Outline Slope Stability Addendum Report

Outline Slope Stability Addendum Report

at Land West of Church Road, Otham, Maidstone, ME15 8SB

for Bellway Homes Ltd Kent

Report Reference: LP1868rev1/add

Report Date: 3rd October 2019

LE/QEMS/Doc 07-5-01 – rev4 LP1868rev1/add Church Road, Otham

TABLE OF CONTENTS

A INTRODUCTION .............................................................................................................................. 6

1 Authority .......................................................................................................................................... 6

2 Objective .......................................................................................................................................... 6

3 Previous Studies .............................................................................................................................. 6

4 Scope of Works .............................................................................................................................. 7

5 Limitations ........................................................................................................................................ 7

B SITE Setting ........................................................................................................................................... 9

6 Environmental Setting .................................................................................................................... 9

6.1 Site Location and Description ............................................................................................ 9

6.2 Geology................................................................................................................................. 10

6.3 Hydrogeology ...................................................................................................................... 11

6.4 Hydrology ............................................................................................................................. 11

7 Previous Investigations ............................................................................................................... 11

7.1 Phase 1 Environmental Assessment by IDOM Merebrook Ltd, reference DS-

21803-18-523 .................................................................................................................................... 11

7.2 LEAP Combined Phase II Geotechnical and Contamination Site Investigation

Report reference LP1868/rev1 ..................................................................................................... 11

C PHASE II - Intrusive Investigation ................................................................................................. 13

8 Investigation Rationale ................................................................................................................ 13

9 Site Work ...................................................................................................................................... 13

9.1 Date and Weather Conditions ....................................................................................... 13

9.2 Site Work Methods ........................................................................................................... 13

9.3 Field Tests ............................................................................................................................ 14

10 Ground Conditions ..................................................................................................................... 14

10.1 Gulls and Fissures ............................................................................................................... 16

10.2 Groundwater ....................................................................................................................... 16

GEOTECHNICAL INFORMATION .................................................................................................... 17

11 Strata Encountered ..................................................................................................................... 17

11.1 Silty Sandy Clay (Weathered Hythe Formation) ........................................................ 17

11.2 Silt and Sand (Weathered Hythe Formation) .............................................................. 17

11.3 Sandstone and Limestone (Hythe Formation)............................................................. 17

11.4 Sand (Hythe Formation/Atherfield Clay Formation) ................................................. 17


D GEOTECHNICAL APPRAISAL .................................................................................................... 18

12 Slope Stability ............................................................................................................................... 18

APPENDIX A – Limitations

APPENDIX B – Figures

APPENDIX C – Trial Hole and Borehole Logs

APPENDIX D – Dynamic Probe Test Results


EXECUTIVE SUMMARY The site is located to the west of Church Road in Otham, Kent. The site comprises agricultural

land covering approximately 16.14ha in area. It is proposed to redevelop the site with circa.

412 plots of two storey residential dwellings and multi-storey blocks of flats with associated

access roads. An assessment as to the impact that this development may have on several offsite

slopes and cliffs of between approximately 3m and 8m in height has been undertaken as an

addendum to the Phase 2 investigation works.

A total of seven windowless boreholes, three dynamic penetration tests and a single rotary

borehole with standpipe installation were excavated to investigate the ground conditions

adjacent to offsite slopes and cliffs, and groundwater levels respectively.

Ground conditions along the northern and western boundaries of the site generally confirm

those encountered during the previous investigation. Some made ground was encountered to

depths of up to 0.75m depth.

Groundwater has been encountered at a depth of 14.6m. Further monitoring is ongoing.

It is advised that allowance is made for sterilising a strip of land at the crest of the slope

extending a horizontal distance twice that of the cliff / slope vertical height from the toe of

the slope (i.e. allowing an angle of repose of 1v:2h). It is recommended that all development

within this sterilised zone is avoided as well as any temporary works that might impose loads

on the slope.

Surface water discharge to deep borehole soakaways below the toe of the slope is

recommended.

It may be possible to further reduce the sterilised zone following detailed slope stability

analysis which would require detailed topographic information and access across the slope /

cliff areas.


Signed:

Oliver Kean BSc Msc DIC FGS

Countersigned:

Richard Brinkworth BEng CEng MICE

Date: 9 October 2019

Revision: Issue 2


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A INTRODUCTION

1 Authority

Leap Environmental Ltd (hereafter referred to as LEAP) has been appointed by Bellway Homes

Ltd (Kent) to provide an outline slope stability assessment at the site referred to as Church

Road, Otham. The instruction was given in an email dated 25th July 2019 signed by Sam Vernon

of Bellway Homes Ltd (Kent). A site location plan is attached as Figure 1 in Appendix B.

2 Objective

LEAP understands that the site is currently privately owned and it is proposed to redevelop

the site with circa. 412 plots of two storey residential dwellings and multi-story blocks of flats

with associated access roads, soft landscaping, private garden areas and several swales as

shown in the proposed site layout drawing (16206 / P101), included in Figure 2, Appendix B.

