Raja Sekhar Madhyannapu

Quality Assessment and Quality Control of Deep Soil Mixing Construction for Stabilizing Expansive Subsoils

This paper presents the process and results of a quality management program performed during and immediately after the construction of two deep soil mixing (DSM) test sections. The quality management program consisted of laboratory, in situ, and mineralogical tests to address the effectiveness of the treatment during and after construction. In situ investigations including the down-hole seismic and spectral analysis of surface waves (SASW) test methods were performed to evaluate the degree of improvement achieved through the measurement of compression and shear-wave velocities of the columns and surrounding soils. Scanning electron microscopy and electron dispersive x-ray analysis were performed on raw, laboratory treated and field-treated specimens for qualitative understanding of the degree of mixing achieved in the field and the compounds formed at particle level during stabilization, respectively. Laboratory tests results on field cores indicated that both field stiffness and strength are about 20 to 40% less than the corresponding laboratory prepared soil samples. The down-hole seismic and SASW tests showed considerable improvement in stiffness in and around the DSM columns. Mineralogical studies indicated the formation of silica and alumina hydrates along with interwoven structure of lime-cement treated clay particles in both laboratory and field specimens, suggesting adequate mixing of the soil and binder in both environments.

Laboratory Procedure to Obtain Well-Mixed Soil Binder Samples of Medium Stiff to Stiff Expansive Clayey Soil for Deep Soil Mixing Simulation

This paper presents a laboratory procedure to prepare well-mixed soil binder samples for simulation of deep soil mixing to stabilize medium stiff to stiff expansive clayey soils. Two natural clays were selected, and stabilized with lime, cement, and combinations of both at various proportions and dosages. Results obtained from tests conducted on identical specimens showed that the current soil-binder mixing and specimen preparation procedures have yielded homogenous and uniform treated clayey specimens. Engineering properties measured on the treated soil specimens were analyzed and ranked to arrive at the optimum binder dosages for field implementation. Concurrent mineralogical studies on selected specimens using X-ray diffraction and scanning electron micrograph studies revealed the presence of pozzalonic compounds and interwoven threads in the treated samples indicative of the mixing and stabilizing phenomenon. The recommended chemical dosages were later implemented in a field deep mixing study to construct two test pads. Several in situ wet grab samples were collected from the treated ground and were subjected to strength and stiffness tests in the laboratory. Comparisons between test results from field cores and laboratory fabricated specimens showed a good match suggesting that the formulated test protocol has provided a reasonable simulation of the DM process.

Design and Construction Guidelines for Deep Soil Mixing to Stabilize Expansive Soils

AbstractThis paper discusses both design methodology and construction procedures for stabilizing expansive subsoils of moderate active depths, using deep soil mixing (DSM) technology/construction. These procedures were derived as a part of the research focusing on the evaluation of effectiveness of DSM technology in mitigating shrink-and-swell behaviors of expansive subsoils under actual field conditions. The design methodology formulated was based on an analytical model proposed for a DSM-treated composite section by modifying the existing heave prediction model for untreated and unsaturated expansive soils. The required area treatment ratio was determined based on the target heave magnitude for the composite section. Design charts were developed depicting estimated heave for increasing treatment area ratios and for various initial swell pressures. Based on this design methodology, DSM construction was implemented under actual field conditions at two test sites. Upon construction, both test sections were...

Small strain shear moduli of lime-cement treated expansive clays

Deep mixing treatments with lime, cement and lime-cement mixtures have been used to stabilize soft and compressible soils, provide excavation support, mitigate liquefaction problems and stabilize steep embankments and slopes. One of the soil properties needed in the characterization of the treated soils is small strain shear modulus, which is used to evaluate the quality control of deep mixing in field conditions, seismic treated site characterization, and numerical modeling of earth structures placed on treated soil columns. A laboratory research study has evaluated small strain shear moduli properties of lime-cement treated expansive soil using Bender Elements. This paper presents these results obtained on both untreated and lime-cement treated soils. Lime-cement treatment was performed by simulating a deep mixing rotary application. Enhancements of small strain shear moduli, G max , with respect to lime-cement dosage levels and curing conditions were addressed. Comparisons of these results with shear moduli results obtained from field DM samples were made to address the quality control issues of field deep mixing application.

