Embankment and cut slope monitoring and analysis

Transcription

1 Embankment and cut slope monitoring and analysis Dr Derek Clarke and Dr Joel Smethurst Introduction - to understand the behaviour of clay slopes (natural/engineered/stabilised) - failure and serviceability problems - climate and changing climate - vegetation - design and performance of stabilisation methods - develop models that will predict the behaviour of the slopes - to inform future design and maintenance - to identify critical actions that may have to be taken 1

2 Introduction - we have a number of instrumented sites used to - monitor slope pore water pressures - understand vegetation effects - climatic conditions - investigate the performance of pile stabilisation structures - we have used these site data to calibrate models of slope behaviour using - simulated soil moisture deficits and surpluses - pore water pressures Introduction Location of field study sites Newbury (cutting) Watford (oak tree) Ironbridge (River Seven valley side) Reading (embankment) Southend (embankment) Hildenborough (pile stabilised embank.) Mill Hill (pile stabilised embankment) Grange Hill (pile stabilised cutting) 2

3 Slope at Newbury Long term continuous monitoring of soil moisture, climate and pore water pressures in a London Clay cutting Neutron probe soil moisture readings TDR probes to measure moisture content Weather station Piezometers and tensiometers Network Rail study sites Embankments suffering serviceability (shrink/swell) problems Inclinometers (Geo-observations) Magnolia Lane, Southend Piezometers (Geo-observations) Extensometers (Geo-observations) Neutron probe soil moisture readings (Southampton) TDR probes to measure moisture content (Southampton) Poundgreen, Reading Rainfall (Southampton) 3

4 Discrete piles study sites Discrete pile sites Strain gauges and inclinometer tubes in the piles Hildenborough, Tonbridge, Kent Mill Hill East Northern Line Grange Hill, Central Line Ironbridge, Telford Inclinometer tubes upslope and downslope of pile row Piezometers Weather stations Clay Research Group study site Watford (Aldenham) effects of individual trees Neutron probe soil moisture readings (Southampton) TDR probes to measure moisture content Weather station Surface level monitoring Resistivity measurements Novel treatment methods to reduce clay shrink swell 4

5 Newbury site Aim: To understand and attempt to model the physical processes that link climate > vegetation vegetation > changes in soil moisture changes in soil moisture > changes in pore pressure/suction Field work Instrumentation of a vegetated London Clay cut slope on the A34 by-pass 4 sensors inserted at 4 locations down the slope Continuous monitoring since 22 Modelling Development & calibration of soil moisture deficit models Slope modelling work using FLAC Newbury site - background London Clay cut slope on the A34 near Newbury, 7.5m high, 16 slope Instrumented section of slope 5

6 Newbury - instrumentation Instruments are installed in 4 groups TDR s EQT Tensiometers Piezometers Curb of road Tensiometers Piezometers EQT Tensiometers Piezometers B C TDR s EQT Tensiometers Piezometers A Weathered clay D London Clay Newbury site Vegetation changes between summer-winter 9 May 3 9 July 3 1 Sept 3 24 Oct 3 6

7 Newbury measurement of volumetric moisture content TDR ThetaProbes Used to monitor soil moisture content Inserted at 3 cm intervals in top 2 metres of the soil profile Small response zone Continuous data-logged readings Neutron Probe Lowered down access tubes during visits to site Measures moisture content of football size zone of soil Newbury measurement of suction Vibrating wire piezometers Flushable water filled tensiometers in top 1.5m of the soil profile Indirect estimates of suction near the soil surface made using Equitensiometers. These Consists of a TDR probe embedded in a ceramic with known SWCC. Readings up to 15Pka possible but calibration is difficult. 7

8 Newbury climate and hydrological monitoring Two climate stations used to measure - Wind speed and direction - Temperature and humidity Runoff collection area - Solar radiation Above data used to estimate potential evapotranspiration Using the Penman Montieth equation - Rainfall Cut-off trench to measure runoff and interflow Newbury measured moisture contents in weathered material Volumetric soil moisture content A.3m A.45m A.6m A.9m A 1.5m Mar-3 Apr-3 May-3 Jun-3 Jul-3 Aug-3 Sep-3 Oct-3 Nov-3 Dec-3 Jan-4 Feb-4 Mar-4 Apr-4 May-4 Jun-4 Jul-4 Aug-4 Sep-4 Oct-4 Nov-4 Dec-4 Jan-5 Feb-5 Mar-5 Apr-5 May-5 Jun-5 Jul-5 Aug-5 Sep-5 Oct-5 Nov-5 Dec-5 Jan-6 Feb-6 Mar-6 Apr-6 May-6 Jun-6 Jul-6 8

