Remediation of Slope Failure by Compacted Soil-Cement Fill

Jason Y. Wu, P.E., M.ASCE1; Kaiming Huang2; and Munira Sungkar3

Abstract: Remediation of a complete destroyed slope is often difficult because of various geotechnical challenges and different constraints at the site. Many solutions have been developed; however, not all stabilization methods are appropriate for every type of slope failure. In addition to the technical considerations, aesthetic concerns and sustainability have also become significant issues during the past decades because of global environmental concerns. The principle of multiple-criteria decision analysis (MCDA) will be helpful in the consideration of these alternatives when all criteria are considered simultaneously. In this study, it was concluded that compacted soil-cement would be the best solution for the site after MCDA procedures. This paper presents a detailed report to demonstrate the benefits of utilizing compacted soil- cement for the restoration of slope failure. Traditionally, compacted soil-cement stabilization has been used worldwide for embankment constructions to mitigate the constraints of problematic sites. When it is applied in a slope remediation, the stability of the reconstructed slope can be improved with the increased strength of the cemented material. The experience described herein indicates that slopes treated with compacted soil-cement not only successfully meet all the site criteria, they also display sustainable performance. Compared to slopes constructed with traditional stabilization methods, the slope in this case demonstrates superior performance. Based on field monitoring to date, the slope in the described case continues to maintain its structural stability and safety. The utilization of compacted soil-cement fill is a valuable and sustainable solution, particularly for those sites that have concerns with the disposal of impracticable silty debris waste. DOI: 10.1061/(ASCE)CF.1943-5509.0000998. © 2017 American Society of Civil Engineers. Author keywords: Slope failure; Remediation; Compacted soil-cement; Sustainability.

Introduction environmental protection and sustainable development also have become significant issues because of concerns about extreme Landslides of unsaturated slopes, triggered by rainfall infiltration and climate and global warming. Ways of reducing negative human im- reduction of matric suction, have become a major worldwide concern pact or using eco-friendly solutions are often mandatory for many over the past few decades (Rahardjo et al. 2001; Wu and Wang environment-sensitive sites (Jefferis 2008; Keaton 2014; Kil et al. 2011). The increase in unit weight and loss in strength due to severe 2016). At times, because of other nontechnical influences, the final rainfall have caused many slopes to disintegrate, leading to piles of decision may not even be made by the design engineer. Therefore, debris and rubble at the site. The rehabilitation of such failures is the determination of treatment for a landslide appears to have no difficult, especially for slopes that consist of silty or clayey soils with definitive rule to follow. Experts recommend that the factors to be high water content. Reconstruction of these materials for the repair of considered for the selection of correct remediation include safety, slopes is also difficult because of fines. The removal and disposal of construction scheduling, availability of materials, site accessibility, the collapsed debris can be particularly troublesome because of envi- equipment availability, aesthetics, environmental impact, political ronmental restrictions in the metropolitan areas. issues, and labor considerations (Abramson et al. 2002; Corkum There are many solutions for landslide rehabilitation. Common and Martin 2004; Brencich 2010; Costa and Sagaseta 2010). A principles used are unloading, drainage, reinforcement, retaining multitude of variables are required to develop the best or the most structures, and surface vegetation (Cornforth 2005). However, appropriate solution, but integrating all variables together generally Abramson et al. (2002) and Brencich (2010) indicate that not all results in an ambiguous or controversial solution. One of the more stabilization methods are appropriate for every type of slope failure; effective methods is using the principle of multicriteria decision the most expensive treatment may not always be the most effective, analysis (MCDA), a valuable method in making important deci- and vice versa. Recently, in addition to technical considerations, sions that cannot be easily decided (Sánchez 2005; Sabzi and King 2015; Kil et al. 2016). Downloaded from ascelibrary.org by Iowa State University on 02/21/17. Copyright ASCE. For personal use only; all rights reserved. Compacted soil-cement fill is a highly compacted mixture of 1Professor, Dept. of Civil Engineering, Chung Hua Univ., Hsinchu, Taiwan 30012, Republic of China (corresponding author). E-mail: soil, cement, and water that has been used for decades to meet en- [email protected] gineering purposes for a variety of infrastructures to improve 2Ph.D. Candidate, Dept. of Civil Engineering, Chung Hua Univ., unsuitable subgrade or to modify soft soil properties (Bergado et al. Hsinchu, Taiwan 30012, Republic of China. E-mail: mkm_huang@rrb. 1996; PCA 2016). By compacting a mixture of cement and soil to a gov.tw designated density, both shear strength and durability of the 3 Ph.D. Candidate, Dept. of Civil Engineering, Chung Hua Univ., Hsinchu, material increase substantially. Cement stabilization improves soil Taiwan 30012, Republic of China. E-mail: [email protected] structure by increasing intercluster cementation bonding and reduc- Note. This manuscript was submitted on May 8, 2016; approved on October 17, 2016; published online on February 15, 2017. Discussion ing the pore space (Horpibulsuka et al. 2010). Therefore, it can be period open until July 15, 2017; separate discussions must be submitted used to construct embankments or rebuild failed slopes (Gill and for individual papers. This paper is part of the Journal of Performance Bushell 1992; Abramson et al. 2002). In comparison with other of Constructed Facilities, © ASCE, ISSN 0887-3828. remedial solutions, compacted soil-cement fill presents many

