RETWALL: An expert system for the selection and preliminary design of earth retaining structures

RETWALL: An expert system for the selection and preliminary design of earth retaining structures Peter J Hutchinson, Michael A Rosenman and John S Gero

This paper describes an expert system for the selection and preliminary design of engineering earth retaining structures. It describes the domain and how the knowledge was acquired from textbooks, questionnaires and interviews. Details of the implementation of R E T W A L L using the expert system shell BUILD are provided, as is a full script of a session. Keywords: expert system, knowledge acquisition, knowledge base, expert system shell, implementation An expert is one who is practised and skilful in a subject. Hence, expert systems are essentially attempts to model the performance of human experts within their respective domains. Traditional computer programs have gained wide acceptance in engineering. They are excellent at storing and manipulating large amounts of numerical information. Databases of items of interest such as buildings, roads and airfields can be stored and manipulated as computer readable models. Simulation and optimization techniques can be employed in ever expanding areas. However, these conventional programs have two major disadvantages, namely the difficulty of non-experts understanding their working and hence accepting their output and the problem associated with altering programs to suit new requirements. These programs are, in addition, constrained by the algebraic nature of conventional programming which limits representation to numerical, graphical or geometric modelling means. Humans converse in natural language and not algebra, and this requires a far richer symbolic representation of concepts than the restrictive algebra. As design and planning are not simply concerned with numbers and analysis of numerical problems but rather use professional expertise and knowledge, ArchitecturalComputingUnit, Departmentof ArchitecturalScience, The Universityof Sydney,NSW 2006Australia

along with accrued experience and applied theory, for decision making, a richer representation than that provided by algebra is required to make computers more useful. This richer representation can be provided by the field of knowledge engineering encapsulated in expert systems and the use of declarative languages developed specifically for the field. This approach moves away from algebraic representations and procedures. Rules are the archetypal structure for use in deductive inference. However, design is not concerned with deduction but with abduction 1. In Coyne et al. 2 the relation of design to abductive reasoning is argued more fully. Whereas deduction commences with a statement and a premise and draws a conclusion, abduction commences with the conclusion, i.e. the design goal. It is, therefore, of interest to demonstrate that design knowledge (as opposed to interpretative or diagnostic knowledge) may be formulated as rules. This paper is concerned with a prototype rule-based expert system for the selection and preliminary design of earth retaining structures. It is shown that the rules are abductive in nature in that the conditional part prescribes some performance requirements and the consequent part prescribes some possible design decisions. For experienced engineers, the task would take only a few hours in most cases when carried out manually, although for larger and more complex projects it would take longer. The task of selection is primarily cognitive and the design of individual types of structure is routinely taught. The selection process for different types is not routinely taught, and in attempting to extract information from the experts it was found that they were not accustomed to explaining their selection process and they found it difficult to define a given selection process. Expert systems share a common fundamental structure even if their knowledge encoding mechanisms differ. They each have the following components: (a) an inference engine;

0950-7051/87/010011-13 $03.00 © 1987 Butterworth & Co (Publishers) Ltd Vol 1 No 1 December 1987

11

(b) a knowledge base; (c) (preferably) an explanation facility; (d) a state description; (e) (possibly) a natural language interface; and (f) (possibly) a knowledge acquisition facility. An expert system shell is an empty expert system. That is, it contains component (a), and possibly components (c), (e) and (f) and may be used to implement a variety of expert systems with different sets of knowledge bases and descriptions. A number of forms of inference engine mechanisms are used in existing expert systems. The control structure or control of inference is often referred to in order to reflect the different controlling strategies of a system 3. For example, the search strategy may be based on forward chaining (data driven) or backward chaining (goal driven) mechanisms, and these may be combined with depth first or breadth first search. The structure of the knowledge base obviously depends on the inference engine which drives the system and the knowledge representation allowed. The explanation facility is a very important part of any expert system because if a system wants to gain acceptance it must be able to explain and justify its answers. The state description gives the state of a consultation with an expert system at the particular time. This is sometimes referred to as the facts base and would contain such things as all the things which are known to be true or false, possibly those items which are still unknown and possibly lists of rules which have been used or have failed. In more complicated systems, the state representation may include such items as certainty factors or, if frames are used, values instantiated for various slots and default values for others. There follows a brief description of the expert system shell used, then a discussion of the knowledge phase, prior to a description of the system itself.

T H E E X P E R T S Y S T E M S H E L L - - BUILD B U I L D is a general purpose rule-based expert system shell useful for a wide range of applications. It requires knowledge about a domain to be stored in the form of inference rules, and values inferred are propagated in an interactive manner. A full description of the system can be found in the User Manual 4. The system uses a restricted English form which can easily be expanded by the addition of extra operators. This makes the knowledge base easily readable. The questions put to the user by the system and the explanation facilities use the rule representation directly. The system supports both goal driven and data driven processes. In the latter case, the given data can be used to infer all that there is to infer (as true) in the knowledge base or be restricted to inferring that which is applicable to a specified topic. The system will ask for any information it requires as needed. Questions are asked in a 'top-down' mode. That is, the user may have information at a higher level than that of the most primitive information level in the system and is thus able to give it at that level. When the system asks the user a question, the user has the option of answering that questions or replying with 'why', 'explain', or 'how'. The reply 'why' (why do you want to know?) causes the system to provide an answer in terms of the rule it is considering at the time. The 12

reply "explain' provides a textual explanation of the line of reasoning. The reply 'how' (how do I answer this?) directs the system to search for any knowledge it may have to satisfy for this request to be inferred. The explanation facility can provide an explanation of how the conclusion to the original request was arrived at, explaining why all other conclusions failed, and explaining why a particular conclusion did not succeed. An important feature of the system is that of domain independent meta-knowledge. This exists in the present system in two forms. The first form of domain independent meta-knowledge provides the user with the capability of determining the scope of the knowledge in the domain of the system. The facility providing this capability is a list of objects each with a list of possible values thai the object may take in the domain of the system By inspecting the set of objects for a particular class of objects and checking whether a particular object is a legal member the user can determine the limits of the knowledge present. The second form of domain independent meta-knowledge provides the user with the capability of determining the nature of the knowledge in the domain of the system. That is, the user can determine whether the value of a particular object can be inferred by the system or whether it needs to be supplied by the user, and whether the object can be used to infer information about other objects. An example of a rule in BUILD form is: r331 (if 'type of application for wall' is__ A and 'type of application for wall' is temporary or emergency or marine and 'height of earth retaining structure (in mm)" is_less or_ e q u a l to 3000 and 'labour and materials are available for sheet pile' and "the ground is suitable for driving sheet piles' and not (~the deflection at the top of the wall is critical') ,~hen 'sheet pile is suitable for this application' and ~design of earth retaining structure' is__ 'cantilever sheet pile'). COLLECTION AND FORMULATION KNOWLEDGE