9 Newbury changes in volumetric moisture content in weathered clay Volumetric water content % End of summer Winter Seasonal change in moisture content over top 1.25m depth of soil 1 Depth from ground surface (m) Aug 3 14 Aug 3 22 Aug 3 3 Sept 3 3 Sept 3 17 Oct 3 4 Nov Nov 3 15 Dec 3 Neutron probe site A : 23 4 Newbury changes in volumetric moisture content in cutting Volumetric moisture content % Seasonal change in moisture content over top.8 m depth of soil Depth from ground surface (m) Aug 3 3 Sept 3 3 Sept 3 17 Oct 3 4 Nov 3 15 Dec 3 Neutron probe site C : 23 9

10 Newbury soil water balance Simple 1-D soil moisture balance model Rainfall and ET Soil moisture deficit (SMD) = amount of water required to recharge the profile Runoff (Runoff only occurs at field capacity) Assume that the major moisture changes occur only in the rooting zone At Newbury =.8m to 1.2m depth Draw up of water from below the major rooting zone Newbury estimated potential evapotranspiration (Penman Montieth) Daily ETo mm/d A34 solarimeter Monthly ETo mm/d XLS solarimeter Monthly ETo mm/d Soton Jan-3 31-Jan-3 3-Mar-3 2-Apr-3 3-May-3 2-Jun-3 3-Jul-3 2-Aug-3 2-Sep-3 2-Oct-3 2-Nov-3 2-Dec-3 2-Jan-4 ETo mm/day 1

11 Newbury soil water balance Infiltration and ET Total depth of available water in the rooting zone = 144 mm RAW = 58mm TAW = 144mm Plants can readily access the initial ~4% of the water in profile Actual ET = full Potential ET The plants become stressed and find it harder to remove the remaining water: Actual ET = Potential ET x f(smd) Draw up of water from below the major rooting zone Max. draw up from measured summer suction gradient at Newbury =.4 mm/day = negligible Newbury estimated soil moisture deficit 23 4 Dailyrainfal (m/day) Rainfall AE < PE when SMD > 58mm Modelled Soil Moisture Deficit Jan 31-Jan 2-Mar 1-Apr 2-May 1-Jun 2-Jul 1-Aug 1-Sep 1-Oct 1-Nov 1-Dec 1-Jan SMD (mm) Readily available water (RAW) = 58 mm Actual ET < Potential ET Total available water (TAW) = 144 mm 11

12 Newbury estimated SMD and measured drying 2 4 Link climate and vegetation with changes in soil moisture Jan-3 31-Jan-3 2-Mar-3 2-Apr-3 2-May-3 2-Jun-3 2-Jul-3 1-Aug-3 SMD (mm) 1-Sep-3 1-Oct-3 1-Nov-3 1-Dec-3 31-Dec-3 SMD calculated from climate parameters Group A neutron probe readings Group C neutron probe readings Group A TDR data Newbury FLAC model Vegetation and climate applied through a series of flow boundaries built into surface rooting zone 7.2m 3.m 4.m 26.3m 24m clay weathered clay top soil silty clay bands 12

13 Newbury - FLAC pore water pressures 4 2 Pore water pressure (kpa) m 2. m 1.5 m 1. m 1. m depth Onset of plant stress 1.5 m depth 2. m depth 2.5 m depth Jan-3 31-Jan-3 2-Mar-3 2-Apr-3 2-May-3 2-Jun-3 2-Jul-3 1-Aug-3 1-Sep-3 1-Oct-3 1-Nov-3 1-Dec-3 31-Dec-3 Pore pressure (kpa) m.6m.9m 1.5m 3.m FLAC pore water pressures at C location, Dec 12 Jan 12 Feb 12 Mar 12 Apr 12 May 12 Jun 12 Jul 12 Aug 12 Sep 12 Oct 12 Nov 12 Dec Modelling London Clay slopes (grass cover) with historical climate data We have shown that the measured soil moisture changes match the modelled Soil Moisture Deficit SMD is dependent on climate and vegetation Historical climate data is available for Southern England at various sites going back to 1885 (Oxford), 1895 (Southampton). The climate data sets contain rainfall and data to estimate Potential Evapotranspiration. These were used to run the soil moisture deficit models from Additionally we have used the BETWIXT future climate data sets to simulate SMD s from

