Proceedings of Softsoils 2014, October, 21-23rd 2014 ASSESSMENT OF THE STRENGTH OF CEMENT-STABILIZED PEAT H. Hashimoto 1, H. Hayashi 2, T. Yamanashi 3 and S. Nishimoto 4 ABSTRACT: Peat covers an area of approximately 2,000km2 in Hokkaido, Japan, where a consolidation method using stabilizers including cement is essential to build road embankments. This paper will discuss important points in carrying out laboratory tests to determine the kinds and amounts of stabilizers necessary to meet the requirements of design strength for peat soil. A chief focus is placed here on the Trencher Mixing Method that is one of the most typical techniques to treat peat soil in Hokkaido, for which the variability of on-site strength of 28-day old materials, their proportion to optimized laboratory strength and on-site strength, and quality-control methods for solidifying and improving Hokkaido peat will be discussed. Keywords: peat, the Trencher mixing method, laboratory mixing method, laboratory strength, on-site strength. INTRODUCTION difficult to project the strength of cement-stabilized soil with high accuracy only from ground investigations Peaty soft ground, which consists of surface peat before construction, because peat is characterized by overlying cohesive soil, is found in Hokkaido, Japan. such properties as high water content, high organic Peaty soft ground is defined as soil with an organic matter content, and inconsistency of these properties. For content of 20% or more accumulated on a soft cohesive this reason, in order to determine the most cost-effective sub-layer maintaining a high underground level. Peat has kinds and amounts of soil improvement materials to be the engineering characteristics of extremely high water added, it is very important to conduct laboratory mixing content, extremely high ignition loss, extremely high tests prior to construction and figure out kinds and void ratio and remarkably low shear strength, which amounts of reasonable stabilizers that satisfy mostly necessitates the addition of soil improvement requirements of the laboratory strength. However, materials to facilitate consolidation when embankments because the mixing efficiency varies among different are constructed on peaty soft ground. methods, the relationship between the design (on-site) The purpose of the consolidation work is to improve strength and laboratory strength changes and variations the shear strength and compressibility of soft ground by of prepared cement-stabilized soil differ. Thus, those chemically consolidating the ground. However, it is factors need to be fully comprehended beforehand. WAKKANAI HORONOBE KUSHIRO TOYOKORO HIDAKA Fig. 1 Investigation spots and distribution of peat ground in Hokkaido, Japan (Noto, 1991) 1 Research Engineer, Civil Engineering Research Institute for Cold Region, 1-3 Hiragishi Toyohira-ku, Sapporo, JAPAN 2 Senior Research Engineer, ditto 3 Team leader, ditto 4 Director, ditto Tab. 1 Properties of the improved ground HORONOBE HIDAKA WAKKANAI TOYOKORO KUSHIRO Peat Organic Clay Clay Peat Volcanic Sand Peat Cohesive soi Peat Cohesive soi Peat Cohesive soil Layer thickness m 3.25 1.75 0.5 1.4 1.55 0.9 1.3 2.8 1.8 0.95 2.35 Wet density g/cm3 1.029 1.214 1.672 0.952 0.961 1.000 1.320 1.048 1.531 1.097 1.111 Dry density g/cm3 0.120 0.372 1.081 0.113 0.563 0.214 0.639 0.203 1.055 -- Soil particle density g/cm3 1.542 2.484 2.68 1.563 2.08 1.685 2.535 1.834 2.506 1.987 2.577 Natural water content % 755.1 226.4 54.7 730.7 62.6 308.9 105.7 290.1 55.6 361.2 89.3 Void ratio 11.850 5.677 1.479 12.792 2.697 6.896 2.994 8.042 1.377 -- Saturation degree % 98.3 99.1 99.1 90.8 54.6 90.2 91 94.8 82.3 -- Ignition loss % 93.4 -- 82.3 - 75.5 11.4 58.4 4.2 33.0 - pH --- 4.9 5.9 5.0 6.1 5.7 6.5 -- Liquid limit % - 113.6 50.7 - NP - 113.6 - 49.8 - 74.8 Plastic limit % - 43.2 24.9 - NP - 43.2 - 35.0 - 41.9 Plasticity index - 70.4 25.8 - NP - 70.4 - 14.8 - 32.9 Tab. 2 Data of the improved ground Improved ground (28-day old materials） Water-cement Number of Improved Survey site Desigh strength Cement type Amount added ratio samples thickness Ratio between Peat and other kN/m2 kg % m HORONOBE 100 For highly organic soil 200 80 52 5.5 5.9 : 4.1 HIDAKA 750 For highly organic soil 350 80 25 10 1.4 : 8.6 WAKKANAI 160 For highly organic soil 76 100 26 4.1 2.9 : 7.1 TOYOKORO 200 For ordinary soft soil 155 100 24 6.0 4.7 : 5.3 KUSHIRO 200 For highly organic soil 221 100 51 3.3 2.8 : 7.2 OUTLINE OF THE SITE SURVEYED of SAC (=(SO3+Al2O3)/CaO) contained in cement solidifier for highly organic soil is relatively higher than Site Condition that of ordinary Portland cement, and its compounding ratio differs among manufacturers. The