1 Introduction and background

1.1 Introduction

This document sets out to record Crossrail’s experience of the use of Geotechnical Baseline Reports (GBRs) in its construction contracts. In reading this report it is important to remember that in all of the major Crossrail Civil Engineering contracts Contractor design activities were limited to temporary works only. There were no Contractor design and build aspects to the permanent works. This greatly simplifies the production of Geotechnical Baseline Reports.

This report briefly sets out what a GBR is; why and where Crossrail used them, how GBRs were implemented and then provides some analysis and commentary on how successful or otherwise they were in practice. In this report example extracts from GBRs are presented in italic Times New Roman, with example Baseline Statements specifically being presented in bold italic Times New Roman.

An appendix containing notes on the drafting of GBRs is also included.

1.2 What is a GBR?

A GBR is a commercial document which formed part of the ITT for each Crossrail contract that included a significant amount of below-ground works.

The purpose of a GBR is to establish the allocation of risk between the Employer and the Contractor in relation to the ground. It does this by including contractual statements (Baseline Statements) that define the relevant geotechnical conditions that the contractor can expect to find during construction. If conditions are found to be equal to or better than a baseline the target cost is unaltered. If conditions are found to be worse than a baseline and the contractor can demonstrate a loss, a compensation event is triggered.

At tender the intention is that the GBR is used as a common basis for pricing geotechnical and geological risk by the tenderers. Post contract award the GBR is used to judge the validity of compensation events for those issues covered by the GBR.

The Baseline Statements establish what is ‘foreseen’ & ‘unforeseen’ in relation to the ground conditions as they affect construction. They should only cover issues which could adversely impact on construction (and hence be the subject of a compensation event).

Baseline Statements must be concise, measurable and clearly defined. They are not necessarily geotechnical ‘facts’ and can be used to position risk boundaries wherever the Employer wishes. In practice Crossrail Baseline Statements are almost all based on what the Employer considered to be a reasonable and realistic view of what was likely to be encountered during the works.

The Crossrail GBRs were NOT the basis for the tender design or any contractor design, nor were they interpretative reports. Post contract award the GBR exists solely to determine whether a compensation event has arisen.

1.3 Why did Crossrail adopt GBRs?

Guidelines on the use of GBR’s were first published in 1997 by the Technical Committee on Geotechnical Reports of the Underground Technology Research Council in the USA. These guidelines were updated in a second edition in 2007. This document sets out the advantages inherent in providing a contractual interpretation of ground conditions as well as providing purely factual data on the ground conditions.

In the UK in 2003 the Association of British Insurers and the British Tunnelling Society jointly produced ‘The Joint Code of Practice for Risk Management of Tunnel Works in the UK’ (the ‘JCoP’). This code states:

7.2.5. Contract Documentation (as well as subcontract documentation for Tunnel Works as appropriate) shall include Ground Reference Conditions or Geotechnical Baseline Conditions prepared by the Client (or prepared on his behalf) or shall require each tenderer to submit with their tender their own assessment of Ground Reference Conditions or Geotechnical Baseline Conditions, the requirements of which shall be defined and fully described in the Contract Documentation.

The terms of Crossrail’s insurance policy require compliance with the JCoP.

The JCoP defines ‘Tunnel Works’ as the following: “Tunnels, caverns, shafts and associated underground structures howsoever constructed and including the renovation of existing underground structures.”

In practice all Crossrail civils contracts that have significant ‘in ground’ works incorporate GBRs even where their scopes do not contain ‘Tunnel Works’ as defined by the JCoP.

The JCoP ‘Tunnel Works’ scope was found in the following contracts:

Stations

• C405 Paddington
• C412 Bond Street
• C422 Tottenham Court Road
• C435 Farringdon
• C502 Liverpool Street
• C512 Whitechapel

Stations advance works

• C411 Bond Street
• C421 Tottenham Court Road
• C430 Farringdon
• C501 Liverpool Street
• C503 Liverpool Street
• C511 Whitechapel

Running Tunnels

• C300 Drive X Royal Oak Portal to Farringdon
• C305 Drive Y Limmo to Farringdon,
• C305 Drive Z Pudding Mill Lane to Stepney Green,
• C305 Drive G Limmo to Victoria Dock Portal
• C310 Drive H Thames Tunnel
• C315 Connaught Tunnel

Station SCL tunnels

• C410, Bond Street/Tottenham Court Road
• C510 Liverpool Street/Whitechapel

Portals

• C330 Royal Oak Portal
• C350 Pudding Mill Lane Portal
• C340 Victoria Dock Portal
• C310 North Woolwich Portal
• C310 Plumstead Portal

Shafts

• C300 Fisher Street Shaft
• C305 Stepney Green Shaft
• C360 Mile End Shaft
• C360 Eleanor Street Shaft
• C305 Limmo Peninsular Shaft

In addition the following non-tunnel works contracts also utilised GBRs:

• C298 Plumstead Depot Enabling Works
• C336 Paddington New Yard
• C520 Custom House Station
• C695 Plumstead maintenance depot

2 How did Crossrail adopt GBRs?

Crossrail used the NEC3 form of contract for its Civils works. In its un-amended form NEC3 does not include provision for GBRs, but in Clause 60.1 it does set out 19 types of compensation event including an ‘unforeseen ground’ compensation event (60.1.12), this states:

The following are compensation events

(12) The Contractor encounters physical conditions which

• are within the Site

• are not weather conditions and

• an experienced contractor would have judged at the Contract Date to have such a small chance of occurring that it would have been unreasonable for him to have allowed for them.

Only the difference between the physical conditions encountered and those for which it would have been reasonable to have allowed is taken into account in assessing a compensation event.

GBRs were incorporated into Crossrail tenders by amending this to state (changes in bold):

(12) The Contractor encounters physical conditions which

• are within the Site,
• are not conditions of a type referred to in the GBR,
• are not weather conditions and
• an experienced contractor would have judged at the Contract Date to have such a small chance of occurring that it would have been unreasonable for him to have allowed for them.

Only the difference between the physical conditions encountered and those for which it would have been reasonable to have allowed is taken into account in assessing a compensation event.

(12A) The Contractor encounters conditions which:

• are within the Site,
• are of a type referred to in the GBR and
• are more adverse than the conditions set out in the GBR

Only the difference between the conditions encountered and those set out in the GBR are taken into account in assessing a compensation event.

This establishes two classes of ‘unforeseen ground condition’ compensation events. One is ‘of a type referred to in the GBR’ and one is where the ‘conditions’ are not ‘of a type referred to in the GBR’.