OF

Classes of knowledge A simple taxonomy of the types of knowledge which one may expect to model in an expert system may be put forward as (a) causal, and (b) experiential. Causal knowledge is knowledge of the form that if one particular thing occurs then another particular thing will be caused to occur (i.e. if A occurs then B will occur). Experiential knowledge is that knowledge based on experience for which no direct causal relationship can be established. This can be mandatory (imposed by some form of authority), conventional or idiosyncratic (the knowledge is particular to individuals or groups of individuals). Generally, in engineering selection and design the class of knowledge required is more experiential than causal There are usually choices involved in any task, and generally if one situation occurs, this leads to the conclusion Knowledge-Based Systems

that a number of other things could occur. The experiential knowledge could then rank the possibilities, or look for other factors which may influence the choice between the possibilities.

Sources of knowledge There are two main sources of expert knowledge in any field: (a) the knowledge contained in the literature; and (b) the knowledge of human experts gained through their experience. With the advent of the electronic age, perhaps the literature area should be extended to include other media for recording information and knowledge such as video tapes, films, audio tapes and so on. Hayes Roth et al. make the distinction between public and private knowledge. "Public knowledge includes the published definitions, facts and theories of which textbooks and references in the domain of study are typically composed. But expertise usually involves more than just this public knowledge. Human experts generally possess private knowledge that has not found its way into the published literature. This private knowledge consists largely of rules of thumb that have come to be called heuristics. Heuristics enable the human expert to make educated guesses when necessary, to recognise promising approaches to problems, and to deal effectively with errorful or incomplete data. Elucidating and reproducing such knowledge is the central task in building expert systems."5 In the domain of earth retaining structure selection the literature is fairly scant. A number of good references exist for the actual design of various types of earth retaining structures6-1°and for the consideration of problems involved with the design of earth retaining structures H-j3. However, these references give the reader few clues as to the steps and factors involved in the selection of one type of earth retaining structure over another. Initial discussions with experts in this field revealed that the final selection of earth retaining structures is generally a heuristic-based decision and, as such, the knowledge of human experts would be more applicable than the knowledge contained in the literature. On the other hand, the literature provides a reasonable coverage of the actual design of individual types of earth retaining structure, such as concrete cantilever and sheet pile walls. Hence for any system which is involved with selection and preliminary design of earth retaining structures, both knowledge from the literature and from human experts will be required. For a field as restricted as earth retaining structure selection, the collection of information from the literature available is not extensive. Most journal articles were found to be too specific, dealing with a given type of wall and a particular problem encountered. For a large area of interest, advice from an expert would probably be required in order to restrict the literature search to a reasonable coverage. The collection of knowledge from human experts is a more difficult proposition in many ways, as first the expert must be identified, next he or she must be persuaded to assist and finally the relevant knowledge must be elicited from him or her. The last part is probably the hardest, as much of the knowledge an expert possesses is used in a subconscious manner. Bramer points out that "Much expert knowledge is of an ill-defined and heuristic nature, frequently at an unconscious level. It is unlikely that even standard textbooks will give suffiVol 1 No 1 December 1987

ciently detailed, precise and accurate information to be suitable for direct use. Much of the expert's skill will undoubtedly be in approximate 'rules of thumb' which are seldom or never recorded. It is the task of the 'knowledge engineer' (system implementer) to elicit this knowledge and organize it, and to achieve this generally involves a period of intensive discussion with one or more subject experts and the analysis of a selected set of test cases. ''14 The methods of collecting knowledge from human experts for use in an expert system include questionnaires, interviews and working together with the expert. Working with the expert can be difficult to achieve, as few experts have the time or the inclination to sit down with the knowledge engineer.

Questionnaire and interviews A detailed review of the literature on earth retaining structures was conducted. Lists of possible factors influencing selection and design were prepared, along with relevant quotes from the literature which, it was felt, could provide clues for the formulation of knowledge for an expert system on earth retaining structure selection. As mentioned earlier, however, this knowledge was not sufficient and the heuristic knowledge of human experts was needed. Hence a questionnaire was prepared, addressing the following areas which could be important in the expert's selection considerations: • • • • • • • • • •

when do you actually require a structure; height restrictions for the various types of structure; applications for the various types of structure; site restrictions; backfill availability; foundation conditions; economics of the various types of structure; aesthetics; material availability; advantages and disadvantages of the various structures; and • any other factors which the expert considered important. Engineers involved with earth retaining structure selection and design from both the public and private sectors were then asked to complete the questionnaire. Retaining wall selection and design is usually only a small part of any engineering project, whether it is a road, harbour, building or any other type of project. Hence it was found that the 'experts' questioned were not solely experts in retaining wall selection and design, but instead had wide experience in this and other areas. The approach used was to contact the individual organization, whether a public authority or a private engineering consultation firm, and attempt to find the most suitable person in the organization to interview and present with the questionnaire. In some organizations, the interviewee consulted others within the organization to answer the questionnaire. In each case, an interview was conducted along with the presentation of the questionnaire in order to introduce the concept of expert systems and the aim behind this project, and to ascertain any other factors not covered by the questionnaire which the 'expert' considered important. While the interviewer had little training in interviewing techniques, he used his own experience 13

and expertise in the domain to prepare the questionnaire, interview the experts and analyse the results.

The knowledge obtained Before the interviews were conducted, it was hoped that they could provide some experiential rules for selection related to easily identifiable and codifiable factors of the form: if

Formulation of the knowledge The knowledge obtained was analysed and formulated into a suitable form for use in an expert system based on the B U I L D expert system shell. To do this it was decided to modularise the knowledge as much as possible into chunks of workable size. The prototype system was 14