The proposed development is currently with internal planning and has been assessed in

accordance with BS EN 19971, as comprising Geotechnical Category 1 structures.

The objectives of this addendum report are to:

Provide further information on the geotechnical quality of the ground along the

perimeters of the site;

Assess the potential for the proposed development to impact the offsite slope or cliff

face along the northern or western perimeters of the proposed development.

3 Previous Studies

The site has been the subject of a previous investigation by LEAP and others. The following

reports have been supplied by the Client, which should be read in conjunction with this report.

Idom Merebrook Phase I Desk Study Report (reference DS-21803-18-523, dated

November 2018).

LEAP Combined Phase II Geotechnical and Contamination Site Investigation Report

(reference LP1868/rev1, dated 13th March 2019)

No detailed topographical survey information has been provided for the off-site slopes.

1 BS EN 1997-1(2004) Eurocode 7:Geotechnical Design - Part 1:General Rules


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A summary of the reports is included in Section 7. The current report is an addendum to the

Phase II investigation report (LP1868/rev1) provided to the client in March 2019.

4 Scope of Works

The current phase of work comprises intrusive investigation, and onsite monitoring. At the

time of writing this report onsite monitoring of groundwater levels was ongoing and will be

reported on separately in the future. The results of this and the previous investigation report

are used to update the initial ground model.

The current ground investigation comprised the following:

1 No. rotary borehole to a depth of 30m.

Installation of a single groundwater monitoring well to a depth of 30m (BH201);

7 No. windowless boreholes (WS201 to WS207) to depths of between 0.50m and

4.00m;

3 No. Dynamic Probe (Super Heavy) Tests (DPT) at selected locations between

boreholes, extending to depths of up to 2.20m;

In-situ geotechnical testing including Hand Penetrometer and SPT tests in the

boreholes;

The intrusive works were completed by contractors who have been scrutinised and are on

LEAP’s approved contractor list. The dynamic probing was carried out by Oaklands SI Limited,

and the rotary borehole was carried out by OT Drilling Limited. The works were supervised

by LEAP.

5 Limitations

This report has been prepared by Leap Environmental Ltd on the basis of information received

from a variety of sources which Leap Environmental Ltd believes to be accurate. Nevertheless,

Leap Environmental Ltd cannot and does not guarantee the authenticity or reliability of the

information it has obtained from others.

Leap Environmental Ltd has used all reasonable skill, care and diligence in the design and

execution of this report, taking into account the manpower and resources devoted to it in

agreement with the Client. Although every reasonable effort has been made to obtain all

relevant information, all potential contamination, environmental constraints or liabilities

associated with the site may not necessarily have been revealed.


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The conclusions reached in this report are necessarily restricted to those which can be

determined from the information consulted and may be subject to amendment in the light of

additional information becoming available. These conclusions may not be appropriate for

alternative schemes.

This report is confidential to the Client and Leap Environmental Ltd accepts no responsibility

whatsoever to third parties to whom this report, or any part thereof, is made known, unless

formally agreed by Leap Environmental Ltd beforehand. Any such party relies upon the report

at their own risk.

Full details of the limitations are provided in Appendix A.


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B SITE SETTING

6 Environmental Setting

6.1 Site Location and Description

The site is formed of agricultural land of approximately 16.14ha in area, located immediately

to the west of Church Road in Otham, Kent. The nearest serviceable postcode is ME15 8SB.

The approximate National Grid Reference of the site is TQ 7875 5393. The site lies at an

elevation of between some 55m and 75m Above Ordnance Datum (mAOD).

The location of the site is shown in Figure 1, Appendix B. The current layout of the site is

shown in Figure 3 in Appendix B.

6.1.1 General Description and Boundaries The site generally comprised a single agricultural field which was fallow at the time of the

investigation. The north eastern section of the site, adjacent to St Nicholas Church grounds,

was vegetated with bushes and a mixture of mature deciduous and coniferous trees.

Hedgerows, trees and fences were present along all boundaries.

The site generally sloped gently towards the north, falling a height of some 20m over 550m.

The site was bounded by residential housing to the north, south and west. Residential houses

of Longham Copse to the north, and Chapman Avenue and The Beams to the west were at a

lower topographic level to the site, defined by a sharp break of slope (northern boundary)

and/or sub-vertical sandstone/limestone cliff face (western boundary).

Slopes and cliff faces were located immediately offsite, located within the rear private gardens

of the adjoining residential properties and were inaccessible for direct observation and

measurement during the investigation due to their location. The slopes / cliffs appear to have

been formed during the historic quarrying activities and ranged in height from between 3m to

5m in the north and 6 to 8m in the south. Cliff faces appear to comprise partly vegetated sub-

vertical scree, with clays over weathered sandstone/limestone rock beds. Slopes to the north

of the site appear landscaped.

