2017-07-24 futurist gdr issue 2 - borough of scarborough · willmott dixon construction ltd...

Willmott Dixon Construction Ltd

Futurist Site

Geotechnical Design Report

REP/GDR/002

Issue | 25 July 2017

This report takes into account the particular

instructions and requirements of our client.

It is not intended for and should not be relied

upon by any third party and no responsibility

is undertaken to any third party.

Job number 251657-00

Ove Arup & Partners Ltd

9th Floor 3 St Paul's Place

Norfolk Street

Sheffield S1 2JE

United Kingdom

www.arup.com

REP/GDR/002 | Issue | 25 July 2017

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Willmott Dixon Construction Ltd Futurist SiteGeotechnical Design Report



Contents Page

1 Introduction 1

1.1 General 1

1.1 Geotechnical category 2

1.2 Scope of report 2

1.2 Assumptions 2

1.3 Previous reports 2

1.4 Use of this report and limitations 2

2 The site 4

2.1 Site location 4

2.2 Site description 4

2.3 Site history 5

2.4 Unexploded Ordnance 6

2.5 Ground level information 6

2.6 Proposed development 6

3 Ground model 8

3.1 Ground investigations 8

3.2 Ground contamination risk assessment 8

3.3 Stratigraphy 8

3.4 Derivation of geotechnical parameters 11

3.5 Key parameter data 11

3.6 Parameters for slope stability assessment 12

3.7 Parameters for new retaining wall 12

3.8 Piezometric conditions 13

4 Global stability 14

4.1 Back-analysis 14

4.2 Existing slope stability 14

4.3 Stability of proposed slopes 15

4.4 Summary of stability analyses 19

5 Earthworks 21

5.1 Site-won fill material 21

5.2 Re-use of site-won fill material 21

6 Demolition of the buildings 23

7 Existing retaining structures 24

7.1 Main section to the Futurist 24




7.2 Section adjacent to King’s Steps 24

7.3 Main section to the Mermaid 24

7.4 Mermaid wall supporting Bland’s Cliff 24

7.5 Terrace walls 25

7.6 Sub-station 25

8 New retaining structures 26

8.1 Design standards & sources of reference 26

8.2 Walls for development platform 27

8.3 King’s Steps 28

8.4 Other Walls to Facilitate the Slope Re-grading 28

9 Drainage 29

9.1 Existing Drainage Infrastructure 29

9.2 Proposed Drainage Strategy 33

9.3 Conclusions 36

9.4 Next Steps 36

References

Figures

Figure 1 Site location

Figure 2 Evidence of Possible Retaining Wall Drainage

Figure 3 Government Guidance on Climate Change Allowances (gov.uk)

Drawings

Drawing FUT-ARUP-ZZ-XX-DR-STR-4010 Sections & Elevations Sheet 4Drawing FUT-ARUP-ZZ-XX-DR-CD-0001 Existing Drainage Network

Drawing FUT-ARUP-ZZ-XX-DR-CD-0002 Existing Catchment Areas

Drawing FUT-ARUP-ZZ-XX-DR-CD-0003 Proposed Drainage

Drawing FUT-ARUP-ZZ-XX-DR-CD-0004 Proposed Catchment Areas

Appendices

Appendix A

Slope stability back-analyses

Appendix B




Global stability calculation output

Appendix C

Yorkshire Water plan

Appendix D

Utility survey




Page 1

1 Introduction

1.1 General

Ove Arup and Partners Ltd (Arup) has been commissioned by Willmott Dixon

Construction Ltd (WDCL) to provide design services relating to enabling works

for the proposed redevelopment at the site of the former Futurist theatre.

The enabling works will involve:

• Demolition of the former Futurist theatre and Mermaid buildings, but leaving

existing retaining structures in place.

• Removal of the surface car park at the top of the slope.

• Re-grading of the slope to ensure that global stability is not adversely affected.

• Backfilling in front of the existing retaining structures at the toe of the slope.

• Construction of a new retaining structure around a new development plot

adjacent to Foreshore Road.

• Preparation of a temporary surface on the new development plot.

• Surface water and land drainage to suit the temporary condition and likely

future development.

• Placing erosion control layer and seeding on the slope.

Future development of the site is likely to comprise a leisure-related building on

the new development plot, with ancillary structures constructed on the slope

above.

This Geotechnical Design Report relates to the following:

• Existing global slope stability.

• Earthworks, including re-use of site-won soils as engineered fill.

• Future global slope stability.

• Future local slope stability.

• Future stability of existing retaining walls.

• Design of new retaining structure around new development plot.

• Design of new drainage to the site.

The future development is not considered in this report, in particular no

assessment is made of building foundations and the impact of new structures on

global and local slope stability.




Page 2

1.1 Geotechnical category

The enabling works scheme has been identified to be Geotechnical Category 3

according to BS EN 1997-1:2004+A1:2013, in that it relates to a steep slope

where instability could directly affect existing residential and commercial

properties and public highways.

1.2 Scope of report

This Geotechnical Design Report (GDR) is prepared in accordance with BS EN

1997-1:2004+A1:2013. It contains the following

• A summary of the findings of the Ground Investigation Report

• A presentation of the ground and groundwater model used in design

• A description of the proposed works

• Detailed assessment of slope stability

• Assessment of the existing and proposed retaining structures

• Assessment of the potential for re-use of soil arisings

• Details of the proposed surface water and land drainage

This Geotechnical Design Report should be read in conjunction with the Ground

Investigation Report (GIR) 251657/REP/GIR/001.

1.2 Assumptions

To prevent uncertainty, the geotechnical investigations and design contained in

this report are based upon the assumptions listed in BS EN 1997-1:2004,

paragraph 1.3 i.e. that all investigation, design and construction works are carried

out by appropriately qualified and experienced personnel.

1.3 Previous reports

In addition to the factual ground investigation reports, this report uses information

from the following references:

Geo-Environmental Interpretative Report, Mermaid/Futurist Theatres, White

Young Green, August 2008 (for Yorkshire Forward).

Phase II Geotechnical and Environmental Assessment, Proposed Town Hall

Development Site, URS, October 2014 (for Scarborough Borough Council).

Site Investigation & Slope Stability Analysis Report, Futurist Theatre, Solmek

Ltd, March 2015 (for Billinghurst George & Partners).

1.4 Use of this report and limitations

This report has been prepared by Arup on behalf of WDCL in connection with the

Futurist site project. It takes into account our Client’s particular instructions and




Page 3

requirements and is not intended for, and should not be relied upon by any third

party and no responsibility is undertaken to any third party in relation to it.

All reasonable skill, care and diligence have been exercised within the timescale

available and in accordance with the technical requirements of the brief.

Notwithstanding the efforts made by the professional team in preparing the report

it is possible that other ground conditions or ground contamination may exist, as

yet undetected, and consequently reliance on this report must be limited

accordingly.




