Transmission And Substation Foundations - Technical Design Manual (TD06088E)

TABLE 4-5 VALUES OF UNIT SIDE RESISTANCE FOR STEEL PILES IN SAND (FROM NAVY MANUAL DM-7)

Friction Angle of Soil φ ’

s’ vo (psf)

20

25

30

35

40

Unit Side Resistance f S (psf)

500

137

175

217

263

315

1000

273

350

433

525

629

1500

410

524

650

788

944

2000

546

700

866

1050

1259

2500

683

875

1082

1313

1574

3000

819

1049

1300

1575

1888

3500

956

1244

1516

1838

2203

4000

1092

1399

1732

2101

2517

develop side resistance. Shaft Resistance in Sand and Mixed Soils φ ’ > 0; c’ = 0

The shaft resistance of steel pipe shaft piles in coarse-grained soils, such as sands and mixed soils is more complex than in clays but can still be determined using traditional deep foundation analyses. The Department of Navy Design Manual DM-7 also gives a simplified method for estimating the unit side resistance for straight shaft steel piles. The value of f S is related to the friction angle of the soil, φ ’, and the effective vertical stress, σ ’ vo , as given in Table 4-5. HELICAL ANCHOR/PILE SPACING & MINIMUM DEPTH Reasonability Check Consideration should be given to the validity of the values obtained when determining the bearing capacity and shaft resistance of the soil. The calculated theoretical ultimate capacity is no better than the data used to obtain that value. Data from soils reports, boring logs, the water table depth, and load information may not accurately represent actual conditions where the helical pile/anchor must function. Empirical values that are used and estimates of strength parameters, etc. that must be made because of lack of data affect the calculated bearing capacity and shaft resistance value. In those situations where soil data is insufficient or not available, a helical trial probe pile can help determine such items as, location of bearing strata, pile capacity, location of soft/loose soil, and the presence of obstructions, such as, cobbles, boulders, and debris. An important step in the process of determining the capacity of a helical pile/anchor is to conduct a reasonability check. The engineer should use the best engineering judgment to perform the reasonability check. This should be based on experience, historical test data and consulting colleagues. This is easily overlooked but must be performed by the designer or by others. Helical Pile/Anchor Spacing Once the capacity of the helical pile/anchor is determined, concern may turn to location of the foundation element with respect to the structure and to other helical pile/anchors. It is recommended that the center-to-center spacing between adjacent anchors/piles be no less than five times the diameter of the largest helix. The minimum spacing is three feet (0.91 m). This latter spacing should be used only when the job can be accomplished no other way and should involve special care during installation to ensure that the spacing does not decrease with depth. Minimum spacing requirements apply only to the helix bearing plate(s), i.e., the pile/anchor shaft can be battered to achieve minimum

DESIGN METHODOLOGY

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