The computer as an interactive geotechnical data bank and analytical tool

The computer as an interactive geotechnical data bank and analytical tool R. Day E. V. Tucker and L. A. Wood DAY R., E. V. TUCKER & L. A. WOOD 1983. The computer as an interactive geotechnical data bank and analytical tool. Proc. Geol. Ass., 94(2),123-132. An evaluation of the feasibility of a computer data storage system has been undertaken utilizing interactive computer techniques that enable a person with minimal geological or computing knowledge to input a borehole record using normal English, in response to a question-answer routine. All data items are internally coded by the computer, reducing effort and minimising the chances of human transcription error. Flexibility of terms input is allowed for by the use of self updating vocabularies. Data is retrieved in response to a question-answer routine that enables complete listings, the selective retrieval of data items, or the analysis of sedimentary successions to be achieved. A routine identifying the changing bulk lithological character of a sequence is presented.

R. Day & E. V. Tucker, Department of Geological Sciences and L. A. Wood, Department of Civil Engineering, Queen Mary College, Mile End Road, London E1 4NS

1. INTRODUCTION The concept of a computer data bank capable of handling geotechnical data derived from Site Investigation Reports is attractive to the construction industry. A number of systems have been devised for commercial and institutional use (e.g. Loudon et al., 1977). These systems have not been widely adopted because in part they rely on coding and the use of mnemonics that must be understood by the operator, necessitating extensive checking on input to ensure the accuracy of the information stored. The excessive cost involved in these operations prohibits the development of an economically viable system because of the large volume of data that should be stored. The need for such databanks is difficult to deny considering the reservoir of information in existence, obtained at high cost but barely retrievable on account of its volume and inaccessibility. C.I.R.I.A. (Tuckwell & Sadgrove, 1977) recognised the desirability of a national registry of site investigation records, especially borehole logs, although the systems envisaged in that report are possibly less satisfactory than some that utilise the large capacity and fast handling speeds of modern computers (Wood, 1981; Wood et al., 1982). The GEOSHARE system described here avoids using external numeric codes or mnemonics but relies on a simplified standard vocabulary readily understood by personnel with only limited geotechnical training. It also provides an interactive computer service that can be used by staff with little computer experience. 2. OBJECTIVES The study is based upon some 400 borehole records for sites along the Essex bank of the River Thames and the Maplin Sands area. The logs detail features of the Drift and parts of the underlying Tertiary strata,

normally London Clay. This paper discusses the feasibility of an interactive computer data bank of site investigation records both as an efficient method of data storage and as a flexible medium for selective data retrieval. Ease of operation is paramount and the following objectives have been defined: (a) The system must be operable by persons with, at most, only limited knowledge of computing; (b) Data must be input in a manner that is both fast and simple; (c) Only factual information should be stored and it must be as complete and accurate as possible; (d) Flexibility of terms must be allowed on input; (e) Retrieval programs must be designed to the users' needs. The pilot study has met most of these objectives although further work is required in some areas to implement ideas fully. 3. COMPUTER GEOTECHNICAL DATA STORAGE SYSTEMS Various systems for storage and retrieval of data have been adopted based largely on a fixed format or 'pseudo-free' format approach and using punched-card technology, implemented in a 'batch' processing situation. These factors limit the extent to which a user orientated environment can be developed, but the widespread introduction of interactive terminals allows further progress to be made enabling a user with limited computer experience to communicate with the computer in normal geological English, avoiding the need to memorise data control instructions. Buller (1964, 1972), Harvey (1973) and Loudon et al. (1977) utilised fixed format systems coding data by reference to a coding manual prior to input. Pre-input