J. Perform. Constr. Facil., -1--1 Fig. 1. Plan view of the sliding area (adapted from Wu et al. 2016, © ASCE)

advantages. It can use on-site materials and therefore reduces the Site Description need for debris disposal. It does not need a retaining wall and thus provides more space and a more attractive appearance. It eliminates As shown in Fig. 1, the site is located in the eastern vicinity of Hsinchu City near Baoshan Reservoir in Taiwan. The hill slope excess moisture problems and promotes easier workability for silty is located at the northwest wing of a residential community. A soils. Its cost and construction time are also competitive compared 4-m-wide access road alongside a curved side hill alignment to other solutions. and a 6-m-high concrete retaining wall along the road were at This study aims to present a solution by using compacted soil- the toe of the slope. A landslide occurred during a strong typhoon cement to successfully rehabilitate a landslide in a congested res- when torrential rainfall hit Taiwan. The slope failure was approx- idential area. Associated with the major study objectives are the 2 imately 2,750 m in plan dimensions, with a height of approxi- remedial measures undertaken for the treatment, such as construc- mately 27 m from the toe to the top of the head scarp. The tion procedures and environmental concerns. The experiences pre- landslide resulted in three to four slumped terraces that caused a sented herein can be beneficial and valuable for cases with similar complete collapse of the upper and lower retaining walls (Figs. 2 conditions. and 3). The slide debris also buried the access road, rendering it completely inaccessible to local traffic. The lower margin of the displaced material just barely stopped in front of several residential Downloaded from ascelibrary.org by Iowa State University on 02/21/17. Copyright ASCE. For personal use only; all rights reserved.

Fig. 2. Collapsed lower slope and retaining wall (reprinted from Wu et al. 2016, © ASCE) Fig. 3. Collapsed upper slope and retaining wall (image by Jason Y.Wu)

J. Perform. Constr. Facil., -1--1 houses. Residents were evacuated to prevent any further life- Laboratory Testing threatening conditions. Laboratory test results indicated that the silty sand contained plastic As an immediate short-term response, the area around the head fines varying from 36 to 43%. Material based on the Unified Soil scarp was covered with a plastic vinyl sheet to minimize ingress of Classification System (ASTM D2487), the sand can therefore be surface water into cracks in the slide area. Several emergency classified as SC-SM (silty clayey sand) material (ASTM 2011a). ditches were installed to divert the surface water from the slide Field water contents ranged from a low of 15.3% to a high of location. 32.6% at the surface. Moisture unit weight varied from 19.6 to 20.5 kN=m3. The quick direct shear strength parameters for samples obtained at depths of 5–9 m ranged from 13 to 39 kPa for Geotechnical Evaluations cohesion and 17–19° for angle of shearing resistance.

