several ground improvement techniques were considered to provide a stable foundation subgrade for the embankment slopes and MSE walls. A stone column test program was performed to evaluate the feasibility of the preferred alternative - vibro-replacement stone columns. 2

3 STONE COLUMN DESIGN The stone column design for this project uses the concept of area replacement ratio (A r ) and includes both static and seismic considerations. The A r is defined as the ratio of the stone column area to the total area of the soil/stone matrix. Barksdale and Bachus (1982) provided guidelines for estimating reduction in subgrade settlements based on A r and stress concentration factor (n). The stress concentration factor can also be defined as the ratio of stone column s shear modulus to the shear modulus of the soil (K G = G stone / G soil ). For improvement of soil liquefaction resistance, Baez and Martin (1993) developed a design approach for determining A r based on SPT data. The design parameters for this project include an earthquake magnitude of 7.5, a peak ground acceleration of 0.32 g, and n (or K G ) = 3. For soils containing 15 percent fines, a minimum A r of 17 percent was necessary to achieve a calculated SPT resistance of 22 or a CPT resistance of 110 tons per square foot (tsf) to prevent liquefaction. For soils containing 35 percent fines, a minimum A r of 17 percent produced a calculated SPT resistance of 18 or a CPT resistance of 75 tsf to prevent liquefaction. For very silty sand or cohesive soils with no densification anticipated, a minimum A r of 35 percent is necessary to achieve the same results. Once the A r is determined, diameter and spacing of the column can be selected based on Eq. 1: Eq. 1 A r = (d/s) 2 where d is column diameter and s is center-to-center spacing between columns. Typical 42-inch-diameter stone columns installed in a triangular pattern at 8-foot center-to-center spacing would have an A r of 17 percent. In addition to the densification and stress concentration effects, dissipation of excess pore water pressure was examined using the one-dimensional model proposed by Seed and Booker (1976). Analysis indicated that the maximum pore water pressure of the soil never exceeded 45 percent of the effective overburden stress during and after ground shaking for the design earthquake. STONE COLUMN INSTALLATION Four test areas were selected for the stone column test programs. Test Areas 1 and 2 are immediately adjacent to each other. Test Areas 3 and 4 are approximately 300 and 500 feet north of the Test Areas 1 and 2, respectively. Figure 1 shows the view looking south to the test areas. Vibro-replacement stone columns were constructed using crushed rock with the gradation that 90 percent passed a 1-inch size sieve and 5 percent or less passed a 3

4 1/2-inch size sieve. The stone was installed using an electric vibrator (165 horsepower) with bottom-feed attachment, as shown on Figure 2. The vibrator has a delivery of 120 kw at 1,800 rpm, a 3-phase current of 380 volts, and a frequency of 60 cycles per second. Free-hanging amperage draw is approximately 120 amps, with maximum amperage draw at 280 amps. The Manitowoc 3900 crane (100-ton capacity) has 80 feet of boom. FIG. 1 View of Project Test Area FIG. 2 Stone Column Vibrator with Bottom-Feed Attachment 4

5 It is commonly accepted that higher amperage is related to denser ground conditions, and in some cases stone column contracts require that a minimum amperage be attained while building the column up from the base. In addition, amperage is used to define probe refusal. During typical stone column construction in soft ground, relatively low amperage (on the order of about 150 to 180 amps) is often observed at the start of column construction. Where the probe encounters dense ground refusal at its tip elevation, amperage could be greater than 200 amps at the base of the column, and increasing to about 250 amps or higher as stone is compacted in successive lifts during column construction. Amperage may increase to over 300 amps during subsequent column construction due to densification caused by the initial column installations. Table 1 shows the geometry of the test sections. Note Test Areas 1A and 2A were created by split spacing smaller diameter columns within a portion of Test Areas 1 and 2, respectively. TABLE 1. Summary of Geometry of Test Sections (1) (2) (3) (4) (5) (6) (7) Test Area 1 1A 2 2A 3 4 Number of Stone Columns in Test Area Column Diameter in Inches Column Spacing, Center to Center in Feet Ar in Percent Column Depth in Feet 17 to to to to 19 9 to to 16 Figure 3 shows a plan of the columns and penetration tests in each test area. Construction Observations During installation of stone columns in Test Areas 1 and 2, water was observed flowing on the ground surface from newly constructed columns. This led to softening of the upper soils, a problem which was exacerbated by rainy weather that soaked the ground around the site. Because the upper soils in the work area became very soft, the front-end loader used during the stone column installation caused deep rutting in the ground surface. The ground surface surrounding the stone columns heaved on the order of about 1 to 3 feet, and this soil was continuously removed during installation to facilitate work activities. In addition to heave at the ground surface, the probe was observed to have substantial cohesive soils sticking to it as it was withdrawn from the 5