Page 4

2 The site

2.1 Site location

The site is located on the seafront close to Scarborough town centre, at approximate

National Grid Reference 504470, 488597. The site location is shown in © Crown copyright and

database rights 2017 Ordnance Survey

Figure 1.

© Crown copyright and database rights 2017 Ordnance Survey

Figure 1 Site location

2.2 Site description

The site is bounded by Foreshore Road to the east, Bland’s Cliff and commercial

and residential properties to the north, King Street to the west and King’s Steps

and St Nicholas Gardens to the south.

The site can be divided into two distinct areas:




Page 5

• The disused Futurist and Mermaid buildings on the seafront, including a series

of terraces on the slope above

• King Street public car park at the top of the slope

The King Street car park is at around 27mOD and Foreshore Road lies at around

5mOD. The Futurist and Mermaid buildings are built into the toe of the slope and

incorporate buttressed retaining walls with retained heights of around 8m.

Retaining structures also retain Bland’s Cliff along the side of the Mermaid

building.

The overall gradient of the terraced slope is around 25°.

2.3 Site history

The GIR includes details of site history. Refer to Table 1 for a condensed

summary.

Table 1: Summary of historical OS maps

Date Site land use

1700s Site is undeveloped. Bland’s Cliff is shown as an elevated track that extends

down to the beach by retained embankment.

The Kings Steps appear to have been built in the early 1700s on a part of the

slope that forms a promontory.

Various buildings are shown at the top of the slope and on the opposite side

of Bland’s Cliff.

1854 The earliest OS map shows buildings in the west and south of the site along

King Street and Kings Steps.

The site is surrounded by buildings (assumed to be a mixture of residential

and commercial properties) to the north and west

1892

One of the buildings adjacent to King Street is labelled as King’s Cliff

Hospital. Buildings adjacent to Kings Steps include a public house and two

buildings marked as Marine Terrace. A public house is present in the north

east corner of the site on Foreshore Road. The remainder of the site is

landscaped and a footpath is shown from the Foreshore to the top of the

cliff.

1903 The Arcadia Theatre building was built on the site of the existing Futurist

Theatre (although did not extend as far into the toe of the slope). At the

same time, Kingscliffe Holiday Camp was established on the hillside above,

with huts arranged along terraces on the slope.

1912 The Palladium Picture House was constructed adjacent to the Acadia

Theatre.

1920 The Arcadia building was demolished and replaced with the Futurist

Cinema, including cutting further into the toe of the slope and incorporating

a substantial retaining wall.

The Palladium was renamed the Arcadia Theatre.

1957 In 1957 the Futurist Cinema was converted for live theatre and from the

1960s became a popular theatre venue.

1966

The Arcadia Theatre was closed and much of its auditorium was used to

extend the Futurist Theatre stage.

The remainder of the Arcadia Theatre was utilised as the Minstels Lounge.




Page 6

Date Site land use

1990 The OS map shows the buildings along the lower terrace level labelled as

Futurist Flats with an electrical substation to the north. The building on the

upper tier of the site is marked as a Council Office.

1994 The Council Office building has been demolished and presumably the car

park was extended onto this area shortly afterwards.

2006 The Futurist Flats were demolished in this year.

2010 The building in the northern tip of the site is no longer shown on the map.

2.4 Unexploded Ordnance

Historical records indicate that Scarborough was affected by naval bombardments

in 1914 (killing 18 people) and in 1941 (killing 137 people).

In 1914, the slope on the site was occupied by Kingscliffe Holiday Camp and it is

understood that the camp offices were hit directly by the bombardment.

The site was similarly occupied in 1941, although it is not known whether the

holiday camp was still in operation. It is understood that the nearby Grand Hotel

was hit directly, but it is not known whether the site itself was hit.

It is recommended that a UXO specialist is asked to prepare a UXO risk

mitigation report for any future site works.

2.5 Ground level information

2.5.1 Topographic Survey

The site and surrounding public highways are covered by a topographic survey

carried out by Landtech Surveys in 2012 (for URS), with supplementary data

collected by APS in 2017.

2.5.2 Lidar data

In order to carry out global stability analyses that considered slopes outside of the

site boundary (in particular the properties and gardens on Prospect Place), it has

been necessary to use ground level information from Lidar data. The Lidar data

used in this assessment was a Digital Terrain Model (DTM) obtained from the

Environment Agency under an Open Government Licence.

2.6 Proposed development

This report is based on the following proposed development:

1. Demolition of the Futurist and Mermaid buildings, but leaving the existing

retaining structures in-situ.

2. Re-grading of the slope above to mitigate potential global slope instability.




Page 7

3. Placing of fill at the toe of the slope to enhance the stability of the existing

retaining structures.

4. Items 1 to 3 above would be coordinated to ensure that global slope stability is

not reduced at any stage.

5. Preparation of a new building development plot adjacent to Foreshore Road.

6. Possible future building development adjacent to Foreshore Road may include

structures built onto the slope above. The potential impact of such structures is

not considered in this report.




Page 8

3 Ground model

3.1 Ground investigations

Ground investigations have been carried out in four phases, starting with WYG in

2008 (covering the whole site), then ESG in 2014 (covering the site and the

existing Council offices to the west), Solmek in 2015 (supplementary boreholes

above and below the Mermaid building) and SEG in 2016 (supplementary

investigation).

The ground investigations have been carried out to obtain information for both

geotechnical and geo-environmental purposes

3.2 Ground contamination risk assessment

Refer to the GIR for a geo-environmental risk assessment, a summary of which is

presented below.

The ground contamination testing recorded only two local occurrences of elevated

PAH concentrations. Ground gas monitoring found very low levels of methane

and low levels of carbon dioxide as well as low gas flow rates. The site presents a

low risk from ground contamination in relation to future end uses.

It is noted that the above risk assessment assumes that the asbestos-containing

materials known to be present within the Futurist and Mermaid buildings will be

removed completely from site as part of the demolition process.

3.3 Stratigraphy

As presented in the GIR, for the purpose of assessing ground conditions, the site is

divided into five areas with similar stratigraphy:

1. Car park

2. Terraced slope

3. Back of Futurist wall

4. Beneath Futurist/Mermaid buildings

5. Foreshore Road

3.3.1 Car park

The stratigraphy beneath the car park is summarised in Table 2 below.

Table 2: Stratigraphy beneath car park

Stratum Description Thickness

(m)

Made ground Tarmacadam, typically over some

sub-base/capping and underlain by

demolition rubble. Occasional soft

clay layers, in particular WYG WS11

between 0.9 and 2.8m bgl.

Min 1.4m (WYG WS4) outside of

historical building footprints

Typically 3.0 to 3.5m within

historical building footprints

(Maximum 3.8m in WYG WS5)




Page 9

Glacial Till Stiff to very stiff clay with varying

sand and gravel content. Occasionally

firm in uppermost part.

(Presumed) discontinuous lenses of

medium dense to dense silty

sand/gravel

Base of Till not proved

Sand/gravel encountered at 8.8 to

11.7m bgl and 23.8 to 27.5m bgl

(ESG CP105).

