212 TRANSPORTATION RESEARCH RECORD 1291

Expert System for Hazard and Risk Assessment on Low-Volume Hill Roads in Nepal

BHASKAR THAPA, BIRENDRA B. DEOJA, AND MEGH R. DHITAL

An exp~rt syst_em that is at the stage of a developmental prototype ts descnbed wtth an emphasis on how it increases the applicability and reliability of hazard and risk assessment on low-volume hill roads in Nepal. The informal and ill-structured knowledge used for hazard and risk assessment is shown to be organized and standardized using a knowledge-based expert system. The use of site- and failure-specific diagnostic procedures is shown to be a significant improvement over current methods of hazard and risk assessment that make use of aggregate weights only. A sample user consultation demonstrates the operation of the expert system.

Hill roads in Nepal service subsistence economies. Although the low level of economic activity results in low traffic vol­umes, the geologically active mountain chain introduces risks to the road that lead to high costs. Thus, hill roads in Nepal are low-volume, high-cost roads. Table 1 indicates this point.

Natural hazards such as rockslides and slumps pose risks of losing investments in hill roads. In order to plan, design, and construct a hill road that accounts for hazards in the form of tolerable risks, it is necessary to incorporate a systematic process of hazard and risk assessment into the practice of road construction. Hazard and risk assessment requires the use of extensive judgment to identify potential dangers, the likeli­hood of experiencing a danger in a given time (the hazard), the vulnerability of engineering structures on different stan­dards of roads to damages from natural or man-induced haz­ards, and the changes in hazard as a result of countermeasures taken to reduce the expected value of loss (the risk). The practice of hazard and risk assessment was introduced only recently in Nepal, because the collection of judgments needed exists only as an ill-structured body of knowledge that is based on specific experiences. To extend this ill-structured knowl­edge into the practice of hill road construction on a national scale, there is a need to produce a standardized approach to hazard and risk assessment that can be readily transferred to pr_ac;ticing engineers ano engineering geologists.

Knowledge-based expert systems (Klf:ESs) provide a robust problem-solving environment to formally organize ill­structured knowledge and to produce a practical tool for car­rying out intelligent tasks performed by highly skilled people. Given the subjective, incomplete, ill-defined, and informal collection of hazard and risk assessment knowledge currently

B. ~hapa, Department of Civil Engineering, 440 Davis Hall, Uni­ver~1ty of California, Berkeley, Calif. 94706. B. B. Deoja and M. R. Dh1tal, Tribhuvan University. POB 3757, Tripureswar, Katmandu, Nepal.

existing in Nepal, a convenient way to standardize the hazard and risk assessment procedure for hill roads is to develop a KBES. A KBES will enable a widespread incorporation of hazard and risk assessments into the practice of hill road construction.

KBES IMPROVEMENTS TO HAZARD AND RISK ASSESSMENT APPROACH

A KBES demonstration prototype was developed with the intent to produce a systematic procedure to hazard and risk assessment while developing a standardized technique. It is believed that the development of a systematic procedure, and specially a computerized one, will enable nonspecialists to address hazards and risks conveniently. Also, the stan­dardization will result in an improved reliability of hazard and risk assessment results.

Existing hazard assessment techniques in Nepal are based on a system of relative weights that express the contribution of natural factors to a potential instability (J). For example, in the case of hazard assessment during the preliminary stage of a road project, all sections of the proposed alignment are assigned a set of ratings depending on the presence or extent of (a) slope attributes-angle, relief, and complexity; (b) climate-annual rainfall, antecedent moisture conditions, and probability of cloudburst; (c) geology-rock type, disconti­nuities, soil type, and soil thickness; ( d) land use-vegetation, roads, and canals; (e) geodynamic processes-existing mass movements, stream undercutting, glacier- or landslide-dammed lakes; and (f) faulting and seismicity. The ratings are then summed along the entire alignment and divided by the length of the alignment to arrive at the relative hazard level per kilometer. A similar approach is taken during hazard assess­ment at the feasibility study stage (J). Weights are assigned to factors that contribute to rock and debris slides. A given site may be rated as high, medium, or low hazard depending on the presence of factors that reflect the structural, litho­logical, hydrogeological, and weathering conditions. A com­puterized version of this approach has been developed as well (2). The ratings or weights are based on field experiences in Nepal (3).

