The Queen Mother Reservoir, Datchet

~eQueen Vo:~er Reservoir, 3H.C Ie. -some aspectsofitsdesignandconstruction by D. B. H. PAWSEY", CEng, BSc(Eng), MICE, MIWES, and A. W. HUMPHREY', CEng, BSc, MICE, MIWES IN NOVEMBER 1969 men and plant of ferent from a "shear" plane discovered outer shoulder of the embankment where W. fk C. French (Construction) Ltd. at less predictable horizons in the London mass rather more than strength is a arrived on the vacated farm land site to Clay approximately 2km away from the virtue. More extensive use of London commence the moving of 14 million ms Datchet site during the construction of Clay as a general bank building material of earth to create the 37700 Ml capacity Wraysbury Reservoir in 1971. was found to be uneconomic. Datchet Reservoir for the Metropolitan To carry water into and away from the Water Board. General design considerations reservoir, extensive use was made of On July 9, 1976, some 6-,'ears later, A continuous earth fill embankment tunnels driven in London Clay at depths Her Majesty Queen Elizabeth, the Queen 5.35km in circumference was formed to below the original ground level over the Mother, inaugurated the reservoir, then build a bunded reservoir for storing the reservoir site of between 24m and 31m. almost filled, and named it the Queen maximum volume of water that could be The tunnels were shield driven and lined Mother Reservoir. Thus for Thames Water contained at reasonable cost on the site. with MWB's patented Wedge Block who succeeded the Metropolitan Water This was taken as being achieved when lining, and were built under separate Board was added a new direct supply all materials above the I ondon Clay contracts prior to construction of the water resource for London at just the available on site within the embankment reservoir, The Wedge Block has success- right time to cope with the present which were suitable for bank building fully carried the increased overburden drought situation. were utilised. An embankment height up pressure under the embankment and the Also added was an important new to 19.8m above the original ground level decreased overburden under the floor water space for recreation, which will resulted. A lower cost per unit of water which resulted from the reservoir con- include sailing and trout fishing. At one stored could have been achieved with a struction. The tunnels terminate in shafts point the embankment has been widened lower embankment but more intensive use directly under the inlet and outlet struc- to provide a "beach" sloping at 1 in 8 of the land was felt to justify the higher tures. from which sailing boats can be laun- cost. ched, and space for the clubhouse built To economise in the use of gravel and Embankment design at the top of the embankment overlook- so increase the volume of water stored, The embankment comprises a central ing the water. For the first time with a layers of compacted London Clay were vertical core wall of compacted London reservoir of this type, provision for incorporated into the upper levels of the Clay taken down and keyed into the recreation has been incorporated into the embankment design. TABLE I. EFFECTIVE STRENGTH PARAMETERS OF LONDON CLAY Site investigation Normal clay of weakness The reservoir lies to the west of Lon- from don Airport not far Windsor Castle, C'b/ins C'b/in' the whole of the site being underlain by London Clay. At this point the London Wraysbury Peak 2.0 20* Clay is thinning rapidly towards the 18'6'2'0.6'lane Windsor upthrust and the strata en- Residual 0 16'8.8'6'.70 countered were: Datchet Peak 1.0 1.5 —300-600mm thick Residual 0 0 —a variable alluvial sandy clay up to 2m thick sand and gravel —1.5-7.6m thick ( ba I last) London Clay —18-36m thick During the initial site investigation by I I ra borehole and auger drilling from which the soil parameters and quantities were determined, a thin plane of weakness in the London Clay was revealed. A dis- I I I + tinctive silvery grey in colour, the plane was 10 38mm thick and lay at a constant interval of about 11m above the base of the London Clay —an influential depth over substantial lengths of the embankment. In a subsequent investigation, the plane was further sampled and its loca- tion verified by sinking a number of trial pits around the proposed embankment line. Both vertical and inclined 102mm samples were obtained. It has been suggested that this feature had resulted from flexure of the clay. In appearance and strength it was quite dif- Construction Engineer, Metropolitan Water Diw- 'a Ilu I uk sion, Thames Water Authority. 