Technical Manual

Inc.

p 30

Chapter 2

Helical Foundation Systems

CHAPTER 2

HELICAL FOUNDATION SYSTEMS

that are cracked, leaning and/or bowing

(Figures

2.8.c1 and 2.8.c2)

. The wall distress may be a

result of changes in soil moisture conditions,

rise in groundwater levels, plugging of the wall

drainage system over time, plumbing leaks,

expansive clay soils, frost-jacking, or surcharge

loads above the wall.

2.8.1 Design Considerations

The helix plates along the tieback shaft must be

located beyond the active wedge or failure plane

to provide proper anchorage. The last helix plate

from the tip (closest to the wall) shall be at least

five (5) times its diameter beyond the failure

plane

(Figure 2.8.1.a)

. The helix plates should

also be located at least five (5) diameters below

the ground surface of the retained soils to model

deep foundation behavior. Multiple tiebacks

shall have a center to center spacing at the

helices of at least three (3) times the diameter of

the largest helix plate to avoid significant stress

overlap within the bearing soils.

Helical tiebacks are often installed at a

downward angle from horizontal, typically on the

order of 5 to 15 degrees. This downward angle is

often considered in order achieve the 5D depth

criteria below the surface of the retained soils,

to increase the vertical effective overburden

stress at the helix depths (in granular soils), or

to extend the helix plates to a deeper, more

competent soil layer. A slight downward angle

may also be considered to simply minimize the

potential for groundwater to follow the shaft and

seep through the wall penetration.

Tiebacks designed with a downward angle

should be installed to a capacity higher than the

calculated/required horizontal tieback capacity

(Figure 2.8.1.b)

. The calculated horizontal

tieback capacity (T

) is determined from

analysis considering the various loads on the

wall. If the tieback is designed for an installation

angle, alpha (

), then the tieback should be

installed to a capacity T

and its corresponding

value of torque if the torque correlation method

is used for capacity verification.

Remember

that the torque-correlated ultimate capacity

should exceed the design working load by an

appropriate factor of safety.

= T

/ cosα

The vertical component of the tieback force

should also be considered so as not to overstress

the wall or the wall bearing soils. The vertical

Figure 2.8.c2

Helical tiebacks and tube steel

walers stabilize concrete retaining wall

Figure 2.8.c1

Helical tiebacks stabilize

sheet pile wall below historic home