123

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R . DAY , E . V . T UCKER AND L. A . W O OD

coding not only requires mu ch effort , an import ant con sideration when storing voluminous archive material (Farmer & Read , 1976) but gives little room for flexibility when inputting data and can intro duce er rors . Both fixed and 'pseudo-free' form at techn ique s were used by Rhind and Sissons (1971), the former consisting entirely of numerical characters, information not in numeric form being transformed by means of codes. Th e 'pseudo-free' form at section used norm al En glish mod ified , for example, by placing characters other th an pr imar y constituents in parentheses. A du al format approac h was adopted by Gover et at. (1971) making use of a 202 word voca bulary against which input word s are checked automa tically and coded, if pr esent. Field trials by geologists (Fa rmer & Read, 1976) indicated the advantages of the 'pseudo-free' form at approach because no fund amental changes in methods of recording data are required . The limitations of the vocabulary proved frustratin g at times when new phenomena needed to be described. The system was regarded as viable only if archive material of more than 30,000 rec ords could be sto red but the cost of effective supervision of the staff inputt ing dat a outweighed the benefits of the syste m . Cr ipps (1978a, 1978b , 1979) found th at due to the restricted range of sediments enco unte red in a project based on Newcastle-upon-Tyne , a three digit number and an op tion al twel ve characte r expression was adequate to describe a single stra tum prior to input. Th e cha racter expression gives flexibilit y to the data sto red but th e method suffers from th e same disadvantages as oth er non-English input syste ms. Simplicity of operation is the key factor if a system is to be ado pted by industry. Th e Saskatchewan Govern me nt Well Data System (Buller , 1972) incorporat ed a retrieval method designed for use by geologists with out previous programming tr aining but the objective was not borne out in practice and training in both programm ing and computing experience was found to be necessary. An interactive syste m designed around stand ard English terms and operable by semi-skilled personnel is essential for the widespread acceptance and use of a data bank. Gover et at. (1971), Harvey (1973) and Loudon et al. (1977) all used retrieval comm and s in near English but the comm ands must be learned before a retrieval can be made . Cripps (1979) devised a re trieval system which works in respons e to a series of programmed interact ive questions en abling the user, with onl y limited computer experience, to communicate with the computer in sta nda rd English , precluding the need to learn data control instruc tions . Th e syste m described here extends the methods of Cripps so th at data , on input , can be typed directly into the computer in a single operatio n as well as being retrieved in response to a series of programm ed qu estion s. Th is dispenses with th e costly procedure of

record ing dat a on punch-cards prior to input. Co ding of dat a is performed int ern ally by the compute r and does not involve the operator. Flexibility of term s input is achieved by a self upd atin g vocabulary producing a simple to ope ra te system.

4. PRESENT STUDY The project is ba sed on sites within the River Tha mes estu ary where mixed sequences of Pleistocen e and Holocene super ficial deposits overlie a highly irreg ular London Clay sur face . Other Terti ary rocks are pre sent locally . In channel ar eas the supe rficial deposits reach 40 m in thickness and show a relatively complex sequence of alterna ting gravels, sands and clays. The thickness away from channels averages 16 to 20 m and successions are comparatively simple, with a tripartite division into basal gravels (partly of Pleistocene age) overlain by silts and clays followed by cover sands. A refined sedim ent analysis has been po ssible for site s at Maplin Sands and Foulness Island (Greensmith & Tucker, 1971) using criteria th at includ e lith ificati on , cohe sion , sediment cyclicity and faun a , showing that a number of ph ases of emergence of th e dep ositional surface followed by transgressive submergence have occurred during the Holocen e . Identific ation of levels of em er gence by direct reading of bore logs is rendered less exact by the general nature of soil description s but progre ss to wards the recogn ition of the se levels, where soils are more consolidated , has been achieve d by computer analysis. A ser ies of programs have been writte n to store the borehole dat a and retrieve it in various form s, utilising an r.c.L. 2980 mainfr ame compute r with the software writt en in Fortran 77.

