Advances in Ground Improvement: Research to Practice in the United States and China

The second Strategic Highway Research Program (SHRP2) was created by the U.S. Congress to address challenges of moving people and goods efficiently and safely on the nation's highways. Geotechnical transportation issues are addressed under the SHPR2 Renewal Focus Area, in which the goal is to develop a consistent, systematic approach to the conduct of highway renewal that is (1) rapid, (2) causes minimal disruption, and (3) produces long-lived facilities. The SHRP2 R02 project is aimed at identifying geotechnical solutions for three elements: (1) construction of new embankments and roadways over unstable soils, (2) widening and expansion of existing roadways and embankments, and (3) stabilization of the working platform. This paper reports the results of the first phase of the project, in which the research team identified 47 ground improvement technologies and processes with application to the three project elements and 32 technical issues, project development pros and cons, and non-technical issues affecting use of the technologies.

Conventional column-type reinforcement elements have been widely used for ground improvement and they can be classified into three categories: (1) sand column and stone column; (2) deep mixed column, lime column, and jet grouted column; and (3) concrete solid piles, concrete pipe piles, and steel pipe piles. The column-type reinforcement elements of different categories exhibit different behavior and failure modes. Recently, new and composite column-type reinforcement elements have been developed, which combine two or three types of conventional column-type reinforcement elements into one column. The composite column is formed by installing a plain concrete, reinforced concrete, or steel pile of a small diameter in the center of the first or second type column to a desired depth. This composite column has much higher bearing and bending capacities than the conventional column-type reinforcement element in the first and second categories. The cost of the new type columns is competitive considering their higher capacities as compared with the conventional columns. This paper summarizes experimental studies and case histories on the behavior and performance of these composite columns used in China.

Pile-supported embankments are increasingly being used for highways, railways, storage tanks, etc. over soft soil. In this paper, a closed-form solution for the one-dimensional loading was presented taking into consideration of the soil arching in the embankment fill, the negative skin friction along the pile shaft, and the settlement of the foundation soil. A case of pile-supported embankments was provided. Field monitored data from contact pressures acting on the piles and the soils, to the settlements of the piles and the soils were reported and discussed. The available design standards and approaches were verified by the case.

Columns in conjunction with or without geosynthetic reinforcement have been used to support embankments over soft clay worldwide. Columns can enhance both load transfer and excess pore water pressure dissipation. So far, no analytical solution is available to evaluate these two effects. Coupled mechanical and hydraulic modeling often becomes one feasible method to analyze this problem. Coupled modeling involves considerable computation time, especially under three-dimensional (3D) situations. To save computation time, the possibility of converting a three-dimensional (3D) problem into a two-dimensional (2D) one is appealing to practice. Two approaches (property and geometry conversions) were adopted and examined in this study to simplify a 3D problem of column-supported embankments into a 2D plane-strain one. A constructed bridge approach embankment supported by deep mixed (DM) columns and reinforced by one layer of geosynthetic was adopted for this examination. The numerical results are compared with the field data to examine the validity of these two simplification approaches. Overall, the geometry conversion approach is better than the property approach to simulate the behavior of the column-supported embankment.

Pile-supported earth platforms provide an economic and effective solution for embankments constructed on soft soils, especially when rapid construction or strict control over deformation of the embankments is required. A system with a piled foundation of equal lengths is referred to as the conventional pile-supported earth platform while the system with non-uniform piles having dissimilar lengths is referred to as the composite pile-supported earth platform. This paper focuses on the numerical investigations of performance and the mechanism of this new type of pile-supported earth platform; particularly the influences of the height of the embankment fill, the capping ratio as well as the length of long and short piles on the settlements, stress concentration, and the efficacy are addressed. Numerical results suggest that the new pile-supported earth platform offer economical design compared to the traditional one and meet the requirement for the bearing capability and settlement.