3.3.2 Terraces

The stratigraphy beneath the terraced slope is summarised in Table 3 below.

Table 3: Stratigraphy beneath terraces


(m)

Made ground The holes on the upper terrace

(WS201 and WS202) encountered

clayey demolition rubble.

The holes on the lower terrace (WYG

WS7 to WS9) encountered more

variable conditions, but generally

typical of demolition rubble. WS8

encountered a void beneath a

concrete slab (former basement to the

Futurist Flats).

2.2m on the upper terrace.

On the lower terrace, a minimum

of 1.1m (WYG WS9) and a

maximum of at least 3.2m (WYG

WS7).

Glacial Till On the upper terrace, both WS201

and WS202 proved stiff clay beneath

the made ground.

On the lower terrace, only WYG

WS9 encountered natural soils,

comprising stiff to very stiff clay to

3.0m, underlain by gravel with sand.


3.3.3 Back of Futurist wall

The stratigraphy behind the Futurist retaining wall is summarised in Table 4

below.

Table 4: Stratigraphy behind Futurist wall


(m)

Made ground South end – WYG WS10

encountered predominantly soft and

very soft clay fill with some very

loose clayey sand.

Central – WYG BH1, BH3 and ESG

CP104 were closely-spaced but

encountered variable conditions. BH1

encountered very dense demolition

rubble over cemented rubble

(requiring rotary drilling to

penetrate). BH3 encountered

predominantly firm clay. CP104

encountered predominantly soft clay.

Around 8.5m




Page 10

North – Solmek BHA encountered

predominantly firm clay fill with a

cemented rubble layer.

Glacial Till Stiff to very stiff clay with varying

sand and gravel content plus

occasional cobbles/boulders.


medium dense to very dense silty

sand/gravel


In ESG CP104 sand/gravel

encountered between 10.0 to

12.2m bgl and 17.8 to 31.2mbgl

In Solmek BHA sand/gravel

encountered between 18.4 to

25.2m bgl and 17.8 to 31.2mbgl

3.3.4 Beneath Futurist/Mermaid buildings

The stratigraphy behind the buildings is summarised in Table 5 below.

Table 5: Stratigraphy beneath buildings


(m)

Futurist

Floor slab Concrete 130mm

Made ground Limestone sub-base 210mm

Slab Concrete/asphalt 20mm

Made ground Brick/concrete rubble, clayey at base.

TP202 encountered obstruction

within this stratum.

340mm

Glacial Till Firm to stiff slightly sandy slightly

gravelly clay

Till proved to 3.45m bgl

Mermaid

Floor slab Concrete 200mm

Made ground Clayey concrete and brick rubble. 130mm

Glacial Till Firm to stiff slightly sandy slightly

gravelly clay

Firm from 1.2m to 3.0m bgl.

Till proved to 3m bgl

3.3.5 Foreshore Rd

The stratigraphy beneath the pavement on Foreshore Rd is summarised in Table 6

below.

Table 6: Stratigraphy beneath Foreshore Rd


(m)

Made ground Tarmacadam and pavement build-up.

WYG WS15 encountered soft clay

fill between 1.0 and 1.9m bgl.

Apparently increases from south

to north from 0.7 to 1.9m thick.

Beach deposits Soils above around 1m OD appear to

be of different nature to the

underlying Glacial Till. The deposits

include soft clay and medium

dense/dense sand. The clay in

Solmek BHB included shell

fragments.

The base of the beach deposits

appears to be relatively consistent

at around 1m OD (approx. 3.5m

bgl).

Glacial Till Firm becoming stiff clay with

varying sand and gravel content.





Page 11


medium dense to dense silty

sand/gravel

3.4 Derivation of geotechnical parameters

Geotechnical parameters for design have been assessed with reference to the data

in the GIR as summarised in Table 7.

Table 7: Derivation of geotechnical parameters

Parameter Symbol Units Fine soils Coarse soils

Bulk unit

weight

γb kN/m3 Direct measurement of

undisturbed samples

Figs 1 and 2 in BS

8002:2015

Effective

angle of

shearing

resistance

φ’ ° CU triaxial tests. Plasticity

correlation (Eqn 7 in BS

8002:2015).

Shear box tests. Section

4.3.1.3 in BS 8002:2015

Effective

cohesion

c’ kPa CU triaxial tests, or zero. Assumed to be zero

Undrained

shear

strength

cu kPa SPT correlation (Stroud,

1975)

Not applicable

Stiffness E’ and

Eu

MPa SPT correlations (Stroud,

1975 and CIRIA 143)

CIRIA 143

3.5 Key parameter data

A summary of the findings of the in-situ and laboratory testing relevant to

derivation of geotechnical parameters is given in Table 8 below.

Table 8: Summary of testing data

Stratum Description In-situ and laboratory data

Made Ground Highly variable. Typically coarse

grained demolition rubble-type

material, but clayey made ground

also identified.

Coarse material generally loose (low

SPT N values).

Beach deposits Generally medium dense sand,

but soft clay also noted.

Minimum SPT N of 7 in the clay. High

SPT N values in the sand.




Page 12

Glacial clay Stiff to very stiff clay with

varying sand and gravel content.

Occasionally firm in uppermost

part.

SPT N values in the glacial clay show

an increase with depth, ranging from 5

to 15 at the top of the site to around 15

to 35 below around 5mbgl.

Predominantly low to intermediate

plasticity.

CU triaxial tests indicate a narrow

range of effective strength properties.

Can be summarised as φpeak’ = 31°,

cpeak’ = 3kPa.

Glacial

sand/gravel

Shallower deposits are presumed

to be discontinuous lenses of

medium dense silty sand/gravel

SPT N values similar to the glacial clay

above.

3.6 Parameters for slope stability assessment

The parameters in Table 9 are used in slope stability assessments.

Table 9: Slope stability parameters

Stratum γb

(kN/m3)

φk’

(°°°°)

ck’

(kPa)

Comments

Existing made

ground

20 30 0 Likely to be lower bound for mixed made

ground deposits.

Beach deposits 20 33 0 Ignored in slope stability models, as beyond

critical slips.

Glacial clay 22 31 2 Most critical stratum for overall stability.

Characteristic parameters chosen.

Glacial

sand/gravel

20 35 0 Some sand/gravel included to represent

lenses in upper part of slope and thick layer

at depth.

Engineered Fill

(granular site-

won)

19 33 0 Assumed as backfill behind new retaining

wall. Unit weight chosen to represent

reasonable degree of compaction.

3.7 Parameters for new retaining wall

The parameters in Table 10 are used in design of the new retaining wall.




Page 13

Table 10: Slope stability parameters

Stratum γb

(kN/m3)

φk’

(°°°°)

ck’

(kPa)

cu

(kPa)

E’

(MPa)

Comments

Beach deposits 20 33 0 N/A 15 Assumes sand, i.e. any soft

clay is removed.