In the 60.1(12) ‘non GBR’ case another clause sets the following test:

60.2 In judging the physical conditions for the purpose of assessing a compensation event under 60.1(12), the Contractor is assumed to have taken into account

• the Site Information,
• publicly available information referred to in the Site Information,
• information obtainable from a visual inspection of the Site and
• other information which an experienced contractor could reasonably be expected to have or to obtain.

This clause only applies to 60.1(12) and not to the ‘GBR conditions’ in 60.1(12A).

This arrangement means there is no hierarchy associated with the GBR in relation to the Site Information in the Contract as there are no situations where 60.1(12) and (12A) can both apply. It also means that there are two completely separate sources of information for assessing an ‘unforeseen ground condition’ depending on whether (12) or (12A) applies.

In other words the Site Information and the GBR cannot be used together in support of a compensation event claim. The Site Information clearly contains ‘facts’ about the ground whereas the Baseline Statements in a Crossrail GBR cannot be presumed to be factually correct.

60.1(12) and (12A) also provide an opportunity for the Client to deliberately choose whether a specific ‘type’ of condition will be addressed using the GBR or addressed by using the Site or other Information.

3 How did Crossrail organise its GBRs?

3.1 GBR Introduction

The Introduction to a Crossrail GBR makes statements on the following:

1. The contractual context and the purpose of the GBR and the Baseline Statements
2. That the Baseline Statements are aligned with the tender design. Alongside this is a very brief statement of the main forms of in-ground construction for that contract.
3. That the Baseline Statements are not ‘geotechnical fact’ and that they do not constitute a warranty that those conditions will be found.
4. That nothing in the GBR changes the Contractors responsibility for the safe execution of the works or for providing the works in accordance with the contract
5. That the GBR isn’t Site Information

In general the Crossrail Civils tender documentation provided a fully engineered permanent works design and typically did not show any temporary works.

One of the consequences of 2) is that temporary works are not covered by the GBR unless they are specifically indicated within the tender documentation. An example of this might be where a tender drawing indicates the minimum extent of ‘ground improvement’ above a shallow tunnel.

The statements in 3) and 5) are very important as they draw a clear distinction between the non-factual (in contract terms) GBR and the factual Site Information. They are also important as the purpose of the GBR was commonly misunderstood once works were underway. A typical misunderstanding would result in the GBR being used by a Contractor as an interpretative report in temporary works design.

For this reason it is worth repeating the point on the purpose of the GBR made above. “The Crossrail GBRs were NOT the basis for the tender design or any contractor design, nor were they interpretative reports. Post contract award the GBR exists solely to determine whether a compensation event has arisen”.

3.2 Baseline Statement Format

In a Crossrail GBR, Section 2 describes in general terms the geological setting for the contract.

None of this text forms part of a ‘condition’ as described in clause 60.1(12A). The 60.1(12A) ‘conditions’ are set out in Baseline Statements in Section 3 onwards. The Baseline Statements themselves are differentiated from the rest of the contextual text in the GBR by being formatted in a particular way. The Baseline Statements are set out in bold italic text.
The only text in the GBR that applies in assessing the merits of a particular 60.1(12A) compensation event is the text in bold italics, the rest is merely context.

Here is an example:

For the purpose described in the Introduction to this GBR, for the Works at **** Station, the Baseline Statements relating to existing boreholes are:

• The boreholes indicated in Table 3.5 will be encountered during the construction of the Works.
• Boreholes identified in the Site Information that are within the limits of the Works will be grouted prior to construction.
• Boreholes will have a maximum grout diameter of 300mm.
• No other boreholes or wells will be encountered.

This is a simple Baseline Statement, it tells the contractor which boreholes will be encountered (in his SCL works), it tells the contractor where these boreholes are (via the Site Information), what form the boreholes will take (grouted, <300mm diameter) and that only those boreholes listed will be found.

The statements are concise, measurable and clearly defined.

Points to note:

The contractor has to encounter an exceedance of the baselined condition during the construction of the works to have a valid compensation event.

The second baseline statement in this example doesn’t add much from a commercial standpoint (there were more boreholes in the Site Information than in the Table) but it does indicate to the tenderer that all the Crossrail boreholes, whether or not they are expected to be encountered, have been rendered relatively safe with respect to any SCL works should the tunnel layout be varied from the Tender arrangements.

What did Baseline Statements cover?

Before setting out the specific Baseline Statements a Crossrail GBR stated how certain conditions were to be measured. Typically this would take the form of a statement like this:

The test methods required in order to provide a valid comparison between conditions encountered during construction and those defined by the Baseline Statements are provided in Table x.y.

 

Table x.y – Valid Testing Methods

Baseline Parameter	Definition	Method of Measurement
Undrained Shear Strength of Fine-Grained Deposits	Undrained shear strength for the intact material not the strength measured on a pre-existing fissure.	Single Stage, Unconsolidated Undrained Triaxial Compression Testing in accordance with BS1377:1990, on a 100mm diameter sample recovered using triple tube rotary coring.
Density of Coarse-Grained Deposits	–	Standard Penetration Testing in accordance with BS EN ISO 22476-3:2005. N values should be reported for uncorrected Energy Ratio.
Unconfined Compressive Strength of Hard Bands and Claystone.	–	Uniaxial Compressive Strength (UCS) testing in accordance with the ISRM Suggested Methods.
Mass Permeability	In-situ permeability of a block of ground, minimum dimensions 10m x10m plan and 0.5m thick	Variable head permeability testing in accordance with BS5930:1999 or Pumping tests in accordance with BS ISO 14686:2003

 

 

In practice applying these statements did not cause any difficulties, but it is recognised that in many circumstances it would not have been practicable to rigidly apply these methods of measurement. It would have been better to add the option of ‘any other method of measurement agreed by the Project Manager’.

Deciding on the ‘types’ of condition to be addressed by Baseline Statements is an important consideration when writing a GBR. The types of condition addressed by the Baseline Statements clearly need to be relevant to the intended or likely form of construction. At the same time the purpose of the Baseline Statements is to foresee the unforeseen.

4 Types of condition used in Crossrail GBRs

A typical high level list of ‘types of condition’ used in a Crossrail GBR is given below. Exactly what was baselined within these categories was determined by the nature of the construction techniques within each contract. Each of these high level conditions is discussed in detail below.