Three types of numerical analyses were performed to investigate net vertical pressures on, deflections of, and strains in geosynthetic reinforcement in column-supported embankments. The analysis types are 3D membrane analysis, 3D cable-net analysis, and axisymmetric membrane analysis. For all three analysis types, the vertical pressures applied on top of the geosynthetic were higher in the areas over pile caps than in areas underlain by soil to represent arching effects in the embankment. Foundation support was represented by springs with high stiffness at the pile cap locations and lower stiffness to represent the soil between pile caps. Square, diamond, and round pile caps were investigated. Geosynthetic strand orientations parallel and diagonal to the pile cap layout were studied. The analyses showed that peaks in geosynthetic strain occur at corners of square and diamond pile caps and at edges of round pile caps, with the peak strains smaller for round pile caps than for other shapes at the same area replacement ratio. Parametric studies showed that maximum deflections and strains increase with increasing vertical stress on the geosynthetic and with decreasing area replacement ratio, soil stiffness, and geosynthetic stiffness.

Finite difference numerical method was adopted to analyze the contribution of piles to the overall stability of pile supported embankments. The stability analysis of a stable pile supported embankment was first performed and the lateral behavior of piles at different locations with respect to the embankment was analyzed. Then by applying a uniform load on the top of the embankment, the pile lateral behavior and possible failure modes were analyzed and compared for piled embankments with different factors of safety. Based on the numerical results, the piles and the soils under and outside the embankment were divided into three zones, namely the extension-bending zone, the shear zone, and the compression-bending zone. The piles located in these three zones exhibit different behavior and have different anti-slip mechanisms. Consequently, the piles at different locations may have different failure modes at the failure of the embankment and have different contributions to the stability of the piled embankment.

Geosynthetic-reinforced pile-supported embankments have been increasingly used as a rapid construction technique to reduce total and differential settlements and enhance load transfer from soil to piles for embankments over soft soils. Most studies have been done so far to investigate this embankment system under static loads. However, highway and railway embankments are subjected to cyclic loading. The behavior of this embankment system under cyclic loading is not well known, therefore, it deserves further research. In this study, the discrete element method incorporated in the Particle Flow Code (PFC) 2D was adopted to investigate the movement of soil particles above pile caps and the geogrid layer. A cyclic load was applied on the top of the assembly and the contact stresses and the tension in the geogrid under cyclic loading were monitored and analyzed. The DEM analysis shows that the geogrid reduced the permanent deformation under cyclic loading and increased the stress concentration ratio as compared with the unreinforced embankment. The flexural stiffness of the geogrid influenced the magnitude and distribution of the tension along the geogrid.

This paper presents an approach based on Reliability-Based Design (RBD) to evaluate area replacement ratios (percent coverage) of columns in a multi-column composite foundation. A geostatistical technique is implemented to estimate the autocorrelation distance of foundation soil along the vertical direction. Instead of using differential and simulation processes to estimated reliability indices from the nonlinear and complex limit state equations, the Immune Algorithm method is introduced as a new and effective technique to provide a more accurate solution to optimization problems. Reliability analyses of both bearing capacity and settlement of multi-column composite foundations are performed to obtain relationships between reliability indices and area replacement ratios of columns. Based on previous studies, a target reliability index of 3.0 is selected as a criterion to estimate area replacement ratios of composite foundations.

The behavior of the composite ground under a flexible foundation is a complicated problem, which includes the interactions among the flexible foundation, the cushion, the composite ground, and the underlying soils. A simplified analytical model is presented, in which the flexible foundation-cushion-composite ground-underlying soil is considered as an interactive system, and two-dimensional deformation is assumed. According to the compatibility of stress and deformation on the interfaces between these four components, and considering the relative movement between piles and soils, the formulas on settlement and stress ratio of pile to soil, which is used to characterize the behavior of the composite ground, are derived.

The composite foundation supported by piles with various stiffness is a recently developed ground improvement method for resisting vertical load in the thick soft soils. In this composite foundation system, the flexible piles are designed to strengthen shallow soft soil, while the rigid piles are used to reduce settlement of the foundation system. A flexible cushion below the rigid foundation cap is used to redistribute the external load and to adjust stress levels acting on the piles and surrounding soils. The soft soil underneath the cushion can also resist a portion of the applied load. As a result, the composite foundation system can resist much higher load with less settlement. Three-dimensional elasto-plastic finite element analyses are conducted to investigate the performance of the cushioned composite foundation system with a combination of both rigid and flexible piles. The effects of several factors, such as, stiffness of the flexible pile, length of rigid pile, stiffness of cushion, that might affect the consolidation behavior of the composite foundation are studied by using the commercial software ABAQUS. The numerical investigation results indicate that there are optimized values existed among these factors. The loading induced pore pressure in the soft subsoil has minimal influence on the stiffness of flexible piles. Whereas, the settlement of composite foundation can be adjusted by changing the length of rigid piles.