__2

Output modules

I

Knowledge base Wall type modules:

height of structure is greater than X and less than Y and type of application is domestic then a possible wall selection is Z

where the form of the knowledge is of the abductive reasoning kind in that the selection of a wall type based on performance requirements is put forward as a possibility. Further knowledge would then be applied in those cases where several possibilities might arise so as to make a final selection. In many cases the 'experts' had very differing views on what the wall selection should be for a given set of conditions. This result raises two questions for consideration in the search for knowledge in expert systems. Firstly, the 'experts' often cannot agree in many areas. Secondly, the questions asked by knowledge engineers in attempting to extract the 'expert's' knowledge require careful formulation to avoid ambiguity, and some knowledge of the area is required by the knowledge engineer to interpret the 'expert's' answers. On many aspects the experts were in general agreement, such as the two main factors involved in selection of structure being cost and aesthetics. Also, the main consideration as to whether an earth retaining structure is required is the geometry of the site. If this is suitable to allow another solution such as an earth embankment or cut then this should be employed, unless there is a good reason not to, as it is a cheaper and easier solution. For certain applications there may be special technical reasons to employ an earth retaining structure. For example, the Department of Main Roads sometimes employs walls to confine noise or to constrain traffic flow. The choice of structure may be related not to the cheapest structure, but to the one which will cause the least problems with industrial relations or will use labour already used in an overall project. For example, if a roadwork project could be completed with a concrete wall or a reinforced earth wall and there are potential problems with formworkers or reinforcers, then the reinforced earth may be favoured as it does not require these skills but uses labour and equipment readily available in roadwork projects. Useful considerations were obtained from the questionnaires to provide a firm basis for a system on earth retaining structure selection. From the knowledge obtained, however, it was clear that generally an individual choice of structure type was unlikely for a given situation, but a number of types should be able to be eliminated and the more likely types then further investigated.

] ~

Build expert system shell

Slope stabilization/ embankment

i I

Railway sleeper wall Crib wall 3abions Selection module

] ]

Reinforced earth 31ockwork wall 3rick wall

High level knowledge lead to selection of appropriat( of earth retaining structur,

] ]

]

3ravity wall ] 3oncrete cantilever

I

Sheet pile

] Low level knowledge involving calculations and output of recommended wall specification

Figure 1. Outline of an expert system for the selection ~?fearth retaining structures then run and the knowledge modified as necessary to achieve the performance deemed satisfactory. From the knowledge acquired and because of the ability of expert systems to deal better with the heuristic type of knowledge required in selection, than the more procedural knowledge involved in the actual design of structures, it was decided to increase the number of structures considered and concentrate on selection of structure type. However, the selection of a structure is only part of the problem and a total expert system on earth retaining structures would need the low level modules as depicted in Figure 1. Numerous procedural programs are available for the design and analysis of specific types of retaining walls. Hutton and Rostron ~5 list about ten programs for sheet pile, concrete cantilever and gravity retaining walls. Newman ~6 has used a procedural computer program to produce the structural design of concrete and masonry cantilever retaining walls for various loading conditions and heights. Rhomberg and Street ~7have used computer generated data to compile a design aid to provide concrete cantilever retaining walls within 5% of absolute minimum cost. Only the high level selection module and one lower level module have been implemented. This allows the selection knowledge gained to be fully incorporated into the system, and also demonstrates a possible way in which the selection module in the expert system can determine the relevant input parameters and use procedural programs or lower level knowledge bases. Knowledge-Based Systems

The system, called RETWALL, consists of approximately 500 rules. RETWALL

Design domain Although only a fairly restricted domain, experts in earth retaining structure selection and design require special knowledge in a number of areas such as: • when an earth retaining structure is required; • the types of earth retaining structures and where they can be applied; • soil properties and their effects on structures; and • design techniques and requirements for the individual structures. It is not proposed to give a detailed explanation of the above areas but instead a brief description of some of the major points is provided below as an introduction to the domain. Earth retaining structures are used when soils have to be retained at slopes greater than those which they would naturally assume; when for some practical reasons abrupt changes in ground levels have to be introduced; or when it is necessary to protect soil banks against destructive agencies. Because earth retaining structures are expensive and may present construction difficulties, unless space or other considerations are paramount, an embankment or battered cut, if feasible, is usually preferable. However, these structures are not always feasible and in many cases, particularly in the urban environment, space saving is important and hence earth retaining structures are required. The type of earth retaining structure employed in a given situation should, according to the Institution of Structural Engineers, "be the most economical in cost, having regard to the materials and labour available, the site conditions, the degree of permanence required and the time available for construction ''~8. In many applications the aesthetic quality of the structure is also of great importance. The selection of the most suitable type of structure for a given situation is largely based on heuristic knowledge of the experts and comparison with previous situations and selections. The design of the structure includes the evaluation of the magnitude and distribution of stresses the soil exerts on the structure, and the design of the structure to withstand these stresses. As mentioned earlier, the system developed concentrates on the selection area.

Capabilities The major capabilities and limitations of the RETWALL expert system are as follows: • It will determine if an earth retaining structure is required. • Options for site geometry for the given application will be displayed graphically. • The applications considered by the system are domestic, commercial, industrial, road, railway, emergency, temporary, marine and heavy vertical load/abutment. • Structures ranging in height from 0 to 20 000 mm are considered. Vol 1 No 1 December 1987

• The following types of structure are considered: earth embankment/cut brick wall blockwork wall crib wall gabions gravity wall railway sleeper wall reinforced earth reinforced concrete wall sheet piling • If one type of structure is markedly better for a given set of conditions than all other possible types, it will be recommended as the design type. • If no structure is markedly better, a number of structures may be found to be suitable and the first of these structures will be recommended as the design type. • If a blockwork wall is determined as being suitable, a more detailed evaluation will be conducted and a scaled, dimensioned drawing showing reinforcing requirements will be displayed. • Soil can be classified according to the Unified Soil Classification System provided the user can give the results of the relevant laboratory tests. • The subgrade allowable bearing pressure can be estimated for low walls given a verbal description of the soil. • Terracing is considered as a suitable method to increase the height to which some wall types may be designed. A major limitation of the system is that, at present, it only considers standard types of wall design or general applications of the walls. For example, a blockwork wall could be designed for a height of six metres but this system will only design blockwork walls to a maximum height of 3.2 m in good soil, and lower in poor soil conditions. If one wished to check whether a brick wall could be used for a height of 2.1 m the system would answer 'no' as it only considers brick walls up to 2.0 m. Another limitation is in the handling of the case where a number of wall types are suitable for a given situation. Because only one lower level module has been written (the blockwork module), the system cannot currently investigate a number of suitable wall types in more detail to come up with alternate recommendation. Currently all the possible types are determined and displayed as being possible, but the first one evaluated as being possible is recommended as the design type. This is because the evaluation for different applications has been ordered, and so the first one evaluated may well be best. However, to make the system more useful, a more detailed evaluation and comparison of the options is required, and this necessitates the lower level modules for all the types to be developed. With the development of these lower level modules, costings should be considered along with the ability of the system to rank alternatives according to weighted criteria, or to allow the user to make a choice based on the data produced by the system.

