MT282 Unit 3 Foundation Method Selection Example

Soft Clay, High Water and an 1890s Neighbor: A Unit 3 Foundation Method Selection for a Podium Building

[Student Name]

Purdue University Global

MT282: Construction Methods and Materials

Unit 3 Foundation Method Selection

[Instructor Name]

[Date]

The site, its borings, the warehouse, loads and costs are composite and illustrative; the sources cited are real.

The Site

The project sits on a narrow urban lot that was once part of a rail yard: one concrete podium story with three wood stories of apartments above it. The geotechnical report's borings show this profile.

0 to 5 feet: fill, mixed sand, cinders and brick fragments from the rail yard

5 to 26 feet: soft to medium gray clay, standard penetration blow counts of 2 to 5

26 to 34 feet: loose silty sand

Below 34 feet: dense sand and gravel, blow counts above 40

Groundwater was measured at 7 feet and rises with the river in spring. On the north side, an 1890s four-story brick warehouse, now offices, stands 9 feet from the new building's north wall. Its foundations are shallow rubble stone, and its owner's engineer has asked that construction vibration be kept low.

The building's columns and bearing walls deliver a total load of about 6,160 kips to the foundation, from the podium, three wood stories and the roof.

Five Conditions for Every Method

Each method is tested against five conditions drawn from the site. Can it reach a suitable bearing layer? Will it control settlement in the soft clay? Can it be built below the groundwater? Will it produce vibration the warehouse cannot tolerate? And is its cost reasonable for a building of this size?

Methods Set Aside

Spread footings. Shallow footings would bear on the fill or the soft clay. The clay's low strength would require very large footings, and its compressibility would allow settlement the geotechnical report estimates at several inches, uneven across the building. Rejected for settlement.

Mat foundation. A thick concrete mat spreads the load across the whole footprint and reduces bearing pressure, but it still sits on 21 feet of compressible clay. Das (2016) notes that mats reduce differential settlement but cannot prevent consolidation of a thick soft layer beneath them. The report estimates total settlement of 3 to 4 inches over several years. Rejected for settlement.

Driven precast concrete piles. Driven piles would reach the dense sand and carry the load well. But impact driving produces strong ground vibration, and the warehouse's rubble foundations, 9 feet away, are the kind most vulnerable to it. Woods (1997), reviewing pile installation effects on adjacent structures, found that older masonry buildings on shallow foundations near driving operations are at high risk of cracking and settlement. Rejected for vibration.

Drilled shafts. Large-diameter drilled shafts could reach the dense sand with little vibration. But below the 7-foot groundwater level, the loose silty sand would collapse into an open hole, requiring full-length steel casing or drilling slurry. That raises cost and slows production for a building of modest load. Rejected for groundwater cost.

Helical piles. Helical piles install with little vibration and are quick. But individual helical piles in this profile would carry modest loads, and the number required to carry 6,160 kips would crowd the pile caps and raise cost. They also bear poorly in loose sand above the dense layer. Rejected for capacity.

The Selected Method: Auger-Cast Piles

Auger-cast piles are formed by drilling a continuous-flight hollow-stem auger into the ground to the design depth, then pumping cement grout under pressure through the stem as the auger is withdrawn, so the grout fills the hole from the bottom up while the soil-laden flights hold the sides. A reinforcing cage is pushed into the fresh grout. Because the hole is never open, groundwater and loose sand do not cause collapse, and because the auger rotates rather than impacts, vibration is low. Brown et al. (2007), in the Federal Highway Administration's manual on continuous flight auger piles, describe the method as well suited to sites with water-bearing granular soils and sensitive neighbors.

The design uses 16-inch-diameter piles about 40 feet long, socketed about 6 feet into the dense sand. The geotechnical report gives an allowable capacity of 80 kips per pile. The load of 6,160 kips divided by 80 requires at least 77 piles. Pile caps under columns and walls need two, three or four piles each for stability and geometry, which brings the count to 96 piles in 34 caps.

The method was chosen because it never leaves a hole open beside a building that cannot tolerate a shake.

Cost

Auger-cast piling for 96 piles, including mobilization, a load test and integrity testing, is estimated at about $310,000, compared with roughly $420,000 for cased drilled shafts and a lower but unusable figure for driven piles. Pile caps and grade beams add the same cost under any deep foundation.

Schedule Effect

Auger-cast piles also fit the schedule. A single rig can install 10 to 12 piles a day in this profile, so production piling takes about two working weeks after the load test, and pile caps can follow the rig across the site. Cased drilled shafts would take roughly twice as long, since each casing must be set, the shaft drilled and the casing withdrawn as concrete is placed. The street permit limits heavy equipment on the frontage to defined hours, and a single compact rig with a grout plant on the site's south side fits inside those limits without blocking the lane.

Risks and Monitoring

Three risks are named. First, auger-cast piles depend on continuous grout delivery; if the grout supply is interrupted while the auger is withdrawn, the pile can neck or contain soil. The contractor's rig must record depth, grout pressure and volume for every pile, and any pile with less than the specified grout volume is rejected. Second, although vibration is low, the warehouse will be monitored with vibration sensors and crack gauges before, during and after piling. Third, the rail yard fill may contain contaminated material, so spoil will be tested and disposed of accordingly.

A test pile will be installed and load tested before production begins, to confirm the 80-kip allowable capacity. If the test shows lower capacity, the piles will be lengthened deeper into the dense sand rather than increased in number, so the caps need not change.

Conclusion

Of six methods, only auger-cast piles meet all five site conditions. Allen and Iano (2019) observe that foundation selection is driven more by soil and site constraints than by any building feature, and this site's clay, water and neighbor leave one sound choice.

References

Allen, E., & Iano, J. (2019). Fundamentals of building construction: Materials and methods (7th ed.). Wiley.

Brown, D. A., Dapp, S. D., Thompson, W. R., & Lazarte, C. A. (2007). Design and construction of continuous flight auger (CFA) piles (Geotechnical Engineering Circular No. 8, FHWA-HIF-07-039). Federal Highway Administration.

Das, B. M. (2016). Principles of foundation engineering (8th ed.). Cengage Learning.

Woods, R. D. (1997). Dynamic effects of pile installations on adjacent structures (NCHRP Synthesis 253). Transportation Research Board.