When embankments are constructed on soft ground, they may encounter settlement and/or stability problems due to low strength and high compressibility of the soft ground. To overcome these difficulties, a controlled modulus column method can be used for the construction of embankments on soft soils. This technology takes advantages of various ground improvement concepts, such as densification, reinforcement, solidification, etc. An experimental study was conducted in the laboratory to investigate the behavior of the controlled modulus columns.

The construction of a large diameter thin-wall cast-in-situ concrete pipe (PCC) pile for ground improvement for embankment construction in Yan-Tong Expressway in China is presented in this paper. PCC piling is a new piling technique which combines the advantages of a few techniques, including prestressed concrete pile, the vibratory drilled pile, and the vibro-piling cased curtain. A PCC pile has an outer diameter of approximately 1 m with a wall thickness about 0.1 m. The construction parameters including the slump, casing pulling speeding, and piling sequence were studied in the field. The possible quality problems are also discussed in the paper followed by preventive measures and solutions.

The effectiveness of Deep Soil Mixing (DSM) treatment method was evaluated in terms of reducing heave movements of expansive subsoils underlying pavements and earth structures. Laboratory mix design study results were used to select a combined binder treatment approach utilizing lime and cement binders. DSM columns were designed and constructed in two pilot scale test sections. Surcharge equivalent to loads from base and surface layers was placed on top of these DSM treated sections through a fill placement. The two pilot test sections along with control sections on untreated soils were instrumented and monitored for period of two years. Monitored results showed that soil shrink-swell related movements in both vertical and lateral directions were lesser than those recorded in the untreated soil sections. Overall, this research showed the potential of DSM treatment method to minimize shrink-swell related movements from expansive subsoils of moderate depths.

Deep mixing method is widely used for soft soil improvement in highway construction in China. Most of past studies have been mainly investigating mechanical properties of soil-cement, while its uniformity has not been well addressed. This study investigated the uniformity of deep mixed columns with electrical resistivity method. A series of electrical resistivity tests, unconfined compressive strength tests, and standard penetration tests were carried out on deep mixed columns. The uniformity difference between single slurry mixed columns and double slurry mixed columns is compared. The results indicate that the electrical resistivity and its derived parameters, such as the formation factor, the shape factor, and the anisotropy factor are effective indicators of the uniformity of deep mixed columns. It is also shown that the double slurry mixing method can significantly improve the mixing uniformity of columns than the single slurry mixing method.

Quality control of ground improvement is a difficult but very important aspect of liquefaction mitigation construction. In this paper, an approach to evaluate the improvement level in liquefiable soils treated by stone columns was developed based on the liquefaction resistance-shear wave velocity-void ratio correlations of sandy soils. According to this method, the required level of ground improvement is supposed to be obtained once the target velocity is reached for a given earthquake magnitude, and this requirement will transfer to the void ratio control during stone column installation. A case study using vibro-stone column is introduced, where field tests including seismic testing (SASW) and SPT were performed before and after ground treatment, and the effectiveness of stone columns for liquefaction mitigation was properly evaluated by shear wave velocity. The high consistency between V_s-based and SPT-N value-based evaluations indicates that shear wave velocity could be used to develop criteria for ground improvement needed to mitigate liquefaction.

Pre-fabricated vertical (wick) drains have been used in conjunction with stone columns to improve treatment effectiveness in sands with high fines content. However, no comparison testing has been performed with and without drains. In this study, side by side comparisons demonstrated improved performance of stone columns with drains relative to that without drains. Despite the presence of wick drains, improvement effectiveness still decreased as the fines content increased. The average increase in SPT (N₁)₆₀ was 114% in a zone with an average of 31% fines, but decreased to about 70% in a zone with an average fines content of 43%. Treatment effectiveness was often minimal in layers containing 15% or more clay sized particles. Significant increases in SPT values were observed with time after treatment in the test areas with drains, but little improvement with time was observed in areas without drains. Increased penetration resistance was similar to that observed at other stone column projects where similar soils were encountered and wick drains were also used.

