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48 passive force contributed by the clay with RAP columns. At relative to driving more piles, some simplified cost estimates a displacement of 1.25 in., the interpreted passive force is were produced for the various approaches. A complete cost about 85 kips. This is about 35 kips higher than the passive assessment is beyond the scope of this investigation and would force (50 kips) obtained when clay alone was acting against the be dependent on a number of factors that would vary from pile cap. This result indicates that nearly all of the increased location to location. In addition, the geometries could poten- resistance provided by the RAP columns was a result of increased tially be further optimized to produce greater resistance rela- passive force against the pile cap and only about 10% of the tive to the geometries used in the field tests. Furthermore, it increase was a result of increased soil-pile resistance below may be possible to reduce the cost of ground improvement the pile cap. This result seems consistent based on the results obtained by jet grouting by using lower strength mixes or by of previous soil improvement tests in which little improve- using more economical approaches such as soil mixing, as will ment in lateral pile resistance was achieved unless the soil be investigated subsequently. Nevertheless, this simple com- improvement extended to the face of the piles. The increase parison provides a first estimate of the economic viability of of 35 kips in the passive force alone represents an increase of the ground improvement approaches for increasing lateral about 70% relative to the passive resistance provided by the pile group resistance. clay alone. One common alternative to soil improvement would be to To produce a more readable report, additional plots, simi- simply add more piles and increase the size of the pile cap. lar to those presented for the pile group in virgin clay are not According to the test results for Cap 1 during Test 2, the max- presented here but are available in Lemme (2010, Appendix 4). imum lateral load resisted by the nine-pile group was about 230 kips. Assuming this load is distributed evenly, each pile would have carried about 26 kips. Therefore, to obtain the 3.11 Summary of Increased same lateral resistance of 500 kips that was achieved through Resistance from Soil jet grouting beneath the pile cap, 20 piles would have to be Improvement Methods added. Similar calculations have been made for each improve- and Cost Considerations ment approach. The cost of the additional piles, neglecting A summary of the geometries of the various soil improve- mobilization costs, were estimated by assuming a typical pile ment techniques and the increase in lateral resistance that they length of 80 ft, pipe pile costs of $30/ft, and driving costs of produced is provided in Table 3-4. The greatest increase in lat- $12/ft. In addition, the cost of concrete fill and reinforcing eral load (400 to 500 kips) was produced by the jet grouting steel cages in the piles was estimated assuming $150/cubic method. Soil mixing and flowable fill also produced significant yard of concrete. Finally, adding more piles will require an increases in lateral resistance (140 to 170 kips). Excavation and increase in the size of the pile cap and an estimate of this cost replacement techniques produced a relatively small increase in also was made for each case assuming 3-ft center-to-center lateral resistance (20 to 40 kips). pile spacing. It should be noted that the treatments producing the great- The cost of jet grouting was estimated to be roughly $475 est improvement were typically those that involved the larger per cubic yard for the 20% by weight cement content used volumes of treated soil and the greatest cost to implement; in this study. Mobilization costs are highly variable and were however, this is not always the case. To provide some indica- not included in the cost because mobilization costs also were tion of the cost effectiveness of the various treatment methods excluded from the pile driving costs. Soil mixing was assumed Table 3-4. Summary of treatment geometries and increased resistance provided by the various soil improvement methods. Treatment Treatment Untreated Increase in Percent Treatment Method Dimensions Volume Resistance Resistance Increase Comments (LxWxD) (cu. Yds.) (kips) (kips) in Resistance Jet Grouting Below Cap 15'x10.5'x10' 58.3 282 500 160 Jet Grouting Adjacent to Cap 6.6'x13'x12' 38.1 214 398 185 Soil Mixing Adjacent to Cap 4'x11'x10' 16.3 282 170 60 Weak Flowable Fill Below Cap 13.5'x8.8'x6' 26.4 232 24 10 Flowable Fill Adjacent to Cap 6'x12'x6' 16.0 265 145 55 Compacted Fill to Edge of Cap 9.6'x8.75'x3.5' 10.9 232 23 10 Compacted Fill 5 ft beyond Edge of Cap 14.6'x8.75'x3.5' 16.6 232 40 18 Rammed Aggregate Piers Adjacent to Cap Top 13-2.5' dia x 13' deep 29.5 285 40 14 Rammed Aggregate Piers Adjacent to Cap Top 13-2.5' dia x 10.5' deep 23.6 50 35 70 1 Note 1: Increase in resistance is for passive resistance only
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Table 3-5. Summary of increased resistance provided by the various soil improvement methods along with cost savings relative to providing additional piles. Increase in Equivalent Add'l Ground Savings Ground Treatment Method Resistance Number of Pile/Cap Improvement Relative to Percent Improvement Comments (kips) Pipe Piles Cost Cost Piles Savings Cost/kip Jet Grouting Below Cap 500 20 $84,200 $28,500 $55,700 66 $57 Jet Grouting Adjacent to Cap 398 16 $69,360 $38,000 $31,360 45 $95 1 Soil Mixing Adjacent to Cap 170 7 $30,345 $10,000 $20,345 67 $59 1 Weak Flowable Fill Below Cap 24 1 $4,335 $3,180 $1,155 27 $133 Flowable Fill Adjacent to Cap 145 6 $26,010 $3,600 $22,410 86 $25 1 Compacted Fill to Edge of Cap 23 1 $4,335 $544 $3,791 87 $24 Compacted Fill 5 ft beyond Edge of Cap 40 2 $8,670 $828 $7,842 90 $21 Rammed Aggregate Piers Adjacent to Cap Top 40 2 $8,670 $4,225 $4,445 51 $106 Rammed Aggregate Piers Adjacent to Cap Top 35 2 $8,670 $4,225 $4,445 51 $121 2 Note 1: Cost of soil improvement doubled to account for increased resistance in opposite direction Note 2: Increase in resistance is for passive resistance only Note 3: Cost/kip for pile/pile cap = $182/kip
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50 to cost about $300 per cubic yard for the 10% by weight cement provided cost savings, although in some cases, the cost savings content, and flowable fill was assumed to cost $75 per cubic are small, as is the increase in resistance. This is particularly yard in addition to excavation costs. For the treatments adja- true for the excavation and replacement approaches. Finally, cent to the pile cap, the cost was doubled assuming that a sim- the cost per kip of increased lateral resistance was computed ilar improvement zone would be required on the opposite side for each case to provide another indication of the cost- to account for load in the opposite direction. effectiveness of the various approaches. For comparison pur- The excavation and replacement cost was assumed to be poses, the cost per kip for the pipe pile alternative was $182/kip. $50 per cubic yard for the small volumes involved and the cost All of the ground improvement methods had lower costs per of RAPs was assumed to be $50 per ft of length. The estimates kips than that for the piles. The lowest cost per kips was pro- of improvement costs associated with each treatment method vided by the excavation and replacement method because of are shown in Table 3-5. In addition, the cost savings associated the low cost of the treatment method. However, it must be rec- with soil improvement compared to providing additional piles ognized that despite the low cost per kip for this treatment is listed for each case along with the percent savings relative to method, the potential for increasing resistance also was quite additional piles. In all cases, the ground improvement method limited.