3 8 3 勿 R A E llison, S J B ooth and P J Strange G eological m apping in urban areas The B G S exp erience in London B ased on a p ap er p resented at the International C onference on G eoscience in U rban D evelop m ent (L andp lan IV) 11- 15 A ugust 1993, B eijing, C hina The B ritish G eological Survey has established the tively leading to better interpretation of ground conditions and pre- L O C U S (L O ndon C om p uterised U nderground and Sur- diction of potential geological hazards. H ow ever, the potential value of sound geological know ledge in underpinning planning and urban face) geological p roject, w hose aim is to p rovide geo- regeneration issues has been largely underplayed, and the benefits of logical m ap s and interp reted datafor London. The resul- detailed understanding of the relevant geological factors, for the tant high-quality m odel has alrea办 p roved valuable in m ost part, have not been fully realised. C ostly rem edial w ork during con s tr u ction caused by unfore- p rojects rangingf rom the site-specif c to strategic over- seen geological factors is not uncom m on in the L ondon region, and v ie i认 the variability of strata below London has been a source ofe ngineer- T h e p erceived requirem entfor geological inform a- Ing difficulty dating back to the first tunnel under the Tham es, built tion in urban areas is to supp ort land-use p lanning, by B runel in 1825. N ow , after careful study of the geology and the processes that have laid dow n the different deposits, there are expla- environm ental m atters, p roject p lanning and civil engi- nations for the form erly "m ystifying，， changes of rock type beneath neering. W ith increasing dem and for urban geoscience L ondon and m any of their variations can be predicted. inform ation, the Survey has develop ed a geologically T he statutory fram ew ork under w hich developm e n t can take place in B ritain puts the onus on local authorities to ensure that a attributed digital m ap-production system . This system is physically safe environm ent is established, and it is a developer's structured to p rovide, on dem and, up-to-date colour- responsibility to clean up contam inated sites. T his m eans that the p rinted geological m aps at any scale. Them atic m aps planning process m ust take account of the risks that geological haz- ards m ight pose. In the London area these hazards include landslip, illustrating sp ecif c asp ects of geological data are pro- dissolution subsidence, chalk-m ining subsidence, ground heave, duced also. The output is m ade available in hard-copy com pressible strata, high groundw ater and rising groundw ater, and digitalform for G IS applications. derelict w orked ground, landfill sites and landfill gas, and high-sul- phate groundw ater. The perceived requirem ent in rapidly developing A dditionally, a digital relational da ta base has been and regenerating urban areas, therefore, is for geological inform ation established using inform ation from 20,000 borehole rec- to support land-use planning, environm ental m atters, project plan- ords for L ondon. B orehole site data and levels, the base ning and civil engineering. The L O C U S (LO ndon C om puterised of m an-m ade ground and three key geological horizons U nderground and Surface) geology project w as established to m eet this requirem ent. T he project is concerned w ith m aking new geolog- are usedfor three-dim ensional com p uter m odelling. F or ical m aps and com puterised geological m odels of L ondon and the L ondon, these horizons are the base of the sup erfi cial deposits, w hich are essentially the sedim ents dep osited F igure L - London and the surrounding region. 勿 the R iver Tham es; the base of t he L ondon C lay, w hich underlies m uch of London and is the best tunnelling 10 2 0 km s m edium ; and the top of the C halk, an im p ortant aqu旅r 砂let1 L- ea1) in the London B asin. Im plem entation of digital m ap V -1〕／ production and the rationalisation 以 p rincip ally, bore- hole ii响rm ation has enabled routine p roduction of c om - 朋 puter-generated geological m odels of London and pro- 。 