Site Investigation Slide Assessment The site is composed of interbedded sandstone, siltstone, conglom- Back calculations were conducted using the computer program erate, and mudstone, typically known as Yangmei formation. GEO 5 and the information gathered from the site investigation Although there are some small facies changes along the direction and laboratory testing. GEO 5 is a software suite providing solu- of the bedding surface, the poorly cemented yellowish sandstone tions for a majority of geotechnical tasks, such as slope stability, and its decomposed fragments are the dominant materials in the settlement, deep excavation, and tunnels (Fine Software 2016). The area. The failure of slopes with similar geological background stability of the slope was analyzed using circular slip surface and in these areas is commonly found in history. automatic search method. The routine procedure includes compar- For remediation, a site investigation program was conducted to ing a number of admissible surfaces that are basically selected by explore the subsurface conditions at the site. It consisted of three random searches. The program continues to compare the factor of borings and four test pits. Based on the subsoil information ob- safety (FOS) by varying the radius of different potential slip surface tained, the site materials comprise yellowish to brownish silty fine until the minimum FOS has been found. sand with alternate gravel-rich layers. The stratum was found at the The FOSs obtained were 1.32, 1.11, and 0.98 for normal, earth- surface and extends to a depth of 12 m, where the borings were quake, and storm conditions, respectively. The marginal FOS and terminated. This material was loose when encountered at the sur- the observed ruptured retaining wall verified that the landslide was face. It then became medium to very dense with depth. The stan- most likely initiated because of the instability of the retaining wall dard penetration blow counts (N values) ranged from 5 to 11 at the under storm conditions. The possible slip surface also indicated the surface and gradually increased with depth to over 50 for depths sliding was predominantly due to a weak surficial layer on the slope below 9 m. Probing tests with N values recorded showed the (Fig. 4). It was apparently caused by the severe surface water depths of the probable slipped surface ranged from 0.5 m at infiltration. The abundance of fines in the site material held up the crest to 3−4 m near the toe. Ground water levels measured the movement of ground water. Observations made at the site in the borings were as high as 0.47 m below the surface. Seepage also found the weep holes of the retaining wall were maintained was also observed migrating along the side of the slope at the toe. so poorly that the ground water could not drain off easily. The Based on visual observation of the site and the interpretations of failure was concluded to be a typical storm-generated shallow the boring information, the surface weakness is believed to be the translation landslide that is very common in the Hsinchu area. effect of rainfall saturation and poor performance of the drainage Hill slopes in this region can become unstable under near-saturation system. conditions as the wetting front reaches the critical shear plane. Downloaded from ascelibrary.org by Iowa State University on 02/21/17. Copyright ASCE. For personal use only; all rights reserved.

Fig. 4. Design scheme of the remedial solution (adapted from Wu et al. 2016, © ASCE)

J. Perform. Constr. Facil., -1--1 Fig. 5. Typical wetting band for torrential rainfall induced shallow landslide in homogeneous soils (data from Savavedra 2016)

Ekanayake and Phillips (2002) indicated that the main reason for such slope failures under near-saturation conditions is most likely the rapid loss of soil strength during the saturation process. Savavedra (2016) studied the patterns of wetting bands for torrential rainfall–induced shallow landslide and, as shown in Fig. 5, for an unsaturated slope subjected to storm conditions. The seepage tends to accumulate and porewater pressure increases near the toe area with time; it therefore easily triggered the slope failure.