Man-made features

• Existing foundations
• Existing Tunnels
• Underground Utilities
• Excavations or filled ground
• The presence of boreholes
• The presence of wells or other shafts

Unexploded Ordnance

Contamination and Waste

Geological features

• Strata Occurrence
• Strata Boundaries
• Strata Description
• Faults
• Hard strata
• Other anomalous features (e.g. ‘drift filled hollows’)
• Ground gas
• Ground Behaviour

Groundwater

• Pore water pressure
• Permeability

Where individual contracts contained several significantly different forms of structure and forms of construction, Baseline Statements for each of these would be collected together in different chapters within the GBR. For example a major tunnelling contract might have different chapters containing Baseline Statements for running tunnels, shafts, cross passages, SCL works and so on. Similar splits would also apply to similar types of structures built in different ways, such as SCL and SGI cross passages. Running tunnel baseline statements would also tend to be grouped by chainage where changes in geological conditions varied along the alignment.

All this variation might mean similar conditions (eg strength of particular strata) could be baselined in different ways in different places or for different construction techniques. This is perfectly reasonably in a GBR as there is no overarching requirement for consistency. Remember that Baseline Statements are not ‘geotechnical fact’.

4.1 Man-made Features

4.1.1 Man-made features – Existing foundations

For all forms of construction the key points that require baselining for these types of obstruction are the dimensions in relation to the extent of the likely encounter, strength and the nature of the materials that will be encountered. Where existing foundations are well understood Baseline Statements are straightforward to draft. Examples would be:

• A maximum of fifty-six (56) reinforced concrete piles of maximum diameter 900mm and with a concrete strength of not more than 60MPa will be encountered during the construction of the Works.

• A 500mm thick basement reinforced concrete slab will be encountered during construction of the Works at ****. The slab covers an area of 30 m2within the footprint of the Works. The concrete will have a strength of not more than 50N/mm2.

• A maximum of forty two (42 No.) piles, of concrete strength not more than 60N/mm2, will be encountered during the construction of the tunnelling Works of which five (5) will be reinforced. The pile diameters will be:

Ten (10 No.) will be of maximum 500mm diameter.

An additional ten (10 No.) will be of maximum 1000mm diameter.

An additional twelve (12 No.) will be of maximum 2500mm diameter.

Where details of existing foundations were not known then Baseline Statements were typically drafted on the basis of experience and/or best estimates.

In some cases the Crossrail Employer wished to have the tenderers make an allowance for as yet unknown obstructions. These statements typically took this form:

• The foundations of four (4) structures will be encountered within the tunnel face at unknown locations along the length of the running tunnels in Drives Y and Z. These foundations will consist of up to six (6) per location, unreinforced concrete piles of maximum diameter 900mm and concrete strength of not more than 60N/mm2.

Note the exact location of the individual piles is not baselined, even though this may well have been known and that information included within the site information.

This was to avoid tenderers pricing for X encounters then being able to claim for X+1 encounters purely because the one of the X number of piles was not in the anticipated location. Of course if the location of particular obstructions is significant then baselining the specific locations may be desirable.

4.1.2 Man-made features – Existing tunnels

Normally the Crossrail works were not expecting to encounter pre-existing tunnels and the baseline statement associated with this type of encounter would typically state:

• No existing tunnels will be encountered during the construction of the Works.

Non-baseline contextual text would typically be used to flag up the presence of nearby tunnels in this part of the GBR as a backup to the site information. This Baseline Statement merely confirms that these tunnels are not expected to be encountered.

It is plausible that Works under a contract could knowingly encounter pre-existing tunnels and if so this could be identified and baselined here, however it is also highly likely that such an expected (foreseen) encounter would be dealt with elsewhere in the tender documentation.

4.1.3 Man-made features – Underground Utilities

Underground utilities proved to be one of the harder types of condition to baseline on Crossrail. There are two principle reasons for this:

• The difficulty of describing/defining the nature of the utilities in measurable ways when the physical nature of the utilities was often unknown. Different types of utilities are often recorded on drawings only by varying line styles.
• The inevitable uncertainty as to the actual location of underground utilities.

These points applied to both live utilities and disused or diverted utilities including those diverted by Crossrail.

The clearest way to Baseline utilities is to refer to specific drawings in the Site Information. However drawings often only record a location in plan, often don’t describe the physical nature of the utility or describe whether it is in use or not.

Ideally tender documentation would include details of all pre-existing utilities and any as-built diverted locations. On Crossrail this information was not always available at the time of tendering (because enabling works contracts were incomplete). The Crossrail GBRs would certainly have been easier to write if they had explicitly excluded encounters with some types of underground utilities and moved these issues to a 60.1(12) approach. However on Crossrail this would probably have led to more compensation events and less cost/programme certainty.

4.1.4 Man-made features – Excavations or filled ground

Typically geological Baseline Statements relating to Made Ground, as described in 4.4 below, dealt with the occurrence and nature of made or filled ground that pre-dated Crossrail.

Baseline statements made under ‘Man-made features – Excavations or filled ground’ tended to cover only filled/excavated ground deliberately created by neighbouring or preceding Crossrail contracts, i.e. encounters that were probably not recorded in borehole logs in the Site Information.

4.1.5 Man-made features – The presence of boreholes

For some forms of construction (e.g. TBM and SCL tunnelling) encountering unexpected boreholes can be a significant hazard (e.g. in-rush of water/soil, metallic obstructions, loss of face support pressure, and mobilisation of contaminants). As a result it is important to identify the location of all known boreholes and to baseline something about the form they will be found in. See 3.2 for an example.

4.1.6 Man-made features – wells and other shafts

Unexpectedly encountering wells or shafts, particularly when tunnelling, has the potential to cause significant delay and extra cost. Crossrail’s consultants were commissioned to exhaustively search for potential encounters. Despite this there were at least 6 encounters with unknown wells during tunnelling, fortunately none of them serious.

As with other conditions these Baseline Statements need to define the number, nature and perhaps location of the encounters. A typical example for a tunnelling contract would be:

• One (1) well will be encountered at the tunnel face at WB Chainage 4390 ± 25m.

• Two (2) additional wells will be encountered at the tunnel face at unknown locations along the length of running tunnels covered by C***.

• Wells encountered will be brick lined and have a maximum diameter of 2.5m.

4.2 Unexploded Ordnance

Crossrail provided significant quantities of background desk study information on UXO risk within each Civils tender pack. In addition all the Civils contracts required the contractor to undertake UXO risk assessments and if required, investigations for UXO.

Crossrail considered the risk of finding UXO to be effectively un-priceable and as a result took on the risk of UXO discovery for every GBR contract via this baseline statement:

• No unexploded ordnance will be encountered during the construction of the Works.