6 hole. This dirty probe could be attributed to either the upper soft soils, or disturbance of relatively stiff cohesive soils at depth. FIG. 3 Plans of Stone Columns and Penetration Tests in Each Test Area 6

7 During construction of the columns in Test Areas 1 and 2, the amperage of the probe was observed to remain relatively steady at about 140 to 220 amps as the column was being built to the ground surface. These amp readings are lower than one normally would expect to see as crushed rock in the column is compacted. The low amp readings are attributed to soft ground conditions encountered during construction, which may have resulted from low confinement and lateral spread as the crushed rock was placed and compacted. During installation of stone columns in Test Area 3, the ground surface remained dry, and very little water was brought to the surface through existing columns. Special caution was taken to define refusal criteria that were intended to limit any disturbance of the stiff to hard cohesive soils that were present below depths of about 12 feet. These criteria were relatively subjective and required close observation of the behavior and sound of the vibrator probe. Although amperage was continuously monitored during the initial probe advance, the presence of the stiff to hard underlying soils was not definitively indicated by consistent higher amperage. Based on the cleanliness of the probe, as well as the lack of heave at the ground surface, we surmised there was not significant penetration of the vibrator probe into the stiff to hard underlying soils. However, this was refuted by comparison of pre- and postinstallation penetration tests. During construction of the columns in Test Area 3, the amperage of the probe was observed to range from about 250 to 300 amps as the column was being built up to the ground surface. These higher amperage readings indicate higher compaction within the stone column, which suggests that the surrounding soils were also being densified. During installation of stone columns in Test Area 4, the ground surface remained dry, and very little water was brought to the surface through existing columns. At some column locations, a cone of depression formed around the probe during initial penetration, indication densification of the sands surrounding the probe. Other observations are similar to those in Test Area 3. The amperage of the probe was observed to range from about 220 to 300 amps as the column was being built up to the ground surface. VERIFICATION TESTING Within each test area, SPT borings and CPT probes were advanced prior to the installation of stone columns. After completing columns in the test area, about 1 foot of ground heave was bladed off the top of the test area to try to locate the top of the stone columns. The ground was allowed to dry for several days to facilitate access for the testing equipment. Table 2 summarizes the before and after SPT and CPT 7

8 explorations in each of the four test areas. The locations of these explorations relative to the stone column layout are illustrated on Figure 3. TABLE 2. Summary of Explorations in Test Areas (1) (2) (3) (4) (5) Test Before After Area SPT CPT SPT CPT 1 B-1 CPT-3 B-1a [6] CPT-5 [7] B-2 CPT-4 B-2a [6] CPT-6 [7] B-5a [6] CPT-9 [15] B-7a [15] 2 B-3 CPT-1 B-3a [5] CPT-7 [6] B-4 CPT-2 B-4a [5] CPT-8 [6] B-6a [14] CPT-10 [14] 3 B-12 CPT-15 B-22a [6] CPT-21 [6] B-13 CPT-16 B-23a [6] CPT-22 [6] CPT-17 CPT-18 4 B-10 CPT-11 B-20a [2] CPT-19 [2] B-11 CPT-12 B-21a [2] CPT-20 [2] CPT-13 CPT-14 Note: Number in [ ] indicates the time interval in days, between completion of columns and post-construction penetration tests. Results of the penetration tests are shown on Figures 4 and 5. The depth axis on Figures 4 and 5 was adjusted where the surface was graded to remove soft soils during construction, to facilitate comparison of penetration resistance before and after construction. In Test Area 1, as shown on Figure 4, CPT data recorded after stone column installation indicate that some densification occurred from 0 to 6 feet below the ground surface where clean sand is present. The CPT data did not indicate significant change in density for the silty sands present from 6 to 10 feet below the ground surface. In contrast to the CPT data, the SPT data indicated little improvement in the 0- to 6-foot-depth range after 7 days, and some possible improvement in this area 15 days after column completion. Below 6 feet, SPT data indicated that density might have actually decreased during stone column installation. Prior to stone column installation, CPT refusal occurred at a depth of 14 feet. After stone column installation, CPT refusal occurred between depths of 16 to 18 feet. This suggests that the deeper soils may have been disturbed due to stone column installation. Also, 8

