January/February 2022 - 19

ground anchors have a design load of 225 kips (1,001 kN), are
up to 140 ft (42.7 m) long, and use 6 in (15 cm) diameter holes.
Wall 2 is 546 ft (166.4 m) long, and it has a 30 ft (9.1 m)
retained height, 80 ft (24.4 m) long piles, and 3 rows of ground
anchors. The ground anchors on wall 2 have a design load of
300 kips (1,334 kN), are up to 125 ft (38.1 m) long, and utilize
8 in (20 cm) diameter holes. Construction of the Westport
Slide Complex retaining walls began in the fall of 2018 and
was completed in the spring of 2020.
This coastal project provides a comparison of using 6 in
(15 cm) diameter drilled holes versus 8 in (20 cm) diameter
holes under similar site conditions. Wall 1 has 4 rows of
ground anchors and utilizes the smaller diameter drilled holes
with a maximum factored test load (FTL) of 225 kips (1,001 kN).
Wall 2 has 3 rows of anchors and uses 8 in (20 cm) diameter
holes with a maximum FTL of 300 kips (1334 kN). Both
contractor staff and Caltrans field staff strongly preferred
drilling more 6 in (15 cm) holes at wall 1 versus drilling less 8
in (20 cm) holes at wall 2 because there were fewer equipment
breakdowns and production numbers were much better with
the smaller holes. The construction timeline and total cost
also favored the smaller diameter holes at wall 1.
In particular, with high-capacity ground anchors comes
the need to use an 8 in (20 cm) drilled hole with larger FTLs.
Higher anchor loads require more post-tensioning steel,
which leads to larger diameter ground anchors; this, in turn,
leads to larger diameter drilled holes. A 6 in (15 cm) diameter
drilled hole is a standard ground anchor size that is seen on
many SPGA projects. This size hole allows 5-strand anchors
to fit easily, while 6 or 7 strands are often required with 8 in
(20 cm) diameter drilled holes. Those larger diameter holes
also require more specialized drill steel, casing and drill rig.
Additionally, the surface area of an 8 in (20 cm) hole nearly
doubles. These factors lead to a substantial increase in time
and effort when drilling the larger versus a 6 in (15 cm) hole.
From our experience working on the Westport SPGA walls
and other similar projects in Northern California, a tipping
point that drives the need for the larger diameter holes seems
to be an approximate FTL of 300 kips (1,334 kN). This is not a
hard rule, as hole diameter is influenced by soil conditions
and the ability of the bonded zone of the ground anchor to
establish a firm bond with the soil strata. For most soil
conditions we experience in Northern California, though,
selecting an anchor spacing that will limit the FTL to
approximately 260 kips (1,157 kN) will likely enable the use of
6 in (15 cm) diameter drilled holes. Post grouting the bonded
zone of the ground anchor after installation is a common
practice with most drilling companies and also helps to avoid
8 in (20 cm) holes with FTLs under 260 kips.
Drilling ground anchors at Westport Slide's wall 1
DEEP FOUNDATIONS * JAN/FEB 2022 * 19
January/February 2022

Table of Contents for the Digital Edition of January/February 2022
