method statement for basement construction - the lyreen view … · 2019. 12. 2. · reviewed this...

IBS2933 – New Development at Mill Street, Maynooth Methodology Statement for ‘Safe’ Basement Construction

IBS2933 – New Development at Mill Street, Maynooth

Mill Street, Maynooth, Co. Kildare RPS Civil & Structural:- Methodology Statement

for ‘Safe’ Basement Construction



Mill Street, Maynooth, Co. Kildare

RPS Civil & Structural

Methodology Statement for ‘Safe’ Basement Construction

Contents

Introduction

Basement Construction / Methodology

Vibration / Noise / Movement

Ventilation / Structure Integration

Conclusions / Caveats

Appendices:

Appendix ‘A’ – Architects Drawings

Appendix ‘B’ – Cross Section at St Mary’s Church & Typical Details

Appendix ‘C’ – Photographs of a major RPS site illustrating similar basement

design and construction techniques



INTRODUCTION

RPS has prepared this ‘Methodology Statement for Safe Basement Construction’ to consider

the most suitable / appropriate form of basement construction for the new residential

development at Mill Street, Maynooth.

This includes particular focus and commentary on the following:-

What RPS considers the most appropriate means of construction / piling works to form

the basement car park.

What RPS considers the most sensitive & appropriate method regarding the

construction of the basement car park access ramp which sits in close proximity to the

old stone boundary wall of St Mary’s Church and the structural body of the Church

itself.

What RPS considers the most sensitive & appropriate method regarding the

construction of the Residential Blocks ‘A’ & ‘B1’ which are located in relative proximity

to the old stone boundary walls of St. Mary’s Church.

The drawing package compiled by Michael Fitzpatrick Architects includes drawn-up sections

through the proposed structures located closest to the St. Mary’s Church and these drawings

are included in the Appendices of this Report complete.



Basement Construction / Methodology

One of the most important initial considerations for any Key Project is to classify the on-site

subsoil conditions and to confirm their properties / characteristics, to ensure the credibility and

robustness of all subsequent structural and civil engineering proposals.

This essential information is generally acquired through a rigorous Site Investigation which

ultimately informs the design and the most appropriate type of basement construction to be

adopted.

It also allows for a clear understanding of the sub-soil performance characteristics and whether

traditional spread footing foundations or a defined piling system is deemed to be most

appropriate and cost effective solution. Any Site Investigation works should take account of

the recommendations contained in British Standards BS5930: 2015: ‘Code of Practice for Site

Investigations’.

The choice of basement construction technique in the case of the Maynooth project is driven

by the following:-

1. Sub-soil / ground conditions

2. Anticipated ground water / water table conditions

3. Proximity to boundary walls

4. Proximity to St Mary’s Church

The latest layout drawings from ‘Michael Fitzpatrick Architects’ clearly illustrate the extent of

the large single-storey basement car park proposed for the new residential development.



Lower Ground Floor Plan & Basement Car Park

The proposed basement car park is an ‘L-shape’ with one leg of the ‘L’ located within the south

western area of the site as far as the boundary with Mill Street. The sides of this leg are

bounded north & south by the stone boundary wall of St Mary’s Church and the car park

belonging to some existing residential apartments respectively. The other leg of the ‘L-shape’

car park extends northwards directly behind St. Mary’s Church and this particular section of

the car park also incorporates the long vehicle access ramps.

The new residential block, ‘Block A’, is located at the western end of the site and the proposed

basement car park wall follows closely the line of the old stone boundary wall of the Church.

The new residential block ‘Block B1’ is located to the north of the site and in this case, the

vehicle access ramps to the proposed basement car park follow the line of the old stone

boundary wall to the rear of St. Mary’s Church.



The Architectural plan drawing, ‘PL-16-164-004’ illustrates the proposed single storey

basement car park layout and the proximity to the existing old stone boundary walls of the

historic St. Mary’s Church. [Note: – the new basement wall structures are positioned

approximately 1m from the old stone boundary walls for much of the their length.]

