Chance Technical Design Manual

• For cohesionless soils ( a method):

RECOMMENDED ADHESION VALUES IN CLAY [NAVY MANUAL DM-7 (1974)], TABLE 5-12 PILE TYPE SOIL CONSISTENCY COHESION (c) (psf)

EQUATION 5-52

Q f = S [ p B(q’)Ktan( ϕ ’)( ∆ L f )]

ADHESION (C a ) (psf)

where q’ = Effective vertical stress on element ∆ L f K = Coefficient of lateral earth pressure ranging

Very soft

0-250

0-250

Soft

250-500

250-480

from K o to about 1.75 depending on volume displacement, initial soil density, etc. Values

Medium stiff

500-1000

480-750

Grout

Stiff

1000-2000

750-950

close to K o are generally recommended because of long-term soil creep effects. As a default, use K o = 1. φ ’ = Effective friction angle between soil and pile shaft

Very stiff

2000-4000

950-1300

Very soft

0-250

0-250

Soft

250-500

250-460

Medium stiff

500-1000

460-700

Steel

• For cohesionless soils (alternate Navy method):

Stiff

1000-2000

700-720

EQUATION 5-53

Very stiff

2000-4000

720-750

Q f = S [ p (B)S( ∆ L f )]

GROUTED PILES IN SAND [NAVY MANUAL DM-7 (1974)], TABLE 5-13

where S = Average side resistance on pile surface area = P o tan( ϕ ’) (see Table 5-13) P o = Average overburden pressure For steel round shaft piles in sand, HeliCAP software uses the alternate Navy method to calculate side resistance with Equa tion 5-46 and f s values from Table 5-4. Tables 5-4, 5-12, and 5-13 are derived from graphs in the De partment of the Navy Design Manual 7, Soil Mechanics, Foun dations and Earth Structures (1974). Later editions of Design Manual 7 limit the depth at which the average overburden pres sure is assumed to increase. The following is an excerpt from the manual regarding this limiting depth: “Experimental and field evidence indicate that bearing pres sure and skin friction increase with vertical effective stress (P o ) up to a limiting depth of embedment, depending on the rela tive density of the granular soil and position of the water table. Beyond this limiting depth (10B± to 40B±) there is very little increase in end bearing, and increase in side friction is directly proportional to the surface area of the pile. Therefore, if D is greater than 20B, limit P o at the pile tip to that value corre sponding to D = 20B.” (D = depth of the pile embedment over which side friction is considered and B = diameter of the pile.) Design Example 8-5 in Section 8 illustrates the use of the Navy Design Manual 7 method to calculate the side resistance capacity of a Chance® Helical Pulldown® micropile. 5.6 APPLICATION GUIDELINES FOR CHANCE HELICAL PILES/AN CHORS The uppermost helix should be installed at least three diam eters below the depth of seasonal variation in soil properties. Therefore, it is important to check the frost depth or “mud” line at the project site. Seasonal variation in soil properties may require the minimum vertical depth to exceed five helix diameters. The influence of the structure’s existing foundation (if any) on the helical pile/anchor should also be considered. Hubbell Power Systems, Inc., recommends helical piles/anchors be located at least five diameters below or away from existing foundation elements.

EFFECTIVE ANGLE OF INTERNAL FRICTION ( ϕ ’) (degrees) 20 25 30 35 40 S = AVERAGE SIDE RESISTANCE ON PILE SURFACE (psf)

P o (psf)

500

182

233

289

350

420

1000 364

466

577

700

839

1500 546

699

866

1050

1259

2000 728

933

1155

1400

1678

2500 910

1166

1443

1751

2098

3000 1092

1399

1732

2100

2517

3500 1274

1632

2021

2451

2937

4000 1456

1865

2309

2801

3356

• The uppermost helix should be installed at least three helix diameters into competent load-bearing soil. It is best if all helix plates are installed into the same soil stratum. • For a given shaft length, use fewer longer extensions rath er than many shorter extensions. This will result in fewer connections and better load/deflection response. • Check the relative economic feasibility of helical pile/an chor options if more than one combination of helix con figuration and overall length can be used. 5.7 LATERAL CAPACITY OF The primary function of a deep foundation is to resist axial loads, but in some cases, they will be subjected to horizontal or lateral loads. Lateral loads may be from wind, seismic events, live loads, water flow, etc. The resistance to lateral loads is in part a function of the near-surface soil type and strength and the effective projected area of the structure bearing against the soil. This section provides a summarized description of the methods and procedures available to determine the lateral capacity of helical piles/anchors in soil. HELICAL PILES 5.7.1 INTRODUCTION

DESIGN METHODOLOGY

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