Modules The overall concept of the system developed is as shown in Figure 1 and discussed previously. The system consists 15

of two main modules, a higher level selection module which provides selection rules and overall control, and a lower level blockwork module which provides a preliminary design for a blockwork wall and demonstrates the possible opera.tion of other lower level modules. The selection module contains the higher level knowledge obtained from the literature review and interviews of experts, which is concerned with the selection of the various types of earth retaining structure. Its rules are formulated in such a way as to control the firing of the lower level blockwork module, only when it has been determined that a blockwork wall is suitable for the given application. Currently if a type of structure other than blockwork wall is determined as being suitable, a message is output that it is suitable and no further investigation of that type is conducted as the relevant lower level modules have not been written. The rules in the selection module can be divided into a number of blocks which provide knowledge on:

earth retaining structure should only be employed if an embankment or cut could not be used, or if there was some general reason for not wanting an embankment or cut. The knowledge block on whether an earth retaining structure is required attempts to establish if an embankment or cut should be constructed, if not, then it is determined by default that an earth retaining structure is required. The knowledge on the types of structure suitable for a given wall application provides a higher level control on the search and determines the order in which the various wall types are considered, and which types are considered for every application. If the types considered by these rules prove to be infeasible, then the system will determine that the design is beyond its knowledge and stop execution of all the other possible, but not feasible, rules for evaluating a design. The knowledge used in this block is formulated as rules such as:

• typical site conditions and geometric parameters of the site for the various applications where an earth retaining structure may be required; • whether an earth retaining structure is required or not; • the types of structure which should be investigated for a given application; • each of the individual types of structure considered and the factors which affect the selection of that type; • various other considerations which affect wall selection such as terracing, surcharge loading and soil properties.

r325(if 'earth retaining structure' i s required and 'type of application for wall' i s A and "type of application for wall' i s marine and evaluated( 'Sheet pile is suitable for this application" and 'Reinforced concrete wall is suitable for this application' and 'Reinforced earth is suitable for this application') and not('Sheet pile is suitable for this application') and not('Reinforced concrete wall is suitable for this application') and not('Reinforced earth is suitable for this application') then 'design of earth retaining structure" i s "beyond knowledge of this system')

Figure 2 shows the flow chart for knowledge which determines whether or not an earth retaining structure is required. As mentioned earlier, one of the main points to emerge from the interviews of experts was that an | I

Yes

I Specialist employment or Technical I reasonsfor choosing an earth I retaininglstructure

I

I I ~-~ f I I Unsuitable or [

I

I

T

I

utilization (including future uses)

I

Geometry of the site, considering size and shape of the site and

I embankment l o t cut

I Suitable for embankment or cut

hut insufficient~pace on site [~ .~ I No land | available

I

I " I m

Availability of suitable land adjacent to site for construction of embankment or cut i u Land available Land available

I

butixpensi . . . .

I Massive

I

d ~heap

I Consequences of failure (including I ground movement on the higher side} I of an embankment or cut

I

I

Moderate

Substantial

I

|

Suitable for embankment or cut and sufficient space on site for construction

I. |

-

Minimal I

roundwater flow through the

proposed alignment of the earth retaining structure

t

I

Moderate

I

Low

I

Very low or Nil

'

Use earth retaining structur

bankment or battered cut ]J

Figure 2. Flow chart for knowledge on the requirement for earth retaining structures 16

The rules on the individual types of structure vary with the amount of knowledge obtained on the structures but generally include a range of heights applicable for the structure, the types of application for which the structure may be used, the aesthetic suitability of the structure and the availability of labour and materials for the structure. A typical example is: r35 I(if 'height of earth retaining structure (in mm)" is...... less_or~equal to1500and "Brick wall is aesthetically acceptable' and 'Labour and materials are available for brick wall" then possible('type of earth retaining structure' is_ 'brick wall') and 'Brick wall is suitable for this application'). The 'possible' predicate demonstrates an interesting extension of the B U I L D expert system shell to allow a number of alternatives to a given predicate (in this case 'type of earth retaining structure') to be found. The final block of rules provide knowledge on such things as terracing, surcharge loading, scale of the project and soil conditions which can then be used by the other rules. Some of these rules may not be required in the case of an experienced user who may give the Knowledge-Based Systems

answers they provide directly. Generally they are employed by the user asking 'how' to the relevant question in one of the selection rules. The blockwork module uses knowledge contained in available design charts to produce preliminary designs for reinforced concrete masonry retaining walls from 1.0 to a maximum, depending on the backfill soil used, of 3.2 m in height. A feature of this module is the output produced, which not only gives wall parameters but also gives a scaled, dimensioned drawing showing reinforcing bar requirements. The design charts used to produce the majority of the rules in this module give footing width, reinforcing bar requirements and wall thickness requirements for given wall height, footing type and backfill soil type. The blockwork module contains knowledge to: • classify the backfill into soil types given by Terzahgi and Peckg; • check that the allowable subgrade bearing pressure is not exceeded; • select the most appropriate wall footing type for the given site conditions; and • select the appropriate reinforced concrete masonry (blockwork) wall design parameters for the given conditions. The effects of backfill soil in exerting pressure on the retaining wall are based on empirical charts for active soil pressure given by Terzahgi and Peck for walls less than six metres in height. The gradings range from granular soil with little or no fines (backfill type 1) to medium or stiff clay deposited in chunks and protected from water penetration (backfill type 5). The lower the type, the more suitable it is for use as backfill and, due to the lower active soil pressures produced, the smaller the section of wall required to retain it. The system uses either verbal descriptions of the backfill soil or the Unified Soil Classification of the soil to grade the backfill as type 1 to 5. For example, a 'backfill type' 2 is 'sand or gravel containing some silt' or Unified Soil Classification GP-GM, GW-GM, SW-SM or SP-SM. To obtain the Unified Soil Classification, a module of about 40 rules (adapted from Burnham et aL n9has been included which gives the classification based on the results of sieve analysis and laboratory tests. The allowable subgrade bearing pressure for the walls given by the design charts used must not be below 125 kPa. To ensure that this restriction is complied with, the rules dealing with footing type selection require that the subgrade allowable bearing pressure is first determined. If the user cannot provide a direct answer in kilopascals, rules giving approximate allowable bearing pressures based on charts given by Carter 7 will be invoked which match verbal descriptions of the subgrade soil with a minimum approximate bearing pressure. These rules are self explanatory and take the form: rf105(if 'soil beneath wall footing' is__ 'firm clay' then 'subgrade allowable bearing pressure (kPa)' is __ 130). A note is included with the display of the question on the 'soil beneath wall footing' to give some rules of thumb for estimating the bearing pressure and matching the verbal description. Vol 1 No 1 December 1987

Base type 1

Base type 2

~\\\',Y///

'