Glacial clay

(formation

level)

22 28 0 60 15 Phi value chosen to

represent φcv’ for base

friction and sliding

calculations.

Wall backfill

(coarse site-won

and imported

fill)

20 35 0 N/A N/A This assumes that only

coarse fill is used in the

critical zone within the

active wedge behind the

wall.

3.8 Piezometric conditions

3.8.1 Existing piezometric profile

The profile used for stability analyses of the current slope is based on all available

information from the site investigation(s) and is considered a representative

condition.

3.8.2 Future piezometric profile

The profile used for stability analyses of the final slope assumes that drainage is

installed as indicated in Section 9 and is considered a reasonable worst-credible

condition that should not be factored further for design.




Page 14

4 Global stability

4.1 Back-analysis

An initial global stability assessment was carried out using assumed original slope

profile to check that the parameters derived from ground investigation are

reasonable.

4.1.1 Assumptions

The following assumptions have been made in the analysis:

• Drained analysis using unfactored parameters. Original slope will have

had a marginal factor of safety (global FOS = 1.0 approx.)

• Original slope profile taken from historical review of the site and

surroundings, in particular estimated from the original profile of King’s

Steps (current profile with adjustments for former arrangement shown on

1892 OS map).

• Crest of slope kept at existing position and elevation, but without made

ground, on the assumption that the original buildings were built on

platforms cut into the slope.

• Cross section is taken across approximate centreline of site. Hence

imposed loading is only accounted from King’s Cliff Hospital (1892)

approximately 12m behind verge. Assumed 2-3 storey Victorian masonry

building with UDL of 25kPa.

• Piezometric conditions within the Glacial Clay are conservatively

assumed.

• The deeper Glacial Sand/Gravel with artesian groundwater is not

considered, as the critical slip surface does not penetrate to this depth.

• Superficial shallow failures are ignored.

4.1.2 Model and findings

Refer to Appendix A for summary output from Oasys SLOPE software.

The results indicate that the chosen parameters for the Glacial Till and assumed

groundwater conditions are reasonable, and compatible with the assumed history

of the site.

4.2 Existing slope stability

Refer to Appendix B for slope stability calculation outputs from Oasys SLOPE

software.




Page 15

4.2.1 Prospect Place section

Slope stability on this section is likely to be dominated by the presence of the

garden retaining wall above Bland’s Cliff and by the retaining structure forming

the rear of the Mermaid Building.

Due to the lack of information on their structure, no attempt has been made to

carry out detailed stability assessment of either of these structures, however refer

to Section 7.

Global stability analysis indicates that the whole slope is likely to have a factor of

safety of around 1.3.

4.2.2 North section

The North section includes the existing sub-station, with the retaining structure to

the rear of the Futurist Building below and terrace walls above. For consideration

of the retaining structures, refer to Section 7.


safety of around unity.

4.2.3 Central section

The central section comprises the terraced slope and retaining structure to the rear

of the Futurist Building. For consideration of the retaining structures, refer to

Section 7.



4.2.4 King’s Steps section

This section comprises the terraced slope and retaining structure to the rear of the

Futurist Building. For consideration of the retaining structures, refer to Section 7.

This part of the slope has the steepest overall gradient.



4.3 Stability of proposed slopes

4.3.1 Assumed drainage and erosion control

When assessing the stability of proposed slopes, it has been assumed that drainage

measures are installed and maintained as indicated in Section 9. Erosion control

measures are also assumed, as indicated in Section 9.

The measures are summarised as follows:

• Erosion control geotextile within topsoil layer




Page 16

• Land drain across crest of slope at interface of made ground and natural clay

• French drain at back of existing Futurist/Mermaid retaining wall, keeping

groundwater a minimum of 2m below proposed ground level.

• Granular drainage layer on top of Futurist/Mermaid ground slab between

existing and new retaining walls (to keep groundwater levels down at the base

of the new backfill).

• French drain at back of proposed new retaining wall, to prevent run-off from

ponding at the back of the wall.

• French drain in front of proposed new retaining wall.

4.3.2 Prospect Place section

The proposed earthworks do not directly affect the properties on Prospect Place,

or the slope immediately below. Groundwater conditions beneath the Prospect

Place properties and gardens are similarly not expected to be affected by the

proposed works. There should therefore be no net change to slope stability above

the existing Mermaid building.

The retaining structure that forms the rear of the Mermaid building is to be

retained, but the building structure in front is to be removed and replaced with fill

placed behind a new retaining structure. The building structure may be

contributing to the stability of the existing retaining structure and therefore the

demolition and filling process will be coordinated to ensure that the existing

retaining structure remains stable at all times.

The surcharge pressure of the new fill will be more than the UDL imposed by the

Mermaid building, but the fill only extends around half way across the existing

footprint, in order to leave space for the proposed development plot. The proposed

works will therefore have an effect on global slope stability. Analyses have

therefore been carried out to confirm that there will be a net improvement in

global stability.

The results of the stability analyses indicate that the current global factor of safety

is around 1.3, which is normally considered acceptable for cutting slopes. When

the Mermaid building is removed, the reduction in toe weight results in a slight

reduction in factor of safety. Placing fill in front of the existing retaining wall

restores the global factor of safety to slightly above the current condition.

The analyses confirm that fill must be placed in front of the existing retaining wall

prior to removal of the building surcharge.

It should be noted that, although the proposed works slightly improve the global

stability, there are no works proposed to the Prospect Place gardens and their

associated retaining structures, and hence their stability will not be improved.




Page 17

4.3.3 North section

The proposed earthworks are not intended to have a significant effect on the slope

directly above the existing sub-station, although there may be minor tie-in of

earthworks to existing terrace walls.

Groundwater conditions beneath the upper slope may be affected slightly by the

proposed drainage works on the remainder of the site to the south. There is

expected to be a slight reduction in piezometric pressures due to this improved

drainage, and there is therefore likely to be a net improvement in slope stability

above the existing sub-station.

The retaining structure that forms the rear of the Futurist/Mermaid building is to

be retained, but the building structure in front is to be removed and replaced with

fill placed behind a new retaining structure. The building structure may be





existing building, but the fill only extends around half way across the existing




global stability.

The results of the stability analyses indicate a current global factor of safety of

close to unity. When the Futurist/Mermaid building is removed, the reduction in

toe weight results in a slight reduction in factor of safety. Placing fill in front of

the existing retaining wall significantly improves the global factor of safety.



It is also noted that, although the proposed works will significantly improve the

global stability, there are no works proposed to the terraces above the sub-station

and hence their stability will not be improved, other than by the effect of new

surface water drainage measures.

4.3.4 Central section

The proposed earthworks will result in a large volume of soil being removed from

the top of the slope, with some fill placed over the terraced slope and a significant

volume of fill placed in front of the existing Futurist retaining wall. There will

therefore be a reduction in surcharge effect at the top of the slope and addition of

toe weight, which should result in a significant improvement in global stability.