survey was carried out in five sites within In each survey site ground stabilized by the Hokkaido where peaty soft ground was improved by the Trencher Mixing Method, a prescribed thickness of Trencher Mixing Method (Figure 1). The Trencher improved soil was sampled with a triple-tube core device. Mixing Method will be detailed later. Table 1 shows Those samples were cut in length twice the sample properties of the improved ground. All of them are diameter to make specimens on which an unconfined characterized by peat surface and accumulations of compression test (JIS A 1216) was carried out on the cohesive soil and organic clay underneath. The sites 28th day after execution at a normal temperature in a soil surveyed were substantially organic, moist and testing room. compressive with thickness of peat layer t=0.5 to 3.25m, ignition loss Li=33.0 to 93.4%, natural water content Consolidation Method w=290.1 to 755.1%, and void ratio e=5.677 to 11.850. These physical properties of peat are typical to Hokkaido. The Trencher Mixing Method is a mechanical mixing method for stabilizing soil properties by applying Tested Items improvement material to in-situ soil and inducing a chemical reaction. Characteristics of the method include Table 2 shows data of the improved ground in the low vibration, low noise and applicability to many types survey sites, where improvement works were given with of soil because of its wide setting range of improved 2 the design strength of quck=100 to 750kN/m for the strength. The method is applicable to the improvement purpose of stabilization of embankments or as a of shallow (1.0 to 3.0 m) and medium (3.0 to 10.0 m) fundamental measure for reinforced soil wall. The type layers, and has overall, strip, zigzag, reticular and other of soil improvement material used in most cases was construction patterns (Fig. 2). The main advantage of cement solidifier for highly organic soil. The proportion the Trencher Mixing Method is its excellent maneuverability. The improvement equipment is based on a backhoe (0.8m3 to 1.9m3 class) and can be used for LABORATORY MIXING TEST FOR CEMENT- a variety of sites. Construction is possible in limited STABILIZED SOIL IN JAPAN spaces, very soft or sloped ground and other sites experiencing difficult conditions for conventional Cement-stabilized soil needs to satisfy the required mechanical mixing methods. Figure 3 shows a standard shear strength, rigidity, and homogeneity to maintain construction system. stability of embankments against sliding failure. Factors In this construction method, ground is improved by that affect the strength of improved peat soil are: (1) stirring cement milk at a slurry plant and pumping it into original properties of peat such as natural water content, the trencher section at the tip of a backhoe. The Trencher organic matter content, and complexity of soil layer of base-machine moves back and forth and up and down structure; and (2) attributes of improvement materials while rotating until its tip reaches the predetermined such as kinds, amounts, age, curing temperature and depth of improvement (Figure 4). mixing method. The accurate strength of cement- stabilized soil is difficult to be figured out only from ground investigations before construction and it is essential to execute laboratory mixing tests (where unconfined compressive strength is investigated by changing kinds, amounts and curing period of Trencher mixing method Surface treatment method improvement materials to determine cost-effective kinds and amounts of materials). Deep mixing method Laboratory Mixing Test In Japan, laboratory mixing tests for cement- stabilized soil are carried out in accordance with Fig. 2 Compaction between the Trencher Mixing Method standards (JGS-0821) set up by the Japanese and other methods Geotechnical Society. A flow chart of the laboratory Construction management system mixing test is shown in Figure 5. The amount of improvement material to be added is expressed with the 3 3 backhoe mass (kg/m ) of improvement material to 1m of humid silo soil with natural water content Most laboratory mixing tests for Hokkaido peat are conducted by using three kinds of improvement Tanker truck Slurry plant materials (blast-furnace slag cement type B, cement Trencher mixing method solidifier for ordinary soft soil, and cement solidifier for highly organic soil) and three mixing amounts. The Fig. 3 Standard construction system specimen making and curing procedures are detailed in Section 4.2. The curing periods are 7 days and 28 days of material age. When a laboratory mixing test is carried out, it is important to set up a laboratory mixing strength.