Development of Remedial Solutions

Sustainability Considerations Sustainable development has become an important issue after the release of Brundtland’s Report by the World Commission on Environment and Development, often referred to as Our Common Future. During the past 20 years, sustainability has become very topical and popular and has passed through an intensively discussed process of development. It is being recognized as an important performance measure that should be considered in the design, construction, and long-term operation and maintenance of civil infrastructure systems (Bocchini et al. 2014; Lounis and McAllister 2016). Pearce and Ahn (2012) recommended two main goals to

Downloaded from ascelibrary.org by Iowa State University on 02/21/17. Copyright ASCE. For personal use only; all rights reserved. sustainable development: (1) enable people to meet essential needs and improve their standard of living and (2) ensure the natural re- sources and systems on which people depend are maintained and advanced for both current and future use (Shillaber et al. 2016). However, current implementations for sustainable development often mislead the engineers into focusing purely on conserving the natural environment. In reality, achieving the goal of sustainability should be a compromise among environment, society, and economy (Parkin 2000; Parkin et al. 2003). Emphasizing any one of these dimensions without consideration for the others can lead to unsustainable designs. Shillaber et al. (2016) considered Fig. 6. Definition of sustainability: (a) strong sustainability; (b) weak the types of sustainability based on the correlations of environment, sustainability (reprinted from Shillaber et al. 2016, © ASCE) society, and economy as depicted in Fig. 6. Strong sustainability

J. Perform. Constr. Facil., -1--1 Table 1. Comparison of Remedial Solutions Evaluation items (and weight of each item) Safety Means of assurance Workability Economy Time Sustainability weighted rating Solution Proposed solutions (30%) (25%) (10%) (15%) (20%) (1.00) preference Cut and fill 4 1 5 3 1 0.52 IV Reinforced earth wall 4 1 3 2 1 0.45 V Compacted soil-cement 5 4 3 3 4 0.81 I Structural piles 5 3 1 1 2 0.58 II Retaining walls 3 2 3 2 3 0.53 III Note: An ordinal rating scale of 1 (worst) to 5 (best) was used to indicate the preference of each proposed solution for the site.

emphasizes that society and economy depend on the environment therefore not on the list for evaluation. The decision making and their growth is limited by the environmental carrying capacity. involved consideration of the alternatives described in terms of Weak sustainability considers the three dimensions in environment, evaluative criteria. The task was to find the best alternative, or society, and economy in separate, overlapping realms; compromise to determine the relative total priority of each alternative when exists when two or fewer dimensions are considered. all the criteria were considered simultaneously. Landslide restorations often involve significant modifications As can be seen in Table 1, an ordinal rating scale of 1 (worst) of natural landforms and hydrogeological conditions and uses of to 5 (best) was used to indicate the preference of each proposed large quantities of materials and energy and therefore present op- scheme for the site based on each variable considered. The final portunities for meaningful reductions in environmental impact. The matrix of relative preferences had to be agreed upon by all members geotechnical engineer should take major responsibility in the selec- of the committee. The individual weighted rating of each proposed tion of different design and construction alternatives by carefully solution per each variable considered could then be calculated us- considering sustainability issues (Shillaber et al. 2016). ing its preference times the weight of that particular variable. For example, the individual weighted rating of Cut and Fill for the safety assurance concern was 4 × 30% ¼ 1.2. Taking the maxi- Multiple-Criteria Decision Analysis mum weighted rating of 5 into consideration, the mean of the The landslide had significantly jeopardized the residential area and weighted rating for each of the proposed solutions can then be cal- the access road. Remediation needed to be conducted immediately culated as the arithmetic mean of the row. For example, the mean of to ensure public safety and for the normal life of the community to the weighted rating of Cut and Fill can be calculated as shown in resume. Examinations of the failed slope indicated that the slide Eq. (1). The preference of all proposed solutions can then be easily mass in its present configuration was minimally stable with only quantitatively determined, and compacted soil-cement was deter- a small margin of safety. mined as the best scheme for this particular site After several emergency meetings, the managing committee of ð4 × 30% þ 1 × 30% þ 5 × 10% þ 3 × 10% þ 1 × 20%Þ=5 ¼ 0 5 the community decided to engage professional experts as a task . force committee to be responsible for the remediation. It consisted ð1Þ of two designers, two independent experts, and one resident. The designers represented a prestigious private consulting firm directly The designer initially proposed using the simplest cut and fill hired by the community. The experts were recommended by the method, which shifted weight from the top of the slope to the national geotechnical society based on their professional back- toe to improve the stability of the slope. However, this method ground. They all have at least 15 years experiences in the area was ruled out because of the typically poor response of silty soil of slope safety. The only nontechnical member was the resident to soil compaction. Other treatments, such as retaining walls and who played the role of task manager to monitor the operation of reinforced earth walls, were also rated unfavorably because of the the task force and also served as a coordinator to ensure good com- difficulty in achieving compaction with the saturated site material. munications between the committee and the community. Structural drilled piles and compacted soil-cement were then con- The decision making for the development of a suitable remedial sidered for relatively easier construction. However, installation of solution would be difficult for this site. The committee thus recom- piles apparently requires much more time and cost compared to mended applying the MCDA approach for this project, and other solutions. the most important items for evaluation were determined based A limited time frame was available for the design and construc-