9 results of boring B-5 suggest a greater degree of disturbance occurred where the stone columns were closer together. 9

10 10

11 In Test Area 2, CPT tests [CPT-7 and CPT-8] show that the density of the upper soil, interpreted to be silty and clayey sand, was not much improved by the stone columns, as shown on Figure 4. Data from CPT-10 [not shown on Figure 4] showed apparent densification of sand from 0 to 5 feet below ground surface where clean sand is present. The CPT tests show good improvement from depths of 5 to 9 feet, no change in the sandy silt from 9 to 12 feet, and apparent reduction in penetration resistance below 12 feet in depth. Overall, the SPT blow counts appear very little changed down to about 12 feet, and somewhat reduced below 12 feet following stone column installation. Prior to stone column installation, CPT refusal occurred at a depth of 14 feet. After stone column installation, CPT refusal occurred at 17 feet. This suggests that the deeper soils may have been disturbed during stone column installation. In Test Area 3, the four CPT tests performed prior to stone column work show inconsistent tip resistance in the depth interval from 2 to 6 feet in slightly silty to silty sand. Comparing CPT results after stone column installation, CPT-21 indicates the tip resistance decreased, while CPT-22 indicates large improvement in tip resistance in the same interval from depths of 2 to 6 feet. In the depth range of 6 to 12 feet, results from CPT-21 and CPT-22 also contrast one another, and alternatively indicate lower tip resistance when compared to the four CPT tests performed prior to stone column construction. [Only two of the CPT tests before construction are shown in Figure 5, for clarity, but all indicated relatively consistent conditions prior to stone column installation]. The SPT results indicate no improvement in density in the depth interval from 0 to 10 feet, while below a depth of 10 feet, blow counts decreased following stone column installation, indicating looser subgrade conditions in the very silty, gravelly sand. In Test Area 4, the four CPT tests performed prior to stone column work show significant variability in tip resistance in the depth interval from 0 to 8 feet. Comparing CPT results after stone column installation, CPT-20 shows no improvement in tip resistance compared with CPT-11 down to a depth of 5 feet, with some improvement from depths of 5 to 7 feet as shown in Figure 5. CPT-19 indicates improvement in tip resistance from 0 to 6 feet when compared to CPT-14, but no change in tip resistance in the silts and clays down to depths of about 13 feet. After stone column installation, CPT-19 had deeper refusal compared to CPT-14 before installation, again indicating that there was apparently some disturbance of the stiff to hard cohesive soils at the base of the stone columns. The SPT results suggest some improvement in density of the surficial sand and silty sand following stone column installation. CONCLUSIONS 1. Stone columns are an effective subgrade improvement technique in clean to slightly silty sands. However, for variably interbedded sands, silty sands, and 11

12 silts or clays, results are not consistent and this may not be an appropriate method of ground improvement where subgrade densification is required. 2. Where stone columns are used in very silty sand or cohesive soils, the engineer must be satisfied that the strength and stiffness of the stone columns is sufficient, even if shear strength of the cohesive matrix soils is reduced due to disturbance. 3. Penetration tests are a good means of quality control for stone column installation. Both CPT and SPT are recommended, as the two methods complement one another and neither method alone is as good as the combination. ACKNOWLEDGMENTS The authors wish to acknowledge their appreciation of the work by the ground improvement contractor on this project, GKN Hayward Baker. The authors also want to thank Phoebe Brandal, David Holmer, and Heidi LeVasseur of Hart Crowser for their assistance in preparation of this manuscript. SYSTEM OF UNITS Conversion from conventional US Units to International System of Units [SI] is as follows. REFERENCES 1.0 cubic yard = 0.76 cubic meters 1.0 foot = 0.30 meter 1.0 ton per square foot = 0.10 mega Pascals 1.0 inch = 2.5 centimeters 1.0 horsepower = 750 watts 1.0 ton = 910 kilograms Baez, J.I. and Martin, G. [1993]. Advances in the Design of Vibro System for the Improvement of Liquefaction Resistance. Proc. Symposium on Ground Improvement, Vancouver, Canada. Barksdale, R.D. and Bachus, R.C. [1983]. Design and Construction of Stone Columns. Research Report, Federal Highway Administration, Contract No. DTFH Seed, H.B. and Booker, J.R. [1976]. Stabilization of Potentially Liquefiable Sand Deposits Using Gravel Drain Systems. Report No. EERC 76-10, U.C. Berkeley. 12