It is assumed that new fill placed over the terraced slope will be granular general

fill and not clayey fill, other than a surface layer of topsoil that will include a

geotextile for erosion control.

Groundwater conditions will change due to the earthworks causing changes in

ground level and also due to the proposed drainage works.




Page 18

The retaining structure that forms the rear of the Futurist building is to be







existing building, but the fill only extends around half way across the existing




global stability.


close to unity.

The removal of the slope crest results in a slight improvement in stability, but then

this would be cancelled out by the removal of the surcharge effect at the toe of the

slope when the Futurist building is removed. Placing fill in front of the existing

retaining wall significantly improves the global factor of safety.



4.3.5 King’s Steps section

Global stability

Similarly to the Central section, the proposed earthworks will result in soil being

removed from the top of the slope, redistributed on the terraced slope and used as

fill in front of the existing Futurist retaining wall. However, the extent of

earthworks are restricted by the need to avoid impacting King’s Steps and

therefore the net improvement in global stability is less than for the Central

section.

It is assumed that new fill placed over the terraced slope will be granular general

fill and not clayey fill, other than a surface layer of topsoil that will include a

geotextile for erosion control.

Groundwater conditions will change due to the earthworks causing changes in

ground level and also due to the proposed drainage works.

The retaining structure that forms the rear of the Futurist building is to be







existing building. The proposed works will therefore have an effect on global




Page 19

slope stability. Analyses have therefore been carried out to confirm that there will

be a net improvement in global stability.


less than unity. There are no obvious signs of global instability and therefore the

assumed ground model must be over-conservative. However, the intention of the

analyses are to demonstrate the relative changes in factor of safety throughout the

proposed sequence of work, and therefore the exact numbers are less significant.

The removal of the slope crest results in a slight improvement in stability, but then

this would be cancelled out by the removal of the surcharge effect at the toe of the

slope when the Futurist building is removed. Placing fill in front of the existing

retaining wall significantly improves the global factor of safety.



Local stability

The Kings Steps section includes the steepest re-graded slope (1v:2h) likely to be

within existing made ground materials. Slope stability analysis indicates that the

re-graded slope will have a factor of safety of around 1.2 (or an over-design factor

of 0.99 using EC7 DA1-2 partial factors), which is slightly lower than would

normally be sought for slopes.

The analyses assume the made ground has a drained phi (φ’) of 30°. The made

ground on the slope is likely to include infilled former cellars and platforms

created for historical buildings. The assumed strength parameter is therefore likely

to be over-conservative and the re-graded slope is considered to be acceptable.

However, it is noted that stability is dependent on groundwater remaining below

around 2mbgl.

4.3.6 Council offices section

The above analyses indicate that there will be an overall improvement in global

stability of the slope within the site. However, it has been suggested that the

removal of soil from beneath the existing car park could have a negative impact

on stability of the existing Council offices that lie uphill of the site.

Additional analyses were therefore carried out to study this potential impact above

the main slope. In terms of global slope instability that could affect the office

foundations, the analyses indicate a small improvement in factor of safety.

4.4 Summary of stability analyses

The above assessments are summarised in Table 11.

Table 11: Summary of stability analyses

Stage Section Council

Offices

Kings

Steps

Central North Prospect

Place

1 Current (SLS) 1.16 0.94 1.05 1.03 1.34




Page 20

2 Removal of slope crest

(SLS)

1.25* 1.01 1.17 N/A N/A

3 Removal of

Futurist/Mermaid building

surcharge

N/A 0.95 1.04 1.00 1.28

4 Placing fill and new

retaining wall (SLS)

N/A 1.44 1.51 N/A 1.36

(1.09 ULS)**

5 Final after regrade of

terrace slope (SLS)

N/A 1.39 1.49 1.29 N/A

Final after regrade of

terrace slope (ULS)**

N/A 1.12 1.20 1.04 N/A

*For slip surface potentially affecting the office foundations.

**ULS analysis uses EC7 DA1-2 partial factors.




Page 21

5 Earthworks

Earthworks drawings can be found in the drawings section at the back of this

report.

5.1 Site-won fill material

The proposed removal of soil from the crest of the slope is targeted on the central

part of the crest, i.e. around exploratory holes WYG-BH2, WYG-16, BH201 and

WS201.

• WYG-BH2 – the arisings will probably comprise around 25% gravelly clay

and 75% sand/gravel (clayey in places).

• WYG-16 – the arisings will probably comprise 25% gravel, 25% sandy gravel

with clay and 50% natural glacial clay

• BH201 – the arisings will probably comprise around 50% sandy gravel with

clay and 50% natural glacial clay

• WS201 – the arisings will probably comprise sandy gravel with clay.

There is expected to be around 3,500m3 of soil arisings in total. On the basis of

the above assessment, the anticipated volumes and classification of materials is

presented in Table 12.

Table 12: Anticipated fill classification and volumes

Material type SHW Series 600 Class Approximate volume (m3)

Sandy gravel 1A (well graded granular general fill) 1,000

Sandy gravel with

clay

2C (stony cohesive general fill) 1,500

Natural glacial clay 2A (wet cohesive general fill) 1,000

The different classes of material should be kept separate as much as possible

when excavating, in particular the natural glacial clay. The different classes of

material shall also be stockpiled separately and kept protected from rain.

The demolition of the Futurist and Mermaid buildings will generate concrete and

brick rubble. This could be processed (crushed and graded) to produce Class 6F2

(or similar) material. However, the contractor may consider that importing fill

materials is preferable to such processing on a restricted and sensitive site.

5.2 Re-use of site-won fill material

Compaction of the site-won fill materials shall be as recommended in SHW Series

600.




Page 22

For the purposes of slope stability design, the compacted fill should be assumed to

have the properties in Table 13.

Table 13: Anticipated properties of engineered fill

Fill class Unit weight

(kN/m3)

φ’

(°)

c’

(kPa)

Undrained shear

strength

(kPa)

Site-won Class

1A

19 32 0 N/A

Site-won Class 2C 20 28 0 N/A

Site-won Class

2A

20 26 0 40

Site-won Class

6F2 (crushed

concrete and brick

from former sub-

structures and

building

demolition)

19 35 0 N/A

Class 2 materials are unsuitable for re-use on the re-graded slope; this includes

reworked glacial clay and mixed clayey made ground. Such materials could be

used as bulk fill in front of the existing Futurist retaining wall.

To ensure an efficient design, only selected granular fill (e.g. Class 6F2) should be

used within the ‘active wedge’ behind the proposed new retaining wall.

As noted in Section 4.3, proposed slopes should be left no steeper than 1v:2h

(26°) in the permanent condition, including the side slopes where the re-grading

ties into existing terraces.




Page 23

6 Demolition of the buildings

In the temporary case some existing Futurist and Mermaid walls are being used to

retain the King’s Steps, Bland’s Cliff and the slope behind. The demolition

sequence and techniques used will be important for ensuring slope stability during

the works until the new retaining structure can be completed and the slope re-

graded.

