Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

The Press-in Method with Augering - Augering Area in Relation to Retaining
Wall DesignTsunenobu NOZAKI
General Manager, International Construction Design & Planning Department, Giken Ltd., Tokyo, Japan
Email: nozaki@giken.com

ABSTRACT
When retaining walls are designed, design parameters of undisturbed soil are normally used. In reality, if theretaining wall is comprised of piles, they are often installed assisted by ancillary equipment, such as water jetting oraugering, to reduce installation resistance. To what extent the impact of these ancillary methods have on the soil andeffect the retaining wall design is not known.
This report features impacts of driving assistance of pile installation on retaining wall design. A project in the UK
was used as a case study to observe if the design of a cantilevered retaining wall is still satisfied, despite a local ground
disturbance by augering in the case of cohesive soil.
Key words: Press-in Method, Retaining Wall, Chalk

The section adjacent to house No 3 has a concrete

1. Outline of the project

retaining wall, which gives way to a short brick and cobble

1.1. Place
Erith is a town in south-east London in the London

retained section at the southern end, adjacent to a garden.

Borough of Bexley. The town is located to the north-east

The remainder of the central and southern sectionof Bexleyheath and north-west of Dartford and lies on thecomprises mainly a very steep unsupported slope,south bank of the River Thames.

overgrown with vegetation but with some zones of bare1.2. Background and objectives of the project
The site was previously used as a warehouse whichsoil.

covered the central and southern part of the site. The front
and northern parts were concrete hardstanding. It wasproposed to develop some area of the site with flats of 5
storey height and of lightweight steel-frame construction.
2. Structural type and piling method
Site2.1. Site condition
The site is located close to the south bank of the River
Thames as shown in Fig. 1. The surface of the site wasopen ground sloping gradually down from West Street,with a steep slope of made ground on the north-westernboundary adjacent to private residential properties. The
steep slope is approximately 5m to 6.5m higher than thesite. It was proposed to build a retaining wall on the slope
shoulder for the construction of a car park.

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Site Location Map

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

2.2. Ground condition
The basic soil makeup is Thanet Sand overlying

Table 1.

Upper Chalk. The site investigation report revealed quite
deep

madegroundincorporating

anold

Preliminary Design Values

Bulk

Effective

Effective

Density

Cohesion

Friction3buried(kg/m )2(kN/m )unreinforced concrete slab, extending to between 1.55m

Angle(degrees)and 2.4m at the southern extent of the proposed building.

ThanetProgressing northwards, the made ground is less thick. AtSand toits thickest part, there was a significant proportion of loose

1.3/1.5m

grey/brown silty sandy clayey soil with frequent brick anddepth

concrete rubble. There were also layers of brick rubble.

ThanetProgressing northwards, the thinner made ground tended

Sand toto be more stable.

+4.75mEach trial pit revealed a chalk layer beneath the madeHard Chalk1800

ZERO331900

ZERO362000

2040ground. Within the southern and central area, there was an
upper zone of highly weathered weak chalk comprisingintact flint fragments and chalk fines. This structureless

The typical borehole log is shown in Fig. 2 with thesheet pile elevation.

material is visually borderline and variable between Grade
Dm and Dc (matrix or clast dominant). However, being
just at or above the water table, it was in a very poor state
when excavated.
The underlying chalk, which was met at each point,
comprises intact blocky chalk, a structured intact hard
chalk with some fines. Once met, it proved to be relatively
difficult to excavate by mechanical excavator. The
laboratory tests have shown this to be a high to very highdense chalk.
Ground water inflows assessed as “medium to fast”
were met in the most southern trial pits in the lowest
ground at 2.15m to 2.6m depth, at or just above the top of
the hard chalk. There was no rise in the water level at thistime of year (April). At the time, the tide was low in theThames.
The following values in Table 1 are suggested forpreliminary design of retention structures: -Grade 1-2 Chalk(Cu=20kPa)

Fig. 2- 502 -Typical Borehole Log

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

2.3. Structural type
The Project was to supply and install a sheet pile
retaining wall, to support adjacent ground, and to allow
the construction of a car park serving the site development.
After excavation the wall eventually stood in cantilever,
and was likely to deflect approximately 40mm at the top.
Giken Ltd. proposed that in order to compensate for thisdeflection, they install the piles off plumb, while still
ensuring that the wall was positioned precisely. Giken’sengineers were consulted on this design before workcommenced.

Augering

The permanent retaining wall was built with 9.5m to
12.5m long crimped pairs of AZ46 and AZ50 sheet piles.
They were installed to form a cantilevered retaining wall

Photo 2.

Piling Work in Progresswith the retained height of 5.4m to 7.18m. The retaining
wall was finished off with a wooden clad for aesthetic

3. Press-in piling

reasons as shown in Photo 1.

3.1. Layout
The sheet pile configurations are shown in Fig. 3, Fig.
4 and Fig. 5.

Fig. 3 Plan View Layout
Photo 1.

Completion with a Wooden Clad

2.4. Piling method
In order to install sheet piles through both the rubbly
made ground and reasonably stiff chalk, the Press-in withsimultaneous augering was utilized. As the retaining wall
was installed close to the boundary line on a sloping
embankment, the passive side of the retaining wall wasaugered during the pile installation to cope with spoils
from the augering. Photo 2 shows the auger set up at thepassive side.

