airport central drained basement

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AIRPORT CENTRAL DRAINED BASEMENT

Terence J. Wiesner1

ABSTRACT

A case study is presented of the geotechnical design, construction and construction monitoring of the

drained basement of the Airport Central Sheraton Hotel building at Mascot, Sydney.

INTRODUCTION

This paper provides brief details of the geotechnical design and construction monitoring carried out for

the basement of the Airport Central Sheraton Hotel at Mascot, Sydney (Figure 1). The building is 12 stories

high with down to 4 levels (depth of excavation about 12.5m) of basement parking and contains commercial

premises as well as a five star hotel.

Figure 1 : Airport Central Sheraton Hotel at Mascot, Sydney.

The design and construct tender to build the hotel was won in 1990 by Concrete Constructions, now

Walter Constructions who, with Douglas Partners, geotechnical consultants, proposed a drained basement.

This nonconforming design eliminated the need for tanking and permanent anchors to resist hydrostatic

uplift. It facilitated an accelerated construction program using the 'top down' method, the first time this

technique was used in Australia. In 'top down' construction the external diaphragm wall, load bearing

barrettes and piles are installed and the basement floors and internal building structure used for lateral

support allowing excavation of the basement and construction of the superstructure to proceed together. This

can greatly reduce building construction time and cost.

1

Principal, Douglas Partners Pty Ltd, 96 Hermitage Road, West Ryde, Sydney, NSW 2114, Australia


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DESIGN CONSIDERATIONS

Analyses of seepage flows into the proposed basement area for hydraulic conductivity of the clays

ranging from 10 -6 to 5 x 10 -8 metres/second were carried out using MODFLOW, a 3 dimensional finitedifference numerical ground water modeling package. The diaphragm wall was assumed to be impermeable

and founded into the clays.Constant head boundaries were set

at the regional groundwater head

at a distance of 70m from the wall.

High inflows initially occurred

in the model due to water from the

subsoils inside the walls and

lowering of the water table to 12m

associated with construction of the

drained basement. Steady state

conditions were achieved in the

model within 50 days with most of

the flow coming from the shalelayer (k = 10 -6m/sec). Estimatedflows into the excavation after 200

days with the wall penetrating to

17m depth ranged from 35 m3/day

to 260 m3/day. Penetration into

the clays to 25m depth reduced

estimated inflow to around

100m3/day for 10 -6m/sec clay

hydraulic conductivity.

The estimated draw down in

the water table around the wall2000 days after construction of

the drained basement is shown in

Figure 2. Even immediately

adjacent to the wall draw down is

less than 300mm indicating a

permanently drained basement is

unlikely to significantly affect

neighbouring structures.

Removal of approximately

12m of predominantly sandy soil

and design of the basement floor

to allow seepage inflows to asump will result in upward

movement of the unloaded strata

due to elastic rebound and

swelling. Pore pressure gradients

causing flow through the clay

layer could lead to additional

upward movements. Accordingly

the effects of construction of a

drained basement on pore

pressures, flows and soil

movements were studied using

FLAC, a finite difference numerical computer program capable of modeling coupled groundwater changesand elastoplastic deformations of the soils during and after basement excavation.

Figure 2 : Predicted drawdown (mm) and instrumentation locations

Figure 3 : Pore pressures and flow vectors 3 days after excavation

Pore Pressures

B = 50 kPa

C = 100 kPa

D = 150 kPa

E = 200 kPa

Scale x 10 = metres


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The construction procedures and subsuface stratification represented in the model were relatively severe

compared with actual excavation and initially produced high pore pressure gradients as well as yield in a

limited section of the clays. Figure 3 shows pore pressure contours and flow vectors approximately 3 days

after rapid excavation to basement level. Pore pressure contour interval is 50 kPa and there are high values

of over 200kPa in the clayey shale and clay. With about 10 metres to basement level in the clayey shale a

simple calculation, neglecting the contribution of material strength, indicates a factor of safety against heave

of close to 1. In the model the strength and weight of overlying material confined this area. As pore

pressures dissipated and approached the long term steady state conditions, the clays returned to the elastic

state and stability increased. In a similar plot at 50 days pore pressures are approaching steady state

conditions and are approximately hydrostatic towards the centre of the excavation.

