National Infrastructure Planning · 2016-05-17 · Page 6 of 13 Our Ref: J14-468-024L-Rev0 TABLE 5:...

Our ref: J14-468-024L-Rev0

Your ref:

22 September 2015

Mr Philip Knipe

National Grid Gas Plc

National Grid House Gallows Hill

Warwick

Warwickshire

CV34 6DA

For the attention of: Mr Philip Knipe

Dear Sirs

Re: Feeder 9 – Mini-Pumping Test – Results and analysis

Status Report

The following Status Report is prepared for the purpose of describing the results of the mini-

pumping testing starting on 24th August, together with associated analysis and finite element

groundwater modelling of the expected flow conditions during excavation.

Introduction

A total of four independent pumping tests were conducted at four boreholes on the Goxhill

side of the Humber Estuary, from existing SI wells L01, L02, L04 and L06.

The tests were undertaken as per the WR32 Consent (32/CONS/193) issued by the

Environment Agency (EA) with the WR32 Consent attached in Appendix A.

The site works were undertaken over 8 days between 24th August 2015 and 3rd September

2015. This included 4 days of testing and 2 days of monitoring and sampling before and

after the tests. No works were undertaken over the bank holiday weekend (29–31 August).

Following the tests, the data collected on site were corrected and the background tidal

influence removed to allow analysis to be undertaken. The tests were analysed (using the

Jacob drawdown method at the pumping well and the Theis recovery method). These tests

determined the transmissivities of the Chalk at the depth of the response zones in each well.

In situ and laboratory sampling were also undertaken to assess the salinity of the

groundwater in each of the pumping wells and at the proximal Fir Tree Farm abstraction to

ensure that the tests were not causing migration of the saline interface.

Data from the vibrating wire piezometers (VWP) were also collected which provided the

background natural tidal groundwater response. Additional analysis is in progress to

measure the speed that the tidal pressure wave from the Humber travels through the

aquifer, from which governing aquifer parameters can also be assessed.

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Our Ref: J14-468-024L-Rev0

Pumping testing and groundwater responses

The order of pumping tests started closest to the Humber at borehole L06, working towards

L01, i.e. L06, L04, L02 and L01. Each borehole had a different response zone depth as

depicted in the drawing below.

Glacial Till

Alluvium

Chalk

River Humber5

60

0

80

0

0

-5

-10

Approximate base of De-structured Chalk

-10

-15

-20

-25

-30

-35

-40

-45

-50

-55

-60

-65

Weathered Chalk – Putty Chalk/Chalk

Bearings

L01 L02

L08

L03

L04

L05

L06

40

0

20

0

0

10

00

12

00

14

00

Vertical Scale

Exaggerated x10

mO

D

Distance (m)

Tide varies between

approximately

+3.4mOD and -3mOD

KEY

Piezometer Response Zone

Groundwater was pumped from the borehole at various flow rates, from a highest of

1.12 litre/sec at L04, to the lowest of 0.09 litre/sec at L01.

Discharge water was clear (see below), with samples collected for laboratory analysis.

The data collected by the data loggers were forwarded to OGI from Soil Engineering, after

which OGI converted the data to water level in metres above Ordnance Datum (mOD).

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To establish the drawdown with respect to the variable background tidal groundwater levels,

engineering judgement was used to create the background data from recorded data either

side of the 240 minute pumping period.

The drawing below depicts, in blue, the measured water levels in L06, with the estimated

tidal groundwater level background reading in dashed red.

00hrs 06hrs 12hrs 18hrs 24hrs

Date: 26-Aug-2015

Gro

un

dw

ate

r le

ve

l (m

OD

)

2.0

1.5

1.0

0.5

0

-0.5

For L06, the impact of pumping at a rate of 1.11 litre/sec results in additional drawdown

below the background tidal data. Subtracting the background groundwater levels from the

levels measured during pumping and recovery gives the specific impact as a consequence of

pumping. This impact is presented below, from both pumping and recovery in L06.

Time since pumping started (mins)

Dra

wd

ow

n (

m)

Borehole L06: Pumping rate = 1.11 litre/sec

Borehole L06: Pumping rate = 1.11 litre/sec

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Analysis of pumping and recovery data

The approach taken of calculating the aquifer transmissivity (T) at the borehole locations

requires analysis of the measured drawdown versus time using two methods of analysis, i.e.

The Jacob pumping testing analysis (See Appendix B)

The Theis recovery analysis (See Appendix B)

These methods have been applied to the corrected pumping and recovery response as

outlined in the book ref. “Analysis and Evaluation of Pumping Test Data” by G.P. Kruseman

and N.A. de Ridder, ILRI publication 47, 2nd Edition (1990).

Following this approach, the drawdown is plotted against time on a log scale, with the

gradient of the best fit line being proportional to the transmissivity. The figure below

illustrates the drawdown measured within L06, plotted against time on a log scale, since

pumping began (blue dots) with the best fit line drawn in red.