4.3 Contamination and Waste

This was an area of weakness in the Crossrail GBRs. This was not because the subject is intrinsically difficult to baseline, but more because of Crossrail specific timing issues around obtaining Wallasea Island acceptance criteria for some natural ‘as dug’ materials that exceeded ‘Inert’ criteria for certain substances. These timing issues meant not all the relevant information on classification of materials was able to be included in the tender documentation.

Wallasea Island is the site of a land reclamation project that took a large proportion of Crossrail’s excavated material. Excavated materials (‘Waste’) were accepted at Wallasea under a Waste Recovery Permit. The processes within the Wallasea Waste Recovery Permit meant that at the point of placement at Wallasea the excavated material was no longer ‘Waste’.

For Crossrail contracts that were to deliver spoil to Wallasea the ideal scenario would have been to define proportions of excavated material that would or wouldn’t meet the Wallasea waste recovery criteria in Baseline Statements.

A non-Crossrail (or non-Wallasea) equivalent approach would be to Baseline proportions of excavated material on the basis of the usual European waste classification systems (Hazardous, Inert etc) or Baseline by using criteria for other known specific disposal sites.

When selecting the best approach for Baselining disposal of spoil much depends on whether the final destination for the spoil is known at the time of tender. If the site(s) are known then the acceptance criteria applicable to those sites can be used to develop Baseline Statements. Clearly different acceptance criteria at several pre-determined sites could cause difficulty.

The status of the spoil in ‘Waste’ terms and whether or not the contractor is incentivised to minimise disposal of ‘Waste’ is also an important consideration and the nature of any incentives may need to be reflected in any Baseline Statements.

Any acceptance criteria used in Baseline Statements might need to incorporate the impact of any additives that might necessarily be contained within the spoil (e.g. conditioners, bentonite or SCL / SCL fibres). Since this is moving outside the normal experience of a Geotechnical Engineer advice should be sought on these matters when writing a GBR.

4.4 Geological Features

4.4.1 Geological features – Strata Occurrence and Strata Boundaries.

These two conditions are typically treated separately in the GBR but are discussed together here.

Before going on to Baseline the relevant properties of geological materials it is necessary to Baseline the geological materials that the works will encounter. This is not always straightforward. Construction works for a Crossrail Station typically take place in a minimum volume of ground 400m x 70m x 40m so there are always variations in the strata encountered. In London these variations are significantly amplified when the works extend below the base of the London Clay into the laterally variable Lambeth Group.
One way to simplify this process is to sub-divide a GBR for strata boundary Baseline purposes, e.g. boxes/shafts and tunnels would have separate sets of Baselines. This approach would also neatly match the differing Baseline requirements for strata properties needed for different forms of construction (see below).

For contracts with below ground construction in relatively low volumes of ground, like shafts or small plan area boxes where there is little variation in strata, a simple table like this usually suffices for both strata occurrence and strata boundary Baselines:

• The elevations of the strata boundaries relevant to the Works are shown in Table x.y.

Table x.y Baseline Stratigraphy at the Site

Stratum	Elevation of base of stratum (m ATD)
	Maximum	Minimum
Made Ground	110	108
Alluvium	109	107
River Terrace Deposits	106	100
London Clay Formation	78	76
Harwich Formation	1 to 2m below London Clay Formation
Lambeth Group clay and silt units	69	60

 

 

For more the complex geology found at Farringdon a slightly more sophisticated approach was adopted to account for differences either side a particular fault. Here Strata Boundaries were separated from Strata Occurrence. The Strata Boundary Statements took this form:

For the purposes described in the Introduction to this GBR, for the C435 Tunnelling Works at Farringdon Station, the Baseline Statement relating to strata boundaries is:

• The elevations of the strata boundaries relevant to the Tunnel Works are shown in Tables x.y and x.z.

Table x.y – Baseline stratigraphy to the west of the Smithfield (or Bh F10) Fault

Stratum	Elevation of base of stratum (m ATD)
	Maximum	Minimum
Made Ground	108	98
Alluvium	104	98
River Terrace Deposits	107	98
London Clay Formation	100	94
Harwich Formation	Maximum 1m thickness below base of LC
Lambeth Group clay and silt units	87	80
Lambeth Group coarse-grained units	84	78
Thanet Sand	74	69

 

 

Table x.z – Baseline stratigraphy to the east of the Smithfield (or Bh F10) Fault

Stratum	Elevation of base of stratum (m ATD)
	Maximum	Minimum
Made Ground	114	107
River Terrace Deposits	113	108
London Clay Formation	93	83
Harwich Formation	Maximum 1m thickness below base of LC
Lambeth Group clay and silt units	82	75
Lambeth Group coarse-grained units	78	72
Thanet Sand	70	62

 

 

 

The location of the “Smithfield (or Bh F10) Fault” was defined by references to particular drawings in the Site Information in the contextual text preceding this Baseline Statement.

The Strata Occurrence statements set out to Baseline additional points about each geological unit and their relationships to the 3D network of tunnels at Farringdon. In particular they Baselined the possibility of not encountering certain Strata within the limits set out above. Such a possibility might arise because of natural variability, the effects of faulting or historic removal or addition by Man.

For the purposes described in the Introduction to this GBR, for the C435 Tunnelling Works at Farringdon Station, the Baseline Statements relating to strata occurrence are:

• The London Clay Formation, the Lambeth Group and Thanet Sand will be present across the site.

• The Made Ground, Alluvium and River Terrace Deposits will be discontinuous across the site.

• The Harwich Formation and the Lambeth Group Sand Channels will occur in discontinuous channels and pockets across the site.

• The London Clay, Harwich Formation, Lambeth Group and Thanet Sand will be dislocated by faulting across the site.

• The SCL Works at Farringdon Station will encounter the London Clay Formation, Harwich Formation, Lambeth Group and Thanet Sand Formation with the exception of the SCL components in Table x.y.

• The SCL Works for ES1/CH1 will encounter the strata listed in Table x.y.

Table x.y – SCL components expected to encounter additional strata

Reference	Structure	Ground Conditions
ES1 / CH1	Escalator Incline 1 – Permeation Grouting and Pipe arches	Made Ground / Alluvium / River Terrace Deposits / London Clay / Lambeth Group / Thanet Sand

 

 

An alternative approach to these issues that also deals with a complex network of tunnels and less complex ground was adopted at Liverpool Street Station. Here a strata boundary table was baselined

• The elevations of the strata boundaries relevant to the Works are shown in Table 3.7.