The site was bound by agricultural land to the east.


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6.2 Geology

Based on the published geology map for the area, the site is anticipated to be underlain by the

Hythe Formation. In addition, deposits of the Atherfield Clay are indicated to be present at

surface immediately to the west of the site boundary, along Chapman Avenue and The Beams.

6.2.1 Hythe Formation The Hythe Formation consists mainly of glauconitic fine to coarse grained sands and

sandstones with some interbedded more strongly cemented sandstones and doggers with

either calcareous or siliceous cement. Beds of clay or sandy clay, and thin seams of fuller’s

other also occur. In Kent and eastern Sussex the formation comprises alternating sandy

limestones ("Ragstone") and glauconitic sandy mudstones (Hassock).

The Hythe Formation in the area has been extensively affected by cambering and valley bulging.

This is a process by which loss of strength in an underlying clay soil causes block movement

of overlying competent jointed rocks. Cambering of blocks and their subsidence into the weak

clay underneath causes the clay to bulge upwards. Rotation and movement of the competent

blocks causes wide fissures or gulls to open up. The gulls tend to be infilled with periglacial

debris i.e. poorly sorted silty clay and locally derived gravel. These gulls can provide an easy

route for the downward passage of surface water, which can lead to washing out of fines and

has in some instances led to the opening up of gulls and subsequently to subsidence at the

ground surface (sometimes referred to as solution features or swallow holes).

6.2.2 Atherfield Clay The Atherfield Clay comprises mainly clay and silty clay with subordinate sandy clay, in shades

of blue brown and yellow. Horizons with clay ironstone concretions up to 300mm think are

common towards the base. The Atherfield Clay Formation generally extends to between 7

and 20m in thickness.

6.2.3 BGS Boreholes The online BGS Geoindex (http://www.bgs.ac.uk/geoindex/) has been reviewed for detailed

local geological and hydrogeological information. Two historic boreholes have been identified

within 500m of the site. Boreholes referenced TQ75SE28 and TQ75SE10 are located circa

230m west and 400m north of the site respectively. Borehole records for TQ75SE28 include

no significant technical information relating to the ground conditions at the site.

Borehole reference TQ75SE10 recorded Kentish Rag down to 32m depth overlying fine sand

to 35m, over rock (not specified on the log) to 36m. Mottled clay was recorded beneath the

rock to the extent of the investigation (46m). Given the location of the borehole at TQ75SE10

(down slope to the site and within proximity of the River Len), the Kentish Rag may be part

of the Hythe Formation, fine sand and rock may well be part of the Atherfield Clay Formation,

while the mottled clay encountered from 35.70m depth onwards could be the Weald Clay

Formation. Groundwater was struck at 4.2m depth and 31.5m depth.


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6.3 Hydrogeology

The site is mapped as being underlain by the Hythe Formation which is classified as a Principal

Aquifer. The Atherfield Clay located offsite to the west is classified as a Secondary B Aquifer.

6.4 Hydrology

The nearest surface water feature is the River Len, located some 400m to the north of the

site.

7 Previous Investigations

7.1 Phase 1 Environmental Assessment by IDOM Merebrook Ltd, reference DS-21803-18-523

The site has been the subject of a Phase 1 Environmental Assessment (Desk Study), by IDOM

Merebrook Ltd, reference DS-21803-18-523, dated November 2018. The reader is referred

to this previous report.

In summary, the site has a history of use as an agricultural field and was mapped as an orchard

between the 1950s and 1980s. Several offsite ragstone quarries and pits are located

immediately north, north east and west of the site are mapped between 1961-1985. The

quarries/ pits immediately north of the site were redeveloped for residential housing from

1985.

The desk study identified potential sources of contamination to include pesticides (associated

with the former orchard) and ground gas migration from adjacent historical quarries.

7.2 LEAP Combined Phase II Geotechnical and Contamination Site Investigation Report reference LP1868/rev1

A total of forty trial pits, twenty dynamic penetrometer tests and three rotary boreholes were

completed by LEAP in February 2019.

Ground conditions comprised topsoil to 0.20-0.50m overlying Hythe Formation (interbedded

silty sands and clays with subordinate horizons of gravel), grading into limestone/ sandstone

with depth. Groundwater was encountered within the Hythe Formation soils at around 15m

below ground level.

An allowable bearing capacity limited to 85kPa was considered for shallow foundations, up to

1m wide on the firm clays below about 1m. However, foundations will require deepening in

the clay soils near trees to NHBC MEDIUM VCP precautions and where loose or soft soils

are found in excess of 1m.


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Low blow counts were recorded at various depths within the DPTs, at a number of locations,

indicating the presence of very loose/ soft soils. Where low blow counts indicate loose or soft

soils alternative foundations are likely to be required (e.g. piled foundations).

Recommendations for further detailed investigations were made.

Suspended floor slabs were recommended on site.

