Performances of SDCM and DCM walls under deep excavation in soft clay: Field tests and 3D simulations

This study presents the results of full-scale tests and three-dimensional finite element analyses of deep cement mixing (DCM) and stiffened deep cement mixing (SDCM) columns under lateral loads and DCM and SDCM walls under deep excavation in soft clay. The DCM walls used in this study comprised one, two and three rows of DCM columns, whereas the SDCM walls consisted of only one row of DCM columns with steel H-beams inserted in either all DCM columns or in alternating DCM columns. The measured and simulated results are presented in terms of profiles of lateral displacement, settlement and bending moment.     

Performance of the bearing reinforcement earth wall as a retaining structure in the Mae Moh mine,Thailand

In this research, a Bearing Reinforcement Earth (BRE) wall with a residual clay stone backfill was successfully implemented as an alternative truck ramp support for an on-site crusher plant in the Mae Moh mine, Thailand. The performance of the BRE wall during and after the end of construction as well as during the service state was evaluated in terms of, settlement, bearing stress, lateral movement, lateral earth pressure and tension force in the reinforcements. Bearing reinforcement is a cost-effective inextensible earth reinforcement, which is composed of a longitudinal member and transverse members. The maximum settlement at the end of construction (20 days) was about 5 mm. The installation of the truck ramp (10 days after the end of construction) resulted in an immediate settlement of about 2 mm. The final settlement due to the backfill, truck ramp and truck load after 270 days was found to be uniform due to the contribution of bearing reinforcement and was approximately 25 mm. The bearing stress which was uniformly distributed was found to increase rapidly with construction time, which was in agreement with the relatively uniform settlements. The lateral wall movement at the front and lateral sides at the end of construction was very small with the maximum movement (at the top of the wall) found to be less than 10 mm. As such, the ratio of lateral movement to height (δ/H) was found to be approximately 0.12%, which was lower than the allowable value of 0.4%. With this low δ/H and the insignificant change in the measured settlement and lateral movement during service, the BRE wall was considered to have a very high stability. The coefficients of lateral earth pressure, K and depth relationship were proposed based on the analysis of measured maximum tensile force in the reinforcements. The maximum tension plane of the BRE wall could be represented by the coherent gravity hypothesis. Using the proposed K and maximum tension plane, the internal stability of the BRE wall was furthermore examined. A proposed method of designing the BRE wall with claystone backfill was also proposed.     

Performance of an earth wall stabilized with bearing reinforcements

This article presents the performance of a fully instrumented test wall reinforced with bearing reinforcement. Bearing reinforcement is an inextensible earth reinforcement. It is composed of a longitudinal member and transverse members. The longitudinal member is a deformed steel bar and the transverse members are a set of equal steel angles. The test wall was 6 m high, 9 m long at the top, 6 m wide at the top, and 12 m long, 21 m wide at the base and was constructed on a hard stratum. The facing panels were made of segmental concrete block which measured 1.50 × 1.50 × 0.14 m in dimension. From the full-scale test results, the bearing stress distribution is a trapezoid shape as generally assumed for the examination of the external stability of MSE walls. The tilt of the bearing reinforcement earth (BRE) wall indicates that the BRE wall behaves as a rigid body. The coefficients of earth pressure decrease with depth and approach the active state at deeper reinforcement level. From the variation in the stiffness factor as a function of depth and lateral earth pressure, the bearing reinforcement has a stiffness factor of K/K_a = 1.7, which is much lower than that of steel grids and metal strips. The lower tension (coefficient of lateral earth pressure) reduces the cross-sectional area of the longitudinal members and hence cost effectiveness. The maximum tension line (possible failure plane) of the BRE wall is bilinear, similarly to the coherent gravity structure hypothesis, which is commonly used for the analysis of inextensible reinforcements. Finally, the suggested method of designing the BRE wall is presented. It has been successfully used to design several BRE walls founded on the hard stratum in different areas in Thailand.     