It may well appear that there is an opportunity in some areas to carry out ‘open-cast’

excavation and ‘batter back’ the ground thus allowing for the construction of a traditional

retaining wall to the basement. However, RPS would highlight that there is a design

requirement for retention in excess of 6m and the proposed line of basement wall is only 2.5m

from the rear of the footpath on Mill Street, some 1m from the car park of the nearby existing

residential apartments, and 1m from the old boundary wall of the Church. Therefore, RPS

would highlight that, for the area of basement car park in the southern area of site, there is

little or no opportunity to construct a traditional retaining wall in this area as the extent of

ground required to ‘batter back’ plus the additional working space required are not available.

Furthermore, the proximity of the Lyreen River and the probable high water table, would prove

extremely problematic with the construction methodology of a ‘traditional wall’ having a

formation level in excess of 6m below existing ground.

St. Mary’s Church as viewed from the Moyglare Road

with the existing Residential Flats visible to the south



To avoid ‘battering back’, it may be argued that steel sheet piling could be used as a temporary

support to ground adjacent to existing properties to allow excavation for traditional wall

construction. However, RPS would state that this can prove to be extremely problematic and

a high-risk option, particularly when deployed adjacent or close to sensitive buildings as

vibrations generated by installing the sheet piles can often cause damage.

Therefore, in the case of this project, RPS does not believe steel sheet piling to enable

traditional construction to be a viable option.

The northern leg of the ‘L’-shaped basement car park also includes the construction of the

vehicle access ramps and these too require in excess of 6m retention depth. The western

retaining wall which effectively houses the ‘Down Ramp’ is only some 1.5m distance from the

line of the old stone boundary wall and the actual body of the Church set another 2.5m inside

that again.

Therefore, assuming a 45 degree load spread, the rear foundation of the single storey element

of the Church plus the weight of the existing stone boundary wall will ultimately ‘surcharge’ the

new basement wall. [In other words, the new retaining wall will not only be required to provide

retention to allow required level changes - it will also have to be designed to resist additional

lateral forces generated by the existing foundations of the Church and boundary walls.]

While it may be considered possible to identify some areas around the perimeter of the

basement where the construction technique adopted could possibly be a traditional reinforced

concrete retaining wall, RPS believes that this should be omitted as an option for the following

reasons:-

Over 6m retention requires considerable engineering for a traditional retaining wall

There is not enough space for ‘batter back’ construction plus working space

The use of steel sheet piles could produce excessive vibration & noise levels

The potential use of different techniques around the site depending on location may

well lead to an over-complication of the construction process

The construction programme timeline may well require to be extended and ultimately

prove more costly.

The proximity of the Lyreen River and information gained through the Flood Risk

Assessment would suggest that ground water / water table issues could prove

problematic in the construction of any traditional wall.



Preferred Basement Design Option

RPS has extensive design experience in basement construction, and consequently, having

reviewed this scheme, would favour the use of a ‘Rotary Cased Continuous Flight Auger Piling’

solution for the entire basement perimeter.

Basements generally can be successfully formed on restrictive sites by deploying in-situ bored

piling techniques. Traditionally this has been done using the ‘CFA’ method of piling

(Continuous Flight Auger) where a hollow stem auger is rotated into the ground to the required

design depth and in so doing forms a hollow cylinder within the ground. As the auger is slowly

withdrawn, concrete is pumped down the shaft via the hollow stem of the auger and is the

delivered under pressure carefully controlled by computerised equipment.

Once complete and with the concrete still ‘wet’, a pre-fabricated reinforcement cage is

positioned over and sunk into the top of the pile to a specified design depth.

CFA Piling can be either of the following:-

‘Contiguous Piling’ - if groundwater conditions are not an issue then individual stand-

alone piles with a slight distance separation between the piles

‘Secant’ Piling – if groundwater conditions are likely an issue, the use of ‘secant piles’

provide an interlock between each pile thus good resistance to potential ground water

ingress during the construction process.