The existing approach to risk assessment is oriented towards the evaluation of road length that will most probably be lost because of damages from hazards over the road life (J). This approach is designed for use during the preliminary stage of road construction to aid in alignment selection and involves risk calculations on successive sections of the alignment facing

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TABLE 1 HILL ROAD COSTS IN NEPAL

Traffic Road

Length (km) Road Standa rd (1990 AADT)

Average Cost per Kil ometer (1990 U.S.$)

Dharan-Dhankuta Mugling-Narayangat Dhangari-Dandeldhura Kohalpur-Surkhet Lamahi-Tulsipur Lamosangu-Jiri

so 36

13S 92 24

110

Double-lane , paved Double-lane , paved Double-lane , dirt Double-lane . graveled Single-lane, graveled Single-lane , paved

197 8S2

< SO <SO

131 138

797.000 445.000 246 .500 498 .000 181.500 214 .300

different types of hazard . Risk assessments during latter stages of road construction are treated with general indications only (J) .

Expert systems have been developed for problems in other areas of geotechnical engineering. CONE ( 4) classifies soils and infers the shear strength from cone penetrometer data; RETW ALL (5) addresses the problem of choosing applicable retaining wall types for different conditions; Shallow Trench (6) aids in interpreting a new soil classification system to plan safety precautions during excavation for shallow trenches; and SOILCON (7) provides advice on how much subsurface inves­tigation is needed given what is known and the requirements of a proposed structure. These expert systems are either at the stage of a development or operational prototype (8).

The KBES demonstration prototype differs from the rating method of hazard assessment in that the KBES diagnoses different types of failures using specific rules along the align­ment , whereas the rating method arrives at an aggregated hazard level per kilometer through a black-box approach. In fact , a major difference between the KBES and present approaches to hazard assessment (both the rating and relative weight methods) is that the former gives explicit justifications for its results, whereas the latter produce results containing implicit judgments that are not amenable to review. Thus, whereas the KBES can be updated or used with unique or new considerations in a systematic way, the current methods are too rigid to allow convenient modifications. The com­puterized approach to hazard assessment, being algorithmic, only automates the existing manual method. It does not improve the method of hazard assessment itself as the KBES does . Also , although the results of hazard assessment using the KBES are designed to fit into the procedure for risk assess­ment, the previous hazard assessment techniques cannot be used for risk assessment because forecasting with respect to potential damage to road elements is not included. The KBES is a significant advance over the current methods of risk assess­ment in that it also introduces currently nonexistent rules to assess the vulnerability of different structures facing different hazards and to modify the likelihood of experiencing a danger given the road design to be implemented and different countermeasures.

The KBES is an addition to the list of existing expert sys­tems for geotechnical engineering. This KBES is most similar to CONE and Shallow Trench, in that heuristics prevalent in the practice of geotechnical engineering are systematized by all three . This KBES, like CONE, is built with a general­purpose representational language, whereas SOILCON, Shal­low Trench, and RETWALL use expert system shells. Unlike CONE, this KBES runs on a personal computer.

KBES STRUCTURE AND USE

The KBES has been implemented in a version of PROLOG called Turbo PROLOG (9). PROLOG has been classified as a general-purpose representation language that uses a logic­?ased representation of knowledge and a backward-chaining mference engine (JO). Turbo PROLOG was chosen because the ?ackward-chaining control strategy is suited to the diag­nostic nature of the hazard and risk assessment procedure , has complete flexibility , and runs on a personal computer. Turbo PROLOG allows calls to algorithmic procedures writ­ten in Turbo Pascal.

. The cont_ext is described to the KBES by the user. Begin­nmg at chamage location zero, the KBES queries the user on various hazard and risk parameters on user-defined segments that may vary according to homogeneous units of data . The query for a given parameter begins at the least-detailed level and proceeds to higher levels of detail until the user's infor­mation is exhausted. For example, in acquiring information regarding terrain composition on a given road section. the KBES starts out by asking the user if the area is composed of resistant or nonresistant rocks. If the user replies resistant , the KBES asks whether the rocky terrain is interbedded or not, what the rock type is, and what the orientation of the rock structure was with respect to the proposed road align­ment . Also, the KBES acquires information regarding such fact?rs as drainage pattern, existing landslides, geology, veg­etation, slope, seismicity, glaciation, road standard, and pro­posed structures. The smallest unit that can be described by the ~ser is 100 m. Depending on the scale of topographic map or a~r photographs, higher units may be used for preliminary studies . The entire alignment is covered and the information is preserved in a data base .