(a) Method of data storage To maximise the util ity of the syste m externa l coding and mnemon ics are avoided and operat ion al simplicity is obtained by inputting data in sta nda rd En glish in response to a series of programmed questions . It should be emphasised that alternative input routines, such as the pre sentation of a pro-forma to the user are also under deve lopment. The data is cod ed internally by th e compute r as well as deciphered when accessed . A twofold division of information pr esent in the Site Inve stigation borehole record can be made:(i) Numerical dat a ; (ii) Descript ive dat a . Th e form er includes numerical borehole refere nce data , sample tests and water level da ta . Th e latt er includes soil descripti ons and non-numer ic borehole refer ence dat a (Table 1). Space for genera l comments exists, up to a maximum of fifty char acters per borehole. Catego ry (i) contains item s esse ntia lly num eric in character th at require no coding by the computer. Som e of this dat a will be of a type that can be chec ked automatically by the computer for logical erro rs, on

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125

illustr ated by referen ce to the vocabularies designed for the soil descriptions. NUMERICAL DATA Th e Pleistocene and Holo cen e soils of the Th am es estuary can be adequately de scribed by reference to NUMERIC BOREHOL E REFERENCE DA TA six qu alitative headin gs:- colour , prim ary soil type , Sheet num ber seconda ry soil type (or formation name if known) , soil G rid referen ce structure (including organic cont ent ) , consistenc y, Ground surface height compaction . Th ese headings fo rm divisions in which Dat e of drillin g vocabularies of relevant term s can be stored. AddiBoreh ole diameter tion al categor ies could have been adde d, for exampl e Casi ng limits the organic content could be cate gorised separate ly if it se rves a useful purpose. Because soil descript ion s TEST DATA are expre ssed using a terminology that is reasonably Samples . tests con sistent it can be shown that the first fou r letter s of a Water level data technical term are usually sufficient for the experienced soil engineer or geologist to identify the comDESCRIPTIVE DATA plete term within the context of the geological environment being considered. 'Gravel' will be repreSOIL DESCRIPTIONS sented in a vocabul ary as 'GRAY', 'sand'. 'clay' and St ratum depth 'silt ' as the complete term and 'mott led' as ' MOTT Co lour with little risk of confusion. If confu sion is liable to Prim ar y soil occur each term could be represented in full. For the Seco nda ry so il or formation name purpose of the present study each vocabul ary has been Structu re or o rga nic co nte nt constru cted using 95 term s, th e numbe r and variety of Co nsiste ncy which being decided from prior knowledge of the Co mpac tio n area's geo logy and the ran ge of descript ions used in the logs. NON-NUMERIC BOREHOLE REFERENCE DATA At the moment , when inputting data , the ope rator D rilling Co mpa ny will be asked to provide information relating to each Client division (catego ry) and will respond to eac h pro gramDrill ing method med question using normal En glish. Either the complete term or the abbreviated first four letter form will GENERA L COMM EN TS be ent er ed to a maximum of three terms. Th is limita50 Ch aracter maximum per bo reh ole tion on the numb er of terms is employed because longer descriptions are uncommon and if used , norinpu t. For example, if the day of the month is input as mally result in a loss of precisi on. Th e terms will be checked by the computer against the appropriate voca32 a correction will be called for by the computer. Non-logical errors can only be found by manu al bulary and. if present , coded and sto red. If a term is checks and the numeric borehole data will be display- input th at is not listed in the vocabulary the two ed on the Y.D.U . after input together with the ques- close st alphab etical terms will be displayed to the user tion 'A RE THERE ANY ERRORS ?' . Mistakes can be as possible alte rna tives. This device draws attention to identified and corrected at this stage . A correction possible spelling and typing errors and provides a facility at this stage is important because it gives the ready mean s of correcting mistakes. The operator can ope rator an opportunity to recogn ise an error on input either accept one of the alternative terms, re-input the that might othe rwise be overlooked by a later manual dat a or upd ate the vocabulary file. If he elects to check. update , the complete vocabulary for that division The de scripti ve data of category (ii) involves terms (category) is displayed on the Y.D .U . together with and phrases having a particular bulk. dependent on the qu estion 'DO YOU STILL WISH TO UPthe completeness of the soil descript ion , and coding of DATE?' . If a suita ble alterna tive term is not avai lable this inform ati on is necessar y for efficient storage. A and the op erator replies ' YES' the new term will be scheme has been devised whereby data is pre sented to automatically stored at the end of the current vocabulthe computer in normal En glish , to crea te a user ar y and the term will be intern ally coded. After the friendly en vironment and the cod ing ope ration is per- deta ils for eac h stratum have been stored there is an form ed inte rna lly by the computer . To achieve this a opportunity to correct errors . again using a simple series of voca bularies are crea ted, eac h one serving a qu estion-answer approach. Table 2 shows part of a specific part of th e borehole record de scripti on. The typical input session. Reducing the vocabulary terms to the first four principl e applies to all cate gories of the non-numeric data th at need to be ident ified but the concept will be lett ers of a word has the adva ntage of redu cing the TABLE I. Data categories stored