RPS’ experience of basement construction design would strongly favour ‘Rotary Cased CFA

Piling’ as the preferred solution for the construction of the basement on this scheme with the

installation of a ‘Secant Piled Wall’.

This type of piling, being an augured solution, will exhibit low values of vibration & noise

(compared to any driven piled or steel sheeter solution) and will ultimately provide the

necessary due care and respect to the structural fabric of the nearby St. Mary’s Church.

‘Rotary Cased CFA Piling’ drills a hollow steel casing into the ground to the required design

depth. An auger counter-rotates in tandem to create a ‘water-tight’ shaft into which concrete

is placed in the normal manner.



This prevents phenomenon called ‘flighting’ ** and the loss of sub-soil ‘fines’ material - hence

greatly reducing the possibility of settlement and damage to sensitive buildings.

** ‘Flighting’ –

If there are sands and gravels present in the sub-soils they are usually water-bearing

and under artesian pressure head. When the auger breaks through the surface and

meets the increased resistance of the underlying sands and gravel, the auger, despite

the computerised control mechanisms, acts as an ‘Archimedes Screw’ and very rapidly

‘fines’ material under this water pressure is flushed to the surface (‘flighting’) thus

creating subterranean voids which can result in local subsidence of the ground and

potential movement of the buildings supported thereon. The use of a steel casing

reduces the potential for this phenomenon to cause damage to adjacent properties.

To limit horizontal deflections (which will be a maximum at ground level) and in order to reduce

any potential for movement in adjacent buildings/roads, the tops of the engineered pile runs

will have to be cross-braced/propped prior to all excavation works proceeding. This will be

the responsibility of the ‘Temporary Works Designer’ who will act for the Piling Contractor and

normally will involve the temporary installation of large diameter hollow steel struts for this

purpose.

RPS have collaborated on several basement projects with specialist civil engineering & piling

company, ‘Dawson-Wam’ who have offices in Belfast, Dublin, Edinburgh, London &

Manchester. ‘Dawson-Wam’ have perfected the use of the ‘Rotary Cased CFA Piling’ system

and are specialists in the installation of secant piled retaining walls and basements. The

secant piled wall is primarily used in ground conditions that are not suitable for other systems

or where there is a high water table. The Rotary Cased CFA system can also, in the right

conditions, offer close proximity piling to within 100mm of existing facades/walls/boundaries

which RPS believes is the ideal solution for this particular scheme in Maynooth.

The system provides relatively silent and vibration-free installation, and seen as a preferred

option near buildings, roads, and other sensitive structures. Secant walls can be used to form

a continuous watertight wall which in this case will benefit the requirements for the

underground carpark.



Selected Pages from Dawson Wam Basement Construction Literature



Vibration / Noise / Movement

Vibration

RPS will insist on specifying the requirement for precision quality vibration monitors and

recording equipment being installed at key locations to constantly monitor vibrations during

the basement excavation/construction process.

This will detect and alert the Structural Engineering Team to any excessive vibrations caused

by the piling operations which could pose a threat to the structural fabric of the St Mary’s

Church or any other nearby structure.

Noise

RPS will also insist on precision Noise Monitors being installed around the site which will have

predetermined trigger levels in accordance with the recognised environmental standards.

These monitors will be alarmed and alert the Construction Team to any site noise levels

exceeding acceptable limits.

Movement

RPS will insist on specifying the requirement for precision instrumentation and recording

equipment being installed at key locations to constantly monitor for any potential movement

on an hour-to-hour basis, during the basement excavation/construction process.

This rigidly enforced procedure will detect and alert the Structural Engineering Team to any

early signs of movement allowing action to be taken.

The existing stone boundary walls of St. Mary’s Church will be regarded as the element most

at risk to movement during the piling installation process. The condition of these stone walls

is unknown to the author and it is also unknown if there is any form of foundation (could be

very unlikely).