The hazard diagnosis knowledge base exists as a series of rules that define the conditions sufficient to produce any of the common mass movements. There may be more than one rule that defines a particular mass movement; such multiple rules are used to account for the different levels of detail contained in the information the user provides. The mass movements diagnosed by KBES are as follows:

Soils

Slump Translational slide Mudflow Surface erosion

Debris

Debris flow Debris slide

Rock

Planar slide Wedge slide Toppling Rockfall

Eac~ . hazard diagnosis rule has an attached probability of reahzmg that danger in a given time interval. The probability of one failure during the time interval (of the road life) is

TABLE 2 CONTEXT OF SAMPLE USER CONSULTATION

Slope Rock Type

Chainage Degree Chainage Type

7.0-7.B 36-450 7.0-7.B Fractured slate

7.B-B.O 45-50° 7.8-8.2 Colluv ium

8.0-9.1 36-45° B.2-B.6 Slate

9 . 1-9.2 46-50° B.6-B.B Colluvium

9.2-9.4 >51° B.9-9.0 Interbedded quartzite + claystone

9.4-10.0 46-so0 9.0-9.1 Eluvium

9 . 1-9.3 Intcrbcddcd quartzite + shale

9.3-9.4 Colluvium

9.4-9.9 Interbedded quartzite + shale

9.9-10.0 Thick-bedded quartzite

Vegetation Landslides

Chainage Type Chainage Type

7.0-9.0 Sparse brush- 7.1-7.2 Medium slump

es 7.2-7.3 Medium planar rackslide

9.0-10.0 Sparse trees e.1-e.2 Two large slumps and one small soil slide

8.8-8.9 A small slump and a large planar rockslide

Debris flow

9.7-9.8 Twa medium slumps

9.J-9.4 Two medium wedge failures

9.4-9.5 A large planar rockslide

9.9-10.0

Drainage Geologic Structure

Chainage Type Chainage Structure

7.0-9.0 Dendritic 7.0-B.O Crushed and fractured rock

9.0-10.0 Trellis 7.8-9.9 Folded area

8.2-8.5 Folded area

9.2-9.4 Crushed rock

Land use Groundwater

Chainage Type Chainage Condition

7.0-10.0 Non - culti- 7.0-7.5 Dry

vated 7.5-8.0 Perennia 1 seepage

8.0-8.4 Dry

8.4-8.5 perennial seepage

8.5-8.9 Dry

8.9-9.l Perennial seepage

9.1-9.5 Dry

9.5-9.7 Perennial seepage

9.7-9.9 Dry

9.9-10.0 Seasonal seepage

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assigned to the diagnostic rules using methods such as the comparison of lotteries (11 ). Also. each rule includes the extent of.the hazard in the diagnosis as small. medium. large . a;1d very large. depending on whether the length of the hazard along the slope or the alignment is between 3 and 10 m, 10 and 30 m. 30 and 100 m. or larger than 100 m. respectively.

The user may specify one of three possible road standards on the proposed alignment during the context definition. The standards are (a) low cost-cut slopes at marginal equilibrium with no mitigations used to control hazards. (b) medium cost­equilibrium cut slopes with partial mitigations. and (c) high cost-equilibrium cut slopes with full mitigation measures taken. For each standard. the vulnerability is defined by rules that estimate the likely road element losses on a given sec­tion subject to a given hazard in terms of an equivalent percent of road length . Risk is estimated by first using rules to mod­ify the recurrence hazard as needed and then as a product of hazard, vulnerability. and net monetary worth appropriate to the first occurrence or subsequent recurrences over the road life .

The hazard and risk diagnoses are made on each 100-m interval along the entire alignment. On completion of these diagnoses. the KBES combines adjacent sections of 100 m that have the same hazard and risk diagnosis . This is the final result that is presented to the user. The user may ask for an explanation of why a particular section has the resulting diag­nosis. The KBES will provide an explanation in terms of the rules that were used to arrive at the diagnosis in question.

Sample User Consultation Session

A small section of the Tulsipur-Salyan road in midwestern Nepal was presented to the KBES for hazard and risk assess­ment. The hazard-related information given to the KBES from 7.0 to 10.0 km is presented in Table 2. A low-cost road standard was specified with a cost of U .S. $140,000 per kil­ometer.