Soil velocity profiles from in-situ seismic tests at deep-mixing sites

The spectral analysis of surface wave (SASW) and the downhole P- wave velocity tests were carried out to characterize the treated and untreated soils at two deep-mixing (DM) sites located on the median of an interstate highway in Texas. These tests are part of a research project evaluating the effectiveness of deep-mixing technology as applied to expansive soils to mitigate the pavement distress caused by subgrade soil swell-shrink movements. Ground treatment for the two sites was performed by mixing in-situ soil with cement and lime to produce DM columns. Results from the initial SASW tests conducted shortly after site construction are consistent with those from shear-wave velocity tests on the laboratory specimens. Results from SASW tests performed one year later indicate certain decreasing in shear-wave velocity for both of the two sites. Results from third-year SASW tests are similar to those from the second-year tests except for the saturated near surface portion. By assuming a proper Poissons ratio, results from P-wave downhole tests are also quite consistent with those for SASW tests.

Closure to “Design and Construction Guidelines for Deep Soil Mixing to Stabilize Expansive Soils” by Raja S. Madhyannapu and Anand J. Puppala

The writers would like to express their appreciation to the discusser for the comments provided on the original paper. Following are the responses to the comments by the discusser. As noted by the discusser in Item 1a of the discussion, predrilling of deep soil mixing (DSM) locations prior to soil mixing was necessary and achieved using a flight auger of diameter similar to the one used for the construction of DSM columns [0.6-m (2-ft) diameter]. Due to the stiff to very stiff nature of unsaturated in situ soils, spoil produced was minimal during the predrilling process. During retrieval of flight auger, few of the loosened soil lumps were reintroduced into the predrilled DSM hole prior to soil mixing step. The extra material shown in Fig. 12 in the original paper was produced during actual soil mixing process and it shows the low amounts of spoil produced during the field mixing operations. The primary objective of this research was to understand the behavior of a treated composite expansive soil test section as a whole system subjected to seasonal climatic changes. Though the writers did not specifically study the zone of influence of soil mixing beyond the DSM columns, the following explanation on the potential zone of influence of DSM columns in the expansive soils is offered. Previous studies based on the laboratory tests on soft soils noted that the zone of influence was close to four times the diameter of the DSM columns. The present expansive soils, on the other hand, are stiff to very stiff in nature and also highly impervious. Hence, the writers opine that the zone of influence due to soil mixing in expansive soils will be less than the ones observed in soft soils. The discusser mentioned the potential heave-related protrusion from isolated rigid inclusions into the soil mass. The writers do not agree with this assessment and offer the following reasons explaining that the heave movements less than the allowable magnitude are anticipated within the treated composite section. Differential soil movements are more prevalent in soft soils than in expansive soils. In present design methodology, the treated composite section is designed based on the treatment area ratio determined using an allowable heave of less than 25 mm for the composite section. Thus, the differential swelling anticipated within the treated area is expected to be less than 25 mm and potentially insignificant. The uplift resistance of DSM columns primarily depends on the depth of treatment, strength and stiffness characteristics of subsoils, and zone of influence beyond the diameter of the DSM column. Chemical modification is the primary method of treatment for expansive soils because alteration of physicochemical properties from chemical reactions result in reduced or insignificant swelling. Also, the placement of a geosynthetic layer over the treated area was to provide additional mechanical reinforcement to transfer any induced swell pressures from untreated soils to treated columns through the inverse of arching behavior. All these factors contribute to minimizing the potential heave across the treated composite area. The writers agree with the discusser in extending the anchoring system into the pavement shoulder area. Further research studies are also recommended on deep soil mixing treatments of expansive soils, which would help in addressing issues such as zone of influence, load transfer mechanism within the composite section, and others.