Drawdown vs Time since drawdown began

With a pumping rate of 95.9 m3/day monitored during the pumping test and with a value of

∆s (drawdown over one log cycle) of 0.37m, transmissivity is calculated as follows:

T =2.3 × Q

4 × π × ∆s=

2.3 × 95.9

4 × π × 0.37= 47.4 m2/day = 5.49 × 10−4m2/sec

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Recovery Analysis

The Figure below illustrates the residual drawdown within L06 against time since pumping

began divided by the time since cessation of pumping, plotted on a log scale.

Residual Drawdown vs Time since start of pumping

÷Time since cessation of pumping

With a pumping rate of 95.9m3/day monitored during the pumping test and with a value of

∆s’ (residual drawdown over one log cycle) of 0.485m, transmissivity can be calculated as

follows:

T =2.3 × Q

4 × π × ∆s=

2.3 × 95.9

4 × π × 0.485= 36.19m2/day = 4.19 × 10−4m2/sec

Performance of all tests Pumping and recovery tests were conducted in four boreholes; L06, L04, L02 & L01. However, tests in L02 and L01 only produced small pumping rates, probably as a result of the well casing/annulus being clogged. There was a suitable recovery response in L02 to make an approximate analysis of transmissivity, but results from L01 were inconclusive. Responses from both pumping and recovery tests in boreholes L06 and L04 demonstrated clear drawdown and recovery curves from which transmissivity was calculated. A summary of the test results are presented in Table 5 below:

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TABLE 5: Transmissivity results

Test Jacob Method using

Drawdown data

Theis method using

Recovery data

L01 Inconclusive Inconclusive

L02 Inconclusive 126m2/d

(1.46 x 10-3m2/s)

L04 264m2/d

(3.06 x 10-3m2/s)

253m2/d

(2.93 x 10-3m2/s)

L06 47m2/d

(5.49 x 10-4m2/s)

36m2/d

(4.19 x 10-4m2/s)

Despite the variable nature of the chalk, the above results demonstrate that all five

calculated values are within a single log cycle and so give confidence to the results.

With L04 and L06 providing the highest flow rates, thus indicating a good connection with

the aquifer, this implies that the highest average transmissivity at L04 (3.06 x 10-3m2/s) is

likely due to the fact that part of the response zone is in the chalk bearings. For the deeper

response zone in L06, the response zone is below the weathered zone and results in a lower

average transmissivity in the order of 4.8 x 10-4m2/s.

Permeability

To establish the horizontal permeability of the aquifer, it is necessary to divide the

transmissivity by the effective thickness, b’, i.e.

K = T/b’

The effective thickness will be greater than the length of the response zone of the borehole,

but less than the entire thickness of the aquifer. Calculations are currently in progress to

establish this effective thickness, however for the purpose of this analysis we have chosen

the effective thickness to be 10m, i.e. 2.5m above and below the response length of 5m.

This assumption results in the calculation of average permeability as:

L04 K = T/b’ = 3.0 x 10-3 m2/s ÷ 10m = 3.0 x 10-4m/s

L06 K = T/b’ = 4.8 x 10-4 m2/s ÷ 10m = 4.8 x 10-5m/s

This calculation demonstrates that, as expected, the chalk permeability decreases with

increased depth, and that the L06 permeability is in line with the lower chalk value estimated

by both OGI and Hyder, i.e. 5.2 x 10-5m/s.

Note that the calculation of flow to the excavation is not sensitive to the permeability of the

chalk bearing aquifer, and that L04 does not have the full response zone in the chalk

bearings.

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Modelling of groundwater impact for recharge and non-recharge conditions

Previously submitted HIA Addendum Report (J14-468-016R-Rev1) uses the conceptual

model below which contains the following permeabilities for the Chalk Bearings and

Fractured Chalk aquifer.

Superficial Deposits K = 2.0 x 10-7

m/s

Chalk Bearings K = 6.5 x 10-3

m/s

Fractured Chalk Kh = 5.2 x 10-5

m/s

Kv = 5.2 x 10-6

m/s

500m28.5m1.2m

+2.33mOD

-7.28mOD

-11.28mOD

-51.28mOD

-26.0mOD

-13.613mOD

-10.613mOD

Fixed Head

-13.613mOD

+5

0

-5

mOD

-10

-15

-20

-25

-30

-35

-40

-45

-50

-55Axisymmetric (Radial) Model

Applying a radial model to the surrounding geology produces the following flow net which

demonstrates a very small drawdown outside the secant pile wall.

CL+5

0

-5

mOD

-10

-15

-20

-25

-30

-35

-40

-45

-50

-55

Note that if a lower chalk bearings permeability is used, this makes very little difference to

the calculated abstraction rate. The flow is mainly sensitive to the vertical permeability

which will be first calculated from the analysis of the tidal responses in the VWPs and then

confirmed by the pumping testing conducted by the Main Works Contractor.

