The research used the seismic synthesis angle to consider the effectiveness of ground improvement work against overturning and sliding during seismic conditions on test specimens that had ground improvement implemented on the back face of the L-type retaining wall (L/H = 0.7). A comparison was made between a conventional L-type retaining wall with no ground improvement and three test specimens with different ground improvement profiles. The main items discovered in this research were as follows.
 1) The trend in the safety factor for overturning on test specimens with ground improvement was that the highest values were on the [θ = 30°] test specimen followed in order by the [θ = 15°] and [θ = 0°] specimens. Furthermore, as the base angle increased (θ1 = 0° to 15°), the safety factor for overturning decreased on all three test specimens and the safety factor when there is slip line occurrence showed almost the same value (1.6 to 1.7).
 2) The safety factor for sliding on test specimens with ground improvement was [θ = 30°] > [θ = 15°] > [θ = 0°], in the same order as the safety factor for overturning. Furthermore, the safety factor trended below that for overturning on all of the test specimens and the safety factor when there is slip line occurrence was around 1.0 on all three test specimens.
 3) The angle ω_T formed between the earth surface and the slip line on test specimens with ground improvement was close to the Mononobe-Okabe theoretical value ω and showed just a slightly smaller value on all of those specimens.
 4) The lateral seismic factor k_h when there is slip line occurrence was greater than the ultimate limit state lateral seismic factor k_h=0.25 on all of the test specimens.
 5) The lateral seismic factor k_h when there is slip line occurrence on the test specimens with ground improvement was the largest on the [θ = 30°] test specimen followed in order by the [θ = 15°] and [θ = 0°] specimens. In the same way, the k_h on the [θ = 30°] test specimen was higher than on the [L/H = 0.7] test specimen and the values for the [θ = 0°] and [θ = 15°] test specimens were smaller than for [L/H = 0.7].