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Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

the retaining wall, a flight narrower than the pile width
was utilized. This left some undisturbed area within thewall profile as described in Fig. 6.

Fig. 6

Augering Area

4. Additional data
After each pile installation, the area disturbed byaugering was backfilled with augering spoils. After pile
installation, the lateral deflection of the retaining wall
was continuously monitored through to the projectcompletion.
Despite the augering on the passive side, theFig. 5

deflection at the top of the retaining wall was only 30mm

Cross Section

or less, while the design deflection is estimated at 44.7mm.
Photo 3 shows post excavation.

3.2. Productivity
The piling work was carried out from November toDecember in 2005 by utilizing the Silent Piler
SCZ675WM with the Pile Auger. The typical production
rate was approximately 100m a day of total pile driving
length, which is equivalent to 116m2 of wall area.
3.3. Encountered difficulties
On the project, minimizing lateral deflection of theretaining wall during excavation was critical, due to thepresence of adjacent private properties. Therefore, interms of lateral deflection of the retaining wall, it was ideal
to position the auger on the active side to minimize groundPhoto 3.

Post Excavation

disturbance on the passive side. At the same time, it wasalso necessary to avoid disturbing the active side, tominimize ground displacement on that side. Furthermore,

5. Design consideration
This report reveals that the augering method was

it was not practical to temporarily deposit and backfill

effectively used to overcome difficult ground conditions.

arisings from the augering on the active side.

At the same time, the retaining wall design requirement

After discussing issues of the effect on the retaining

was met, despite the ground disturbance as a consequencewall design and wall integrity, a cantilevered Z sheet pileof the augering. It is thought that the following aspects

retaining wall was installed utilizing augering on themay have contributed to achieving the design requirement.

passive side. In order to minimize the lateral deflection of- 504 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

1) Ground-arch effect
When a retaining wall is loaded laterally,distribution of the soil stress can be simulated basedon the Theory of Elasticity using the Boussinesq
equation that considers a point load on the surface ofa semi-infinite, homogeneous, isotropic, weightless,elastic half-space. The concept of the soil arch effectprepared from the Boussinesq's equation by Bowles[1996], as shown in Fig. 7.
Fig. 8

Pile width, D

Effective passive area

The effective passive area is described as
Pile contact pressure, q0

follows: Effective passive area = nW
n: Passive mobilisation factor
W: Undisturbed area on pile surface
In general, a passive mobilisation factor of 2-3 isused depending on soil conditions. There is nosimple relationship between the characteristics of theeffective passive area (nW) and soil conditions,because any relationship is dependent on the pile sizeand on the nature and sequence of the strata. "nW" ata certain distance (H) in low strength cohesive soil isgenerally greater than that in dense cohesion less soil.
2) Confined effect of disturbed soil
After pile installation, the auger is extracted andthe augering spoils are backfilled. In general, thestrength of the backfill is ignored for retaining wall

Pressure distribution formed on the passive side ofdesign, due to its uncertainty as a result ofa pile, showing the intensity of pressure q/q0, based on thedisturbance. However, the augered areas are not left

Boussinesq equation (after Bowles [1996])

void, but are filled with augering spoils. As such, theFig. 7

augered areas are sufficiently confined and theOn the project, the piles were installed withbackfill should transfer the active earth pressure toaugering, leaving an approximately 250mm ofthe surrounding undisturbed soil to some extent. It is

undisturbed area between adjacent piles. It wasthought that this “confined effect” contributed to aconcluded that the lateral earth pressure loaded onlateral deflection, which is smaller than the design

the disturbed area was radially transferred to thedeflection. However, this effect is not covered in thispassive soil. This gave an effective passive area

report, and the following aspects should be observed

wider than the 250mm as shown in Fig. 8.

if checking this issue at another point in time.
a)Measuring density or stiffness of backfillb) Measuring shear strength of backfillc)- 505 -

Measuring compressibility of backfill

Proceedings of the First International Conference on Press-in Engineering 2018, Kochid) Measuring stress on pile surface on passivesidee)

Measuring stress on surface of undisturbed
soil on passive sidef)

Measuring lateral deflection of retainingwallg) Making speculation on linking above aspects.
6. Concluding remarks
When augering is required to install retaining walls,
it is prudent to give retaining wall design carefulconsideration to this aspect, especially if the retaining wall
is a cantilevered wall. This is because the impact on thesoil parameters by augering is not scientificallyascertained and it is difficult to evaluate characteristics ofthe disturbed soil. Therefore, augering may cause
unexpected large horizontal deflections of retaining walls
if an overoptimistic retaining wall design is used. On theother hand, if over pessimistic retaining wall design isused, unnecessary remedial works, such as grouting or thelike, may be required to stabilize the retaining wall. This
will make retaining wall construction less economical.
Unless retaining walls are installed with a complete
underreamed auger (larger auger diameter than the pilewidth), there are some undisturbed areas on the retaining
wall surface, which have decent horizontal passive
strength for the retaining wall. Based on this, the retaining
wall can be designed with a reasonable passive
mobilization factor, which determines the effective
passive area. With this approach, retaining walls can bedesigned

rathereconomically,

avoiding

overestimated/underestimated design.
References
Bowles, J.E. 1996. Foundation Analysis and Design. 5thedition, McGraw-Hill, Singapore.

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