126

R . DAY . E . V . T UCKER AND L. A . W OOD

TABLE 2. Part of a typical input session 14:49:20 14:49:27 14:49:27 14:49:27 14:49:34 14:49:34 14:49:42 14:49:43 14:49:57 14:49:57 14:50:13 14:50:13 14:50:22 14:50:22 14:50:26 14:50:26 14:50:29 14:50:29 14:50:29 14:50:29 14:50:34 14:50:34 14:50:37 14:50:37 14:50:39 14:50:39 14:50:47 14:50:47 14:50:47 14:50:54 14:50:54 14:51:00 14:51:00 14:51:08 14:51:08 14:51:15 14:51:15 14:51:20 14:51:20 14:51:26

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4

INPUT DEPTH TO STRATUM BASE NO 3 0390 7.2 DEPTH TO STRATUM BASE= 7.20 INPUT COLOUR 0391 BLUE GREY INPUT PRIMARY SOIL 0392 SILT CLAY INPUT SECONDARY SOIL 0393 PEAT INPUT STRUCTURE 0394 WOOD PEAT BAND INPUT CONSISTENCY 0395 SOFT TO FIRM INPUT DEGREE OF COMPACTION 0396 UNKN ARE THERE ANY ERRORS?-YES OR NO 0397 YES IS DEPTH, COLOUR. PRIMARY SOIL, SECONDARY SOIL, STRUCTURE, CONSISTENCY OR COMPACTION INCORRECT? ENTER ONE CATEGORY ONLY 0398 SECO INPUT SECONDARY SOIL 0399 UNKN ARE THERE ANY FURTHER ERRORS?-YES OR NO 0400 NO INPUT DEPTH TO STRATUM BASE NO 4 0401 9.3 DEPTH TO STRATUM BASE= 9.30 INPUT COLOUR 0402 DARK BROW INPUT PRIMARY SOIL 0403 CLAY PEAT INPUT SECONDARY SOIL 0404 UNKN INPUT STRUCTURE 0405 AMOR AND FIBR INPUT CONSISTENCY 0406 FIRM INPUT DEGREE OF COMPACTION 0407 UNKN

typing on input. It suffers from the disadvantage of the abbreviated form being produced on output but this has proved adequate for easy identification within the framework of the current project and has not presented ambiguous terms when re-reading the record . The use of mnemonic codes to reduce typing was considered but experiments using the Franklin derived code method (see Brisbin & Edigar, 1967) showed that although the more common terms might soon be memorised, less common terms required reference to a manual causing inconvenience and delay . The use of recognisable character strings is also advantageous as shown by Rhind and Sissons (1971) who, after conducting experiments on data preparation , concluded

that operators found the work less dem anding, proceeding more. quickly and fewer mistakes were made. Despite the overall simplicity of the input operation certain procedures must be followed to ensure that the strict meaning of the data is retained . For example, it is normally accepted that to be of use a log must distinguish between primary and secondary characters and that the colour will refer to the primary soil. Hence the question-answer style of data input used in this pilot study is directly applicable to industrial exploitation ; however it is appreciated th at for a commercial system a style of input calling for a less well defined knowledge of the subject matter by the user is necessary . The simplistic soil descriptions used by