Ll Base type 3

~N\\xXZZ/

' Base type 4

Figure 3. The different wall footing (base) types used

Four different wall footing types, as shown in Figure 3, are considered by the blockwork module. The most economical and preferred one is type 1, while type 4 is proffered if the space available for excavation and construction behind the face of the wall is limited. Type 2 and 3 wall footings are applied in boundary wall situations where all the available space on a site is required, and the wall footing cannot pass beneath some boundary or site restriction. The knowledge on site geometry and restrictions required by the rules which determine the wall footing type ('base type') is obtained by the selection module and is thus already in the facts base of the expert system. These rules take the form: r271(if 'subgrade allowable bearing pressure (kPa)' is___ greater than 125 and 'site case most applicable (as shown in the diagram)' is__ 1 and 'horizontal distance shown (d) (in mm)' is___ greater or e q u a l t o 500 then 'base type' is__ 1). The 'subgrade allowable beating pressure (kPa)' has already been discussed and these rules ensure that it is instantiated and checked before the design for a blockwork wall can be produced. The 'site case' and 'horizontal distance' refer to a drawing produced by the selection module about which the user would already have answered questions by the time this rule is 'fired'. Hence the user would only have to provide the subgrade allowable bearing pressure and the system would automatically deduce the 'base type'. The final block of rules in the blockwork module forms the major part of the module, providing design parameters for the wall and invoking the C language graphics procedure to produce a scaled, dimensioned drawing 17

showing reinforcing bar requirements. A typical example of these rules is: r 136(if 'height of earth retaining structure (in mm)' i s greater__than 2600 and 'height of earth retaining structure (in mm)' i s less than or__equal t o 3000 and 'base type' i s 1 and 'backfill type' i s 3 then 'biockwork wall type' i s 300 and 'footing width' is__ 2000 and 'V-bars' i s '$24 at 200' and 'X-bars' i s '$24 at 400' and draw)

Features Quintus PROLOG (in which BUILD is written) provides tools for loading and then calling C programs from within the PROLOG. R E T W A L L uses three C language files to produce graphical displays in two separate windows on the screen. An example of the screen displays produced is shown in Figure 4. The C language files are linked to the expert system. The other drawing created by the system uses a procedure and is invoked from the blockwork module. To produce the drawing of the blockwork wall, the procedure requires the values of height, footing width base type and blockwork wall type as integers and V__bars and X bars as character strings. The original BUILD expert system shell incorporates a built-in 'why' predicate which the user can invoke by asking 'why' to any question asked by the system. This predicate then causes the rule which is currently being investigated by the system to be displayed. In some cases this gives the user few clues as to what the system is really doing and why it needs to prove the particular rule shown. Willey and Thornley 2° employ an explanation facility which is based on the association of an explanation with each question asked. In the BUILD expert system shell this has been incorporated by the optional association of notes with predicates on which questions are asked. i~ n t

145

d

>i

rOfa . . . . . . . .

: SITE ( n o

bdildir, gs) ,

~

site

CASE 1. ~ i l i . l g s i t e t o w i t h i o 9 i v e ~ dTsloe~¢e o f b o u n d o r y o r r e s t r l c t i o r

s]Ie b o ~ n d o r y / r e s t ,

h

'.11

bour,dary/,estr~ct;,.,r,

ict{on

; : : : : i n to ofc . . . . . . . . . . .

b o ~ n d.Jr ~ / , e s t riot',: r,

C~SE 2 FII : n g b u i l d n g ~ i t e tO w i t h i n the dlstance of hot~ndory o" restr:ctlon

~ - ~

d 2 ~1

bourldo'y/r

e s t r T c t o~

i SITE >~\\\v////

extent of required level o r e a CASE 3 Cutting to within given distonce of boundery or rest,:ction ~lay have building£ on site

extent of required ; level o r e a CASE ~ CutTing Io wlttln given d i s t a r , ce o f k or re£trlct~c,r B d n g / r o a , - J a[:,o~e s i t e

Figure 4. Example screen display showing site options to assist input 18

This note facility has been extensively used in the RETWALL knowledge base with a 'notes list' at the end of the knowledge base showing all of the notes. Not all predicates have a note associated with them, however, and where a note is given it appears automatically when the question is asked. Even with the 'why' and note facilities it was still felt that further explanation could be required in some instances and so an 'explain' predicate was added to BUILD. This predicate allows an explanation to be associated with a list of rules to help explain why a question is being asked. As these explanations appear throughout the knowledge base at the head of each block of rules, they also provide additional documentation within the actual knowledge base to explain the rules. An example is: exp([r321, r322, r323, r324, r325,r325], ['This rule directs the search for a design to the most suitable types', 'of earth retaining structure for this application. If none of the ', 'types proves suitable then the system cannot find a design.']). When the user asks the system to 'explain', the explanation will be output if it exists and, if not. the 'why" explanation will be automatically invoked. Although it may appear that the knowledge and the control in the R E T W A L L expert system has been separated by using the BUILD expert system shell to provide control and the RETWALL knowledge base to provide the knowledge, this is not the full story. The BUILD expert system shell does contain the mechanisms to consult files, add and remove information, display information, process to find the value of, or prove a goal and explain its reasoning. However, the formulation of rules in the knowledge base and the ordering of rules provides a higher level of control by allowing a directed search. In attempting to find the value of or prove a goal, BUILD will search the knowledge base 'objects list' to find which rules have the goal as a consequent, and then look at the rules to see if it has the knowledge to prove any. If not, it will attempt to prove the first rule in the list by questioning the user on the antecedent of the rule. Hence, by ordering the rules in the 'objects list', the user directs the search of the system by determining which rules are evaluated first. This method of control is used extensively in the R E T W A L L expert system. The rules which are affected by order or which cause more efficient execution by their ordering have been specially ordered. An example is the set of rules which prove 'earth retaining structure'. The options for this predicate are 'earth embankment/cur or 'required'. In order to prove that an earth retaining structure is 'required' it is first attempted to prove that it is an "earth embankment/cur. If it is not then it must be 'required'! Hence a number of rules are used to try and prove that the 'earth retaining structure' is 'earth embankment/cur. If these rules fail then the following rule will succeed: r62(if not('earth retaining structure' i s 'earth embankment/cut') then 'earth retaining structure' is required). Knowledge-Based Systems

? list

goale.