Table 3.7- Baseline Stratigraphy at the Site

Stratum	Elevation of base of stratum (m ATD)
	Maximum	Minimum
Made Ground	111	104
Alluvium	110	106
River Terrace Deposits	109	98
London Clay Formation	80	75
Harwich Formation	Maximum 2m thickness below base of LC
Lambeth Group (clay and silt units)	69	60
Lambeth Group (coarse- grained units)	61	56
ATD is Above Tunnel Datum.

 

 

After this the Strata encountered by individual elements of the tunnel network were tabulated in another Baseline Statement (only an extract from this particular table is shown below).

The names and codes for each tunnel element matched those defined in the Works Information.

• The Works at Liverpool Street Station will encounter the London Clay Formation, with the exception of the components shown in Table 3.8.

Table 3.8 – Components Expected to Encounter Additional Strata

Reference	Structure	Ground Conditions
	Finsbury Circus Shaft	Made Ground / Alluvium / RTD / London Clay Formation / Harwich Formation / Lambeth Group clay and silt units / Lambeth Group Sand Channels
PTE	Platform Tunnel East	London Clay Formation / Harwich Formation / Lambeth Group clay and silt units/ Lambeth Group Sand Channels
PTW	Platform Tunnel West
CH1	Lower Concourse Chamber 1
CH2	Lower Concourse Chamber 2
CP1	Cross Passage 1
CP2	Cross Passage 2
CP3	Cross Passage 3
VD4	Ventilation Duct 4

 

 

• The Made Ground, River Terrace Deposits, London Clay Formation and Lambeth Group will be continuous across the site.
• The Alluvium, Harwich Formation and the Lambeth Group Sand Channels will occur in discontinuous channels and pockets across the site.

A different approach to these conditions was adopted for running tunnels. For the EPB TBMs, which mined mainly in clay, sand or mixed clay sand strata, contextual text was used to list areas of common geology using tunnel chainage. A Baseline Statement was then used to set out three different classes of face condition is this way:

• Clay/silt conditions are defined as greater than 35% (by weight) passing the 0.063mm sieve.

• Coarse material is defined as less than 35% (by weight) passing the 0.063mm sieve.

• Mixed face conditions are defined as where greater than 10% of the material at the face is of different material (e.g. greater than 10% sand (by weight) in a predominantly clay face).

Baselined tables were then used to set out face conditions on a chainage basis in this way:

• Face conditions as detailed in Table x.y will be encountered within the tunnel face along the length of the eastbound running tunnel in Drive *.

Table x.y – Drive * eastbound face conditions

Chainage from	Chainage to	Face conditions
Limmo Peninsula Shaft	EB 15350	clay / silt
EB 15350	EB 13240	mixed
EB 13240	EB 12800	coarse
EB 12800	EB 11750	mixed
EB 11750	EB 9670	clay / silt
EB 9670	Farringdon Station	mixed

 

 

Additional Baseline Statements were also used to add further detail as required. Examples include:

• The mixed face conditions that will be encountered between EB Chainage 15760 and 16180 will consist of clays, sands (as Baselined in Section 3.2) and gravels.

• For 70% of the route between chainage EB 15760 and chainage EB 16180, the sands encountered at the face will be up to 3m thick.

• The sands are defined as less than 35% (by weight) passing the 0.063mm sieve and more than 65% (by weight) passing the 2mm sieve.

Portal approaches

• Within the 150m prior to entering Victoria Dock Portal, River Terrace Deposits will be encountered within the crown increasing in thickness towards the portal.

• Within the 15m prior to entering Pudding Mill Lane Portal, River Terrace Deposits will be encountered within the crown

Strata expected at individual running tunnel cross passages was separately Baselined.

The contract for Crossrail’s Thames Tunnel used slurry TBMs to mine a weak Limestone (Chalk) with some overlying sands and gravels for short distances at the portals. The GBR also used contextual text to list areas of common geology along the tunnel chainage but did not include Baseline Statements on strata boundaries as the encounters with these were very limited in extent, and they were very well defined in the Site Information. As a result the risks associated with TBM operation and unexpected variations were low.

An alternative approach for strata boundaries and strata occurrence Baselines would have been to reference strata boundaries drawn on Geological sections + or – a value. This approach was not adopted in any Crossrail GBR because:

• It is awkward to apply in practice (i.e. potentially constantly varying baseline conditions).

• Boundaries on geological sections are almost always broad approximations based on off-set boreholes (especially so in the case of tunnels where boreholes deliberately avoid the tunnel alignments). This uncertainty and the fact that a single line has to represent the full width of the works means any reasonable + or – element is likely to end up being relatively large. This means it is hard to fine tune any element of risk transfer.

4.4.2 Geological features – Strata Description

In this section geotechnical properties for each stratum are baselined. The properties or parameters that are baselined are determined by their relevance to the construction techniques involved in that part of a contract (i.e. what exceedance could cause a problem to that construction technique). As an example, there is little point baselining London Clay discontinuity spacing for piling, but it could be important for SCL tunnelling.

The table below summarises the typical properties baselined for different classes of ground and construction methods used on Crossrail. Sections on baselining more anomalous geological conditions (in a London context) follow this section, e.g. hard strata, faulting.

Typical Baselined Properties for strata encountered by Crossrail

Strata / Parameter	Piling / Deep Foundations	Bulk excavation	SCL tunnelling	TBM tunnelling (including Cross Passages)
Made Ground Heterogeneity	Yes	Yes	N/A	N/A
Made ground Max particle size	Yes	Yes	N/A	N/A
Made Ground Water Bearing*	Yes or No	Yes or No	N/A	N/A
Alluvium Organic content	Yes	Yes	N/A	N/A
Alluvium Strength**	Max & Min Su. Range of N if applicable	Max & Min Su. Range of N if applicable	N/A	N/A
Alluvium Plasticity	Yes	Yes	N/A	N/A
Alluvium Lithology variation	Opportunity to baseline significant high K (e.g. sand) or low strength (e.g.peat)	Opportunity to baseline significant high K (e.g. sand) or low strength (e.g.peat)	N/A	N/A
River Terrace Deposits – N value**	Max	Max	Yes	Yes
River Terrace Deposits – grading	Fines content and max particle size	Fines content and max particle size	Fines content and max particle size (prior to grouting)	Fines content and max particle size
River Terrace Deposits – Water Bearing*	Yes or No	Yes or No	Yes or No	Yes or No
London Clay & fine grained Harwich Fm / Lambeth Gp – Plasticity	N/A	N/A	Yes	Max PI
London Clay & fine grained Harwich Fm / Lambeth Gp – Su**	Max & Min	Max & Min Max, excludes Claystones	Max & Min Max, excludes Claystones	Max & Min Max, excludes Claystones
London Clay & fine grained Harwich Fm / Lambeth Gp -fissure spacing	N/A	N/A	Yes Vary near known faults ?	Yes Vary near known faults ?
London Clay –seepages * especially near Claystones	Yes or No	Yes or No	Yes or No	Yes.  Vary near known faults ?
Coarse grained Harwich Fm, Lambeth Group and Thanet Sand Fm – strength**	Max & Min N	Max & Min N	Max & Min N Min Ø’	Yes Vary near known faults ?
Coarse grained Harwich Fm, Lambeth Group and Thanet Sand Fm – water bearing	Yes or No	Yes or No	Yes or No	Yes or No
Harwich Fm – water bearing	Yes or No	Yes or No	Yes or No	Yes Vary near known faults ?
Lambeth Group – seepages*	Yes, from fissures, laminations and near nodules and hard and/or cemented layers	N/A	Yes, from fissures, laminations and near nodules and hard and/or cemented layers	Yes Vary near known faults ?
Lambeth Gp sand channels	See below
Thanet Sand – grading	Fines content	Fines content	Fines content	Fines content
Chalk – Strength**	Max UCS	Max UCS	Max UCS	Max UCS
Chalk – Water bearing	Yes or No	Yes or No	Yes or No	Yes or No