GEOTECHNICAL DATA BANK

drillers should not lead to ambiguity although it is recognised that records made by trained geologists may be elaborated to levels of detail where confusion can exist between dominant and subordinate elements of the soils due to the poor ordering of the characters described. In our experience jumbled descriptions are uncommon and methods could be introduced to sort such records prior to input. In these operations some

minor pieces of information could be lost but this is compensated for by the much larger quantities of information that can be consulted through a versatile retrieval system than by traditional manual techniques. Following the input of several boreholes it is advisable to display a decoded listing of the data in order to conduct a manual error check (Table 3). Mistakes can

TABLE 3. A typical decoded listing 11:33:55 11:33:55 11:33:55 11:33:55 11:33:55 11:33:55 11:33:55 11:33:56 11:33:56 11:33:56 11:33:56 11:33:56 11:33:56 11:33:56 11:33:56 11:33:56 11:33:57 11:34:16 11:34:16 11:34:16 11:34:16 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:17 11:34:35 11:34:35 11:34:35 11:34:35 11:34:35 11:34:35 11:34:35 11:34:35 11:34:35 11:34:35 11:34:35

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4

SHEET NO= TQ GRIDREF-EASTING= 997 GRIDREF-NORTHING= 866 -0.75 ACTUAL GROUND SURFACE LEVEL= DATE = 9081973 DRILLING METHOD=SHEL AND AUGE DIAMETER AND LINING= 0.20TO 13.00 DIAMETER AND LINING= 0.15TO 17.00 CLIENT=BRIT AIRP AUTH DRILLING COMPANY = FOUN ENG! LTD

DEPTH TO STRATUM BASE= 4.00 COLOUR = GREY PRIMARY SOIL = FINE SAND SEC SOIL OR FORMATION NAME=SILT FINE SAND STRUCTURE, SHELLS OR PLANT MATTER = OCCA SHEL DEGREE OF COMPACTION = MEDI DENS LOOS

DEPTH TO STRATUM BASE= COLOUR = GREY PRIMARY SOIL = SILT CLAY CONSISTENCY = VERY SOFT

9.00

4 DEPTH TO STRATUM BASE= 4 COLOUR=GREY BROW

10.00

4

4 PRIMARY SOIL = CLAY SILT 4 CONSISTENCY = SOFT TO FIRM 4 4 4 4 4 4 4 4 4 4 4 4 4

127

DEPTH TO STRATUM BASE= 16.00 COLOUR = BROW BUFF PRIMARY SOIL= FC SAND SEC SOIL OR FORMATION NAME = FINE MEDI GRAV DEGREE OF COMPACTION = MEDI DENS DENS

DEPTH TO STRATUM BASE= 20.00 COLOUR = BROW GREY PRIMARY SOIL = SILT CLAY 4 STRUCTURE, SHELLS OR PLANT MATTER = FISS 4 CONSISTENCY = STIF 4 4 4 REMARKS = WATE ADDE DURI BORI FROM SEA BED TO 16.00M

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R . DAY . E . V . T UCKER AND L. A . WOOD