The stability of the existing wall could be affected by removal of parts of the

building superstructure or ground slabs and/or excavations at its toe. However, the

removal of mass at the top of the slope and replacement of mass at the base of the

slope prior to demolition allows them to be used in the temporary case as

described in Section 8. Through review of existing building drawings, site survey

and discussion with the demolition contractor, the following basic demolition

sequence has been established:

• Undertake asbestos removal and soft strip.

• Add ballast locally to rear of existing retaining wall in the form of mass

concrete to improve global stability.

• Demolish Mermaid structure and install temporary support to Bland’s

Cliff wall. It is anticipated this will take the form of temporary propping.

These two operations will be completed in tandem working from the front

of the building to the rear.

• Demolish the flat roof section of the Futurist down to first floor level.

• Remove roof sheets to main Futurist building.

• Remove steel roof trusses to main Futurist building.

• Demolish walls to Futurist down to first floor level, starting at Northern

end of the building working towards the central stage beam. The

buttressed wall forming the rear of the building will need to remain in-situ

including the buttresses.

• Demolish the level one tiered seating.

• Remove central stage beam to ground.

• Continue with demolition of building from central stage beam towards

Kings Steps. The buttressed wall forming the rear of the building will need

to remain in-situ including the buttresses.

• Install temporary sheet piling to support the wall to Kings Steps until the

new retaining structure can be completed and backfilled.

During the detailed design stage the demolition sequence will be developed along

with the necessary temporary works required to maintain the stability of the slope.




Page 24

7 Existing retaining structures

The existing Futurist and Mermaid buildings currently retain the Kings Steps, the

slope to the rear and Bland’s Cliff. Refer to the drawings in the back of this report

for the line of the existing retaining structures.

7.1 Main section to the Futurist

The retaining structure at the rear of the Futurist Building is likely to be of a

combination of mass and reinforced concrete construction and appears to be

buttressed in several places by concrete structures, including shear walls. There is

no reason to suspect that it is currently unstable or even marginally stable, but

there is evidence of water ingress and the wall’s integrity could have been affected

by movement of water.

The stability of the wall could be affected by removal of parts of the building

superstructure or ground slabs and/or excavations at its toe. Considering the past

use of the site, the retaining structure should be capable of tolerating construction

plant loading behind.

7.2 Section adjacent to King’s Steps

The Futurist building wall retaining the Kings Steps is constructed entirely from

brick and varies in thickness from 0.7m to 1.2m. The stability of the wall could be

affected by removal of parts of the building superstructure or ground slabs and/or

excavations at its toe and as such suitable temporary works will be required until

the new wall can be constructed and backfilled.

7.3 Main section to the Mermaid

The retaining structure at the rear of the Mermaid Building is likely to be of mass

and/or reinforced concrete construction. There is no reason to suspect that it is

currently unstable or even marginally stable. However, its stability could be

affected by removal of parts of the building superstructure or ground slabs and/or

excavations at its toe. Considering the past use of the site, the retaining structure

should be capable of tolerating construction plant loading behind.

7.4 Mermaid wall supporting Bland’s Cliff

The Mermaid wall retaining Bland’s Cliff is constructed from a brick internal skin

with a brick aggregate concrete outer skin. It varies in thickness from 0.7m to a

minimum depth of 1.3mm. The stability of the wall could be affected by removal

of parts of the building superstructure or ground slabs and/or excavations at its toe

and as such suitable temporary works will be required until the new wall can be

constructed and backfilled.




Page 25

7.5 Terrace walls

The terrace walls are typically of brick construction, with occasional concrete

sections. Many of the sections of wall are in poor condition and exhibiting

evidence of damage, e.g. bulges, cracks, leaning.

For the purpose of this report, it is assumed that the terrace walls are potentially

marginally stable and could become unstable if undermined by excavations at

their toe, surcharge loading or by removal of mature trees.

7.6 Sub-station

From a review of historical mapping, it is considered most likely that the

substation was constructed by cutting into a former terrace and then backfilling

around. It is assumed that the substation building walls are retaining soil on three

sides. The arrangement of retaining structures at the rear of the building is

unclear, but the next terrace wall is located immediately behind and may therefore

impose load on the building. The stability of the substation building walls have

not been considered in detail for this report. However, it is assumed that any

increase in surcharge above or to the sides of the building could adversely affect

their stability.




Page 26

8 New retaining structures

In order to create a development platform at the base of the re-graded slope, a new

retaining structure is required to support the slope and limit its extents. It is

proposed that this will take the form of a reinforced concrete cantilever wall on a

reinforced concrete base. Because of the site constraints and varying retained

heights the wall construction varies along its length.

8.1 Design standards & sources of reference

8.1.1 CDM Regulations

The design and construction of the building will be in accordance with the

Construction Design and Management (CDM) Regulations 2015. The structural

design takes into account the responsibilities of designers stated in the CDM

Regulations. Refer to the Arup Hazard Risk Register for details of residual risks.

8.1.2 Codes of Practice & References

Document

number

Year of

publication Description

BS EN 1990 2002

+A1:2005

Eurocode: Basis of Structural Design

NA to BS EN

1990:

2002

+A1:2005

UK National Annex for Eurocode – Basis of structural

design.

BS EN 1992-1-1 2004 Eurocode 2: Design of concrete structures. General rules

and rules for buildings

NA to BS EN

1992-1-1

2004 UK National Annex to Eurocode 2 - Design of concrete

structures. General rules and rules for buildings

BS EN 1997-1 2004 Eurocode 7: Geotechnical design. General rules

NA to BS EN

1997-1

2004:2007 UK National Annex to Eurocode 7 - Geotechnical design.

General rules

BS EN 206-1 2002 Concrete Part 1. Specification, performance, production and

conformity.

BS8500

Complementary

BS to BS EN

206-1

Part 1: 2006 Concrete. Methods of Specifying and Guidance for the

Specifier.

Part 2: 2006 Concrete. Specification for constituent materials and

concrete.

2008 +

A1:2010

National Structural Concrete Specification for Building

Construction, 4th Edition, as expanded and amended by the

Project Specification

2010 National Structural Steelwork Specification for Building

Construction, 5th Edition, as expanded and amended by the

Project Specification




Page 27

8.2 Walls for development platform

The design of the new walls has been completed based on the geotechnical

parameters described in section 3 of this report, and on the basis that the drainage

described in section 9 is installed and maintained.

Drawings for the new walls can be found in the back of this report.

8.2.1 Bland’s Cliff

Bland’s Cliff falls from a level of approximately 12m at the rear of the Mermaid

building to a level of 5.2m at Foreshore Road. The wall has been split into four

sections and it is anticipated that these can largely be constructed with the

temporary works to Bland’s Cliff remaining in place. Careful consideration will

need to be given to the sequencing of this work and a clear construction strategy

developed during detailed design, particularly where the new wall clashes with the

assumed existing structure.

This wall is generally a 1m thick stem with a base 7.5m in length and 1m thick.