****~i**illooi~,i~io,~,i~oili,~lli**i,oi~i,l,llli.~,,iooo,,,Iooi

gl : g2 : g3 : g4 : 95 : gO : !g7 : g8 : gg : gl~ gll g12 g13 g14 915

design of earth retaining structure earth retaining structure Crib uill ts suitable for this application Black.ark ~all ts suttable for this application Brick wall ts suitable for this ippltcation Gravity ~all is suitable for this application Sheet p i l e I s s u t t a b l s f o r t h t s a p p l i c a t i o n R e i n f o r c e d e a r t h te s u i t a b l e f o r t h t 8 a p p l i c a t i o n R e i n f o r c e d c o n c r e t e uell i s suitable f o r t h i s a p p l i c a t i o n : Gabtons a r e s u i t a b l e f o r t h i s a p p l i c a t i o n : maximum number o f t e r r a c e s a l l c ~ e d : black.ark ill1 type : base t y p e : backfill type : soil classification of backftll

! i x + . ~ ot" reclulrlcl iii~ 14 a l l ~

r~--~

' ,,,v/C/,j/ 5¢n[ no b~flllln,.~

,l ~ ', +~-~,-ta, i CI

1.

bo..do,,/,.+l,idio.

~'~:lill

Filling i h l £o w~thln ¢3h~1

+ IIP~IN

o'l' rKlulr~l arlma

boundc, ytrelrtr re.ties

c~mE 2. Flll|n0 buhdln 9 mite to wlihln the dlitonee Of bouetatary oe rglitlctlon

tlrmton¢o t:~ b<~undory Or rlm++rletlor,.

site taounaory/r +~r/c4ron

h'

~

111~41bouP~dory/r Ildr r e l l o l ~

~

sneer command find gl. d *****************************************************************

h

i

~_/

earth retaining

structure

,

SITE

.~ al ~k.xxxv#///

: mekocd o¢ r~ImlPoa ; llrem Orlm

I e~t~.A

of

; required

is_ ? - enter

o p t i o n s f o r v a l u e s ape : t e m p o r a r y o r m a r i n a o r Jtamrgency o r domestic o r commercial trial or road or r a i l w a y o r heavy v e r t t c a l l o a d / a b u t m e n t

: li

i,j dq ~..,~XxXXV////~

r+ The t y p e o f a p p l i c a t i o n f o r value (howA0hy/expialn)

h

;

+ ll.v~

cl•

m3

CASE 4. Ciiltlrl 9 t6 wRhl~ q~,41~

Cd~t: 3. CuttJ~q to wlthlm 9k, l'~ d|llrtonc'lP ot" boundory or rmltr|c't~on May h a i l bu]d;ngm on site+

diiton~! oi bounOary or reltr|d|on Bufld|ncJ~toacl o~*ovl she.

or indue

? domestic. type of tppltcetlen

for wall

I s _ domestic

The s t t e case most a p p l i c a b l e er value (hou/why/e~platn)

( a s shown i n t h e diagram) i s _ ? - ant

? 1. The h o r i z o n t a l why/explain)

d i s t a n c e shown ( d ) ( I n ~ )

is

? - enter value (ho~/

? 50BB. The h e i g h t d i f f e r e n c e y/explain)

shown ( h i

( i n mm) i s

q - e n t e r v a l u e (hou/uh

? 288B,

Figure 5. Screen dump of text and graphics dialogue during run By ordering the rules in this case, the control imposed will allow more efficient processing as by the time rule 62 is 'fired' it has already been proven that the 'earth retaining structure' is not 'earth embankment/cut'. R E T W A L L - - E X A M P L E I N USE

The example below gives a complete consultation with the system (running on Sun Microsystems workstations) to select a wail type and then produce a preliminary design for a blockwork wall. Screen dumps taken during the running of this example are given in Figures 5 and 6 and a brief explanation of some points of interest

27~0

O0

is given after the example. As an idea of the time taken for an average consul-

tation, the following example took about 10 minutes, with the longest wait of about one minute occurring when the system was searching the blockwork module to determine if it could find the 'blockwork wall type'. This occurs when the selection module has found the possible types of earth retaining structure and then tries to find the 'blockwork wall type' in order to find a recommended design (rule318). To illustrate some of the features of the BUILD expert system shell, the result of the command to explain proof is given at the end of the example. Vol 1 No 1 December 1987

leo

Figure 6. Screen dump of a selected and designed blockwork wall enter c omma nd *~*iilii*ai~**a~a0e*m*iiiiiiilililili*e*e*i*a~i**li,i****,i,

? find 'design of each retaining structure'.

The type of application for earth retaining structure is? value (how/why/explain)

--

enter

19

options for values are: temporary or marine or emergency or domestic or commercial or industrial or road or railway or heavy vertical load/abutment

possible(type of earth retaining structure i s _ blockwork wall) is true Blockwork wall is suitable for this application is true

?industrial. type of application for wall is

industrial

The site case most applicable (as shown in the diagram) is__'? enter value (how/why/explain) ?2. The horizontal distance to level area(dl)(in mm) is__'? - - enter value (how/why/explain) ?2000. The horizontal distance to buildings (d2)(in mm) is 9 enter value (how/why/explain) ?3000. The height difference shown (h)(in mm) is_'? enter value (how/why. explain) ?1700. earth retaining structure i s

required

Gravity wall is aesthetically acceptable'? - - yes/no (how/why/explain) '?yes. Labour and materials are available for gravity walt? - - yes/no ( h o w why/explain) ***NOTE*** This includes consideration of the specialist (if any) labour skills available and required, and the materials available and required. If you ask how, you will be questioned individually on these aspects. If other forms of construction are also applicable you will be asked about them later. '?no. A t t h i s p o i n t t h e s e l e c t i o n m o d u l e h a s c o m p l e t e d its evaluation and found that a possible type of earth retaini n g s t r u c t u r e is b l o c k w o r k w a l l . T h e b l o c k w o r k m o d u l e is t h e n f i r e d b y r u l e 318: The backfill type i s ?

height of earth retaining structure (in mm) i s Crib wall is aesthetically acceptable? ? explain.

1700

yes/no (how/why/explain)

***BRIEF EXPLANATION OF RULE r362"** This rule checks the suitability of crib walls for this application and if suitable, recommends them as a possible type of structure. Other types of structure will then be checked by different rules. If crib walls are markedly better than other types for the given conditions then they may be recommended as the design above the other types. Crib wall is aesthetically acceptable? ?yes.

yes/no (how/why/explain)

Labour and materials are available for crib wall? explain)

yes/no (how/why/

***NOTE*** This includes consideration of the specialist (if any) labour skills available and required, and the materials available and required. If you ask how, you will be questioned individually on these aspects. If other forms of construction are also applicable you will be asked about them later. ?how. local material available(crib units)? - - yes/no (how/why/explain) ?yes. local material available(suitable granular fill for crib wall)? yes/no (how/why/explain) ?yes. trade skills available(labourers)? - - yes/no (how/why/explain) ?yes. Labour and materials are available for crib wall is true There is a user preference or other special reason for using crib wall? - - yes/no (how/why/explain) ?no. Blockwork wall is aesthetically acceptable'? yes/no (how/why/ explain) ?yes. Labour and materials are available for blockwork wall? - - yes/no (how/why/explain) ***NOTE*** This includes consideration of the specialist (if any) labour skills available and required, and the materials available and required. If you ask how, you will be questioned individually on these aspects. If other forms of construction are also applicable you will be asked about them later. ?yes.