be rectified by means of a question-answer ed it routine . Err or checks similar to those referred to previou sly are available. An y necessar y updating of th e vocabulary can be achi eved dur ing th e session. For any computerised dat a system to be viable the time required to input the dat a and validate the entries must lie within acceptable limit s defined by the project th at eventually draws on the dat a base. Expedi en cy must not be allowed to redu ce the accuracy or completeness o f infor mation sto red. It follows from th is that altho ugh the basic philosoph y of the system described here could be maintained for othe r projects, different dat a records that differ in style might requ ire a mod ified versio n of the scheme to satisfy their particular requirements. Deep boreholes will prov ide considerably longer logs than a normal site investigation drilling programme and the time required to input a record will be grea ter. When a hole is cored and subsequently logged by a professional geologist a more detailed subdivisi on of form ations is likely and the accompanying descript ion might bulk larger and be more comprehensive than in a log produced by a driller. With in cert ain industries (e.g. mining, quarrying) colloqu ial term s will be used commonl y and the vocabulary of the system would need to be mod ified accordingly. Th e emphasis given to certain parameters can vary fro m on e project to another but all these qu alification s can be compensated for either by crea ting additiona l categorie s of data in the dat a file or by extending the vocabulary. A compromise might eve n be reached because some catego ries of d ata essential in a site investigation record would not be regarded as essential in a syste m designed for mining or petroleum investigations. Th e time taken to input a typical S.1. borehole record (excluding sample and test data) into the curre nt syste m is about fifteen minutes and the manu al check is completed in under four minut es. A furt her two minut es might be necessary to ed it the file. The whole operation can be compl eted in twenty minutes. With further development of different input routines it is envisaged that this time will be substantially reduced . The development of que stion-answer routines en able s a person with no computing experience to become fully competent at inputting a borehole record within a few hours. Som e understanding of common soil and geo logical te rms is beneficial but not essential. Th e assumption is made that the logs being input have been compiled pro fessio nally and that the recorded data is present in a reason abl y ordered scientific form .

(b) Retrieval Retrieval of inform ation is also in response to a que stion - answer routin e , data bein g retrieved by area , by borehole or by stratum horizon . A suite of program s has been develop ed includ ing: complete listings, dep th to formation boundar ies, depth to occurrence of

a specified soil type, soil type at a specified depth and the identification of different types of cyclic seque nces . Output data can be processed and linked to program s develop ed by othe rs to perform tasks such as contouring (Wood et al.. 1982). The exampl e detailed in the following sectio n is chosen becau se it demon str ate s how the system can be used as an effective interpret ative tool.

5. STRATUM ANALYSIS AND IDENTIFICATION OF FORMATION BOUNDARIES Th e form ation is the fundament al unit in lithostr atigraphy and a prime requirement in geo logy and soil engin eer ing is to establish the boundaries that separate form ations with in which lithological unity exists. Many formati on boundaries are gradational and are not represented by a sharp lithological change from one soil or rock type to another. The position at which a boundary is dr awn can be subjective and traditi onall y will be decided by a geologist by reference to his experi ence of the stra tigraphy of the area. A computer on the other hand ne eds a defined meth od pro grammed into its memory before it can pro vide a decision. Th is has the disad vantage of losing 'geologica l intu ition ' but the adva ntage of rep eatability. Accuracy can be maintained by quantifying the instructions to an extent that would be impracticable for manu al procedur es. Th e routin e adopted anal yses the boreh ole record by scanni ng blocks of str ata at unit thickness inte rvals to identi fy the cha nging bulk lithol ogical character of the sequence and draw formatio n bound aries at the point s of change . Each unit is then classified acco rding to the domin ant soil type present.

Routine SIGSED Fou r variables are present , unit thickne ss defining the block size. extent of block overlap , critical percentage of a sediment type needed to classify a block. and the interval of backtrack needed to define the point where a particular sediment exceeds the critical per centage. Block size and critical percentage can be controlled by the op erator in response to two questions at the beginn ing of -eac h pro gram run. The critical per centage values will be arrived at throug h expe rience of the local geology accompa nied by a sho rt testi ng prog ra m. If the numb er of soil types present is small the percentage value is likely to be high. If a more varied seque nce of sediments exists then lower values will prob ab ly prove acceptable. A mod ification has been made to the routine th at enabl es specified formation boundari es to be produced in isolation . Th e program developed for the study are a recognises fou r main soil types: gravel and coarse sa nd, fine and medium sand , silt and clay. One of these soils referred to as the 'required sediment', will be selected

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129

TABLE 4. Simplified flow diagram of SIGSED INPUT CORNER CO-ORDINATES OF AREA REQUIRED