Downloaded from ascelibrary.org by Iowa State University on 02/21/17. Copyright ASCE. For personal use only; all rights reserved. on particular considerations for the site. They were: safety assur- tion of treatment for the failed slope. The local community needed ance, workability, economy, time, and sustainability (aesthetic and to quickly regain passage through the access road. The potential of environmental constraints such as vibration, noise, waste dumps). further sliding was also heightened by the weather forecast of an- Then, the committee further weighted each variable based on its other incoming typhoon to the area. In addition to the schedule con- significance to the residents of the community (Table 1). For ex- straints, there were additional sustainability considerations imposed ample, safety of the completed slope was the priority concern for on the treatment proposal. The traffic patterns and the adjacent res- everyone at the site, and it was therefore rated as high as 30%. Life- idential buildings precluded the use of heavy and noisy equipment. cycle cost was relatively less important; therefore, it was rated as The disposal of construction waste materials would also cause only 10%. unacceptable environmental impact. Furthermore, the residents re- Almost all available slope remediation methods on the market quested that the slope treatment be functional and economical in had been discussed, and the five most probable solutions were se- addition to ecologically sustainable and aesthetically pleasing. lected for final evaluations. The use of a drainage system and sur- As described earlier, multiple methods of slope stabilization are face vegetation are common additions for all these schemes and applicable to any given landslide, and the one chosen to be used