If the old stone boundary walls are found to have no foundation and exhibit some movement

during the pile installation process, it may be deemed necessary to install a concrete footing

using traditional sequential underpinning techniques.



Site Investigation / Geotechnical Engineer

RPS would strongly recommend that before design commences on the project that a

comprehensive Site Investigation is undertaken which yields sufficient information to establish

the properties and characteristics of the subsoils and all a qualified Geotechnical Engineer to

assist with the engineering requirements and design of the extensive basement car park.

Ventilation / Structure Integration

Ventilation of the basement car park areas may well require to be mechanically assisted.

Careful liaison with the Architect and Building Services Consultants will be needed to ensure

that ventilation requirements are adequately integrated into the structural grid and to

coordinate design of openings, which for water tightness will be better positioned through the

podium slab rather than through the ‘Secant Piled Wall’.

To reduce the excavated depth necessary to accommodate the car park ducted ventilation

systems, RPS would consider the use of ‘impulse fans’ which would omit the need for ducts,

reduce the overall basement depth, and be more cost efficient.



Conclusions

The site on Mill Street, Maynooth, poses engineering challenges for the construction of an

extensive single storey basement car park with formation level at times in excess of 6m depth.

Having reviewed the site location, constraints and boundaries, RPS is of the opinion that the

most appropriate and cost effective solution to constructing the basement structure (which will

be located under the new residential accommodation blocks) is to use a ‘Secant Piled Wall’

installed using a ‘Cased Rotary Continuous Flight Auger’ system. This particular installation

system greatly reduces vibration and noise and more importantly best mitigates against any

potential movement of nearby structures by removing the risk of ‘flighting’.

The system is completed with the installation of a structural ‘Capping Beam’ and ‘Liner Wall’.

The ‘Capping Beam’ effectively allows all the installed piles to share vertical loads generated

by the superstructures above and also lateral loads generated by the retained earth and any

surcharging of the earth e.g. road vehicles etc. The internal ‘Liner Wall’ provides the final

defence to any potential water ingress and will generally incorporate a vertical drain system.

The installation method of the Secant Piled Wall using a well tried & tested ‘Rotary Cased CFA

Piling’ system will permit the basement to be constructed close to any neighbouring buildings

and roads with little or no impact. This is a key issue for this particular site where RPS

clearly recognize the sensitivities / requirements to reduce/eliminate any potential

construction risks whatsoever with regard to the maintaining of the structural fabric

and integrity of the historic St. Mary’s Church.

Caveats

Site Investigation Report

This Structural Report on ‘Safe’ Basement Construction is compiled without access to a site

specific Site Investigation Report and assumes that there is no rock at or close to the surface,

which could affect the preferred choice of construction for the basement.

Existing Stone Boundary Walls

The existing stone boundary walls to the St. Mary’s Church may have no foundation

whatsoever and may be susceptible to some minor movements during secant pile wall

installation. If deemed problematic, a concrete footing should be installed by underpinning.



Appendices

Appendix ‘A’ - Architects Drawings

Sitemap


Site Section Drawing 03



Appendix ‘B’ - Cross Section at St Mary’s Church & Typical Details

Appendix ‘C’ - Photographs of a major RPS site at Titanic Quarter



APPENDIX ‘A’

Architects Drawings

Sitemap







SITEMAP



APPENDIX ‘B’

Cross Section at St Mary’s Church & Typical Details



Cross Section at St Mary’s Church & Typical Details



Appendix ‘C’’

Photographs of a major RPS site at Titanic Quarter, Belfast

illustrating similar basement design and construction techniques



RPS Site at Titanic Quarter, Belfast

Basement Secant Pile Wall & Capping Beam

RPS Site at Titanic Quarter, Belfast




RPS Site at Titanic Quarter, Belfast –

General View across Basement Car Park Slab

with Secant Pile Wall & Capping Beam in distance

RPS Site at Titanic Quarter, Belfast -


at Ramp Location into Car Park

method statement for basement construction - the lyreen view … · 2019. 12. 2. · reviewed this...

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