I

INPUT SOIL TYPE TO SEARCH FOR = REQSED

I

INPUT CRITICAL AMOUNT OF REQSEDIBLOCK = CRIT INPUT BLOCK SIZE END OF

STOP

+-(~---FINDBOREHOLE

FILE

NEXT BOREHOLE WITHIN REQUIRED AREA + - - - - - - - - - - - - - - - , ON FILE

I

FIND BASE OF ANY MADE GROUND OR TOPSOIL PRESENT

Nlo BLOCKS OVERLAP AT A PRE-SET INTERVAL (50%)

I

..--.---_+_-- HAS END OF BOREHOLE BEEN REACHED

-----~-----YES ---'

I

BLOCK TOP

=N + 1

BLOCK BOTTOM

=N + 2

I

I

CALCULATE AMOUNT OF REQSED IN BLOCK = AMOUNT

I NO - - - IS AMOUNT

??;CRIT YES I FIND POINT AT WHICH REQSED IS INITIALLY> CRIT IS N = 1 YES - - - - - - - + - - - - - - - - - , NO ADD =0 r----------------~ADD =ADD + 1 BLOCK TOP =(N - 1) + ADD

I I ??;CRIT YES I

CALCULATE AMOUNT OF REQSED IN BLOCK = AMOUNT NO +-(---IS AMOUNT

FIND TOPMOST LEVEL OF REQSED IN BLOCK + - - - - - - - - - - - - - - - - - - '

I

CALCULATE REDUCED LEVEL

I

OUTPUT GRID REFERENCE, REDUCED LEVEL

I

SIMILAR PROCEDURE TO FIND BOTTOMMOST LEVEL OF REQSD NEXT BLOCK

~(-------,I

for input in response to a programmed question. The 'critical percentage' of the soil type is pre-determined by a geologist familiar with the regional stratigraphy. In this particular exercise a value of 50 per cent is used. The bore log is scanned from the base of any topsoil or made ground present (Table 4). An initial

block is calculated from this datum surface to examine each stratum in turn to determine whether it constitutes the 'required sediment' and whether it falls partly or completely within, or lies outside the block. Thickness calculations are made and if the required sediment is found to constitute less than the critical

130

R. DAY, E. V. TUCKER AND L. A. WOOD

fine sand 5.5

7

fine sand 9.5

TOP

11

silt 13 16 16.45

fine sand silt gravel

8 BOTTOM

f~ f~ 16

17

5 13

f:

}~

18

21

(e.g. for slit 6m block 50% critical level)

Fig. 1. Diagrammatic representation of routine SIGSED. To find Top: Blocks 1 to 3 contain < critical % of silt. Block 4 contains :3 critical % of silt causing backtrack to block 5. Block 5 contains :3 critical % of silt. The first identification of silt from this block represents the top boundary of the Silt formation. To find Base: Block 6 contains < critical % of silt causing backtrack to block 7. Block 7 contains < critical % so block 8 is created just below block 7. Block 8 contains :3 critical %. The lowest silt horizon in this block represents the base of the formation. The routine continues, examining deeper strata for further significant percentages of the required sediment.

percentage a new block, overlapping with the previous block, is created. This process is continued until the required sediment attains the critical percentage level. This approximately defines the upper boundary of the 'required sediment'. To locate this boundary precisely, blocks with greater than 50 per cent overlaps (~ in the case of a six metre block), are then calculated working down from a position high in the previous block (backtrack). The first identification of the 'required sediment' in a unit block exceeding the critical percentage will represent the boundary between the two formations under consideration (Fig. 1). The routine continues in a similar fashion, first finding the bottom

boundary of the formation before exarrumng deeper strata for further significant percentages of the required sediment. The basic procedure could be modified in several ways; for example, the log could be scanned downwards from a selected stratum. This would prove particularly useful in deep boreholes such as those sunk through Coal Measures, where certain horizons are named. A routine has also been developed that works upwards through a borehole from a known stratum. This gives added flexibility in stratum information handling, Additional criteria or completely new criteria can be incorporated to change the descrip-