 

 

*Ground water levels, pore pressures and permeabilities are baselined elsewhere – see below.
** Max/Min strengths were typically given reasonable and realistic values rather than values that repeated the maximum and minimum strengths indicated by the Site Information. Specific instances of hard strata (e.g. Claystones) were baselined separately (see below).

4.4.3 Lambeth Group Sand Channels

Baselining the extent of encounters with sinuous and/or discontinuous Lambeth Group Sand Channels within a 3D network of SCL tunnels is particularly challenging, especially in faulted ground. Various methods were adopted with no obvious ‘best way’ being identified.

The following is a typical set of Sand Channel Baseline Statements, in this case from Farringdon, where there were many faults, many sand channels and very few boreholes within the station footprint because of the layout of buildings above:

• Sand Channels will be present within the clay and silt units of the Lambeth Group.

• 25% of the length of the tunnels listed in Table 3.1 that are excavated in the Lambeth Group clay and silt units will encounter Sand Channels up to 3m deep and up to the full width of the tunnel during construction of the Tunnel Works. The length of tunnel that encounters any single Sand Channel will vary according to the geometries of the Sand Channel and the tunnel. For the purposes of this document the length of tunnel that encounters a Sand Channel will be taken to be the average length of tunnel where that Sand Channel is encountered.

• During Temporary Access Shaft construction, Sand Channels totalling up to 3m thickness, and extending over up to the full area of the shaft will be encountered. These Sand Channels will extend horizontally beyond the zone of influence of the SCL Works.

• Sand channels within the Lambeth Group encountered within the Tunnel Works will exhibit a minimum relative density corresponding to an SPT N value of 40 and a maximum relative density corresponding to an SPT N value of 110.

• 90% of the Lambeth Group Channel Sands will have a fines content (particle size <63µm) of less than 30%. The maximum particle size encountered will be 60mm in diameter.

• The minimum drained friction angles of the Lambeth Group Channel Sands encountered by the Tunnel Works will be 32°.

• The Lambeth Group Sand Channels will be water bearing.

The second bullet point refers to 25% of the total length of SCL tunnels “within the clay and silt units of the Lambeth Group”. If there had been more confidence in the location of sand channels within the Station this statement could have said 25% of the length of each of the tunnels listed in Table 3…

Note “will encounter Sand Channels up to 3m deep”, this means 3m deep in the encounter, (i.e. in the tunnel), it doesn’t mean the total channel depth will be no more than 3m.

4.4.4 Ground Behaviour for TBM interventions

Ground Behaviour for TBM interventions was baselined on a chainage basis for TBM contracts. During normal TBM operation with a closed face these statements have little relevance as it isn’t possible to encounter these conditions. However any interventions that involve access to the face (planned or otherwise) would encounter one of these conditions. Ground behaviours in this circumstance were baselined using the ‘Tunnelling Ground Classification System’ (Modified After Terzaghi, 1950):

• Firm Ground – A heading may be advanced up to a metre or more without immediate support. Hard clays and cemented sand or gravel generally fall into this category.

• Ravelling Ground – After excavation, material above the tunnel or in the upper part of the working face tends to flake off and fall into the heading. Slightly cohesive sands, silts, and fine sands gaining their strength from apparent cohesion typically exhibit this type of behaviour. Stiff fissured clays may be ravelling also. In fast ravelling ground the process starts within a few minutes, otherwise the ground is slow ravelling.

• Squeezing Ground – Ground squeezes or extrudes plastically into tunnel without visible fracturing or loss of continuity and without perceptible increase in water content.

• Running Ground – Granular materials without cohesion or unstable at a slope greater than their angle of repose. When exposed at steeper slopes they run like sugar or dune sand until the slope flattens to the angle of repose. Clean, dry granular materials generally fall into this category. Apparent cohesion in moist sand or weak cementation in any granular soil may allow the material to stand for a brief period of ravelling before it breaks down and runs. Such behaviour is cohesive-running.

• Flowing Ground – A mixture of soil and water flows into the tunnel like a viscous fluid.

4.4.5 Hard Strata

Specific instances likely of Hard Strata were baselined for all types of contract involving excavation, mining and piling or diaphragm wall construction where they were likely to be encountered. These instances were:

• Claystone layers within the London Clay

• Cemented bands within the Harwich Formation and Lambeth Group

• Gravel layers within the Lambeth Group

• Flint bands (including the Bullhead Beds)

For TBM contracts the locations of these encounters were defined by chainage. For station contracts (including boxes and shafts) the locations were baselined according to geological and layout complexity and this could include both elevation limits, and plan limits, either by chainage or tunnel reference names. Baseline statements would typically include thicknesses, maximum strengths (usually defined as a UCS) and in the case of Claystones the number of layers present.

Where Lambeth Group hard strata were thought to be in the form of discrete nodules maximum size and maximum strength were baselined together (as only large and strong nodules were considered to be problematic).