Long term swell movements resulting from the removal of approximately 12.5m of sands and water were

estimated using the model and other calculation methods to be of the order of 60 to 100mm. Approximately

one third to one half of this movement was expected to occur within a few days of removal of the overburden

materials.

Based on the FLAC, MODFLOW and other calculations it was therefore considered that conditions inside

the wall would eventually approach hydrostatic with very small upward flows. Significant heave was

considered unlikely provided the wall and underlying clays formed an effective seal. Monitoring during

construction was considered essential to ensure design requirements were met and pore pressures and groundresponses were as expected.

CONSTRUCTION MONITORING

After satisfactory completion of

design details and contract

negotiations, construction of a

diaphragm wall around the perimeter

of the site was commenced, followed

by barrettes to carry internal loads of

up to 13000kN, and driven steel H

piles to carry smaller loads of 1000to 3000kN. Line loadings on the

wall were estimated to range from 85

to 210kN/m although some sections

carried column loads of up to

3600kN. Full time geotechnical

monitoring was carried out during

slurry wall construction. Each wall

panel was logged during excavation

to ensure it was keyed at least 2

metres into low permeability clays

and the founding material was

adequate for the design loads.Piezometers, borehole

extensometers and inclinometers

were also installed to monitor ground

water and soil response during

excavation. The locations where

instrumentation was installed are

shown in Figure 2. Frequent

problems were experienced with the

excavation contractor damaging or

destroying the monitoring points.

Some success was eventually

achieved by encasing the upper section of the piezometers with heavy drill casing.

Figure 4 : Plot of measured and predicted excess pore pressures


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Piezometers were initially measured weekly then monthly then 3 monthly. Selected results are

summarised on Figure 4, which was a simple way of presenting the internal piezometer results and the

associated risk of heave. Measured pore pressures above the 'heave' line would have meant the excess

pressure head at the location of the piezometer screen was greater than the effective weight of overlying

material below final basement level. A factor of safety of 1.2 was applied for the 'marginal stability' line.

Excess pore pressures were at a relatively safe level by February 1991 when final excavation level was

achieved. Subsequently pore pressures slowly approached the theoretical long term predictions.

During construction, in order to facilitate excavation, a deep pumping bore was installed to lower

groundwater levels inside the excavation. Flows were initially higher than expected however they dropped

rapidly to well below 20 m3/day after a small leaky section of the western portion of the wall was located and

plugged. Water inflows measured after the project was completed were generally minimal and it was not

possible to distinguish between actual draw down and natural groundwater fluctuations in water level

measurements around the building.

The excavated sands were fine grained and silty in part. Problems were experienced with excessively wet

spoil, perched water tables, and traffickability due to the partially saturated sands and speed of excavation.

These were addressed by draining to and pumping from shallow sumps.

Excavation was initially carried out to the first basement level and the outer sections of the floor slab cast

to connect and brace the walls to the internal columns. This left a large central hole through which plant waslowered and further excavation carried out to the third basement floor level. This was also partly cast as it

was required to provide further bracing to the walls before the lowest basement level was excavated. A

subfloor drainage system and sumps were installed beneath the lowest basement floor slab. Construction

proceeded on the superstructure at the same time as the basement. Inclinometers just outside the basement

walls indicated lateral movements of less than 30mm in the perimeter walls confirming the adequacy of the

method of lateral support.

Unfortunately the borehole extensometers were destroyed early in the excavation and could not be located

or recovered despite the use of coloured grout around the holes. Observations after final excavation level

was reached indicated no discernible basement heave.

The building was successfully completed and the drained basement has operated for almost 10 years with

very low inflows and no known problems apart from normal routine maintenance to clean out iron oxide and

iron hydroxide precipitates in drains, sump and pumps.

ACKNOWLEDGEMENTS

All the work described in this paper was carried out by Douglas Partners personnel as geotechnical

consultants to Concrete Constructions Pty Ltd, now part of the Walter Construction Group. The permission

and assistance of Stephen Tamone, project director for Concrete Constructions, to publish details of the

project are gratefully acknowledged. Emged Rizkalla, site engineer for Douglas Partners at the time, made a

substantial contribution to the success of the project as he carried out much of the fieldwork and site

monitoring.

airport central drained basement

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