This paper presents a case history on the efficacy of Rodin Jet Grout (RJP) construction in the soft deposit of Shanghai. The RJP columns were employed to improve the soft deposit in a deep excavated pit. The excavation depth was about 43 m. Jet grouting reached to the depth of 50 m, in which the soil types varied from soft clay, silt, stiff silty clay (SSC), and sand. Field construction was performed with different construction parameters such as jetting pressure, nozzle diameter, and retrieving speed. The diameter and mixing uniformity of the columns was measured and checked layer by layer during the excavation of the pit. The results showed that except for the layer SSC, the column diameter is over 1.8 m with the largest diameter of 3.3 m in soft clay layer at a depth of 15.3 m. In cohesive soil layer, mixing quality is good. However, in cohesiveless soil, although the eroding range reached 2.0, the soil and cement was separated and mixing uniformity was poor.

With the fast development of subways, tunnels, and high-rise buildings in Shanghai in the past 10 years, jet grouting technology has been widely used in various projects which also further promote its development. In addition to the single, double and triple pipe methods, an engineering method similar to RJPI has been used. The treatment depth has increased to approximately 50m from 20–30m. The main objectives of using jet grouting technology in the above projects are: foundation reinforcement and formation of water insulation curtain. Since soil layers in Shanghai are soft, adopting jet grouting technology to form vertical or horizontal water-resisting bodies of different depths and thicknesses has become an important means to achieve this purpose. In combination with the actual examples of application, this paper presents the status of jet grouting applications in Shanghai area in the past 10 years and looks into the prospects for technological developments in the future.

Rigid pile composite ground is a type of artificially improved ground widely used for high rise buildings in northern China. Its seismic behavior is of concern to many engineers. This paper summarizes the study carried out by the author's group. Both shaking table tests and dynamic finite element analyses were carried out to investigate the seismic behavior of this type of composite ground. It is shown that the earthquake resistance capacity of this type of composite ground is sufficient for usual situations. However, for buildings of more than 30 storeys on layered soils with larger difference in modulus there can be damage to the piles under strong earthquakes. In addition, a simplified calculation method is proposed to analyze the seismic response of the piles, and its applicability is validated through a number of comparative calculations.

This paper presents a post-grouting technology specifically developed for slurry bored piles, including its principles, strengthening mechanism, empirical formula, and a case study. This technology was successfully used for more than 18000 bored piles in the Terminal 3 project of the Beijing Capital International Airport. Static load test results from 72 preliminary test piles installed in 14 test zones in the T3 site area are summarized and analyzed in this paper. The results demonstrate that post-grouting significantly improve the settlement behavior and bearing capacity of bored piles.

A new warehouse building was planned for Home Depot in Camarillo, California. The geotechnical investigation at the site indicated the liquefaction hazard in loose sandy soil layers under the design earthquake and static settlement problems in soft clay layers under the building loads. The specialty geotechnical contractor designed and performed soil improvement at the site. Vibro stone columns were installed to mitigate the site liquefaction by providing densification, reinforcement, and drainage in the loose sand soil layers. Deep soil mixing columns were installed to support heavy building column loads. Liquefaction analyses were performed on the CPT records before and after the vibro stone column treatment. FLAC analysis on the soil mixing treatment was performed to optimize the mixing column penetration depth. Full size static load tests on soil mixing column groups proved that the FLAC analysis correctly simulated the site condition. This paper covers design methodology, production work, and quality control and testing, with supporting figures.

This manuscript provides insight into a new approach to chemically-based soil improvement with organosilanes (OS). In particular, OS creates a hydrophobic surface on virtually any silica-based material through covalent bonding. In contrast to ion exchange techniques, grafting OS on soils results in near permanent modification. As an illustration, laboratory testing was conducted to evaluate the influence of OS modification on the compaction, strength, swell, erosive and hydraulic properties of several soils. OS modification resulted in modest changes to strength and swell potential and a dramatic reduction in infiltration capacity. Likewise, use of OS on a 2H:1V slope reduced the mass of eroded soil by a factor of nearly 50. Overall, these results suggest that OS modification may have wide application in geotechnical and geoenvironmental engineering.