20

--- enter value (how/why/explain)

***NOTE*** Backfill is graded from very good to unsuitable for use with retaining walls from type 1 to 5 respectively. If familiar with the gradings. you may type the appropriate number, otherwise type "how". options for values are: Ior2or3or4or5 ?how. The backfill to be used is

enter value (how/why/explain)

***NOTE*** The backfill to be used can be classified by using one of the verbal descriptions given, or if unsure by entering "'other" the system will question you to obtain the "Unified Soil Classification" of the soil. options for values are: sand or gravel with little or no fines or sand or gravel containing some silt or sandy or gravelly soil with considerable % of silts and clays, or silts and clays that are thoroughly broken into small pieces or medium or stiff clay deposited in chunks or other '?'sandy or gravelly soil with considerable % of silts and clays'. backfill type is

3

The subgrade allowable bearing pressure (kPa) is (how,,why/explain)

'? -

enter value

***NOTE*** This may be determined accurately by testing and calculation and then input, or, by asking "'how", the system will provide verbal descriptions which allow typical values of allowable bearing pressure to be determined. Some testing will however be required by the user to match the verbal descriptions given. options for values are: 0 to 10000 ?ho~. The soil beneath wail footing is _ ?

enter value (how/why/explain)

***NOTE*** The verbal descriptions provided below allow typical values of allowable bearing pressure to be determined, based on ultimate bearing capacity for clays and settlement considerations for sand and gravel. The clay calculations use a rule-of-thumb that for shallow foundations for light walls, allowable bearing capacity is twice the unconfined compressive strength. This can be estimated on site by taking numerous pocket penetrometer readings of the clay (most pocket penetrometers are graduated in terms of compressive strength). For sands and gravels, if SPT N-values are not available, densit~

Knowledge-Based

Systems

may be estimated by using a pick and shovel. If it can be excavated by a shovel, it is loose; if a pick is required, it is medium dense. For saturated or submerged conditions specify the next lower sand or gravel category (eg if dense but saturated, specify as medium dense). hard clay .... (cohesive strength > 300 kPa) very stiff clay ... (cohesive strength 150-300 kPa) stiff clay ... (cohesive strength 80-150 kPa) firm clay ... (cohesive strength 65-80 kPa) soft clay ... (cohensive strength < 65 kPa) very dense sand/gravel ... (SPT N-value > 50) dense sand/gravel ... (SPT N-value 30-50) medium dense sand/gravel ... (SPT N-value 13-30) loose sand/gravel ... (SPT N value 5-13) options for values are: rock or hard clay or very stiff clay or stiff clay or firm clay or soft clay or very soft clay or very dense sand/gravel or dense sand/gravel or medium dense sand/gravel or loose sand/gravel or other ?'stiff clay'.

subgrade allowable bearing pressure (kPa) i s base type i s

160

I

blockwork wall type i s footing width i s

200

1400

V-bars i s

S l6 at 400

X-bars i s

S16 at 400

design of earth retaining structure i s

blockwork wall

************************************************************

enter command ************************************************************

? explain proof.

possible(type of earth retaining structure i s blockwork wall) and Blockwork wall is suitable for this application proved true by rule No. r341 because type of application for wall i s industrial type of application for wall i s heavy vertical load/abutment or temporary or marine or emergency is false as type of application for wall i s industrial not(type of application for wall i s heavy vertical load/abutment or temporary or marine or emergency) is true height of earth retaining structure (in mm) is less o r e q u a l t o 3200 as height of earth retaining structure (in mm) i s 1700 Blockwork wall is aesthetically acceptable is true Labour and materials are available for blockwork wall is true Labour and materials are available for crib wall proved true by rule No. r361 because local material available(crib units) is true local material available(suitable granular fill for crib wall) is true trade skills available (labourers) is true earth retaining structure i s required and site type and height of earth retaining structure (in mm) i s 1700 proved true by rule No. rl0 because type of application for wall i s industrial type of application for wall i s marine or road or railway is false as type of application for wall i s industrial not(type of application for wall i s marine or road or railway) is true site case most applicable (as shown in the diagram) i s 2 height difference shown (h) (in mm) i s 1700 horizontal distance to level area (dl) (in mm) i s 2000 horizontal distance to buildings (d2) (in mm) i s 3000 1.17647 is 2000/1700 1.17647is less t h a n 3 type of application for wall i s industrial and showsite(industrial) proved true by rule No. rl because type of application for earth retaining structure i s

industrial

************************************************************

design of earth retaining structure i s blockwork wall proved true by rule No. r318 because possible(type of earth retaining structure i s blockwork wall) is true blockwork wall type i s 200 blockwork wall type i s 200 and footing width i s 1400 and V-bars i s S16 at 400 and X-bars i s S16 at 400 and draw proved true by rule No. r133 because height of earth retaining structure (in mm) i s ~ r e a t e r than 1400 as height of earth retaining structure (in mm) i s 1700 height of earth retaining structure (in mm) is less or equal to 1800 as height of earth retaining structure (in mm) i s 1700 base type i s 1 backfill type i s 3 base type i s 1 proved true by rule No. r272 because subgrade allowable bearing pressure (kPa) is__.greaterthan 125 as subgrade allowable bearing pressure (kPa) i s 160 site case most applicable (as shown in the diagram) i s 2 type of application for wall i s industrial horizontal distance to level area (dl) (in mm) is__greater or equal to 500 as horizontal distance to level area (d 1) (in mm) i s 2000 subgrade allowable bearing pressure (kPa) i s proved true by rule No. r104 because soil beneath wall footing i s stiff clay

160

backfill type i s 3 proved true by rule No. r265 because backfill to be used i s sandy or gravelly soil with considerable % of silts and clays