8 Agent, W. & C. French (Construction) Ltd. Fig. l. A 600mm inlet jet in action during the reservoir's first filling in January f975 October, 1976 27 underlying London Clay stratum, and sup- ported on either side by inner zones of ballast with outer zones or berms of overburden material. Under the inner zones, the overburden material being a ~ Inst relatively weak and compressible material rrrrr was first stripped off. tail 'f Nrf The line of the embankment followed Ilfff closely the boundary of the site and the II I II heights and widths of the outer berms were varied so as to maintain overall stability sensibly constant at all cross- sections. Both circular and non-circular modes of failure were considered, using among others the method of Morgenstern and Price 0 In addition to the height of the embankment, two significant factors were the depth below the ground level to the surface of the London Clay and the presence or otherwise at an influential INLET depth of the weak plane. An exceptionally wide embankment resulted on the north- west side of the reservoir where the / plane coincided with a peak in the London / Clay. For considerations of stability, the core wall was made as narrow as was con- / sistent with mechanical compaction and Limnological towar ~ / having a slight taper in width towards OUTLET OUTLET T the top. This taper was provided to assist TOWER InOEF in maintaining compressive stresses in Shaft r I Limnological tower ~ - r~ Yy ri rng the core during settlement of the embank- r iz i~ n ment due to consolidation of the underlying I ondon Clay. Being vertical and in a central position in an approximately symmetrical embankment, shear distor- C iH/4'fff I tions in the core were minimised. The natural moisture content of the ///// if/ff HILLHiiLH London Clay as dug for the core was fff ///// lliiiiiLLL" giitiii6 ffffffff /I//j(jjjigiii'ihii qttttttt i'o close to the plastic limit and about 3% frffff Ifffff11111111 of water was added to facilitate its compaction into a completely remoulded and TOHHEE homogeneous mass. 'IHS~ WRAYSBURY 0 DATCHET RS. Y From measurements of pore pressure SCALE dissipation in the underlying London Clay TUNNELS fROM INTAKE SIPHON FEET two similar reservoirs —Queen Elizabeth ~ FEET 0 500 1000 1500 2000 at II (1960) and Wraysbury (1971)—more METRES 0 100 200 of consolidation strengthen- . rapid rates WINDaon trrra taken into ac- r n ing were anticipated (and count in the design) than were predicted rr from conventional laboratory oedometer In addition sand drains were in Fig. 2. Plan of the reservoir; Top Water Level = 35.1m OD tests. stalled in the regions of highest excess pore pressure to accelerate consolidation. The predicted rates of consolidation were confirmed by the measurement of pore pressure dissipation during construction: Firstly, under four sections of the outer n-GROUND LEVEL NATURAL BALLAST SAND DRAINS~ ~~ Fig. 3. Cross-section of the Datchet reservoir embankment LONDON CLAY 28 Ground Engineering berm which, at the initial commencement of the construction contract, were raised as "trial" embankments, later to be incorporated into the finished profile. In two of the four sections, sand drains were also installed. Secondly, under the main embankment as it was raised. Six cross-sections here were instrumented, four of them being at the same chainages as the trial berms, and two of them elsewhere. The trial berms were of course some- what less than full height trials and they increased effective stresses in the underlying London Clay to a correspondingly less extent than the main embankment. It was therefore necessary to extrapolate ~~OR the results to allow for the decrease in the rate of consolidation which occurs with increasing effective stress. Records of Bm, behaviour at Queen Elizabeth II and more particularly at Wraysbury where measure- ~~~~M .I ' ments were more extensive were utilised in this respect. Construction of the embankment The topsoil was first excavated and then the overburden over the area of the Fig. 4. 0 & K bucket wheel excavator digging London C/ay and loading into a Cat 631B core and inner zones, to reveal the underlying ballast. This ballast was used to make haul roads above original ground level, so that materials could be moved effectively in wet weather. Carefully routed haul roads were built to and from the clay borrow pits and from the ballast and overburden strip areas. An average 6m deep cut-off trench was dug along the centre line of the embankment and then a key was formed into the London Clay, 3m deep. To dig ballast from the cut-off trench use was made of 38-RB and 3W walking dragline excavators. Later, Caterpillar 6318's were employed to excavate this ballast. The clay core trench was dug using DB's and Cat. 463 boxes, while batters of the key were formed by a 14E grader with an angled blade.

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