Large amount of dregs has been generated during the manufacturing of pure alkali in Tianjin Alkali Works. The urban environment is seriously affected by this material. So studies on the behavior and the utilization of the alkali residues and its influence on the environment were conducted by using simulated tests in laboratory and field as technical measures. Meanwhile, the influences on environment are evaluated through specific projects. This research shows that alkali residue may be used as soil of engineer. Not only to be used in large scale for filling low land areas and beach zones in Tanggu, but also to be used to improve the ecological environment. This utilization would solve the problem associated with the storage of alkali residue. It also has remarkable social, economic, and environmental benefits.

Compaction grouting is a ground improvement technique that involves injection of a thick-consistency soil-cement grout under high pressure into the soil mass, consolidating, and thereby densifying targeted soil in-place. This technique is often used to lift or re-level any settled or tilted structure. Inclined grout injection is necessary to perform grouting below any existing foundation. For an efficient inclined grouting operation, prior estimation of heave at the ground surface is important. In this study, we estimate the upheaval of the ground surface just above the inclined grout column based on the elastic continuum approach. Influence factors are derived for the estimation of ground heave. Using these influence factors, vertical displacements due to the radial pressure exerted by the grout column are estimated at different points. A parametric study is performed to investigate the effects of grout column length, depth of injection of grout and inclination of the grout column on the heave at the ground surface.

New opportunities for utilizing biological processes to modify the engineering properties of the subsurface have recently emerged at the interface of microbiology, geochemistry, and civil engineering. This paper presents an overview of bio-mediated soil improvement systems in the context bio-mediated calcite precipitation of sands. Micro-scale and macro-scale investigations of microbial-induced calcite precipitation (MICP) identify fundamental material properties and mechanical characteristics of bio-cementation. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) techniques reveal micro-scale calcite formation and degradation characteristics. Calcite minerals are found predominantly near the silica sand grain contacts, strengthening particle bonds. Fracturing of the cemented matrix is found to occur within the calcite phase. A 1-g scaled shallow foundation model is developed to evaluate how MICP treated sands may improve the performance of geosystems at the macro-scale. MICP treatment targets a passive treatment zone of loose sand directly beneath a footing. The spatial distribution of calcite within a sand matrix at the micro-scale translates to a reduction in settlement (or increase in load capacity) at the macro-scale. Comparison of load tests on the footing for MICP treated and untreated experiments reveals up to a five-fold reduction in foundation settlement.

The total pullout resistance of geogrid in a soil includes two components: (1) the interface friction resistance that takes place along longitudinal ribs and (2) the passive resistance that takes place against the front of transverse ribs. A series of pullout tests and analyses were conducted to quantify the contributions of the above two components to the overall pullout resistance. It was found that the mobilized passive resistance is much higher than the friction resistance under the same test conditions. Furthermore, the values of these two components are influenced by both pullout displacement and normal stress to a large extent.

Drinking water treatment plants annually produce significant amount of lime sludge during water purification procedures. They are typically stored in lagoons and disposed as solid waste. This presents a significant economic burden to daily operations. This study investigates a sustainable development strategy from beneficial utilization of this material. An experimental program is implemented to address the technical issues related to application of this material in the construction of soil embankment, where lime is commonly used as soil stabilizers. Specific topics associated with the engineering applications of lime sludge treated soils include: the effects of dry versus wet mix procedures for the introduction of lime sludge into soils, procedures to determine the optimal content of lime sludge, and the long term durability of lime sludge treated soils under freezing-thawing cycles. This study aims to help develop a sustainable strategy to utilize lime sludge for routine construction activities.

The conventional slurry trench methods of constructing low-permeability vertical barriers have been widely used, namely, soil-bentonite (SB), cement-bentonite (CB) and slag-CB. More recently, in situ mixing methods to construct vertical barriers, such as Deep Soil Mixing (DSM) and the Trench Remixing and Deep wall method (TRD) have found increased usage. The TRD method was has been widely employed in Japan for nearly 15 years but has only recently been used in the United States. The TRD method is a one-phase process that involves the simultaneous, full-depth cutting and mixing of in situ soils with additives to create, in place, a continuous soil mixed wall. A blend of cementitious materials (granulated ground blast furnace slag and Portland cement) and slurry are injected as the milling/cutting proceeds horizontally resulting in continuous vertical mixing of in situ soils with the injected materials, without creating the open trench typical of other walling methods. To date, three TRD projects have been completed in the United States. The focus of this paper is the most recent project completed on the Herbert Hoover Dike (HHD). Other projects include 1) the first TRD project in the United States where the method was used to construct a test cell under saline ground water conditions for use as a soil mixed barrier wall to prevent the intrusion of salt water into fresh ground water aquifers and 2) a cutoff wall to prevent leakage of secondary treated sewage from a containment pond into surrounding ground water.