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Recharge Conditions

Under the conditions that external boreholes are drilled and installed into the permeable

chalk at suitable locations at distance from the secant pile wall, this will enable the

abstracted groundwater to be recharged back to the aquifer.

This approach substantially reduces the overall net abstraction from the aquifer so mitigates

the impact on the water resource.

The drawing below depicts a plan view of the site where a series of recharge wells are

located. Note these are example locations with final locations being selected by the MWC.

In the above drawing, the green flow paths demonstrate that the recharged groundwater

ultimately flows back to the drive pit where it is abstracted from inside the secant pile wall

structure. This approach enables the contractor to control the rate of recharge to the wells.

Note that in this simulation, the area in pale green represents where there is zero drawdown

below the original average ambient piezometric level of 1.1mOD (At L06 on 26 August).

The drawing below depicts a section model where the water level in the wells can be set at a

predetermined level by introducing the water to the recharge well head.

The simulated flow paths below demonstrate that the groundwater is effectively recirculated

from the recharge well to the abstraction well. This simulation demonstrates that the overall

abstraction, i.e. pumping minus recharge, is substantially lower than when pumping alone.

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Chalk bearings

Fractured chalk

Superficial deposits

Recharge

wellSecant pile

wall

Abstracted groundwater pumped into recharge wells

Conclusions

The mini-pumping tests were conducted successfully and provided a range of values of

transmissivities at various depths into the aquifer.

Engineering judgement of effective thickness at the response zones resulted in calculated

values of permeability which have been incorporated into a finite-element groundwater

model of the aquifer local to the drive pit excavation at Goxhill.

Based on aquifer parameters assessed from previous studies, together with those

parameters derived from the recent mini-pumping tests, the finite-element model has

simulated groundwater flows to the internal wells and drawdown surrounding the excavation

for conditions of (i) internal pumping only and (ii) where there is both internal pumping and

external recharge through a series of recharge wells.

The model has demonstrated that for the range of aquifer conditions expected, a strategy

utilizing a series of pumping wells located inside a concrete secant pile wall structure,

combined with a series of external recharge wells, demonstrates that the impact on the

aquifer is low.

Furthermore, the net overall abstraction of groundwater can be controlled by the Main Works

Contractor, as they can control the proportion of abstracted groundwater that is recharged

back to the aquifer from the external recharge wells.

Yours faithfully

Dr Stephen Thomas

Technical Director & UK Registered Ground Engineering Adviser

APPENDIX A: WR32 CONSENT

APPENDIX B: METHODS OF PUMPING TEST ANALYSIS

Ref. “Analysis and Evaluation of Pumping Test Data” by G.P. Kruseman and N.A. de Ridder,

ILRI publication 47, 2nd Edition (1990).

The following example demonstrates this analysis, based on the drawdown observed at a

piezometer at increasing time during a well pumping test.

Following the example depicted on the graph above, for a pumping rate Q = 13.392m3/day

and a drawdown per log cycle Δs= 0.71125m, transmissivity (T) is calculated as follows:

T =2.3×Q

4×π×∆s=

2.3×13.392

4×π×0.71125= 3.446m2/day = 3.988 × 10−5m2/sec (Example values only)

Based upon t0 = 2.78× 10-3 days we can obtain the Storage Coefficient (S) from:

S =2.25×T×t0

r2 =2.25×3.446 ×2.78×10−3

5.04122 = 8.48 × 10−4 (Example values only)

Extend the line until it intercepts the time axis where drawdown s = 0.

In this case, t0 = 4min = 2.78× 10-3 days

A straight line best match fit time through the late time data points on the log scale is required to produce a drawdown per log cycle (Δs).

Δs = 0.71125m

EXAMPLE ONLY

Transmissivity = T =2.3 × Q

4 × π × ∆s

Storage Coefficient = S =2.25 × T × t0

r2

Q = pumping rate r = radial distance from pumping well

PUMPING TEST ANALYSIS: Theis recovery method.

When the pump is switched off after a pumping test, the water levels in the well and the

piezometer will start to rise. This rise in water levels is known as residual drawdown, s’.

The analysis is based on the plot of residual drawdown (s’) in metres versus time since the

start of pumping divided by time since cessation of pumping (t/t’), on a log scale.

The following example demonstrates this analysis, based on the residual drawdown

observed at a piezometer during recovery period after a pumping test.

Following the example depicted on the graph above, for a pumping rate Q = 166.74m3/day

and a residual drawdown per log cycle Δs’= 0.2775m, transmissivity (T) is calculated as

follows:

T =2.3×Q

4×π×∆s=

2.3×166.74

4×π×0.2775= 109.97m2/day = 1.27 × 10−3m2/sec (Example values only)

EXAMPLE ONLY

Transmissivity = T =2.3 × Q

4 × π × ∆s′

Q = pumping rate

Δs’ = 0.2775m

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