vided the m eans to visualize autom atically a range oJ 司古 司 inform ationf rom point data to 3-D m odels. 孙 毛公内白0 内人 YkkkES 13ARFIAGE LA C K W A LL T U N N E L W IN D S O R 褚认轰竺 P U R F L EE T In tro d u ctio n ＋ 产牲 :-:-A 3A T T E R S E A ： T he London region, with。population in excess of 6.3 million, is one of the m ajor conurbations in w estern Europe. T he processes of 街介少 份T呼}} ORRIDOR planning and developm ent in the region, in com m on w ith all urban areas, can benefit from a know ledge of the ground conditions w hich 犷 are encom passed in geological inform ation. Such inform ation, w hen 才 used appropriately, w ill ultim ately result in cost benefits by effec- Episodes, Vol. 16, no. 3 3 8 4 surrounding region (figure 1). T his w ork utilises recent advances w ith lim b dips generally less than 2'. In central and southeast Lon- m ade by the B ritish G eological Survey (B G S) in the developm ent of don sm all periclines, w ith axial-plane traces roughly parallel to the digital borehole databases and digital production of geological m aps m ain synchne and with limb dips up to 5', are superim posed on the and draw s heavily on the w ide range of geological expertise and larger structure. T he C halk crops out around the m argins ofL ondon, experience of the Survey staff. form ing the relatively high relief (over 130 m above sea level) of the T he project w as set up w it h t h ree m ain aim s and objectives: ．to produce geological m aps of London at scales of 1:10,000 and 1:50,000 Nfsoeordr eitmhxae Dmnotpswl wens Per uaernfl doe Cledthe idnlt etwhrineth eH atihslltes a C,n ahdnad Il一nian ntdhdseo f croo irrnme tsh th oeef w sceosmrte.e Po pafel tarhieceol Ligneoense-, ．to establish a digital relational borehole database don B asin (figure 3). Few faults are know n, the principal ones being norm al, w ith a curving northeast trend. M inor flexures in structure ．to produce com puter-generated geological m odels of the m ain contours suggest also northw est-trending flexures or faults. sub surface ro ck un its. In early to middle Quaternary (Pleistocene) times, a rive r a n臀 － tral to th e T h am es fl o w ed no rtlleastw ard to tn e no r h or L o nu on (G ibbard, 1985). D uring the A nglian stage, som e 350,000 years ago, G eology of L ondon a m ajor ice sheet advanced southw ard across B ritain as far as the northern part of the L ondon region, leaving a till deposit w hich has T he London region lies astride the valley of the River Thames on subsequently been dissected. T his ice sheet blocked the valley of the C retaceous C halk and unlithified Palaeogene clays and sands. T he ancestral Tham es (G ibbard, 1977), diverting the river to its present overlying superficial deposits principally consist of Q uaternary course. Subsequently, fluvial sands and gravels assigned to seven gravels laid dow n by the T ham es. river terraces have been deposited in the I 0-km -w ide present valley. T he C halk w as deposited acro s s t he entire region. It w as folded They are about 2 to 5 m thick and overlie benches cut largely in L on- and eroded, leaving about 220 m thickness in the London region, don C lay, form ing broad, m ore-or-less flat, tracts of land on w hich m o-t nf ei-m trM L ondon is situated (fiRure 4). prior to deposition of the Palaeogene strata. T he P alaeogene sequence, up to 180 m thick, w as laid dow n at the m argin of the con- tem porary N orth Sea and records a succession of transgressive enndeedD黔玺搬忽紫esveean-lseivane)l忠丫念nBdr i1ta0i0n 1mw ehtircehs events caused largely by eustatic sea-level changes. T he low est beds low er than the present level. D uring this period the Tham es in the (T hanet Sand F orm ation and U pnor F orm ation) are m ainly shallow London region cut dow n to a m axim um of 15 m below present sea m arine sands. They are succeeded by brackish-w ater sands and level. T he subsequent rapid sea-level rise in the H olocene led to lagoonal clays (W oolw ich Form ation) and alluvial m uds,that w ere ilnofiwlsli"n gno o fm tohries tvhaalnle y0．.a5 n kdrn th ien deivameloetpemr tehnatt o efro lodceadli uset)d to "s 5c0ou mr hinotlo- subjected to pedogenesis (R eading Form ation).