Using various hazard diagnosis rules, the KBES made the hazard diagnosis presented in Table 3. The 100-m unit that was subject to the same hazard has been combined by the KBES so that Table 3 indicates chainage sections subject to different hazards. The risk diagnosis results indicated in Table 4 indicate the most likely amount of loss in terms of monetary value caused by one occurrence of a given hazard sometime

TABLE 3 HAZARD DIAGNOSIS RESULTS

Probability of One Failure

Chainage During Road Extent of (km) Hazard Type Life (20 years) Failure

7.0 to 7.3 Plane failure (rock) 0.5 Small 7.3 to 7.7 Wedge failure 0.9 Medium 7.9 to 8.2 Slump 0 .9 Medium 8.2 to 8.4 Wedge failure 0.7 Small 8.8 to 9.0 Plane failure (rock) 0.9 Large 9.2 to 9.3 Wedge failure 0.6 Small 9.3 to 9.4 Slump 0.7 Small 9.4 to 9.5 Wedge failure 0.9 Medium 9.5 to 9.6 Wedge failure 0.7 Medium 9.6 to 9.7 Wedge failure 0.9 Medium 9.8 to 10.00 Plane failure (rock) 0.9 Large

215

TABLE 4 RISK DIAGNOSIS RESULTS

Chainage Vulnerabilitv Risk (km) (% road len.gth) (U .S.$)

7.0 to 7.3 2.5 524 7.3 to 7.7 2.0 1.006 7.9 to 8.2 3.3 1.248 8.2 to 8.4 2.5 490 8.8 to 9.0 25.0 6.29J 9.2 to 9.3 5.0 420 9.3 to 9.4 5.0 490 9.4 to 9.5 10.0 1.258 9.5 to 9.6 10.0 980 9.6 to 9.7 10.0 1.258 9.8 to 10.0 25.0 6.29J

during the 20-year road life. Two examples of explanations given for the hazard diagnosis results are presented in Table 5 and interpretations of the explanation may also be requested by the user. The explanations shown in Table 5 indicate the reasoning followed by the KBES. The diagnosis was made because various parameters had the values presented in Table 5. The total risk for the 3-km section, because of the certain occurrence of each hazard at least once over the 20-year life, is U.S. $20,260. The sample run can easily be extended to the entire alignment to arrive at total alignment risk .

The example, which has shown the simplest scenario the KBES can analyze, illustrates the improvement made by the KBES over previous methods in that (a) it uses a formal and computerized method, (b) it diagnoses specific failure modes instead of producing a general hazard level, (c) it uses logic to diagnose hazards instead of aggregated weights or ratings. (d) it can diagnose multiple hazards at the same site that the old manual method could not do, (e} it assigns an extent of failure and an explicit probability of failure that were absent from the previous method, (f) it explains why a particular hazard and risk was diagnosed, (g) it operationalizes the con­cept of vulnerability in terms of road standard and corre­sponding design, and (h) it performs hazard assessment in a

TABLE 5 EXPLANATION OF HAZARD DIAGNOSIS

Location: 7.0 to 7.3 km Type of failure: Planar rock slide Extent of failure: Small

Explanation

The slope angle is between 36° and 45°

The dip slope is the natural slope

The alignment is parallel to the strike of beds

More than three joints sets present

The rock is fractured slate

The main boundary thrust is nearby

A medium rock slide is at 7 .2 to 7.3 km

Vegetation is sparse brushes High rate of precipitation is the

main trigger No soil cover Seismically active region

Location: 7.3 to 7.7 km Type of failure: Wedge failure Extent of failure : Medium

The slope angle is between 36° and 45°

The natural slope is not the dip slope

The alignment is perpendicular to the strike beds

More than three joint sets present

The rock is fractured slate

The main boundary thrust is nearby

Vegetation is sparse brushes High rate of precipitation is

the main trigger No soil cover Seismically active region

216

way that enables risk assessment. The KBES can also assess risk when huzard recurrences are of concern and when dif­ferent countermeasures may be taken in the event of a hazard.

CONCLUSION

A KBES. which is currently at the stage of a developmental prototype. has been developed for hazard and risk assessment of low-volume hill roads in Nepal. This KBES, by formally organizing ill-structured hazard and risk assessment knowl­edge. provides a convenient way for nonspecialist practition­ers to incorporate the important considerations into the prac­tice of hill road construction in Nepal. Also. the KBES has contributed a standardized method of integrating hazard and risk assessments to the state-of-the-art of this field in Nepal.

Work is proceeding to expand the knowledge base and to adjust the control strategy accordingly. This work is oriented towards the production of an operational prototype and a commercial level system.

REFERENCES

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TRANSPORT A TIO;\' RESEARCH RECORD 1291

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9. Borland International. Turbo Prolog Owner's Handbook. Scotts Valley, Calif., 1986.

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