Environmental impacts of utilizing waste steel slag aggregates as recycled road construction materials

Slag is an industrial waste generated during the steelmaking process. Electric arc furnace slag (EAFS) and ladle furnace slag (LFS) are both produced at different stages of steelmaking process, respectively, in electric arc furnaces and refining ladle furnaces. As part of this research, an extensive suite of engineering and environmental tests were undertaken on steel slag aggregates to evaluate their potential usage as road construction materials. The engineering assessment included particle size distribution, hydrometer, organic content, flakiness index, Atterberg limits, particle density, water absorption, pH, minimum and maximum dry densities with a vibrating table, modified compaction, California bearing ratio (CBR) and Los Angeles abrasion tests. In addition, a suite of environmental tests comprising total and leachable heavy metal tests were undertaken on both types of steel slag aggregates. From an environmental perspective, EAFS and LFS were found to pose no environmental risks for use as aggregates in roadwork applications. The engineering properties of LFS aggregates with its satisfactory geotechnical and environmental results, particularly its high CBR values, indicated that the material was ideal for usage as a construction material in roadwork applications such as pavement base/subbases and engineering fills. EAFS, with its comparatively lower CBR value, was found to be only suitable to use as a construction material for pavement subbases and engineering fills. The usage of steel slag aggregates in roadwork applications would bring about a practical end-of-life alternative for their sustainable reuse and possibly divert large amount of these waste materials from landfills.     

Compaction behavior of fine-grained soils,lateritic soils and crushed rocks

This paper studies compaction characteristics and California Bearing Ratio, CBR values of fine-grained soils, lateritic soils and crushed rocks. All test data were collated from the Bureau of Rural Road 6, the Department of Rural Roads, Thailand. The Ohio's and the modified Ohio's curves can predict satisfactorily the compaction curves of the fine-grained soils, and lateritic soils and crushed rocks consistent with the grade B of the American Association of State Highway and Transportation Officials (AASHTO) requirement. The CBR value of a specific soil is directly related to the relative dry unit weight (the ratio of dry unit weight to maximum dry unit weight, γ_d/γ_d,max). The field compaction result of a fine-grained soil at the optimum water content, OWC, shows that initially the dry unit weight increases rapidly with the number of roller passes and the relationship between dry unit weight and number of roller passes is represented by the logarithm function. Finally, the dry unit weight reaches a constant value, which is close to the laboratory maximum dry unit weight. Even with a large number of roller passes (compaction energy), the dry unit weight cannot be enhanced further because the soil state approaches the zero air void state. In practice, the excess roller pass is thus not economic. Based on the analysis of the test data, the field compaction procedure for road embankment and pavement constructions, which includes the material selection and the construction control, is suggested. It is useful in terms of both engineering and economic viewpoints.     

An Approach for Assessment of Compaction Curves of Fine Grained Soils at Various Energies Using a One Point Test

Compaction curves of soils are essential for establishing practical and reliable criteria for an effective control of field compaction. This paper deals with the development of a practical method of assessing laboratory compaction curves of fine-grained soils. It is found that for a given fine-grained soil compacted at a particular compaction energy, the relationships between water content (w) and degree of saturation (S) are represented by power function, which are w=A_dS^B_d and w=A_wS^B_w for the dry and the wet sides of optimum, respectively (where A_d, A_w, B_d and B_w are constant). The B_d and B_w values and optimum degree of saturation (ODS) are mainly dependent upon soil type irrespective of compaction energy. The A_d and A_w values decrease with the logarithm of compaction energy and the decrease rates are practically the same for any compacted fine-grained soil. This leads to a simple and rational method to assess the compaction curve wherein the compaction energy varies over a wide range using a one point test (a single test). Assuming that fine-grained soils compacted under standard Proctor energy behave in agreement with Ohio's curves, the modified Ohio's curves for the other three compaction energy levels (296.3, 1346.6 and 2693.3 kJ/m³) are developed based on the proposed method. These curves can be used to assess the entire compaction curves at the required compaction energy based on a single set data of dry unit weight and water content.     

A Case History on Underpinning for a Distressed Building on Hard Residual Soil Underneath Non-Uniform Loose Sand

This paper presents a case history on the failure of Suranivet 9, a student dormitory in the campus of Suranaree University of Technology (SUT), Thailand. The dormitory encountered excessive differential settlement due to the variation in soil profile. Part of the building was underlain by very stiff to hard SUT silty clay and part by loose clayey sand. Underpinning to extend the foundations down to stable stratum was employed to strengthen bearing capacity and minimize settlement. The underpinning design and procedure were summarized. In practice, the static formula was used for the preliminary micro-pile design (selection of pile section and length for different loads and soil profiles). The undrained shear strength (S_u) of SUT silty clay was approximated using the Stress History and Normalized Soil Engineering Properties (SHANSEP) technique and standard penetration number (N). The finite element method was employed to predict the load-settlement curve of the micro-pile. Modified cam clay model was proved as a suitable model for this prediction. The measured settlements of the underpinned foundations after one year service were less than 0.5 mm. This small settlement guarantees the stability of the underpinned structure. It is also found that the settlement ratio (ratio of the measured settlement of underpinned foundations to the predicted settlement of single micro-pile) varied from 0.7 to 3.0.     