Vol 1 No 1 December

1987

The 'explain proof given above shows the rules that were proved in determining the design. An 'explain fail' listing was also obtained but was not included as it was over 20 pages long. The selection procedure in the above example follows the same lines as discussed previously. The blockwork module is 'fired' at the point indicated above and immediately tries to find the 'backfill type'. As the user does not know the type from the numerical options given, verbal descriptions are given which match the various types. If the user could not match one of these verbal descriptions the rules proving the Unified Soil Classification could be invoked by an 'other' response. However the verbal description is matched and so the 'backfill type' is determined. Finally the system determines the 'subgrade allowable bearing pressure' and with the geometry of the site already known (from the selection module), the 'base type' is determined and the appropriate design parameters output. At this point the typical graphical output shown in Figure 6 is produced. DISCUSSION Extensions to R E T W A L L The system at present is an example of the possible application of expert systems in engineering design. For the system to become a useful design tool the following must be implemented: • write the remaining lower level modules; 21

• incorporate checks for infeasible or unlikely data input; • allow modification of data; • provide a better user interface which makes more use of windows and the ability to use the mouse to selection options; • provide more rules for the comparison of options after the processing of lower level modules has occurred; • further improve the explanation capabilities of the system; • allowing general rules to be applied a number of times to different cases in the one consultation; and • allowing the consultation state to be saved at any time so that soil tests or other investigations can be carried out to provide the answers required by the system. Some of the lower level modules could be written to include procedural programs which perform such tasks as the optimization of the sectional dimensions of a given wall type such as concrete cantilever or gravity wall. Programs to produce bills of quantities and costings would also be required to allow better comparison of options. These programs would be used by the lower level modules and would only be invoked when a suitable preliminary design was obtained.

which are based on 'learning' concepts have only a very rudimentary concept of learning. While they may extract some general rules from those case studies presented to them they cannot induce the causes for the various behaviours of the cases and then apply them in other specific cases. Expert systems, however, are in their infancy. This paper has shown that useful applications are possible in the field of engineering design. ACKNOWLEDGEMENTS This work is supported by continuing grants from the Australian Research Grants Scheme and the Australian Telecommunications and Electronics Research Board.

REFERENCES 1 March, L 'The logic of design and the question of value' in March, L (ed) Architecture oJi/brm Cambridge University Press, Cambridge (1976) pp 1 40

2 Coyne, R D, Rosenman, M A, Radford, A D and Gero, J S 'Innovation and creativity in knowledgebased design' in Gero, J S (ed) Expert systems in Computer-Aided Design, North-Holland, Amster3 4

Expert systems generally This paper has demonstrated the applicability of a rulebased expert system to engineering design. However, in general, one form of knowledge representation is not sufficient to adequately allow for the process of design. Other forms, e.g. frames, are much more suitable for describing the state of a design and for the transfer of information through inheritance properties. Rosenman, Manago and Gero 2~ working at the Architectural Computing Unit, Department of Sydney, have developed a system linking a frame-based system to BUILD and also to the Eagle Computer Modelling System, thus greatly extending BUILD's capabilities. Gero, Oxman and Manago 22 have demonstrated the capabilities of expert systems to integrate various aspects of the design process, such as interactive graphics. Current systems apply in very restricted domains but in the future it is likely that controlling or planning systems23 could be implemented which actually select other lower level systems for a task and pass knowledge between different systems in the same sort of modular concept to that used in the RETWALL system. There are still many problems to be overcome with the use of expert systems. The availability of really useful expert system shells and other software tools is still quite limited, knowledge acquisition is a substantial problem and there is a fundamental lack of understanding of the human thought process. There is a great deal of knowledge currently available but not in a form readily usable by expert systems. More work is necessary with regards to the integration of expert systems and databases. In general the level of the knowledge in current expert systems is shallow. They have no knowledge of what they know or about what they do not know and thus cannot efficiently access or reorganize their knowledge according to circumstances. Even those expert systems 22

dam (1987) pp 435-465 Buchanan, B G and Shortliffe, E H Rule based expert systems Addison-Wesley, Reading, UK (1984) Rosenman, M A BUILD expert system shell Users manual Architectural Computing Unit, Department of Architectural Science, University of Sydney (1987)

5 Hayes-Roth, F, Waterman, D and Lenat, D Building expert ,sTstems Addison-Wesley, Reading, UK 6 7 8

(1983) Bowles, J E Foundation analysis and design McGraw Hill, New York (1982) Carter, M Geotechnieal engineering handbook Pentech Press, Plymouth, UK (1983) Cornfield, G M 'Sheet pile structures' in Winterkorn,

H F and Fang, H-Y (eds) Foundation engineering handbook Van Nostrand, New York, USA (1975) pp 418--444 9 Terzahgi, K and Peck, R B Soil mechanics m engineering practice Wiley, New York (1967) 10 Wu, T H 'Retaining walls' in Winterkorn, H F and

Fang, H-Y (eds) Foundation engineering handbook, 11

12

13 14

15 16

Van Nostrand, New York (1975) pp 402-417 Aggour, M S and Brown, C B 'The prediction of earth pressure on retaining walls due to compaction Geotechnique Vol 24 No 4 (1974) pp 489-502 Casagrande, L 'Comments on conventional design of retaining structures' J. Soil Mechanics & Foundations Div. ASCE, Vol SM2 (February ! 973) pp 181 198 Johnson, S J 'Analysis and design relating to embankments' in Anal. Design in Geotechnical Engineering ASCE, Vol 2 (1974) pp 1 47 Bramer, M A 'A survey and critical review of expert systems research' in Michie, D (ed) Introductory readings in expert systems, Gordon and Breacl~, London, UK (1982) pp 3-29 Hutton, G and Rostron, M Computer programs/or the building industry Architectural Press, London, UK (1979) Newman, M Standard cantilever retaining walls McGraw Hill, New York (1976) Knowledge-Based Systems

17 Rhomberg, E J and Street, W M 'Optimal design of retaining walls' J. Struct. Div. ASCE (May 1981) pp 992-1002 18 'Earth retaining structures' Civil Engineering Code of Practice No. 2 Institute of Structural Engineers, London (1951) 19 Burnham,G, Gaskell, A, Hutchinson, P and White, P Knowledge base for an expert system - - the unified soil classification system Computer Applications Research Unit, Department of Architectural Science, University of Sydney (1984) (unpublished) 20 Willey, D S and Thornley, R An expert system for

Vol 1 No 1 December 1987

heating and ventilation sketch design, Plymouth Polytechnic School of Architecture, Plymouth (1985)

21 Rosenman, M A, Manago, C and Gero, J S 'A model-based expert system shell,' 1AAAI'86 (1986) pp C:1:1-15 22 Gero, J S, Oxman, R and Managa, C 'Graphics and expert systems' A USGRAPH86 (1986) pp 2529 23 Coyne, R D and Gero, J S 'Semantics and the organization of knowledge in design' Design Computing Vol 1 No 1 (1986) pp 68-89

23