The soils were classified as sulphate design class DS-1, and, assuming a mobile groundwater

table, the ACEC class is AC-1.

Soakaways constructed within the intact competent rock at depths in excess of 5m below

ground level and remote from slopes were considered to be feasible.

A geo-environmental risk assessment was carried out. Contamination testing of the soils

identified localised asbestos (chrysotile fibre bundle) within the topsoil at one position, and

marginally elevated arsenic within the natural soils at >1.0m depth.

Gas monitoring has recorded low concentrations of carbon dioxide (up to 0.3%) and methane

(up to 1.9%). No flow has been recorded on site. The site has provisionally been classified as

a NHBC GREEN site, whereby no protective measures are considered necessary.

Recommendations are given for additional topsoil sampling in the vicinity of the trial hole

where asbestos was identified and for completion of the monitoring regime.

As with any redevelopment site, there is always the risk of hitherto undetected contamination,

and it was recommended that further investigations should be carried out prior to

redevelopment.


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C PHASE II - INTRUSIVE INVESTIGATION

8 Investigation Rationale

A total of seven windowless boreholes (designated WS201 to WS207) were excavated along

the northern and western boundaries of the site to depths of between 0.50m and 4.00m.

Three additional penetrometer tests (designated DPT201 to DPT203) were undertaken to

provide additional information on the ground strength. The exploratory hole logs and dynamic

probe results are included in Appendices C and D, respectively. A plan showing the site

investigation locations is attached as Figure 3 in Appendix B.

A single rotary borehole (designated BH201) was excavated to depths of 30m to undertake

long term groundwater level monitoring beneath the site.

9 Site Work

9.1 Date and Weather Conditions

The intrusive investigations were undertaken between 10th and 12th September 2019. At the

time of the investigations, the weather was dry and overcast.

9.2 Site Work Methods

9.2.1 Borehole Drilling Boreholes were drilled using a rotary rig. The boreholes were excavated at 115mm diameter.

Due to the nature of the borehole (monitoring well installation) simplified driller descriptions

have been recorded on the logs and have been provided in Appendix C.

9.2.2 Soil Logging and Sampling Soil samples were recovered from the boreholes and trial pits for field screening, logging and

sampling.

Boreholes were logged in general accordance with the requirements of BS 5930:20152 and BS

EN ISO 146883.

Visual and olfactory evidence of contamination was noted if encountered.

2 BS5930:2015 Code of Practice for Ground Investigation

3 BS EN ISO 14688 Parts 1-2 Geotechnical Investigation and Testing. Identification and classification of

soil


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All sample containers were sealed and labelled with a unique location identity, depth and date

of sampling.

9.2.3 Monitoring Well Installation A single deep groundwater standpipe was installed in BH201.

The deep groundwater standpipe consisted of 50mm diameter HDPE pipe. The response zone

within the deep groundwater well was typically targeted to intercept the groundwater surface

and was surrounded by washed filter gravel. The plain zone was surrounded with bentonite

to provide a seal. BH201 was finished with a bung and a raised steel cover.

The monitoring well installation is shown on the borehole log and summarised in the following

table.

Table 1: Borehole installation details

9.3 Field Tests

9.3.1 Standard Penetration Tests Standard penetration tests were undertaken at regular intervals within the windowless

sampler boreholes as drilling progressed, in accordance with BS EN ISO 22476-3;2005.

Uncorrected blow counts, ‘N values’, are recorded on the borehole logs in Appendix C.

9.3.2 Dynamic Probing Three dynamic probe (super heavy) tests were undertaken at infill locations between the

windowless boreholes, in accordance with BS EN ISO 22476-2:2005 +A1;2011. The results

have been correlated with soil density/undrained shear strength directly from the

corresponding SPT N blow counts on site, taking into account the site conditions, and Cearns

10 Ground Conditions

The ground conditions are described in detail in the logs attached in Appendix C. In summary

the soil conditions encountered during this and previous investigations were as follows:

Borehole Type

G-gas

W-water V-

vapour

Plain Well Screen

(mbgl)

Response Zone

(mbgl)

Bentonite Seal

(mbgl)

BH201 W GL-29.00 30.00-20.00 GL-20.00


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Table 2: Summary of soils encountered

Depth

From (m)

Depth To

(m)

Soil Type Description

G.L 0.10-0.50 TOPSOIL Brown gravelly silty clay. Gravel is variable quantities of

flint, chalk, sandstone and brick.

(Glass was noted previously within TP125 and TP126.)

GL – 0.15 0.35 – 0.75 MADE GROUND Brown sandy clay with gravel of brick, charcoal and

siltstone (in WS202, WS203 and WS204 and WS206

only).

0.10-0.75 0.60-6.50 SANDY SILTY

CLAY/ SILTY

CLAYEY SAND

Comprising variable sandy silty CLAY and SAND with a

variable proportion of gravel and subordinate horizons

of SANDSTONE / LIMESTONE.