This paper presents a case study using dynamic compaction to improve fill in a mountainous area in China. This fill is composed of weathered rock and soil with grain sizes mostly between 5 and 200 mm, but with the largest size over 500 mm. The fill, which has been dumped and placed without compaction over the past 2 years, ranges from 2 to 8 m thick across the site. The bearing capacity of this fill was about 70 to 100 kPa. Dynamic compaction consisting of two high-energy passes and two low-energy passes were applied to improve the ground. The compaction energy of each high-energy pass was 3000 kN⋅m, and the grid spacing was 5 x 5 m. The compaction energies of the first and second low-energy passes were 1500 kN⋅m and 1000 kN⋅m, respectively. After the dynamic compaction, plate loading tests, field density tests, ground penetrating radar explorations, and other tests were performed. The tests show that the ultimate bearing capacity after improvement is over 440 kPa, the average secant elastic modulus is 34.50 MPa, the depth of influence of the dynamic compaction is at least 6 m, and the relative compaction is more than 95%. These values satisfied the design requirements for a building that was constructed on the fill after dynamic compaction. The building's foundation elements consist of reinforced concrete strip footings. The observed settlement of the building became stable rather quickly after construction, and the total building settlement was only 12 mm. Dynamic compaction effectively improved the fill in this mountainous area in China.

Explosion has been used in several soil improvement methods in granular fill densification and in removing and replacing soft soil layer. These methods are briefly reviewed and discussed. A new method, the so-called explosive replacement method, is described. The new method uses energy of explosion to remove and replace soft clay with crushed stones. Explosive charges are placed in the soil to be improved according to a certain pattern. Crushed stones are piled up next to the area where charges are installed. The explosion creates cavities in the soil and causes the pile of crushed stones to slide into the exploded area. A case study on the use of this new method to the soil improvement works for a highway is present.

In areas where river sediments are thick, construction of earth dams by excavating these sediments and replacing with compacted fill causes significant cost increases. River sediments consists of, in terms of soil mechanics, either granular (sand, gravel) or fine-grained (silt, clay) materials. Granular sediments can be improved with relative ease against instability and compressibility; however, because such deposits are highly permeable, additional measures are required to prevent seepage loss through the dam foundation. Fine-grained deposits, if they do not contain extensive sand or gravel lenses, offer fewer problems relative to seepage loss and may not require seepage control measures. However, such materials typically have low strength and high compressibility and present stability and settlement concerns. The objective of this article is to present examples of earth dams founded directly over fine-grained river sediments. Available methods for improvement of highly compressible, low strength deposits are limited. Staged construction (although it is time-consuming, it can be accelerated to reasonable levels with prefabricated vertical drains) and column-supported earth fill (although more expensive it enables accelerated construction) can be cited as two main approaches for this purpose. Because it is generally more economical, the design principles of staged construction and the relevant methods of analysis are presented herein. Alibey Dam for slow staged construction and Hamzadere dam (not yet constructed) for accelerated staged construction are cited as examples of this approach. The results of analyses and observations indicate that earth dams can be constructed directly on soft fine-grained river sediments with appropriate ground improvement.

The vacuum preloading method has been used in a number of countries for soil improvement and land reclamation works. Prefabricated vertical drains are normally used together with the vacuum preloading technique. When a surcharge load larger than the vacuum load is required, a combined vacuum and fill surcharge preloading method is also adopted. The mechanism of the vacuum preloading method is explained by examining the pore water pressure and effective stress changes in both the fill surcharge and vacuum preloading cases. Two case studies are presented to illustrate the applications of the vacuum preloading method and the combined vacuum and fill surcharge preloading method. Discussions on the effective depth of the vacuum preloading method and the methods to estimate the degree of consolidation are also made.