J. Perform. Constr. Facil., -1--1 should be the most economical and effective in addressing the prob- to counterbalance the driving force of the slope. An additional able needs of the particular site. soil-nailing system installed at the upper and lower slopes provided Based on the previous discussion, although MCDA may not be further reinforcement. To enhance the sliding resistance of the persuasive from the academic perspective, it compared much better toe berm, a shear key, 1 m deep and 2 m wide and using the same than the traditional solution that was predominantly developed by compacted soil-cement, was built underneath the toe berm. The designers only. The MCDA procedures described in this case suc- main purpose of a shear key is to force the critical slip circle deeper, cessfully developed a specific solution for the site in a relatively thereby increasing the resistance along the slip surface. The hori- short time for an emergency slope remediation. The task force man- zontal and cutoff drainage system was installed to effectively inter- dated the designers give more attention to elaborating on their sol- cept and safely divert surface and groundwater. ution that completely met the needs of the project. Design of Compacted Soil-Cement Analysis and Design The shear strength of the compacted soil-cement increases with the increase of cement in the soil. The mix design is determined in Design of Slope Stabilization Scheme the laboratory for a given soil and desired strength. Review of case histories and laboratory test results indicates that the mix Based on the results of MCDA and a careful evaluation of all the designs used for slope stabilization generally consist of 1 to constraints on site, the solution of compacted soil-cement fill, com- 10% cement by weight of the soil to produce cohesive strength bined with soil nailing and horizontal drainage, was selected for the of 172 to greater than 862 kPa (Abramson et al. 2002). Other stud- rehabilitation. Treatment with cement has been mainly used in ies for the mechanical behavior of soil-cement mixtures also found highway, railroad, and airport construction to improve the mechani- similar results (Mun et al. 2012; Yilmaz and Ozaydin 2013). cal properties of the bearing layers, and has only more recently The properties of compacted soil-cement that would produce been examined as a viable type of soil reinforcement (Gill and the desired strength for the site should be determined before Bushell 1992; Diyaljee et al. 2000). For the case described herein, construction. However, because the remediation had to be com- the shear strength and the workability of the site material would be pleted promptly, only limited experiments were conducted for significantly improved with the addition of cement. Therefore, the the purpose of verification. Experiences with compacted soil- treatment of the soil with cement would eliminate the difficulty of cement with similar site material at a nearby highway subgrade working with silty soils with high water content. A simple unload- improvement suggested a mix design containing 8–9% cement ing method was also presented to greatly reduce the driving force of by weight of the sand at a density greater than 90% of the maxi- the slope. mum standard Proctor density (ASTM D698) for this project To further ensure the slope’s additional safety, soil nails were (ASTM 2008). The working moisture content used was 11−19%, recommended to install on the upper part and the toe of the slope. depending on the site condition. The observed values of 28-day A soil-nailed toe berm with sufficient weight and strength would compression strength ranged from 2,605 to 2,750 kPa with an counterbalance the driving force of the slope, further enhancing the in-place dry density of 16.5–17.9 kN=m3. safety and stability of the slope (Chu and Yin 2003; Corkum and To verify the engineering properties of the compacted soil- Martin 2004). The soil nailing technique was chosen in this project cement mixtures, moisture-density compaction tests and uncon- because of its ease and speed of construction, minimal preparation fined compression strength (UCS) tests were performed following requirements, and decreased traffic disruption during construction. the ASTM D558 and ASTM D1633 procedures, respectively Based on the site history and evidence of seepage on the side slope, (ASTM 2007, 2011b). Type I portland cement (PC), the most uni- water was a major concern for the future stability of the rehabili- versal stabilizing agent, was used in the study. Based on the pre- tated slope. The installation of an integrated drainage system was vious, a mixture was prepared using 8.6% cement by weight of the included to eliminate any future deleterious effects of excess water. sand with a moisture content of 19%. The sample was compacted to The proposed system included surface cutoff trenches, intercept a dry density of 17.9 kN=m3, approximately 92% of the maximum underdrains, and slope horizontal drains. standard Proctor density. Results of UCS tests showed that samples The stability of the proposed scheme was examined in detail by with such a design mix would achieve strengths of 1,075, 1,565, computer analyses during its design stage. The computer program and 2,672 kPa following curing periods of 1, 7, and 28 days, re- again searched the most critical circle of potential failure plane by spectively. The design mix and the observed UCS values agree well varying the radius of each trial center point until a critical radius, with those found by others. corresponding to the minimum FOS for center, had been found. In general, geotechnical engineers are allowed to consider com- The shear strength required along a potential failure surface just pacted soil-cement a hard soil and take 50% of the UCS value as the to maintain stability was calculated and then compared to the mag- cohesive strength for stability analysis. However, the toe berm has been designed as a solidified structure subjected to shear and flexu-