Borehole WS206 encountered poor recovery and very

low N-values between 2.00m-4.00m.

Previously probe results recorded very low blow counts

between 1.80m-5.70m (TP110) and 1.60m-4.20m

(TP119).

0.45-6.50 12.00->20.00 SANDSTONE/

LIMESTONE

Weak SANDSTONE/ LIMESTONE.

(Recorded as fractured between 3.30m and 18.75m

previously in BH101).

12.00-14.00 >30.00 SAND Grey SAND (encountered in base of boreholes BH201

and previously at BH103 only).

The ground conditions encountered along the northern and western boundaries of the site

generally confirm those encountered across the wider part of the site during the previous

investigation, with the exception of a shallow thickness of made ground encountered in

boreholes WS202, WS203, WS204 and WS206.

As part of the current investigation exploratory holes were excavated through a surface cover

of topsoil and/or made ground some 100mm to 750mm in thickness. The topsoil and or made

ground consisted of gravelly sandy clay with gravel of brick, charcoal and siltstone.

Underlying the topsoil and/or made ground, the upper soils generally consisted of interbedded

silty sands and clays with subordinate horizons of gravel and thin beds of sandstone/ limestone.

The depth to the base of these soils was found to be variable, extending to the depths of

between 0.50m and 6.50m. These soils are considered to be representative of the weathered

Hythe Formation or periglacial debris infilling the gulls.


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Within borehole BH201, sandstone and limestone was encountered beneath the clay and sand

deposits to a depth of 12.00m (52.18mAoD). The borehole then progressed into grey sand to

the full depth of excavation of 30.00m (34.175mAoD). These soils are considered to represent

either the Hythe Formation or, possibly, the uppermost section of the underlying Atherfield

Clay Formation.

Based on the results of the in situ and DPT tests, the strength of the upper soils was found to

be variable both laterally and vertically. The clays were generally recorded as stiff to very stiff

and the sands were generally loose and medium density in condition. Poor recovery and loose

soils were encountered within WS206 at depths of between 2.00m and 4.00m indicating the

potential presence of an infilled or partially infilled gull.

10.1 Gulls and Fissures

Previous investigation results may indicate the presence of infilling of gulls/ fissures across the

site at depth. The general E-W orientation of these features, indicated from aerial photos,

would suggest any features also outcrop within the rear gardens located to the west of the

site. Although direct observation of these cliff sections was not possible.

10.2 Groundwater

No groundwater was encountered during the excavation of the windowless and rotary

boreholes.

Groundwater monitoring results are summarised as follows:-

Table 3: Groundwater Monitoring Results

Monitoring

well

Depth to groundwater (mbgl)

18/09/19

BH201 14.61m

Groundwater has been encountered at 14.61m (49.57mAoD) below ground level within

BH201 on the first of three monitoring visits undertaken as part of the most recent scope of

works. 117l of groundwater was purged resulting in a groundwater level of 15.45m below

ground level. At the time of writing this report the monitoring program is ongoing.

It is noted that groundwater was encountered in BH101 as part of the previous investigation

at a depth of between 15.27m (41.8mAoD) and 15.20m (41. 73mAoD) during two monitoring

visits carried out during the winter months on 19th and 27th February 2019. This borehole was

subsequently decommissioned 07/03/2019 on completion of the monitoring program.


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GEOTECHNICAL INFORMATION

11 Strata Encountered

11.1 Silty Sandy Clay (Weathered Hythe Formation)

The topsoil was underlain by soft to stiff plastic clay and sandy clay soils with subordinate

horizons of gravel and thin beds of sandstone/ limestone to depths of up to 6.50m below

ground level. The results of the in-situ testing, including those from the previous and current

stage of works (UCS values 25kPa to 600kPa), indicate that the clays can be considered to be

locally soft to firm, but in general stiff and very stiff in consistency.

11.2 Silt and Sand (Weathered Hythe Formation)

The clay soils generally transitioned into sand variably across the site, both laterally and with

depth. The results of the in-situ testing (N values 3 to 65) indicate that the granular soils can

be considered to be generally very loose to very dense in consistency.

11.3 Sandstone and Limestone (Hythe Formation)

Where penetrated, the clay and sand were underlain by sandstones and limestones to depths

of 12m in the vicinity of borehole BH201 and in excess of 20m across the wider site. Based

on the field tests the sandstone is classified as weak.

11.4 Sand (Hythe Formation/Atherfield Clay Formation)

Within boreholes BH201 and BH103 (previous investigation), grey sand deposits were

encountered beneath the sandstone and limestone beds. It is possible that these deposits may

represent the upper part of the Atherfield Clay Formation.


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D GEOTECHNICAL APPRAISAL

The foregoing geotechnical appraisal does not constitute a Geotechnical Design Report in

accordance with BSEN1997. The following recommendations are for preliminary design

purposes only.