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GEOTECHNICAL DATA BANK

tion of the required sediment, for instance firm clays could be distinguished from soft clays. Block size is an important variable. If all other variables are held constant an increasing block size results in greater generalization, drawing attention to the major sediment types present. Too large a block size results in a loss of all detail, whilst too small a block size merely reproduces the borehole record in its original form. Consequently, a balance has to be reached and this can only be found by subjecting several test records to a range of block sizes and examining the results. The other variables in 'SIGSED' were tested by studying the boundary between the cover sands and the central unit of clay and silt. Varying the extent of block overlap had little effect providing there was at least 33 per cent overlap. The 50 per cent sediment critical amount proved to be a realistic value for the study area where usually only two or three soil types are present within anyone block. If a greater number of soil types commonly occurred together a lower percentage value might need to be set. The degree of backtrack depends upon the minimum stratum thickness considered important. This stage affects only the fine detail of significant sediment limits. The method described is very much an automation of the process by which a geologist might group sediment types and define formation boundaries. The skills of the geologist are still needed, however, to bring background knowledge and experience to the procedures. This is necessary when examining the effects of changing the variables against the succession under consideration. Once suitable values have been ascertained, automatic retrieval of formation boundary depths can be carried out. The method can be considered both as a labour saving tool for the geologist and as a device to analyse a record in a quantitative rather than qualitative manner.

6. DISCUSSION AND CONCLUSIONS The pilot study has demonstrated the feasibility of storing and retrieving site investigation borehole records in both a fast and simple manner, utilising interactive computer techniques. The methods developed could be applied to other geological settings and geographical areas with little modification. The vocabularies could be enlarged if required, by modifying the internal coding method of the system. More detailed descriptions, or descriptions having a different emphasis from the site investigation records utilised here, could be catered for by creating additional categories, each with its own vocabulary or by redefining the vocabularies used in this study. The terms held within the vocabularies can be selected to suit the area of interest. The question-answer style of data input employed to date has proved acceptable. However, it is anticipated that other forms of data input, perhaps based upon word processor technology, would be necessary in a commercial system. The retrieval routines developed can be broadly divided into two categories, those specific to the study area, e.g. depth to London Clay basement, and those with more general application, e.g. SIGSED, which allow sedimentary successions to be analysed in various ways, including the identification of formation boundaries, the grouping of strata into sedimentary cycles and facies analysis.

ACKNOWLEDGEMENTS The authors are grateful to Dr. R. A. B. Bazley of the Institute of Geological Sciences and Binnie and Partners for making available the borehole logs on which this study is based. One of us (Robert Day) acknowledges a research award from the Science and Engineering Research Council.

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19,290-9. FARMER, D. G. & W. A. READ, 1976. A Minimal Effort for Maximal Return Philosophy Applied to IGS Onshore Borehole Records. Computers and Ceosciences, 2,365-74. GOVER, T. N., W. A. READ, & A. G. ROWSON, 1971. A pilot project on the storage and retrieval by computer of geological information from cored boreholes in Central Scotland. Inst. Geo!. Sci. Rep. No. 71/73: 30. GREENSMITH, J. T. & E. V. TUCKER. 1971. The effects of late Pleistocene and Holocene sea-level changes in the vicinity of the River Crouch, East Essex. Pr~c. Ceo!' Assoc., 82, 301-22. HARVEY, B. 1. 1973. A Computer System for Storage and Retrieval of Hydrogeological Data from Well Records. Rep. Inst. Geo!. Sci., No. 73/18, 34pp. LOUDON, T. V., J. M. CUBITI, M. R. HENSON. & S. V. DUNCAN, 1977. South Essex geological and geotechnical survey, Part 4: Computing. Inst. Geo!. Sci. RHlND, D. W. & J. B. SISSONS, 1971. Data banking

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