4.4.6 ‘Drift Filled Hollows’

The presence or absence of ‘Drift Filled Hollows’ (or ‘Scour Hollows’) were baselined for all types of contract involving excavation, mining and piling or diaphragm wall construction. Where the locations of these features were known the extent and nature of the feature would be baselined in the ways described above. Some TBM contracts contained speculative baselined encounters with Drift Filled Hollows phrased in this way:

• One (1) scour hollow/infilled pingo scar feature will be encountered within the tunnel face at an unknown location along the length of running tunnels in Drives * and *. The scour hollow feature shall comprise highly unstable mixed full face conditions over a distance of 50m with high ground water inflows.

4.4.7 Fault Zones

Where appropriate conditions relating to Fault Zones were baselined, either when faulting could lead to changes in the type of the strata encountered or where this did not occur changes in the nature of the strata encountered, for instance reduced strength or increased discontinuity frequency.

4.4.8 Ground Gas

Differences to normal atmospheric gas composition that might be found during construction were baselined as “Ground Gas”. These statements did not state the specific proportions of gases that might be encountered but merely that they would be different to normal atmospheric proportions.

4.5 Groundwater

Groundwater levels, permeability and pore pressures were baselined for every contract with a GBR. These baseline statements were in addition to those in the strata description statements which addressed the presence or absence of water in each strata unit. Typical examples of baseline statements are: (Note these are not a ‘set’ of Baseline Statements and they are not necessarily internally consistent with each other):

• The Upper Aquifer groundwater level will be 4m below ground level ± 1.0m.

• Pore water pressures in the London Clay Formation follow a hydrostatic profile from 105mATD ± 1.0m to ***mATd. Below ***mATd pore water pressures will be between hydrostatic and hydrostatic minus 80kPa.

• The maximum porewater pressure that will be encountered by the Works will be ***kPa

• Pore water pressures in the Lambeth Group will be less than 130kPa.

• The Lower Aquifer water level will be a maximum of 69mATD.

• The range of permeability for the ground conditions encountered by the running tunnels in Drive * is provided in Table x.y.

Table x.y – Baseline permeability values

 

Strata	Minimum permeability (m/s)	Maximum permeability (m/s)
London Clay Formation	1×10-11	1×10-7
Harwich Formation	1×10-8	1×10-5
Lambeth Group	1×10-11	1×10-4

 

Ranges for permeability values would normally be baselined for all strata expected to be encountered, including superficial deposits such as Made Ground, Alluvium and River Terrace Deposits.

4.6 GBR Appendix

All the Crossrail GBRs also contained an Appendix which listed the relevant geotechnical information found in the Site Information (Volume 3 of the contract). This is not a necessary component of the GBR but fulfilled two other useful functions. Firstly the procurement teams compiling the ITT document could use it to check they had incorporated only the relevant factual geotechnical information and secondly it provided a useful shortcut to the relevant data for the tenderers and the eventual successful contractor.

5 How successful were Crossrail’s GBRs?

5.1 Value of GBR compensation events

The total value of the below ground works in Crossrail civils contracts that utilise GBR’s (i.e. those that contain clause 60.1.12A) is not simple to assess, as some ‘GBR’ contracts with large values had modest below ground scopes. Nevertheless an estimate has been made and it is approximately £4.25 billion.

An analysis of most of the clause 60.1.12A compensation events (CEs) arising from these contracts has been undertaken. The dataset analysed covered the period from the start of construction to the end of January 2016. Almost all civils contracts were either complete or substantially complete at this point, although some excavation works continued until spring 2017. Despite this the dataset is considered to be representative of the overall Crossrail experience of GBRs.

In all 321 60.1.12A CE notifications were received.

Of these 182 were rejected* and 7 had no outcome recorded at the time the data was captured. This leaves 132 valid CE notifications.

However a number of these were repeat occurrences of a single issue within a contract. If these are taken together as single incidents the total reduces to 102 valid CE notifications.

The total value of these 132 CEs was £15.25M** (or 0.36% of the approximate value of the ‘GBR’ scope). Six of the CEs had values >£1M, the highest being £1.6M. The mean and median values of a CE were £159K and £25K respectively.

*There are dispute resolution arrangements by which a contractor can continue to pursue a rejected CE if he wishes.
**Some significant GBR type issues were resolved as part of wider supplemental agreements between Crossrail and Contractors and the value of these is not included here, if they were it is considered to be unlikely the proportional value of GBR NCE’s would be significantly greater than 0.5% of the total ‘GBR contract value’.

5.2 Nature of compensation events

The causes of the valid CE’s were also analysed and of the 132 CEs:

• 72 related to obstructions, with almost all of these being man-made obstructions. This figure includes unforeseen obstructions and known obstructions with greater than baselined dimensions or strengths. The 72 included 1 CE with a value of >£1M

• 23 related to bentonite loss during Dwall construction. All these CEs occurred on one contract with each CE relating to a single wall panel.

• 16 related to the discovery of contaminated material, mostly relating to the discovery and subsequent disposal of buried asbestos.

• Only 9 related to encounters that could be described as geological or geotechnical. These included 5 of the 6 CEs with values >£1M

5.3 Discussion

Internally within in Crossrail the subjective view is that the GBRs have been a success in three ways:

1. They were relatively straight forward to write.

2. They were relatively straight forward to apply.

3. The cost outcomes struck a reasonable balance between cost certainty & risk transfer.

Comments made by senior figures from a number of Crossrail major civils contractors suggest the form of the GBRs provided a clear guide to the assumptions that the Client presumed would be taken in during the pricing of the tenders.

Objective views of the success or otherwise of the Crossrail GBRs are difficult to make in the absence of other published data for similar schemes in broadly similar ground.

Whilst this report focusses on the content and format of the Crossrail GBRs a significant contributor to any success the GBRs achieved is the quantity and quality of the information gleaned from the ground investigations carried out for the project.

In practice the ground presented very few surprises, despite much of the work being in the variable strata below the traditional tunnelling medium of London Clay. The few ‘surprises’ that were uncovered were in small volumes of ground in locations where the drilling of ground investigation boreholes etc. was impracticable.

This is reflected in the statistics on the nature of the successful CEs presented above. Very few of these were geological or geotechnical. However those that were geological or geotechnical in nature tended to be the higher value CEs.

Appendix A

Notes on writing GBRs and baseline statements

These notes are based on the Crossrail experience of GBR’s, i.e. no contractor permanent works design and no contractor involvement in the writing of the GBRs. They are however expected to be generally valid.

General points

When writing baseline statements you are attempting to foresee the unforeseen and in doing so you are setting boundaries for what the contractor can claim additional time or money for.

In order to be able to do this you need to understand the client’s views on risk taking versus price certainty. These views will depend on many things including the quality and quantity of the geological and geotechnical data available to the project and the level of the client’s technical knowledge.