For the detailed design, the short-term and long-term design situations must be considered.

Where relevant, the following limit states should be considered:-

Loss of equilibrium of the structure or the ground, considered as a rigid body, in which

the strengths of structural material and the ground are significant in providing

resistance (EQU)

Internal failure or excessive deformation of the structure or structural elements in

which the strength of structural materials is significant in providing resistance (STR)

Failure or excessive deformation of the ground, in which the strength of soil or rock

is significant in providing resistance (GEO)

Loss of equilibrium of the structure or the ground due to uplift by water pressure or

other vertical actions (UPL)

Hydraulic heave, internal erosion and piping in the ground caused by hydraulic

gradients (HYD)

The following factors should also be considered.

Overall stability and ground movements

Nature and size of the proposed construction including the design life

Conditions with regards to the surroundings (e.g. neighbouring structures, traffic,

utilities, vegetation, contamination etc.)

Ground and groundwater conditions

Influence of the environment.

12 Slope Stability

A proposed site layout drawing (16206 / P101) has been provided by Bellway Homes Ltd

which at the time of the current reporting is understood to be undergoing internal planning

review.

The current proposal comprises access roads forming the infrastructure close to the site

boundaries with residential homes no closer than circa 15m from the site boundary. Several

swales are also located across the site, two are located close to the northern boundary of the

site.

The ground investigation has recorded cohesive clay soils with sandstone bands to a depth of

2.45m along the site boundary, overlying sandstone rock to depths in excess of 12.0m.


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Underlying the sandstone rock sands of the Atherfield Clay Formation are present to depths

in excess of 30m (34.175mAoD).

At this stage, a safe long-term natural slope angle of 1v:2h (i.e. 26 degree) should be assumed

for the granular soils. Therefore, assuming a maximum slope height of 8m, a slope failure could

impact a horizontal distance of up to 16m from the toe of the cliff. Clay layers have not been

identified within the Atherfield Clay nor Hythe Formation as part of this or previous

investigations to date, although they are noted to be present across the wider area and have

led to large scale failure in the past. The presence of either superficial clays or at depth would

reduce the recommendations herein.

The areas of the site that fall within the potentially affected distance from the slope toe (1v:2h)

should be considered sterilised for development or temporary works. Failure of the slope

within these areas could lead to settlement/displacement of the surface which could impact

any above and below ground infrastructure present.

To mitigate against groundwater impacting the stability of the offsite slope all drainage and

trench excavations should also be avoided in the sterilised zone.

Surface water should be discharged to deep borehole soakaways well below the level of the

toe of the off-site slope. Shallow discharge of water from say leaks in drains on site could lead

to water entering the rear gardens of the adjoining properties and has the potential to increase

pore water pressure increasing the risks of slope failure.

During the construction phase care should be taken with regards to placement of stockpiles

and/or tracking of heavy plant to avoid direct loading of the offsite slopes and cliffs. The

placement of any temporary additional loads and vibration should be avoided at all times within

the sterilised zone.

It is noted that failure within the offsite slopes and cliffs has the potential to occur at any time,

and that these will impact offsite residents where measures to limit failure of the existing slope

and cliffs have not been undertaken.

This work does not constitute a full detailed slope stability analysis. Further detailed slope

stability analysis could be carried out to further refine the safe distances from the toe of offsite

slopes and cliffs. For this work to be carried out further detailed topographic information

would be required together with access across the slope areas.


APPENDIX A – Limitations

Limitations


LIMITATIONS This report is confidential to the Client, and Leap Environmental Ltd accepts no responsibility

whatsoever to third parties to whom this report, or any part thereof, is made known, unless

formally agreed by Leap Environmental Ltd beforehand. Any such party relies upon the report

at their own risk. Unless explicitly agreed otherwise in writing, this report has been prepared

under LEAP’s standard terms and conditions, as included in the quotation for this works.

This report has been prepared by Leap Environmental Ltd on the basis of information received

from a variety of sources which Leap Environmental Ltd believes to be accurate. Nevertheless,

Leap Environmental Ltd cannot and does not guarantee the authenticity or reliability of the

information it has obtained from others.

Leap Environmental Ltd has used all reasonable skill, care and diligence in the design and

execution of this report, taking into account the manpower and resources devoted to it in

agreement with the Client. Although every reasonable effort has been made to obtain all

relevant information, all potential contamination, environmental constraints or liabilities

associated with the site may not necessarily have been revealed. LEAP cannot be held

responsible for any disclosures or changes in regulation that are provided post production of

this report, and will not automatically update the report.

The conclusions reached in this report are necessarily restricted to those which can be

determined from the information consulted, and may be subject to amendment in the light of

additional information becoming available. These conclusions may not be appropriate for

alternative schemes.

The extent of the exploratory holes, laboratory testing and monitoring undertaken may have

been restricted due to a number of factors including accessibility, the presence of buried or

overhead services, current development and site usage, timescales or clients specification.