8.2.2 King’s Steps

The King’s Steps are currently retained by the Futurist building. To maximise the

size of the development platform the new wall has been located adjacent to the

existing Futurist retaining wall at Foreshore Road. The wall is perpendicular to

the main retaining wall to the rear of the site. This will mean that the temporary

works stabilising the existing will likely encroach on the construction of the new

wall, particularly at the rear of the site. It is therefore proposed that this section of

new wall will be constructed in short sections to limit the amount of temporary

works being removed during construction. Careful consideration will need to be

given to the sequencing of this work and a clear construction strategy developed

during detailed design.

Similar to Bland’s Cliff, the King’s Steps fall from a level of 16.5m at the rear of

the Futurist to a level of 5.2m at Foreshore Road. The wall has been split into two

sections, though these do not necessarily coordinate with any sequencing strategy

at this stage.

This wall is generally a 1.5m thick stem with a base 6m in length and 1.5m thick.

8.2.3 Main Slope

The wall to the rear of the site retains an approximately uniform height of 5.3m.

The wall is set back approximately 15m from the existing wall. This wall is

generally a 1m thick stem with a base 7.5m in length and 1m thick.

Weepholes will be provided close to the base of the wall at approximately 3m

centres. In addition, free-draining granular materials will be placed behind and

below the wall to ensure that water pressures do not build up in the event that

weepholes become blocked in the future.




Page 28

8.3 King’s Steps

From the rear of the Futurist building to King Street, the King’s Steps and slope

are retained by an existing brick wall. Where possible it is proposed to re-use this

wall, however new walls similar to the existing may be required. During the

detailed design stage a survey of the existing structure will be undertaken. This

will identify areas of the wall requiring repair or replacement, with appropriate

solutions being designed as required.

8.4 Other Walls to Facilitate the Slope Re-grading

To the northern side of the site, the new slope profile has been tapered in to the

existing site levels providing an offset of approximately 10m from the site

boundary where no significant works are being undertaken. This will require a

number of the existing terrace walls on the site to be reduced in height and re-used

to retain the edge of the new slope. Similar to the wall along the King’s Steps it is

proposed to re-use these walls where possible, however new walls similar to the

existing may be required. During the detailed design stage a survey of the existing

structures will be undertaken. This will identify areas of the wall requiring repair

or replacement, with appropriate solutions being designed as required.




Page 29

9 Drainage

9.1 Existing Drainage Infrastructure

9.1.1 Desk Study

9.1.1.1 Statutory Undertaker Assets

Yorkshire Water sewer records have been provided to Arup indicating the

position and details of public sewers within the vicinity of the site (refer to

Appendix C). The relevant assets are summarised below:

• A 2000mm concrete public combined sewer within Foreshore Road

• A 225mm vitrified clay public combined sewer within Blands Cliff,

discharging to the sewer within Foreshore Road.

• A 675/500mm brick culvert within King Street.

• A section of public combined sewer is present within the north-western

area of the site (within the car park). This sewer appears to receive flows

from two adjacent properties outside of the site boundary on King Street

and Newborough. The status of this sewer and any easements or

wayleaves affecting it is not known. The outfall is also not shown on the

records, but it is assumed to discharge into the site drainage network.

9.1.1.2 Scarborough Borough Council Assets

An enquiry was made to Scarborough Borough Council (as Local Authority for

the area) to confirm whether there were any records of drainage assets in this area.

No drainage records were available for the site area.

9.1.1.3 North Yorkshire County Council Assets

An enquiry was made to North Yorkshire County Council (as highway authority

for the area) to confirm whether there were any records of highway drainage

assets in this area. No drainage records were available for the site area.

9.1.2 Site Observations

Although there are visible gaps within retaining wall masonry, it is not known

whether there are any functioning weep holes or back of wall drainage systems to

existing retaining structures.

A grated drainage channel is present within the Futurist basement which suggests

that some groundwater may permeate through the wall to be collected on the

internal face (see Figure 2). There are however no records or surveys to confirm

the condition or the outfall from this system.




Page 30

Figure 2 Evidence of Possible Retaining Wall Drainage

9.1.3 Drainage Survey

A drainage survey has been undertaken by L&M Survey Services to determine the

presence and condition of existing drainage infrastructure within the site. Full

details can be found within the drainage survey report, and a copy of the site

drainage plan is contained within Appendix D.

The survey findings are illustrated on drawing FUT-ARUP-ZZ-XX-DR-CD-0001

and summarised below:

• The survey did not establish full connectivity of the site drainage network.

Some of the outfall routes have been assumed based on the available data,

but some outfall routes couldn’t be determined.

• Surface water and combined water drainage systems are present along the

frontage of the Futurist building. These appear to receive flows from the

building roof drainage system and sanitary appliances (although an

internal drainage survey was not undertaken as part of this survey). These

networks appear to connect to the Yorkshire Water sewer within

Foreshore Road.

• A sewer was surveyed to the north of the site coinciding with the position

of the public combined sewer indicated on Yorkshire Water sewer

records. This extends to adjacent properties to the north and north-west.

• The car park drains via four road gullies. Two of these gullies discharge

into the public combined sewer and two gullies discharge into a private

surface water network. The outfall destination could not be determined for

either of these networks.

• A small section of surface water drainage system accepts downpipes from

the substation and hardstanding to the rear of the Futurist. This network

outfalls into the public combined sewer within Bland’s Cliff.

Manhole cards were not provided to allow verification of the condition and

connectivity of the manholes.




Page 31

9.1.4 Existing Site Catchment Areas

An assessment of the existing site catchment areas has been undertaken and is

shown on drawing FUT-ARUP-ZZ-XX-DR-CD-00002.

The catchment areas and assumed outfall destinations are summarised below:

Table 14 - Existing Site Catchment Areas

Catchment Area Assumed Outfall

Futurist Building 2,245 m² Yorkshire Water sewer in Foreshore Road.

Hardstanding 1 270 m² Yorkshire Water sewer in Bland’s Cliff

Hardstanding 2 310 m² Yorkshire Water sewer in Foreshore Road via King’s Steps

private sewer.

Landscaped

Terracing 1,115 m² No known drainage systems. Assumed to infiltrate.

Car Park Area 1 750 m² Yorkshire Water sewer within the site.

Car Park Area 2 750 m²

Private surface water sewer (assumed to connect to private

sewer in King’s Steps and ultimately connect to Yorkshire

Water sewer within Foreshore Road).

Third Party Plot 1* 220 m² Yorkshire Water sewer within site.

Third Party Plot 2* 190 m² Yorkshire Water sewer within site.

TOTAL 5,850 m²

* Third Party Plot catchment areas are assumed.

The assumed outfalls require verification to validate the drainage strategy.




Page 32

9.1.5 Existing Flow Rates

The average rainfall intensities for a 15 minute duration storm for three return

periods within the Futurist locality are provided in Table 15.