The author of the GBR needs to:

1. Have in depth knowledge of the design, the ground conditions and the required construction methods.

2. Understand broad geological setting for the project and any possible variation that might arise over and above that indicated by the ground investigation data.

3. Understand the limitations of the factual information provided by the ground investigation data.

4. Understand the GBR applicable construction scope and the circumstances in which the GBR would be applied.

5. Understand the GBR related compensation event mechanisms set out in the contract.

6. Have knowledge of the defined terms used in the contract.

7. Understand the client’s appetite for risk versus price certainty.

It is unlikely that any individual or group of individuals from a single organisation (client, designer etc.) will be able to meet all these criteria. In addition on a large project like Crossrail there are likely to be many separate contracts that require GBRs (thirty four on Crossrail). So consistency of approach across several contracts could become a problem for a client if consistency was required.

Crossrail’s approach to this conundrum was to provide the designers with a skeleton GBR format and ask them to complete the GBR in draft. This dealt with items 1 to 3 (or 4) above.

This draft GBR would then be reviewed, revised (often substantially) and issued by a very small group of Crossrail technical staff such that items 2 to 7 above were covered.

In finalising the GBR the Crossrail technical staff would be liaising closely with both the designers and Crossrail’s specialist contract formulation teams.

Notes on drafting Baseline Statements

These are presented as a series of bullets points

• Where contracts include several significant construction techniques Baseline Statements should be divided up into chapters specific to each construction technique. This is to avoid confusion arising from irrelevant or misleading conditions being inadvertently baselined for some techniques. For example problematic strengths may be vary across different construction techniques. This division also allows greater flexibility in setting risk boundaries as Baseline Statements do not necessarily have to be factually correct.

• In deciding which conditions to baseline start from a consideration of how each construction technique could be compromised or constrained by an encounter with an ‘unforeseen’ condition. Then baseline these conditions. This is attempting to forsee the unforeseen. E.g. piling – high strengths, hard bands etc; bulk excavation – excessively high and excessively low strengths (plant support).

• Encounters #1. Remember when drafting Baseline Statements to consider the works the GBR is intended to cover (e.g. permanent or temporary and permanent) together with the content of Clause 60.1.12, which says:

The Contractor encounters conditions which:

• are within the Site,

• are of a type referred to in the GBR and

• are more adverse than the conditions set out in the GBR

The introduction to a Crossrail GBR says the Baseline Statements are aligned with the tender design, which in Crossrail’s case was a fully engineered design which typically did not include any specified temporary works.

This meant for Crossrail that the GBR only applied to encounters with conditions within the space occupied by the permanent works (if no specific temporary works were prescribed in the tender design). So it was not possible for a 60.1.12A CE to be raised against an exceeded baselined condition where the location of the exceedance cannot be encountered within the permanent works, e.g. in a temporary shaft, not shown in the tender design that was outside the envelope of the permanent works.

• Encounters #2. Consider the pros and cons of being very specific about the locations of encounters when drafting Baseline Statements. In some cases being specific can be very cost effective as the tenderer can price for just the specified location. In other cases it can be counterproductive. For instance if the GBR baselines X number piles as man-made obstructions at specific positions A , B and C and X number are encountered, but at locations A, B and D, an opportunity for a CE is provided even though the number of piles is as defined in the GBR. An alternative approach could be to be less specific by saying ‘in this particular part of the site X number of piles will be encountered’.

Good practice would be to include any relevant location information on obstructions in the Site Information to give the tenderers the opportunity to provide the best price. Much will depend on the quality of the information available.

• Encounters #3 Baselining continuously variable locations, such as strata boundaries. The advice here is to never baseline strata boundary lines drawn on geological sections, as this effectively says the location is exactly this single value at this specific location. In almost all cases this single value will be not be the actual value in practice. The recommended approach is to tabulate ranges of locations, if necessary appropriately sub-dividing the site into sub-units based on the structures being built or variations in the ground conditions or both.

• If the contract has a similar arrangement to Crossrail re. Clauses 60.1.12 and 12A then remember GBR does not have to baseline every ‘type’ of condition. It is possible for the GBR to remain deliberately silent on a ‘type’ of condition so as to only use Clause 60.12 for any CEs that arise. Take care if this approach is adopted because any mention of a ‘type’ of condition anywhere in a Baseline Statement could allow a 60.1.12A CE to be presented.

• Take care to check that any terms defined elsewhere in the contract have the exactly the same meanings in the GBR. An example might be the terms ‘works’ or ‘site’. This should be reinforced by adopting the formatting set out in the contract for any defined terms (e.g. Capitalisation or italics)

• Take care with the use of contractual, technical or geological ‘shorthand’ in Baseline Statements. If this is unavoidable then define the terms fully within the GBR in a contextual section.

• Geological terminology #1. Always use proper strata names rather than shorthand versions. For example use ‘London Clay Formation’ rather than ‘London Clay’. This helps to avoid assumptions being made about the nature of the strata based on the shorthand name, e.g. River Terrace Deposits rather than Thames Gravels.

• Geological terminology #2. Ensure the terminology used in the GBR is compatible with that used in the Site Information borehole logs. This could be challenging as desk study sourced historic logs may use outdated terminology. This can be dealt with by adding a ‘bridging’ contextual section in the GBR.

• When drafting Baseline Statements on geotechnical parameters it is common to quote ranges of values or separately quote maximum or minimum values. Best practice would be to only quote the data limit(s) relevant to the encounter. For example there is little point baselining a minimum strength for a hard strata or nodule, alternatively it is beneficial to baseline strength ranges or max/min values for bulk excavation in a shaft where the excavation plant stand on the excavated material.

• Consider carefully how the term ‘and’ is used (or not used) in Baseline Statements. Some encounters require two or more conditions to be met to be problematic, for example hard nodules in SCL excavation need to be both excessively strong and excessively large to be a problem. In a Baseline Statement these two parameters need to be linked with and for the Baseline Statement to be effective. Where no linkage is required the use of ‘and’ could inadvertently invalidate the intention of individual statements. Best practice would be to keep Baseline Statements as short and simple as possible with the use of formatting to separate them (bullet points for instance).

• Within any group of specific construction technique Baseline Statements avoid inadvertently baselining the same condition more than once as this can lead to errors. This situation could arise when tabulating strata boundary elevations for one Baseline Statement and for another, separately listing strata units to be encountered within specific sub-units of a structure.

• When baselining the groundwater levels or pore pressures that will be encountered consider the values adopted in the Baseline Statement in relation to abstraction or depressurisation activities of third parties and their timing.