The exploratory holes only assess a small proportion of the site area with respect to the site

as a whole, and as such may only provide an overall assessment of ground conditions on site.

The presence of hotspots of undisclosed contamination or exceptional and unforeseen ground

conditions cannot be discounted.

Eurocode 7 gives guidance on the type of sampling, sample quality, number and spacing of

intrusive investigations, and number of laboratory tests required. It is intended that the

Geotechnical Information section of this report will fulfil the general requirements of the

Ground Investigation Report as set out in section 6 of Eurocode76, although this is subject to

the restrictions imposed on the investigation as listed above. For geotechnical design,

Eurocode 7 requires the Geotechnical Design Report to address both the geotechnical and

structural aspects of the geotechnical design for both the limit and serviceability states. The

6 BS EN 1997 Eurocode 7- Geotechnical Design - Part 1: General Rules (2004) and Part 2: Ground

Investigation and Testing (2007)


Geotechnical Appraisal section of this report will not meet the requirements of a Geotechnical

Design Report (GDR), and should therefore be used for preliminary guidance only.

The presence of asbestos may be noted during the site walkover survey, intrusive

investigations and/or from the results of contamination testing. However, this report does

not constitute an asbestos survey. On this basis, the presence of asbestos on site cannot be

discounted and a full asbestos survey should be undertaken.


APPENDIX B – Figures

Figures

Site

Site access

Client: Bellway Homes Kent

Site: Church Road, Otham Date: 30/09/2019

Title: Proposed

Development Plan

Project ID: LP1868

Figure No.: 2

Client: Bellway Homes Kent

Site: Church Road, Otham Date: 17/09/2019

Title: Borehole Location

Plan

Project ID: LP1868

Figure No.: 3

WS201

WS202

WS203

WS204

WS205

WS206

WS207

DPT201

DPT202

DPT203

BH201


APPENDIX C – Trial Hole and Borehole Logs

Trial Hole and

Borehole Logs

Leap Environmental LtdThe Atrium, Curtis RoadDorking, Surrey RH4 1XATel: 01306 646510www.leapenvironmental.com

Borehole LogBorehole No.

WS201Sheet 1 of 1

Project Name: Church Road, OthamProject No.LP1868

Co-ords: 578705 - 154135Hole Type

WS

Location: Maidstone Level: 56.57Scale1:25

Client: Bellway Homes Ltd (Kent) Dates: 12/09/2019 - 12/09/2019Logged By

TK

RemarksNo further progress below 0.70m depth - too dense. Borehole remained dry and stable on completion.

Well Water Strikes

Samples and In Situ Testing

Depth (m) Type ResultsDepth

(m)

0.35

0.70

Level(m)

56.22

55.87

Legend Stratum Description

TOPSOIL: Brown sandy silty clay with rootlets.

Hand Penetrometer UCS at 0.3m depth = 600 kPa.Light grey and orange brown clayey SAND interbedded with sandy CLAY with frequent cobbles of sandstone.

End of borehole at 0.70 m

1

2

3

4

5

0.60 D0.70 N=71 (9,7/7,8,11,45)



WS202Sheet 1 of 1



WS



TK


Well Water Strikes



(m)

0.10

0.75

1.30

Level(m)

55.96

55.31

54.76


TOPSOIL: Dark brown slightly gravelly sandy clay with rootlets. Gravel is fine to medium flint. MADE GROUND: Brown slightly gravelly clay. Gravel is fine to medium brick, charcoal and siltstone.

Dense light grey clayey SAND with frequent bands of sandstone.


1

2

3

4

5

0.40 D

0.80 N=31 (17,9/7,6,6,12)

1.25 N=50 (25,30/50 for 75mm)



WS203Sheet 1 of 1



WS



TK


Well Water Strikes



(m)

0.15

0.450.50

Level(m)

55.67

55.3755.32


TOPSOIL: Dark brown sandy clay with rootlets.

MADE GROUND: Brown slightly gravelly sandy clay. Gravel is fine to medium brick.

Light grey and brown sandy CLAY with frequent bands of sandstone.


1

2

3

4

5

0.50 N=62 (8,27/62 for 150mm)



WS204Sheet 1 of 1



WS



TK


Well Water Strikes



(m)

0.10

0.40

0.80

Level(m)

60.94

60.64

60.24


TOPSOIL: Dark brown sandy clay with rootlets. MADE GROUND: Brown slightly gravelly sandy clay. Gravel is fine to medium charcoal and brick.

Orange brown slightly silty slightly sandy gravelly fine to coarse SAND with occasional bands of sandstone. Gravel is fine to medium sandstone.


1

2

3

4

5

0.50 D

0.80 N=65 (6,16/65 for 225mm)



WS205Sheet 1 of 1



WS



TK


Well Water Strikes



(m)

0.20

1.00

Level(m)

63.76

62.96


TOPSOIL: Brown sandy clay with rootlets.