Table 15 - Average Rainfall Intensities

Return Period 1 Year 5 Year 30 Year

Average Rainfall Intensity 28 mm/hr 47 mm/hr 69 mm/hr

Based on the rainfall intensities in Table 2 and the assumed outfall routes, the

peak flow rates discharging to Yorkshire Water sewers have been determined

based on the Modified Rational Method. The flow rates are shown in Table 16.

Table 16 - Estimated Existing Peak Flow Rates

Outfall Area (m²)

Peak Flow Rate (l/s) & Return Period

1 year 30 year 100 year

YW Sewer in Foreshore Road 3,305 25.7 43.1 63.3

YW Sewer in Bland’s Cliff 270 2.1 3.5 5.2

YW Sewer within site 1,160 9.0 15.1 22.2

TOTAL 4,735 36.8 61.8 90.8




Page 33

9.2 Proposed Drainage Strategy

Drawing FUT-ARUP-ZZ-XX-DR-CD-0003 illustrates the proposed drainage

strategy.

The demolition of the futurist building and re-modelling of the terracing and car

park results in an overall reduction in impermeable area on the site. The strategy

therefore seeks to capitalise on this reduction to reduce the flow rates discharging

to public sewers by allowing for some infiltration through landscaping.

Although the re-graded slope is to be landscaped, the gradient of the slope is

likely to result in a degree of surface water run-off. The percentage run-off from

areas of soft landscaping has therefore been taken as 30%. This represents the

upper limit of the recommended rates provided in BS 752 Table E.3. A filter drain

is proposed behind the new retaining wall to collect run-off from the landscaped

areas. The flows from this system are to discharge into the sewer in King’s Steps

as this most accurately reflects the existing flow regime.

The new retaining wall requires a drainage system to relieve the build-up of pore

water pressures against the wall. However, a back-of-wall drainage system would

present an ongoing maintenance burden and the deep inspection chambers would

present a hazard to those maintaining the system. It is therefore proposed that a

weep-hole system is installed, discharging onto the development platform.

A temporary drainage system shall be installed on the development platform until

the construction of the future development. The details of this system are pending

confirmation of the intended temporary uses. However, at this stage the area is

assumed to be 100% impermeable and is assumed to utilise the existing building

drainage connections discharging into the Foreshore Road public sewer (note that

the final development shall be subject to a separate planning application).

The remaining area of car park and adjacent third party plots shall continue to

drain to the sewer network within the site. Upon confirmation of the full extents

of this network, modifications or diversions may be required to maintain

connectivity following the re-grading of the site.




Page 34

9.2.1 Proposed Site Catchment Areas

The proposed site catchment areas, percentage impermeability (PIMP) and outfall

destinations are shown in drawing FUT-ARUP-ZZ-XX-DR-CD-0003 and are

summarised below:

Table 17 - Existing Site Catchment Areas

Location Area PIMP Proposed Outfall

Development Platform 1,230m² 100% To continue to discharge to Yorkshire Water

sewer in Foreshore Road.

Hardstanding area

adjacent to sub-station 140m² 100%

To continue to discharge to Yorkshire Water

sewer in Bland’s Cliff.

Landscaped Terracing 230m² 0% Not to be positively drained.

Landscaped Slope 3,135m² 30% New connection to private surface water sewer

in King’s Steps.

Remaining Car Park 600m² 100% To continue to discharge to Yorkshire Water

sewer within the site.

Third Party Plot A* 190m² 100% To continue to discharge to Yorkshire Water


Third Party Plot B* 220m² 100% To continue to discharge to Yorkshire Water


Landscaped Area 1 105m² 0% Small area adjacent to retaining wall assumed to

infiltrate.

Landscaped Area 2 70m² 0% Small area adjacent to retaining wall assumed to

infiltrate.

TOTAL 5,850m²

* Third Party Plot catchment areas are assumed.




Page 35

9.2.2 Proposed Flow Rates

The design rainfall intensities for the site have been determined by applying a

climate change uplift factor to the existing rainfall intensities as described in

government guidance on flood risk (see Figure 3).

Figure 3 Government Guidance on Climate Change Allowances (gov.uk)

The upper end confidence level has been adopted for design up to 2115,

representing an uplift in rainfall rates of 40%.

Table 18 - Design Rainfall Intensities

Return Period 1 Year 5 Year 30 Year

Existing Average Rainfall Intensity 28 mm/hr 47 mm/hr 69 mm/hr

Design Average Rainfall Intensity 39 mm/hr 66 mm/hr 97 mm/hr

Based on the rainfall intensities in Table 18 and the assumed outfall routes, the

peak flow rates discharging to Yorkshire Water sewers have been determined

based on the Modified Rational Method. The flow rates are shown in Table 19.

Table 19 - Proposed Peak Flow Rates

Outfall Area

(m²)

Impermeable

Area (m²)

Peak Flow Rate (l/s) & Return Period

1 year

+CC

30 year

+CC

100 year

+CC

YW Sewer in Foreshore

Road 3,185 2150.5 24.1 40.7 59.8

YW Sewer in Bland’s

Cliff 140 140 1.5 2.6 3.8

YW Sewer within site 960 960 10.4 17.6 25.9

TOTAL 4,735 3250.5 35.2 59.6 87.6




Page 36

9.3 Conclusions

Based on the site information available, the majority of the Futurist site is

assumed to discharge to the Yorkshire Water sewer in Foreshore Road, either via

King’s Steps, Bland’s Cliff or directly from the Futurist building.

The proposed drainage strategy seeks to maintain the current regime by

continuing to discharge flows to the sewers in King’s Steps and Bland’s Cliff.

The demolition of the Futurist building and remodelling of the site results in a

reduction in impermeable area, which reduces the peak flow rates reaching the

Yorkshire Water sewer in Foreshore Road.

Run-off from landscaped areas shall be collected using a simple filter drain

discharging to the sewer in King’s Steps.

A back-of-wall drainage system shall be installed behind the new retaining wall

with a weephole system discharging onto the development platform. The

discharge through this system is negligible for design purposes.

A temporary drainage system shall be installed on the development platform

utilising the existing building connections to the public sewer in Foreshore Road.

The drainage system for the permanent development works shall be subject to a

separate planning application.

The small section of Yorkshire Water sewer within the site and the incoming

connections from the adjacent plots shall be maintained as part of the permanent

works, and the route to outfall retained as part of the permanent works.

Modifications to this network may be required to ensure the route is compatible

with the new site level strategy.

9.4 Next Steps

Further assessment of the existing site drainage system is required to confirm the

outfall routes and enable the design of any modifications to existing networks

remaining as part of the permanent works.

The ownership of the sewer in King’s Steps should be determined to obtain a

consent to make a new connection and agree details.

The drainage strategy is subject to approval from Yorkshire Water. Further

consultation with Yorkshire Water is required to determine the status of the public

sewer within the site and confirm any easements or wayleaves affecting the sewer.

Works affecting public sewers also require approval from Yorkshire Water.

2017-07-24 futurist gdr issue 2 - borough of scarborough · willmott dixon construction ltd...

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