Downloaded from ascelibrary.org by Iowa State University on 02/21/17. Copyright ASCE. For personal use only; all rights reserved. nitude of available strength. Because the compacted soil-cement structure and soil nails presented a much higher strength, the po- ral stress. It should be considered a concrete-like member, and its tential slip surface therefore extended backward until it moved be- shear strength should be determined based on ACI 318 (ACI 2011) yond the margin of the shear key and soil nails where the minimum as shown in Eq. (2) FOS was found (Fig. 4). The estimated FOSs for the final design pffiffiffiffiffi scheme were 1.82, 1.55, and 1.47 for normal, earthquake, and 0 vc ¼ 0.17 fc ð2Þ storm conditions, respectively. Although the results appeared to be conservative, the task force agreed that the remediation must 0 be strong enough to ensure future safety. where fc (MPa) = 28-day unconfined compression strength. Fig. 4 indicates the final design scheme for the rehabilitation of To account for the possible site uncertainties, the engineers fi- the slope. The weight of the slope was reduced by shaping it to an nally designed a conservative cohesive strength of 250 kPa for the angle of 35° and cutting it into three tiers, with each tier shifted toe berm analysis. Table 2 shows the design parameters for slope backward by 2 m. A compacted soil-cement toe berm was used stability analysis.

J. Perform. Constr. Facil., -1--1 Table 2. Design Parameters for Slope Stability Analysis Moisture Angle of shearing unit weight Cohesion resistance Soil type (kN=m3) (kPa) (degrees) Silty sand 19.6 39 19 Compacted soil-cement 21.3 250 0

Remediation Construction

Construction of the toe berm was initiated with the demolition of the collapsed concrete retaining wall. As described earlier, the disposal of construction waste was restricted to the site. Therefore, the demolished concrete was crushed and sieved to 25–75-mm pieces, which were then wrapped in pieces of geofabric to serve as an intercept underdrain behind the compacted soil-cement toe berm. To accommodate the constraints of the construction site, the size and quantity of construction equipment were limited. The on-site Fig. 8. Compaction of the soil-cement toe berm (reprinted from Wu construction of soil-cement fill was accomplished using only two et al. 2016, © ASCE) medium-sized PC-200 backhoes. The excavated soil from the shaping of the slope was placed in a steel batch container 8 m3 in vol- ume. As seen in Fig. 7, bulk cement was then loaded in and thoroughly mixed by backhoe. Additional water was unnecessary because field moisture content measured 23.8–32.6%. The mixed soil-cement was then transported and spread in layers at the fill location by backhoe. Placement of the soil-cement fill was limited to a loose thickness of 30 cm per layer. As shown in Fig. 8, the mixed materials were then compacted with kneading efforts applied by the backhoe bucket. Conventional roller compaction was not allowed in this case because of site constraints. Galvanized soil nails, each 9 m long, were installed upon completion of the toe berm (Fig. 9). The upper slope was covered with an erosion protection system using welded wire mesh and coconut matting. Additional short soil nails, 3 m long, were installed to minimize any possible surface movement of the upper slope (Fig. 10). A cutoff trench system made of high-density polyethylene (HDPE) was installed to intercept the surface water. Soil nails and HDPE trenches were selected predominantly because they permit fast and easy construction. During construction, the properties of the compacted soil- Fig. 9. Installation of soil nails at the toe berm (image by Jason Y. Wu) cement were constantly monitored at the site. The moisture content Downloaded from ascelibrary.org by Iowa State University on 02/21/17. Copyright ASCE. For personal use only; all rights reserved.

Fig. 7. Mixing of the soil-cement fill on site (reprinted from Wu et al. Fig. 10. Welded wire mesh and coconut matting surface protection at 2016, © ASCE) the upper slope (image by Jason Y. Wu)

J. Perform. Constr. Facil., -1--1 economic, and environmental factors, and by constraints imposed by the site conditions. MCDA procedures were very helpful in the consideration of these alternatives when all the criteria were evalu- ated simultaneously. The stabilization of landslides with compacted soil-cement is not common. However, the case study described herein has demonstrated that this solution meets all the site criteria. Among all possible alternatives, it presented the least environmental impact, the best public safety, and an acceptable construction performance. Compacted soil-cement fills are a valuable and sustainable solution, particularly for sites that have issues with the disposal of sliding debris.

References

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