Very stiff orange brown slightly silty slightly sandy gravelly CLAY. Gravel is medium to coarse sandstone.

Hand Penetrometer UCS at 0.4m depth = 600 kPa.

With a band of sandstone at base of borehole.

End of borehole at 1.00 m 1

2

3

4

5

0.70 D

1.00 N=50 (5,20/15,15,10,10)



WS206Sheet 1 of 1



WS



TK

RemarksNo further progress below 4.00m depth - too dense. Borehole remained dry on completion. Borehole collapsed to 2.20m depth on completion.

Well Water Strikes



(m)

0.40

2.00

4.00

Level(m)

67.43

65.83

63.83


MADE GROUND: Brown slightly gravelly sandy CLAY.

Hand Penetrometer UCS at 0.3m depth = 600 kPa.


No recovery between 2.0m and 3.0m depth.

Limited recovery (5%) between 3.0m and 4.0m depth.

SANDSTONE/LIMESTONE encountered at base of the borehole.


1

2

3

4

5

1.00 N=27 (4,8/6,6,9,6)

2.00 N=11 (5,6/3,4,2,2)

3.00 N=3 (1,0/1,1,0,1)

4.00 N=55 (7,13/10,13,17,15)



WS207Sheet 1 of 1



WS



TK

RemarksNo further progress below 2.45m depth - too dense. Borehole remained dry on completion. Borehole collapsed to 2.20m depth on completion.

Well Water Strikes



(m)

0.10

1.00

2.45

Level(m)

72.61

71.71

70.26


TOPSOIL: Dark brown gravelly clay with rootlets.Stiff brown sandy slightly gravelly CLAY. Gravel is fine to coarse ironstone and siltstone.

Hand Penetrometer UCS at 0.8m depth = 280 kPa.Becoming firm below 0.8m depth.


Poor recovery between 1-2m depth.


1

2

3

4

5

1.00 N=8 (3,2/2,2,2,2)

2.00 N=23 (2,2/23 for 227mm)

2.45 N= 50 (50 for 75mm)


Rotary Core LogBorehole No.

BH201Sheet 1 of 3



RO

Location: Maidstone Level:Scale1:50


TK

RemarksBorehole cased to 30m. Borehole remained dry on completion. Standpipe installed to 30m, top 29m plain with gravel surround between 20m and 29m, arising surround between 1m and 20m and bentonite seal between 0m and 1m. Remainder slotted with gravel surround. Completed with cap and an upstand cover.

Well Water Strikes

Depth(m)

Type/ FI

Coring

TCR SCR RQDDepth

(m)

0.50

3.00

6.50

8.50

Level(m) Legend Stratum Description

TOPSOIL.

Brown CLAY.

Brown SAND.

Grey LIMESTONE.

Grey LIMESTONE.

Continued on next sheet

1

2

3

4

5

6

7

8

9

10



BH201Sheet 2 of 3



RO



TK


Well Water Strikes

Depth(m)

Type/ FI

Coring

TCR SCR RQDDepth

(m)

12.00


Grey SAND.

Continued on next sheet

11

12

13

14

15

16

17

18

19

20



BH201Sheet 3 of 3



RO



TK


Well Water Strikes

Depth(m)

Type/ FI

Coring

TCR SCR RQDDepth

(m)

30.00



21

22

23

24

25

26

27

28

29

30


APPENDIX D – Dynamic Probe Test Results

Dynamic Probe Test

Results


Probe LogBorehole No.

DPT201Sheet 1 of 1



DP


Client: Bellway Homes Ltd (Kent) Dates: 12/09/2019 -Logged By

TK

RemarksNo further progress at 3m - double bouncing.

Fall Height

Hammer Wt

Probe Type

760

63.5

DPSH

Cone Base Diameter

Final Depth

Log Scale

50

2.10

1:25

Depth(m)

1

2

3

4

5

Blows/100mm10 20 30 40

01

24

577

61

6333

22

110

1515

2550

Torque(Nm)



DPT202Sheet 1 of 1



DP



TK

RemarksNo further progress at 1.7m - double bouncing.

Fall Height

Hammer Wt

Probe Type

760

63.5

DPSH

Cone Base Diameter

Final Depth

Log Scale

50

1.70

1:25

Depth(m)

1

2

3

4

5

Blows/100mm10 20 30 40

14

916

99

71010

511

1016

201212

50

Torque(Nm)



DPT203Sheet 1 of 1



DP



TK

RemarksNo further progress at 3.2m - double bouncing.

Fall Height

Hammer Wt

Probe Type

760

63.5

DPSH

Cone Base Diameter

Final Depth

Log Scale

50

2.20

1:25

Depth(m)

1

2

3

4

5

Blows/100mm10 20 30 40

01

23

44

311

126

4333

414

2216

1112

1850

Torque(Nm)

Outline Slope Stability Addendum Report

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