14 Simulated daily soil moisture deficits (London) SMD (mm) Grass cover, TAW = 18 mm, RAW = 9 mm Betwixt medium high scenario, London Heathrow Long term modelling of maximum summer Soil Moisture Deficit 25 Maximum Soil Moisture Deficit (mm) Grass cover, TAW = 18 mm, RAW = 9 mm Betwixt medium high scenario, London Heathrow 14

15 Increased stress on vegetation : Number of days SMD > RAW Days/Year Long term modelling Excess rainfall that becomes Runoff Runoff (mm/year)

16 Modelling London Clay behaviour Pattern shows - increase in maximum Soil Moisture Deficits in summer - increase in summer drought - higher frequency of dry winters when soil does not re-wet Implications - greater clay shrinkage for longer periods in summer - shrink-swell cycles will increase in magnitude - vegetation is likely to die off or change to other (semi arid) plants - vegetation cover may decrease (exposure to surface erosion in winter) - winter runoff reduces (however storminess is not accounted for) 16

17 Newbury measured pore water pressures near the surface (kpa) Suction (kpa) Mar-3 Apr-3 May-3 Jun-3 Jul-3 Aug-3 Sep-3 Oct-3 Nov-3 Dec-3 Jan-4 Feb-4 Mar-4 Apr-4 May-4 Jun-4 Jul-4 Aug-4 Sep-4 Oct-4 Nov-4 Dec-4 Jan-5 Feb-5 Mar-5 Apr-5 May-5 Jun-5 Jul-5 Aug-5 Sep-5 Oct-5 Nov-5 Shallow tensiometers site C&D: Dec-5 Jan-6 C.3 C.6 D.3 D.6 EQT C.3 Feb-6 Mar-6 Apr-6 May-6 Jun-6 Jul-6 Newbury measured in pore water pressures (kpa) 1.m 2.5m Nov-2 Jan-3 Mar-3 May-3 Jul-3 Sep-3 Nov-3 Jan-4 Mar-4 May-4 Jul-4 Sep-4 Nov-4 Jan-5 Mar-5 May-5 Jul-5 Sep-5 Nov-5 Jan-6 Mar-6 May-6 Jul-6 Pressure (kpa) C 1. C 1.5 C 2. C Day Piezometers site C:

18 Newbury envelope of pore water pressure changes Equitensiometer 44 kpa m depth Pore pressure (kpa) September Approx. maximum pore water pressure envelope Limit of measurement for tensiometers 1.5 Depth (m) Group A Group B Group C January 23 Group D Approx. minimum pore water pressure envelope Newbury site London Clay properties Soil Water Characteristic Curve (SWCC) Oven dry 8 7 LC drying LC wetting LC drying LC wetting Taken from Croney (1977) 6 14 Wilt point 15 kpa Soil Suction pf , kpa 1, kpa 1 kpa Soil Suction (kpa) kpa kpa Gravimetric Moisture content % Gravimetric moisture content % Permeability: Triaxial ~ 1x 1-1 m/s; field (bail tests) ~ 1x 1-8 m/s 18

19 LC drying LC wetting Gravimetric moisture content % Pore pressure (kpa) Approx. maximum Equitensiometer September.5 pore water pressure 44 kpa sucti on 23 m depth 1. Limit of measurement for tensiometers Group A Group B 2.5 January Group C Group D 3.5 Approx. minim um pore water pressure envelope Newbury - moisture contents Soil Suction (kpa) SWCC.5 1 Volumetric moisture content % Link measured pore water pressures with measured moisture contents Envelope of pwp s Depth (m) Depth from ground surface (m) Aug 3 3 Sept 3 3 Sept 3 17 Oct 3 4 Nov 3 15 Dec 3 Neutron probe measurements Upper bound from measurements of suction Low er bound from measurements of suction Newbury developing models to predict soil moisture deficit and pore water pressures The long run of observed data has provided information on soil moisture and pore water pressure - in wintertime - in summer time and shows behaviour in - a very wet winter (22-3) - a typical damp summer (24) - a typical dry summers (25) - an exceptionally dry summer (23) We have developed soil moisture deficit models (linked to climate) and we can use historical climate data sets to estimate the likely behaviour of the site over many years 19