Effects of industrial by-product based geopolymers on the strength development of a soft soil

Portland cement is traditionally used as a binder in ground improvement projects on soft soil foundations. The use of cement in ground improvement projects, however, is fraught with both, financial and environmental concerns due to its relatively high cost, the use of natural resources and the high carbon footprint from cement production. Attempts are being made to find alternative environmentally friendly binders with a low carbon footprint using industrial by-products such as fly ash (FA) and slag (S). Using waste by-products such as FA and S to produce geopolymer binders, as novel green cementitious materials, may provide an environmentally friendly and effective ground improvement option. In this study, the effect of adding geopolymers to a soft soil was investigated for usage in deep soil mixing (DSM) applications. The soil was a soft marine clay known as Coode Island Silt (CIS). Different combinations of FA and S with six combinations of sodium and potassium based liquid alkaline activators (L) were added to the soil to study the effects on its engineering and chemical properties. These changes were evaluated via an unconfined compression strength (UCS) test, scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy (EDS) tests. The tests were conducted after 3, 7, 14 and 28?days of curing. Based on the results, the important role of L in strength development was studied, and the combination of 30% NaOH with 70% Na₂SiO₃ was found to achieve the highest strengths. Furthermore, increasing the S content was found to result in significant improvements in strength. The excellent correlation between strength and stiffness shown in the results are expected to help in the development of relationships for strength prediction of these green binders in geotechnical applications. This study shows that FA and S based geopolymers can be used as sustainable binders in DSM projects, with significant environmental benefits.     

Pullout Resistance of Bearing Reinforcement Embedded in Sand

Mechanically stabilized earth (MSE) structure has been widely accepted as a retaining structure. Its construction cost is mainly controlled by backfill materials, which are generally coarse-grained soils, and reinforcement type (steel volume). The present paper introduces a new cost-effective reinforcement, designated as “Bearing Reinforcement”. It is composed of a longitudinal member and transverse (bearing) members. The longitudinal member is made of a deformed bar, which exhibits a high pullout friction resistance. The transverse members are a set of equal angles, which provide high pullout bearing resistance. The maximum pullout bearing resistance of a single isolated transverse member, σ_bmax, can be determined by using the plasticity solution based on the modified punching shear failure mechanism. Influential factors governing the mobilization of pullout bearing resistance are spacing, S, leg length, B, and numbers, n of transverse members. The larger the S/B, the lower the transverse member interference. The S/B ratios of <3.75 and >25 are referred to as full and free interference, respectively. The relationship between normalized average pullout bearing stress, σ_bn/nσ_n and pullout displacement, d, where σ_bn/n is average pullout bearing stress of the bearing reinforcement with n transverse members and σ_n is applied normal stress, is practically identical for the same level of transverse member interference. This relationship can be modelled by hyperbolic function. From this finding, a suggested procedure for estimating pullout characteristics (maximum pullout resistance and pullout force versus displacement relationship) of the bearing reinforcement for any level of transverse member interference (any S, B, and n) based on a one point test on the bearing reinforcement with a single isolated transverse member is proposed. Good agreement has been obtained between the predicted and the measured pullout characteristics. This suggested method is useful for the internal stability analysis of MSE wall in terms of engineering and economic viewpoints.     

Strength development in silty clay stabilized with calcium carbide residue and fly ash

Calcium carbide residue (CCR) and fly ash (FA) are waste products from acetylene gas factories and power plants, respectively. The mixture of CCR and FA can produce a cementitious material because CCR contains a large amount of Ca(OH)₂ while FA is a pozzolanic material. Soil stabilization by CCR is classified using three zones: active, inert and deterioration. In the active zone, the natural pozzolanic material in the soil is adequate to produce a pozzolanic reaction. Hence, the input of FA into this zone does not significantly improve strength. Strength in the inert zone can be significantly increased by adding FA. FA improves the densification and pozzolanic reaction. The deterioration zone is not recommended for use in practice, even with the input of FA. The unsoundness due to free lime hinders strength development. Although the soaked and unsoaked strengths depend mainly on the CCR and FA contents, most of the ratios of soaked strength to unsoaked strength vary between 0.45 and 0.65. It is proved that a mixture of CCR and FA can be used for soil stabilization instead of ordinary Portland cement. The possible mechanism regarding the control of strength development presented in this paper can be applied to other clayey soils stabilized with different cementitious materials produced from Ca(OH)₂-rich and pozzolanic materials. This putative mechanism is also fundamental for further studies involving the development of rational dosage methodologies.