1 Shallow Helical Pier Foundation in Claystone, C.S. Russell, 2012

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Nov 26, 2013 (3 years and 8 months ago)

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1

Shallow Helical Pier Foundation in Claystone, C.S. Russell, 2012

Abstract

An economic novel foundation system on swelling clays was successfully used on 15 new houses in
Grand Junction, Colorado. The foundation system is a concrete grade beam set on a roadbase footing
with helical piers embedded into bedrock shale
(clay
stone) three

feet. The system has not registered
any failures in fifteen years whereas other foundation designs in the area have. These failed systems
have included grade beam over spread footing and pad over structural fill. The helical
pier

system h
as
proven more economic to the successful concrete pier systems.


Figure
1
. Side View of Helical Pier Foundation


Piers are set 20 foot on center

Introduction

The subdivision owner of “The Legends” wanted a more economic foundat
ion system than the
successfully used grade beam on concrete pier. These foundations typically were 3’ x 8” concrete grade
beams with (2) #5 t&b over 12” x 20’ reinforced concrete piers. The owner further stipulated that grade
beam over spread footing and
pad over structural fill was not acceptable because of the failures that
had been encountered to date. The subdivision lay on the Mancos Shale. A
well cemented
swelling
claystone covered by nearly one foot of
decomposed shale. This decomposed shale was mos
tly illitic clay
with low to moderate swell pressures.

2

Shallow Helical Pier Foundation in Claystone, C.S. Russell, 2012

The foundations were problematic due to a nearby large irrigation canal and subdivision irrigation.
These features introduced water into the soil subsurface which inturn caused swelling and settlement

of
the clays which inturn affected the foundations. This foundation movement most often caused unsightly
interior drywall cracks.

Drill tests showed the
subdivision
b
edrock showed an average N of
140

on SPT tests
. This N exceeds the
linear portion given
on the Terzagi and Peck

correlation for unconfined compressive strength
(
Lindberg,
M, 1997).
Experience has shown
, for this area type soil, concrete piers are designed for 1500

psf skin
friction and 20,000 psf end bearing pressure.
No failures of concrete
pier foundations have been noted
here. Surface clay swell pressures averaged 1500 psf with low movement.

Helical Pier Design

The well cemented shale bedrock material initiated just one foot under the ground surface. Uplift tes
ts
on 10” helical piers burie
d 3

foot subsurface were subjected to a force of
3
0 kips without movement.
This force exceeds the weight and the friction of a right circular cone of the type soil (14 kip).
Here
, on
the pier, the SS5 1 ½” square rod has a 70 ksi yield strength

Figure 2. M
athcad Program Calculating
Strength of Reinforced Concrete Grade Beam

the weakest link. The 10” pier was chosen after tests showed that a torque of 5,500 ft
-
lbs could be
encountered consistently

at 3

feet of penetration into the soil. This is the recommend
ed installation
torque by the pier manufacturer (Chance Piers, 2012).

A standard 7’ pilot helical was used. This allowed
for full coverage of the grade beam. Tension would be caused by the claystone being uplifted by the
bottom of the grade beam. Therefore

the helical was tied into the top of the grade beam by welded
rebar. This left 4’ of helical penetrating the 8” thick road base and +/
-

3’ of claysto
n
e material.

Comp
ression tests were not run. The empirical end bearing pressure of 20 ksf was used (see a
bove). This
allowed for 11 kip to be put on each helix. In order to not depend on end bearing pressure to negate all
settlement, a strip of 8” thick by 16” wide road base was compacted to 95% ASTM D
-
698 under each
grade beam. Tests and experience showed th
e underlying soil had an allowable bearing pressure of 1000
psf. This allowed for 1.33 kip per linear

foot of 8” grade beam. It
assumes that the force is transmitted
from the grade beam through the roadbase at a 45 degree angle.

Th
e average factored load
on

each grade beam was 2000 plf. This included both the roof and floor load.
Given the end bearing pressure of the helix and the bearing pressure of the decomposed shale each
helix was spaced out 16 foot to 20 foot on center under bearing walls.

The swell

pressure of 1500 psf was also negated by the helix. After allowing for deadloads, each helix
could span
33 feet on nonbearing walls. The weak link here is the grade beam. A Mathcad program
generated off of first principles (Popov, 1976) shows that the
standard

reinforced

concrete beam can
span
26 feet.

3

Shallow Helical Pier Foundation in Claystone, C.S. Russell, 2012

in
Diamet erRebar
5
8

in
NumberofRebar
2

AreaofRebar
NumberofRebar


Diamet erRebar
2






2


AreaofRebar
0.614

in2
c
.0001

kd
root
Est eel
Econcret e
AreaofRebar

Dept ht ocent erRebar
c

(
)

inWidt hWallDesign
c

(
)
c
2


c








kd
4.992

jd
Dept ht ocent erRebar
kd
3


jd
31.336

YieldSt rengthConcret e
2
MaxMoment

12

inWidt hWallDesign
kd

jd


YieldSt rengthConcret e
1783

psi
YieldSt rengthSt eel
MaxMoment
12

AreaofRebar
jd


YieldSt rengthSt eel
58011

psi
Concrete Design
Simply Supported
Est eel
29000000

Econcret e
5000000

psi
Legends Subdivision
YieldSt rengthConcret e
3000

psi
w
1100

plf
L
26

ft
YieldSt rengthSt eel
60000

psi
MaxMoment
w
L
2

8

MaxMoment
9.295
10
4


ft
lbs

inWidt hWallDesign
8

Dept ht ocent erRebar
36
3



Figure 2. Mathcad Program Calculating Strength of Reinforced Concrete Grade Beam

This
program
assumes a simply supported beam and is more conservative than the fixed end support
beam.


Conclusions

Uplift tests show that the uplift capacity of the helical pier system exceeds the weight and the friction of
a right circular cone of soil screwed into nearly 3’ of claystone. This is termed the cementation force.
This force can equal and exc
eed the weight and frictional forces.
The helicals are unaffected by surface
water at burial depths of nearly 3’ of claystone.

Compacted road base creates an adequate footing for grade beam over pier construction.

Summary

A shallow helical pier system wa
s successfully used to resist uplift and support a typical grade beam over
swelling claystone.

This system proved more economical that conventional grade beam over concrete
pier foundations.

References

1.

1997, Lindberg, M.,
Civil Engineering Reference Manu
al
, Sixth Edition,
Professional Publications.
Belmont, CA

4

Shallow Helical Pier Foundation in Claystone, C.S. Russell, 2012

2.

2012, Chance Piers,
http://www.abchance.com/resources/literature/helical/TM
-
10
-
2006_SS5
-
series.pdf

3.

1976
, Popov,E.P., Mechanics of Materials, Second Edition, Prentice
-
Hall, Englewood Cliffs, New
Jersey

Terminology

t&b = top and bottom, e.w. = each way, o.c. = on center

kip = kilopound, ksf = kilopound per square foot, plf = pound per liear foot, psf = pound
per square foot

List of Figures

Figure
1
. Side View of Helical Pier Foundation


Piers are set 20 foot on center

Figure 2. Mathcad Program Calculating Strength of Reinforced Concrete Grade Beam

Bibliography

Chris Russell is a consultant with a BS, MS and P
hD in engineering. He is also a registered professional
engineer.